IMPACT RESISTANCE Introduction Impact resistance is a measure of the ability of a material, specimen, or structure to withstand a sudden load without failure. The impact resistance of a specimen or structure is therefore a complex function not only of intrinsic factors such as the mechanical properties of the material but also of extrinsic factors such as geometry, mode of loading, load application rate, environment, and, quite importantly, the definition of failure. The issue of stress geometry or mode of loading is all-important: a material highly resistant to failure in one mode of loading can fail catastrophically in another. Therefore, the impact resistance of a structure is relevant only with respect to a particular mode of loading. A soundly designed structure can fail unexpectedly if the geometry of a structural component is altered by the gradual change of shape, material wear, or crack growth, or if the mode of loading is changed, as when a force is applied in an unforeseen manner. Thus, it is often necessary to test the material or structure under conditions more severe than those of actual use. It is also prudent to follow certain well-established design guidelines. Last but not least, the use of damage tolerant materials may contribute to the avoidance of catastrophic failure. Impact often implies a high loading rate, but the type of failure usually associated with impact can also occur under apparently innocuous conditions, even at low rates of loading. In addition to high strain rates, low temperatures and sharp notches tend to reduce fracture resistance. When the rate of strain energy accumulation is higher than can be contained in or dissipated by the material in the vicinity of a crack or a flaw, sudden and unstable fracture can occur, often entailing crack speeds of hundreds of meters per second. Higher 528 Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
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speeds are more likely to cause unstable fracture, other conditions being equal, for then even small flaws, less severe geometries, or milder temperatures can become critical. This is partly due to the viscoelastic nature of polymers. Under truly high speed conditions, ie, when inertial effects are involved, the deformation and fracture behavior are complicated by wave propagation and constraint effects (1–4). In contrast to impact resistance, the fracture toughness of a solid polymer is much better defined for a given set of quasi-static low strain rate testing conditions (see also FRACTURE; FATIGUE and ASTM D5045-99). Toughness can often be improved by the incorporation of a soft, elastomeric phase into the rigid polymer matrix. The art and science of material modification for the purpose of toughening is extremely complex (5). Enhancement of toughness improves impact resistance. The performance of a material or a structure also depends on local variations due to processing conditions, such as cooling rate, shear stress, and melt-flow paths, which result in orientation and residual stress. Multiphase materials can exhibit even greater local variations in material properties than those observed in single phase polymers. A structure produced by a given process may possess significant morphological differences from test specimens. Ultimately, it may be necessary to test the impact resistance of components. Since the impact toughness of a material, as it is commonly called, depends on the technique of measurement, the testing instruments and the technique must be considered first.
Testing Machines and Techniques Several types of machines are in use for different types of tests, offering advantages and disadvantages as well as different information. Impact strength is measured by many different empirical methods, some of which may not be appropriate for the performance evaluation of finished products. More sophisticated and meaningful testing devices are under development but remain mostly in research laboratories. The following is a description of the machines and techniques that have gained wide currency. However the foregoing should serve to warn the reader to be cautious about the validity of the results for applications. For this and other reasons a number of standardized test methods are used to approximate actual use conditions. Standard Test Methods. Many impact tests are sufficiently standardized to have ASTM designations (see Table 1). Pendulum-Type Instruments. Pendulum-type machines are used for notched or unnotched specimens that may be of different sizes and supported as a cantilever (Izod) or as a bar supported at its ends (Charpy). These tests are also referred to as flexed-beam impact tests. The Izod impact test is based on an old, established test originally designed for metals in which a notched-bar specimen is tested in cantilever fashion with an excess energy pendulum machine. Izod (ASTM D256), Charpy (ASTM D6110 and Research Report D20-1034, to become a new standard), and the tensile-impact (ASTM D1822) tests can be performed with pendulum machines.
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Table 1. Standard Tests for Impact Resistancea Test
Designation
Brittleness temperature
ASTM D746
Falling weight
ASTM D3029
Falling weight
ASTM D1709
Falling weight
ASTM D2444
Fracture toughness High rate stress/ strain (tension)
ASTM D5045 ASTM D2289
Izod impact
ASTM D256c
Charpy impact
ASTM D6110 (also, Research Report D20-1034)
Tensile impact
ASTM D1822
a Ref.
Description The temperature is determined at which plastics and elastomers exhibit brittle failure under impact. Impact resistance indicated by energy to break or crack rigid plastics by means of a falling weight (tup); constant drop height and variable weight (tup)b are recommended. Similar to D3029 but for measuring impact resistance of polyethylene film by free-falling round-headed dart. For impact resistance of thermoplastic pipe and fittings by falling weight (tup).b For plane-strain fracture toughness. Area under stress–strain curve measures impact resistance at testing speeds up to 254 m/min. Energy to break a notched cantilever beam specimen upon impact by a pendulum. Notch tends to promote brittle failure. Unnotched impact strength is obtained by reversing the notched specimen in the vise. Notch sensitivity can be determined by using Method D. Similar to Izod impact test. Notched specimen is supported on two ends and struck by a pendulum in the middle, a three-point-bend setting. Recommended for plastic materials too flexible, too thin, or too rigid to be tested by ASTM D256. Measures energy to break by “shock in tension” imparted by a swinging pendulum.
6.
b Designation c Includes
of the weight. Charpy impact testing.
In these machines a mass is attached to the end of an arm that rotates about a pivot point; a striker is attached to a mass. The arm is raised to a predetermined position. When the arm is released, it swings downward and strikes the specimen, which is mounted rigidly in a vise or a fixture. The precise mounting position and the type of fixture depends on the test. The mass, length of the arm, and angle at the raised position determine the amount of available energy. The total energy must be high enough to give a complete break. Typical pendulum machines of the Izod and Charpy types are depicted in Figures 1 and 2, respectively; specimen geometries are shown in Figure 3. After the striker breaks the specimen at the bottom of its swing, it continues to swing upward until all the kinetic energy is
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Point of impact
90o
Fig. 1. Izod-type pendulum impact machine, ASTM D256.
converted to potential energy. The energy absorbed by the specimen, Es , is found by ES = EI − ER − EF − Ew where EI is the initial energy available, ER the energy at the maximum angular travel, EF the energy dissipated by friction, and Ew the kinetic energy to remove the broken, detached half-specimen as it is thrown forward by the pendulum. The last one is usually small; EF can be found by performing the test without a specimen in place. Machines are usually equipped with a dial with calibration marks mounted in the plane of rotation in such a way that EI and ER can be read directly. An “instrumented” pendulum machine is equipped with a load cell, usually a piezoelectric quartz device for high frequency response, mounted on the striker or on the mounting block to which the specimen vise is attached. The electric signal from the load cell is amplified, digitized, and recorded by electronic memory. The energy is calculated from the load–time relationship by a computer connected to the instrument. The total energy can be displayed by a digital readout, or the
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90o
Fig. 2. Charpy-type pendulum impact machine, ASTM D256.
entire load–time trace can be displayed on a cathode-ray tube screen. In the latter instance, a microcomputer is attached and the information can be processed as desired. The Izod test is used mostly in the United States and the United Kingdom [British Standard (BS) 2782, Method 306A]. The notched specimen is firmly clamped in a vertical position in a vise fixed in the base of the apparatus (Fig. 1). A pendulum fitted with a striker head of 3.2-mm radius, falling from a height of 0.61 m (ASTM) or 0.31 m (BS), hits the specimen horizontally at a point above the notch. At least 10 samples are required to obtain a satisfactory assessment of impact strength. The U.S. test method has now been accepted as an International Standardization Recommendation ISO/R180, 2000. The material from which the test pieces are cut must be prepared under carefully controlled conditions to ensure development of full strength and freedom from strain caused by uneven heating or rapid cooling. The cutting of the notch requires special care, particularly with notch-sensitive materials. Small variations in the notch can give wide variations in impact values. The notch should be cut with a tool in good condition and shaped smoothly and accurately to the dimension called for in the test specification. Errors are caused
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Fig. 3. Geometry of testing and specimen support arrangement for (a) Charpy and (b) Izod tests specified in ASTM D256 (7). Courtesy of Applied Science Publishers Ltd.
by variations in clamping pressure, failure to strike the specimens squarely, and the condition of the cutter and the cutting techniques for machining notches (8,9). Impact strength decreases linearly with increasing clamping pressure. A taper of 13 min of an arc on the specimen induces errors up to 25% of the expected impact strength of a perfectly parallel specimen. Variations in excess of 10% in expected impact strength were observed from differences in the notch cutter. Torsional stresses caused in the Izod specimen by skewness between the pendulum striker head and the surface of the specimen increase impact strengths even for small angles of skewness (Table 2). Table 2. Effect of Skew Striking on Impact Strength of Poly(methyl methacrylate) Sheeta,b
Angle of skewness 10 50 1◦ 10 4◦ 50 5◦ 10 a Ref.
Impact strength, J/mc of notchd at clamping pressure 16.5 MPae
49.6 MPae
26.05 27.75 28.02 29.78 29.78
22.42 25.78 26.95 29.36 29.52
8.
b Perspex,
Imperial Chemical Industries. convert J/m to ft·lbf/in., divide by 53.38. d British standard. e To convert MPa to psi, multiply by 145. c To
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Table 3. Energy for Crack Initiation and Propagation for Standard Cast PMMAa,b
ASTM BS a Ref.
Energy, %
Notched impact strength, J/mc
Crack propagation
Crack initiation
Energy ratiod
17.08 24.02
69 49
31 51
2.22 0.96
8.
b Perspex,
Imperial Chemical Industries. convert J/m to ft·lbf/in., divide by 53.38. d Crack propagation to crack initiation. c To
The total breaking energy as measured by the Izod test comprises the following: energy to initiate fracture; energy to propagate fracture across the specimen; energy for plastic deformation; energy to throw the broken fragment away from the test piece; and energy lost through friction and vibration (9). The energy to initiate and propagate fracture depends largely on the notch geometry. The energy for plastic deformation is important in materials that, even when notched, break by ductile failure. The energy to separate the broken fragments is important in materials of low impact strength and high density, eg, mineral filled phenolic resin. The effect is corrected by placing the fragment on the striker head of the machine, striking it again, and subtracting the energy absorbed from the original apparent impact strength. However, in being thrown forward the fragment may adopt different modes of movement, and may rotate only under the first strike. For high impact, low density materials, this source of error is less important. Points of increased stress or of stress concentration can be introduced during machining; their effect is less for the ASTM specimen than for the BS specimen (8). As can be seen in Table 3, in the ASTM test the energy required for crack propagation in PMMA is more than twice of that required for crack initiation; in the BS test they are about equal. Therefore the ASTM test results are much less likely to be affected by the specimen preparation than are BS test results. The BS specimens are more susceptible to imperfections in the notch surface and to the direction of machine marks (ie, stress-concentrating imperfections). Moreover, since a machined surface is never as smooth as a molded surface, molded (notched) specimens are expected to give higher values. This has been confirmed with specimens prepared from PMMA moldings where molded notch values are higher than machined notch values by a factor of 1.8. In addition, specimens with molded notches, especially those from crystalline polymers, exhibit higher impact strength than those with machined notches because of the difference in morphology or thermal stresses due to rapid cooling near the surface and slower cooling in the interior. In the Charpy impact tester the specimen is mounted on a span support and struck centrally with a swinging pendulum (see Fig. 2). The results are expressed in terms of breaking energy per unit of cross-sectional area. The Charpy pendulum impact test is most commonly employed in the European continent. Both the standard test bar and the small standard test bar are specified in the German Standard DIN 53453. The sample is notched and is
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supported horizontally against the stops at either end. The pendulum striker hits the sample centrally behind the notch and the excess energy of the pendulum is measured by the angle of the subsequent swing. A large number of tests is necessary to give an average value. The Charpy-type test used in the United States is ASTM D6110 (Research Report D20-1034); in the United Kingdom, it is BS 2782, Methods 306 D and E. Although the Charpy impact test is accurate and reproducible, significant errors may occur (10). Testing and machine variables tend to produce abnormally high values, since they retard the pendulum swing or otherwise cause excess energy losses. The recorded energy may be in error by as much as 100–200% at low energy and 15–20% at high energy levels. Differences are usually caused by the condition of the test machines; methods of machining and finishing the specimens; and cooling and testing (5). With strict controls and carefully prepared specimens, an average impact value should be accurate to within 5% for values up to 27.1 J (20 ft·lbf). Pendulum-type machines are inexpensive and simple to use and maintain. Data can be produced from which fracture toughness GIc (more correctly known as the critical strain-energy release rate in Mode I) can be calculated (see FRACTURE and ASTM D5045-99 and later in this article). However, these machines are limited in the range of impact speeds, with a maximum of 3.4 m/s. Furthermore, the common practice of reporting only the total energy does not allow easy interpretation of the data, which can be overcome by using the instrumentation described previously. The use of instrumentation as described previously is often helpful for this purpose. Falling-Weight Impact Instruments. Falling-weight methods are usually employed for sheet specimens. The weight may be gradually increased in mass, or dropped from increasing heights on the same specimen or onto a series of specimens. Like the pendulum tester, the machine is driven by gravity. The specimen, ordinarily in the form of sheet, is subjected to a direct blow from a falling weight. The weight is raised to a known height and is allowed to fall almost freely inside a guide tube or along a set of guide rails (see Fig. 4). A striker is mounted beneath the weight or in a guide resting on top of the specimen; the Gardner impact tester, usually preferred, is of the latter type. In recent years drop towers of considerable height have become commercially available. However, as the terminal velocity depends on the square root of the drop height, the maximum velocity that can be conveniently obtained is limited. In the case of the drop tower an arrangement similar to that described for the instrumented pendulum impact tester is necessary to determine the load applied to the specimen and the terminal velocity. Since the maximum velocity achieved with this type of machine is considerably higher than that available from the pendulum-type machine, vibration and inertial effects can interfere with the recording of the load signal. Therefore all supporting components must be as rigid as possible, and the piezoelectric load cell assembly must be of low mass and high rigidity to produce a high natural frequency. In addition to the load, the velocity of the falling weight must be determined. Frictional effects can result in significant errors when calculating the velocity by assuming free fall. The velocity must therefore be measured directly by a linear array of, for example, photoelectric sensors positioned near the end of the fall, just above where the specimen is struck. An accelerometer attached to the falling weight
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Vol. 6 Control housing
Hoist chain Height limit switch
Release latch housing
Hammer
Guide post (2)
Support post (2)
Height indicator scale
Brake switch Stop block (2)
Base Anvil specimen support area
Fig. 4. Typical drop-weight testing machine (11). Courtesy of ASTM.
measures load and velocity separately. The electric signal from the accelerometer is recorded digitally as before. The absorbed energy and the force are calculated from the change in velocity caused by the impact. The results are obtained by processing the acceleration signal in the computer. The accelerometer must be mounted near the tip of the striker to minimize interference from reflected shock waves. An accelerometer is preferred over a load cell at high test velocities. In the falling-weight test (BS 2782, Method 306B) also known as a dartdrop test or Gardner impact test, a spherical ball striker, 12.7 mm in diameter, is fastened to a load-carrying device to which weights can be attached. The striker assembly slides freely in vertical guides and is released from a predetermined height to strike centrally on a specimen, which is supported on the base of the equipment. In the so-called staircase method the load is increased by fixed increments on successive specimens, each struck once, until a specimen fails. The weight is then reduced by fixed increments until a specimen withstands the impact, after which it is increased again by fixed increments until a specimen fails again. This procedure is repeated on at least 20 samples to determine the energy
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Drop Impact, height t, J m 2.74
366
2.64
352
2.54
339
2.44
325
2.34
312
2.24
298
2.13
285
2.03
271
1.93
258
1.83
244
1.73
230
1.63
217
F50 = 268 J
o x
o o
x
o x
x
o x
o o
o
o o
x x
x
x x
Failure
o
o x
o
o
o x
x
537
x x
o x
o o
o x
o x
o o
x x
o x
x x
o x
o x
No failure
9
0
14
9
2
14
0
2
Fig. 5. Typical results of a dart-drop test using the “staircase” method of analysis (12). To convert J to ft·lbf, divide by 1.35. Courtesy of the Society of Plastics Engineers.
level at which 50% of the specimens break; the result is quoted as the impact strength for 50% failure (F 50 ) (see Fig. 5). An important source of uncertainty in this test is the criterion used to determine failure. Sometimes failure is obvious, as when the specimen shatters; other times failure is indicated by a small crack. As this is usually a visual test, whether or not failure has occurred depends on the judgment of the testing technician. In the Probit method (13), a set of energy levels for the falling weight is selected. A series of specimens is tested at each level, and the failure–impact energy relation determined. With the help of a detailed failure–impact energy curve a simple control test can be carried out by testing a small number of specimens at a single fixed energy level. This procedure is far more reliable and economical than the F 50 values. In the common drop-weight test, force and displacement are not recorded by electronic instruments. On the other hand, in fully instrumented procedures a weight–height combination at which the specimen is consistently broken is found by trial and error, or by choosing a height that produces a desired terminal velocity and adjusting the weight to break the specimen consistently. A number of drop tests are then performed under identical conditions. Each test is recorded by an electronic instrument. The exact number of tests required depends on the divergence of the data and the confidence limit desired. This method requires fewer specimens. The velocity can be varied as desired (within the limits of the tester), and an entire set of data can be obtained at the same velocity. The deformation behavior, including the point of crack initiation, can in many cases be deduced from the load–deflection curve, reducing operator errors. Furthermore, the ability of a material to decelerate a moving mass can be assessed. Many materials that give low Izod impact values on notched specimens fail in a ductile manner when tested in sheet form. Notched impact strength and the impact properties of sheet are not correlated because the stress states and the material responses differ. The cause of failure of sheet material in practice is more likely to be revealed by a falling-weight impact test than by an Izod test (14). Falling-weight tests thus
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give a better guide to practical behavior unless the molded article contains ribs, sharp corners, and similar geometry. Any anisotropy present, such as excessive orientation leading to weakness in a given direction, is revealed in the test. For example, an oriented injection-molded specimen may fail with a single straight crack at a low energy, whereas a substantially unoriented compression molded specimen of the same material would fail with cracks in several directions after having withstood much higher impact energies. In falling-dart testers impact is produced by means of a pointed weight or dart. This method assesses the toughness of plastic film under certain specified conditions, such as sample geometry, dimensions of the dart head, and velocity. In one such apparatus, a perfectly flat specimen is clamped by a vacuum and held in uniform tension. The flat test film is placed over the specimen holder and kept wrinkle-free by the vacuum. The dart is automatically released from an electromagnet as soon as the correct pressure differential is reached. Darts of various weights are dropped, and the percent failure is plotted against dart weight on probability graph paper. A straight line is drawn through the plot. The intersection of the line with 50% failure is the weight in grams for F 50 impact failure. ASTM D1709 gives a formula for the calculation of impact strength without a graph. In the examination of tubular film the edge fold as well as the face of the film may be tested. The U.S. specification for the falling-dart method (ASTM D1709-01) refers specifically to polyethylene film, whereas the U.K. standard (BS 2782, Method 306F) covers all flexible plastic film. In an equipment modification two electromagnetic counters are provided: one records the number of specimens tested and the other, which is operated by a push switch, the number of breaks. For tests of thick film, a clamping ring is provided in addition to the vacuum clamping to prevent film slippage, which is especially important in this case. In both specifications the dart, which has a 38-mm-diameter hemispherical head, falls freely 0.66 m to strike the specimen. The weight of the dart is adjusted with loose weights to determine the limit required to fracture 50% of the specimens. Other more recent versions are commercially available, usually equipped with digital recording devices and computer analysis of the results. Tensile-Impact Instruments. A standard Izod impact-testing machine, a variation of the swinging pendulum, is easily adapted at little cost to tensile impact testing. The conventional Izod vise is replaced by one capable of holding the fixed end of a tensile specimen and a metal crosshead is secured to the other end of the specimen (see Fig. 6). The swinging pendulum strikes the crosshead removing both the crosshead and half of the specimen. Development of this instrument led to the specification ASTM D1822-99 (tensile impact energy to break plastics and electrical insulating material). This method can be used to measure the impact resistance of test pieces cut from actual moldings, and help to assess directional differences in stress that arise from flow-induced molecular orientation produced during mold filling. It is also very useful for the evaluation of very small samples. The energy required to produce failure can be obtained by varying the height of fall or the mass of the falling weight or by measuring the excess swing of the pendulum. Transducers and electronic instruments can provide load–deflection curves. The results show reasonable correlation with the falling-weight test of film and with the performance of moldings and sheet. Tensile impact instruments require less material than the usual falling-weight impact test. At relatively low
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Tossed crosshead clamp Anvil
Pendulum
Anvil Pendulum
Pendulum head
Anvil Crosshead clamp
(a)
(b)
Fig. 6. Tensile impact machines. (a) Specimen in base; (b) specimen in head.
speeds they often give good correlation with falling-weight tests. This correlation is due to the fact that in both tests the tensile mode of stress is generated by the testing geometry. The falling-weight test, however, generates a biaxial tensile stress that can more easily give rise to failure. This is a consequence of the higher component of hydrostatic tensile stress in the biaxial tensile test, which favors the cohesive mode of failure. Special Test Machines. Hydraulically and pneumatically driven machines have found increasing use in research and quality control laboratories; ASTM standards have not yet been established. These machines require higher operator skills and are more expensive, but offer better control and produce more information. Hydraulic and pneumatic impact testing machines differ in the way in which the impact energy is derived, in the amount of energy and maximum velocity that can be developed, and in the amount of deceleration suffered by the striker. A hydraulic fluid or a gas under high pressure is stored in an accumulator. Upon receiving an electrical signal, a valve is opened, and the fluid or gas is transferred to a cylinder, wherein a low friction piston is connected to the load application device. The pressure difference between the two sides of the piston causes acceleration to a high velocity. Different designs in the valve and pressure return arrangement, and, increasingly, high speed digital electronics and signal processors allow the velocity to be controlled to varying degrees of success. The maximum load capacity of the testing machine depends on the pressure of the fluid, piston area, and pressure drop in the valve and upon expansion. The maximum velocity depends on the pressure, the flow rate allowed by the valve, the size of the accumulator, the inertia of the moving mass, and the distance allowed for acceleration. Commercial hydraulically driven impact testers are now capable of generating a maximum speed of 22 m/s at a maximum load of 900 kg (see Fig. 7); different designs are available including computers for control and data acquisition and analysis. Possible tests include tensile impact, puncture (flexed-plate) impact, and flexed-beam impact. A pneumatic machine is shown in Figure 8; anvils for various types of impact tests are shown in Figure 9. The high compressibility of gases means that the striker could decelerate significantly if the force required to deform the test specimen is high.
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Fig. 7. Hydraulic high speed impact tester. (a) Specimen enclosed in environmental chamber; (b) hydraulic drive unit at the end of the rails for component testing. Courtesy of Instron Corp., Canton, Mass.
A principal advantage of hydraulic pneumatic machines employing feedback control is that a higher fixed velocity can be maintained. Much depends, of course, on the specifics of the design. In pendulum machines the velocity upon striking the specimen is initially constant at 2.4 m/s in the Izod test, and 3.4 m/s in the Charpy test. After the specimen is struck the velocity decreases, depending on the amount of energy absorbed. This problem is more serious in tough specimens. In drop-weight tests, the velocity upon impact depends on the drop height, and yet the test results often report only the product of weight and drop height, ignoring the impact velocity. Since polymeric materials are rate sensitive in their mechanical behavior, tests using gravity-driven machines could produce misleading results because the terminal velocity just prior to striker contact with the specimen could vary widely. The use
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Pneumatic cylinder Three-way viewing through transparent safety panels Position sensor Door sefety interlock Height 210 cm
Force transducer Adjustable stripper plate
Impactor
Anvil
Adjustable anvil height Firing button
Pressure regulator impactor velocity control
Impact absorbing feet Stabilizer
Fig. 8. Pneumatic impact tester. Various types of impactors, anvils, and force transducers are available for different applications. Courtesy of ICI Australia Operations Pty Ltd., Dingley, Victoria, Australia.
of the special machines described here avoids such problems. (This test standard has been discountinued in 1992.) Product Testing. Standardized specimens are specially prepared for falling weight tests, swinging-pendulum tests, tensile impact tests, and others. However, frequently results from such tests fail to predict the performance of actual fabricated components in service. For this reason, the conventional forms of impact testing are more useful as control or material comparison tests. This limitation has become more serious in recent years with the rapid introduction of many new polymers and their blends. One reason for the difference in performance is the fact that the fabricating process can influence impact strength. A typical example is the effect of flow-induced orientation in thin-walled moldings. The flow behavior of the material during the filling of the mold may be more important than the basic impact properties of the unoriented material in governing the impact behavior of the thin-wall moldings. Specimen geometry is also important because the stress state and rate of stress buildup are governed by the size and shape of the article. Therefore the impact strength of the finished product may need to be tested under appropriate conditions in addition to the usual control and quality tests.
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Izod
Flat plate
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Charpy
Plaque test
Film test
V block
Fig. 9. Accessory anvils for gravity, hydraulic, and pneumatic testers. Courtesy of ICI Australia Operations Pty Ltd., Dingley, Victoria, Australia.
Mechanical Behavior Tensile Impact. The tensile impact test subjects a specimen to uniaxial tension at a strain rate of approximately 102 s − 1 . This strain rate is 3–6 orders of magnitude higher than that encountered in conventional uniaxial tension testing (7,15,16). The effect of strain rate on uniaxial tensile behavior depends on the scale of deformation and the temperature range. At small strains the tensile modulus E increases with strain rate, but the magnitude of change depends on the temperature range. If the relaxation process is significant, eg, the glass transition or a strong secondary relaxation occurs near the test temperature, the modulus would also be rate dependent. The change can be about a factor of 2 for a secondary relaxation, and an order of magnitude or more for a temperature somewhat below T g . If no significant relaxation occurs near the test temperature, E can be unaffected by the strain rate. At room termperature, PMMA has a strainrate senstitive modulus whereas PC has not, although at sufficiently high rates strain-rate sensitivity develops. At strains beyond 1–2%, some materials become brittle even at low strain rates. An example is commercial PS, which typically contains small amounts of foreign material that serves as nucleation centers for craze initiation. Pure PS without surface flaws may be susceptible to shear yielding; increasing strain rate reduces flaw tolerance. If the material is not brittle enough to fracture, it eventually reaches a yield point defined as the maximum in the load–elongation curve. This yield point is not necessarily due to shear yielding. Impact-modified polystyrene, for example, exhibits a yield point independent
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100 90
Nylon-6,6
Teflon FEP
Delrin Lexan
Teflon TFE
80 Poly(vinyl chloride) True stress σ, MPa
70 60 Polypropylene Polyethylene
50 40 30 Polypropylene 20 TFE 10 FEP
0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 True strain ε
Fig. 10. True stress–true strain behavior of polymers. Delrin is an acetal resin; Lexan, a polycarbonate; Teflon TFE designates the tetrafluoroethylene homopolymer; Teflon FEP is a copolymer of tetrafluoroethylene and hexafluoropropylene (17). To convert MPa to psi, multiply by 145. Courtesy of Polymer Engineering and Science.
of shear yielding. If the load maximum is due to shear yielding, further straining results in a neck because of strain softening (16). Some crystalline polymers do not exhibit a maximum in the load–elongation curve but merely a knee because of the strengthening effect of the spherulites. Yield strength usually increases with strain rate (15,16). The strain-rate sensitivity of the shear yield stress behaves in much the same fashion as the modulus; it also depends on a significant relaxation process near the test temperature. After neck formation the strain-rate sensitivity stabilizes and neck extension occurs (16). On the other hand, thinning of the neck results in ductile fracture, which can also be due to high strain rates. Examples of uniaxial tensile true stress–true strain behavior of polymers deformed at low strain rates are shown in Figure 10. Common engineering stress– strain curves cannot be used to determine the elongational strain because the shape for a given set of testing conditions depends on the specimen geometry. Some polymers show ductile deformation after yield by a process of necking and cold drawing. The apparent neck extension depends on the length of the specimen gauge section if the conventional method of calculating engineering strain ε = l/l0 is used, where l is the elongation and l0 the original gauge length. The true value of the extensional strain can be ascertained only by direct measurement of the strain in the necked section. If the specimen dimensions are identical and
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Load
0
Displacement
Fig. 11. Typical load and energy data from tensile impact test on polypropylene using an instrumented pendulum tester. The dotted line shows the accumulated energy as the deformation increases. Courtesy CEAST SpA, Torino, Italy.
the extents of necking comparable, the apparent maximum engineering strain can be used as a relative ductility index. Ductility usually decreases with strain rate or decreasing temperature. At a given strain rate and temperature, ductility depends on molecular weight and amount of filler or impurity in the material. Stresses applied orthogonal to the principal stress direction can also affect ductility. Tensile impact toughness, a measure of the total energy absorbed by a dogbone-shaped specimen in uniaxial tension, is the integrated value of the typical force–elongation curve. It is highly sensitive to the ductility of the material at the testing rate. The tensile impact behavior of polypropylene is shown in Figure 11. The curve is typical of a pendulum impact tester equipped with transducers and digital storage instruments. The strain-softening behavior is clearly discernible even at such a high strain rate. The advantage of electronic instrumentation can be seen immediately. Flexed-Plate Impact. In flexed-plate impact, a ball, dart, or striker is driven by gravity or pressure into a thin plate supported by or clamped over an annular area. The nose of the dart is usually hemispherical, though sometimes square. A flex-plate impact test corresponds closely to conditions of actual use. However, geometric factors, such as specimen thickness and dart-to-annular opening ratio, cannot be readily standardized, and furthermore, the geometry itself can cause a change in the deformation mode (14). The progression of deformation processes associated with the load–deflection curve can be seen in Figure 12. In Figure 12A the specimen failed in a ductile manner and the plastic deformed extensively around the dart nose. The remaining unsupported material was cold drawn. Partially deformed specimens and finite-elements analysis reveal yielding
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A
Load
B
C
Displacement
Fig. 12. Typical load–deflection data from puncture impact tests. (A) ductile failure; (B) cracking started at about 50% of the maximum load; (C) brittle failure (18).
under the dart early in the deformation process, at about 25% of the maximum load. This initial yielding is marked by a slight decrease in the slope. Subsequently a yielded zone in the shape of a hemispherical dome develops around the dart nose. As deflection continues, the net area undergoing yielding also increases, which results in geometric hardening, ie, the load increase is due to the change in specimen geometry as described above. The processes of material softening and geometric hardening eventually combine to result in a slight increase in slope. At this stage of the deformation the unsupported area of the specimen is akin to that of a membrane rather than a plate. The stress in the membrane is essentially uniaxial tension in the radial direction. Finally, excessive thinning takes place somewhere in the specimen, the exact location being dependent on the geometry, and rupture causes the load to drop precipitously (18,19). Thus, the total energy absorbed, which is the area under the load–deflection curve, depends in a very complex manner on both the testing geometry and the yield and deformation behavior of the material. The curve of Figure 12B is obtained when the specimen develops a crack during the membrane-stretching process. The sudden release of elastic energy upon cracking causes the load-cell assembly to vibrate at its natural frequency and results in the “ringing” signal. The crack is stabilized by the
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specimen geometry and does not propagate. Consequently the specimen is able to carry an increasing load even though, by some definitions, the specimen would be considered to have failed (18). The curve in Figure 12C is obtained when the specimen fails in a totally brittle manner; no decrease in the slope of the load– deflection curve is discerned. Cracking occurs on the tension side opposite the area where the dart or striker contacts the plate. Little or no plastic deformation occurs anywhere on the specimen. Brittle fracture of this type occurs more readily in thick plates even for materials normally considered to be ductile. This observation serves to warn against treating such test results as material properties. Clearly, the test specimen and specimen holding fixture contribute importantly to the test results. In addition to plate thickness, decreasing temperature, increasing deformation rate, or environmental aging can transform ductile-type failure (Fig. 12A) to brittle-type failure. In the case of a ball or a hemispherical dart impact test, it is commonly assumed that the resultant stress state at the center of the plate is biaxial tension. This assumption is erroneous because the normal force on the plate is significant and could lead to over optimistic values on ductile-to-brittle transition conditions, whether expressed as a critical temperature or aging time. This behavior is caused by normal stress suppressing brittle failure (15,18,20). Flexed-Beam Impact. Flexed-beam impact tests are sometimes carried out on unnotched specimens, though notched specimens are preferred. The Izod and Charpy tests are examples of flexed beam tests. These two testing geometries are different, but both types of notches create a severe stress concentration. Many materials that are tough in the tensile and flexed-plate impact tests become brittle in notched flexed-beam tests because the stress state near the base of the notch favors a brittle response due to the high component of hydrostatic tensile stress mentioned above. For a given specimen thickness, the impact toughness depends on the notch tip radius (see Fig. 13). Subjecting a thin, notched specimen to bending creates a tensile stress concentration along two orthogonal directions: the main one along the specimen length, the other parallel to the notch or crack direction. The magnitude of the former depends on the notch sharpness. The minor stress is due to constraints created by notch-free sections of the specimen, which are at a lower stress. Because the specimen is thin, it contracts freely in the thickness direction; a state of plane stress is said to exist in this instance. The biaxial tension state is more favorable than the uniaxial tension state for craze or crack formation because of the larger hydrostatic or dilatational stress component in the former case. Many polymers that are ductile in uniaxial tension exhibit brittle behavior in this stress state; other polymers are able to shear-yield. Shear yielding creates a large plastic zone at the notch tip, and subsequent failure is due to ductile tearing of the material. This mode of failure usually absorbs a large amount of energy. Some impact-modified materials are able to form a plastic zone ahead of the notch containing of a high density of crazes. Considerable energy can be absorbed by this mechanism. The load–deflection behavior of tough plastics is illustrated in Figure 14. Subjecting a thick notched, notch-sensitive specimen to bending creates a tensile stress component that is perpendicular to the specimen faces. This third stress is due to constraint in the thickness direction because the material ahead of
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Fig. 13. Impact strength as a function of notch tip radius for different polymers (21). To convert kJ/m2 to ft·lbf/in.2 , divide by 2.10. POM = polyoxymethylene. Courtesy of The Plastics and Rubber Institute.
the notch is unable to contract freely in this direction; the constraint effect is said to be due to plane strain. The triaxial tension state is even more favorable than the biaxial tension state for the formation of crazes, again because of the larger dilatational stress component in the former. With sufficiently large thickness and sharp cracks few unmodified polymers are able to deform in a completely ductile manner owing to the intrinsically low cohesive strength of polymers relative to their yield strengths. The fracture is due to craze formation and rapid crack propagation. At intermediate thicknesses or notch radii the specimen can deform in mixed mode: plane-strain in the specimen center and plane-stress near the surfaces. In the plane-stress region, plastic deformation zones are often found. Their size determines the amount of energy absorbed by the specimen; the plane-strain zone, by contrast, absorbs little energy.
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500
Supertough nylon 400
HIPS
Force, N
300
Polycarbonate
200
ABS 100
0 0
5
10
15
20
Displacement, mm
Fig. 14. Load–deflection behavior of a number of tough polymers tested in the notched Charpy geometry at low rates (22). To convert N to kgf, multiply by 0.102. Courtesy of the Society of Plastics Engineers.
The impact toughness of a material, as measured by a flexed-beam test on a notched specimen, depends on the thickness of the specimen and the radius of the notch, and therefore cannot be considered a material property. Furthermore, the transition from predominantly plane-stress to predominantly plane-strain is unpredictable when complex geometries are encountered in structural components. Because this transition can create unexpected and catastrophic failures, some polymers normally considered to be very tough and ductile may benefit from impact modification. The modified versions are less sensitive to geometric variations. The transition from plane-stress to plane-strain can also be brought about by impact rate (23), temperature (Fig. 15), molecular weight, and thermal history (24). The effects of thickness and rate are shown by an example in Figure 16. In this example notched polycarbonate (PC) specimens are tested at various bending speeds. The thick (6.4 mm) specimens are uniformly brittle, whereas the thin (3.2 mm) specimens are uniformly ductile. The intermediate thickness (4.4 mm) specimens exhibit a ductile-to-brittle transition at about 0.3 cm/s. In toughened PC even the 6.4-mm-thick specimens are tough up to about 40 cm/s (see Fig. 17). Fracture Toughness from Flexed-Beam Impact Tests. As stated above, the so-called impact toughness values obtained from Izod and Charpy tests are not material properties because they depend on specimen thickness, notch depth, notch radius, and other factors unrelated to material properties. These
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40
Unnotched
Impact strength, kJ/m2
30
ρc = 2 mm
ρc = 1 mm
20
10
ρc = 0.25 mm
0 −80
Tb −60
−40
−20 0 Temperature, °C
20
40
60
Fig. 15. Effect of test temperature on the impact strength of PVC specimens containing various notch radii (21). T b = brittle temperature. To convert kJ/m2 to ft·lbf/in.2 , divide by 2.10. Courtesy of The Plastics and Rubber Institute.
values are therefore useful only for the comparison of different materials, and are useless for design calculations. By contrast, fracture toughness (ASTM D5045-99), which is a material property, is useful in design calculations. Consequently it is desirable to have fracture toughness values that are taken under impact conditions, and especially if they could be obtained from the Izod or Charpy test machines. The fracture toughness GIc can be determined from the impact energy U if the geometry of the specimen is known (25) and if the specimen fails in plane-strain. To determine if the latter condition is satisfied one could examine the fracture surface. If the top edges of the fracture surface remain in the planes of the sides of the specimen and no sign of large-scale plastic drawing in front of the notch tip is observed then this condition is probably satisfied. Another signature for planestrain fracture is the load–deflection curve, which would have the shape of a right triangle. This points out the desirability of using an instrumented tester. In this analysis it is assumed that linear elastic fracture mechanics (LEFM) applies. When a specimen containing a sharp notch of depth a is subjected to
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800 700
Fracture energy, J/m
600 500 400 300 200 100 0
10−1
100
101
102
cm/s
Fig. 16. Effect of test speed on the fracture energy of polycarbonate specimens of various thicknesses tested in the notched Charpy geometry. ◦, 3.2 mm; , 4.4 mm; , energy up to load maxima for 4.3 mm; , 6.4 mm (22). To convert J/m to ft·lbf/in., divide by 53.38.
bending with a load P, a deflection x results. The energy U stored is given by U = Px/2
(1)
When the crack grows, compliance C = x/P increases as the stored energy is released. Thus, if it is known how C changes with a the strain-energy release rate G can be calculated from equation 1
U = P2 C/2
(2)
For a specimen of uniform thickness B, G=
1 dU 1 dU dC = B da B dC da
(3)
When fracture occurs, the critical strain-energy release rate Gc is calculated: Gc =
P2 dC 2B da
(4)
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800
Fracture energy, J/m
700 600 500 400 300 200 100 0
10−1
100
101
102
cm/s
Fig. 17. Effect of test speed on the fracture energy of an impact-modified PC as in Figure 16. ◦, 3.2 mm; , 4.4 mm; , energy up to load maxima for 6.4 mm (22). To convert J/m to ft·lbf/in., divide by 53.38.
In an impact test, usually only the energy U is determined. Combining equations 2 and 4 gives U = Gc B
C dC/da
(5)
In practice, U is determined for a series of specimens with various notch depths a. It is therefore more convenient to express equation 5 in terms of Gc =
U dC BDC d(a/D)
(6)
where D is the specimen width. The dimensionless compliance function φ=
C dC/d(a/D)
can be determined experimentally from quasi-static bending tests, and has been calculated analytically for this geometry (26). Equation 6 can then be rewritten as U = Gc BDφ
(7)
predicting that a linear relationship should exist between the impact energy U and BDφ, the slope of which should be equal to Gc . G can then be obtained by
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Table 4. Strain-Energy Release Ratesa Gc , J/m2b Material Polystyrene General purpose HIPS PVC Darvic 110 Modified PMMA Nylon-6,6 Polycarbonated PE Medium densitye High density f Low density ABS Lustran 244
Charpy
Izod
0.83 × 103 15.8 × 103c
0.83 × 103 14.0 × 103c
1.42 × 103 10.05 × 103 1.28 × 103 5.30 × 103 4.85 × 103
1.38 × 103 10.00 × 103 1.38 × 103 5.00 × 103 4.83 × 103
8.10 × 103 3.40 × 103 34.7 × 103
8.40 × 103 3.10 × 103 34.4 × 103
49.0 × 103c
47.0 × 103c
a Ref.
25. Courtesy of Polymer Science and Engineering. convert J/m2 to ft·lbf/in.2 , divide by 2.10 × 10 − 3 . c J as defined in Ref. 1. c d Specimens cut in the extrusion direction. e Density = 0.940; MI = 0.2. f Density = 0.960; MI = 7.5. b To
determining U for a series of specimens with various notch depths if the relationship is indeed linear. If the impact tester is instrumented, a further criterion would be a rapid drop in load P after reaching a peak, ie, the load–deflection curve should have the shape of a right triangle. If the notch is too blunt and a significant amount of plastic flow occurs, which would cause the top of the load–deflection curve to round off, a correction factor may be needed, ie, the crack length a is increased by the plastic zone size rP (25,27): af = a + rP The plastic zone size can usually be determined from the appearance of the fracture surface and is distinguished by a whitened or very rough area directly ahead of the original notch tip. The Gc , values for some plastics for both the Izod and Charpy geometries (25) are in Table 4. With the same method, the GIc of a number of polymers was determined (3) at different rates (Fig. 18). The procedure outlined above can be useful if the specimens are prepared carefully to ensure that LEFM applies. The specimen should be as thick as is practicable, and the notches should be very sharp. If a razor blade is used for cutting, a compressive plastic zone must be avoided at the notch tip. This can be accomplished by slicing the notch tip rather than by pressing the blade into the notch. If the fracture surface contains plastic flow, the crack size most likely has
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600
100
50
0 0 20 40 60 80
BD, 10−6 m2 (a)
500 400 300
Gc = 5.6 kJ/m2
200 100 0
Fracture energy, mJ
2000
Gc = 1.6 kJ/m2 Fracture energy, mJ
Fracture energy, mJ
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Rate Increasing
1000
0 0
20 40 60 80
BD, 10−6 m2 (b)
0
50
100
150
BD, 10−6 m2 (c)
Fig. 18. Fracture energy as a function of BDφ for (a) PMMA at 20◦ C; (b) PTFE at 25◦ C (◦, Charpy; •, Izod); (c) HDPE at 20◦ C and several rates (, 26 s − 1 ; , 122 s − 1 ; •, 620 s − 1 ) (3). To convert J to ft·blf, divide by 1.355. To convert kJ/m2 to ft·lbf/in.2 , divide by 2.10. Courtesy of The Institute of Physics, Great Britain.
to be corrected. The method for determining fracture toughness can be extended to tensile impact of a notched specimen (28).
Improving the Impact-Resistance of Materials Impact resistance is not an absolute term. It depends on the design of the structure as much as it does on the intrinsic material toughness. Nevertheless the impact resistance of a material can be increased by modifying it with tougheners. This increase is sometimes at the cost of reduced modulus and strength, and perhaps fatigue resistance. The impact toughness of commercial plastics is shown in Table 5.
High Impact Polystyrene (HIPS), Acrylonitrile–Butadiene–Styrene Copolymer (ABS), and Modified Poly(xylenol ether) (PXE). Polystyrene is a transparent, rigid, easily processable, inexpensive material. Its chief disadvantages are extreme brittleness and high susceptibility to crazing by organic liquids. It was also among the first materials to be produced in a toughened form, commonly known as HIPS, in order to allow the material to be used in applications with moderate loadings such as electrical applicances. In the most common production method rubbery polybutadiene (PBD) is precipitated from a styrene–polybutadiene solution in styrene during the polymerization of styrene. The precipitated phase itself typically contains 2–10-µm particles consisting of a rubbery sphere containing spheroidal polystyrene inclusions that are separated by thin membranes of PBD rubber (see Fig. 19). It is believed that this type of composite particle has an advantage over solid rubber spheres because for a given weight fraction, it maximizes the effective volume fraction of the rubbery phase, thus enhancing the impact toughness without causing undue reduction in modulus. The rubbery phase can also be introduced by blending PS with
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Table 5. Typical Notched Izod Impact and Drop-Weight Impact Toughnessa Notched Izod, J/mb Thermoplastic Acetal Nylon, amorphous Nylon-6g Impact modified nylong Nylon-6,6g Toughened nylon-6,6g 33% glassg Nylon/ABS Nylon/PPO Polyarylate Polycarbonate Polycarbonate/ABS Polycarbonate/PBTg,h Polycarbonate/PET Polyester, aromatic (LCP)i Thermoplastic polyester, PBTh Impact modified PBTh Impact modified PBT/PET 30% glass-reinforced PET, thermoplastic polyester Toughened 30% glass PET Polyetheretherketone Polyetherimide Polyethersulfone Poly(phenylene oxide) modified Poly(phenylene sulfide) Poly(phthalate carbonate) Polysulfone Polyurethane, engineering thermoplastic
22◦ C 53–53 587–1121 53–81 961 27–64 907–1174 219 998 213 294 641–854 454–641 694–854 907 53–534 43–53 801–854 801–854 101 139–235 59–81 32–53 53–81 267–534 27–75 534 69 53–213
−40◦ Ce
Falling dart,c N·md 22◦ C
−40◦ Ce
294 f
65
57 f
96 f 133 f 213 133–598 267–614 f 160–641 694 f
73 51
35 f 40
38–61 56–57
30–46 f 49 f 50 f
43–54 54
43 f
213–480 f
747–801 f 96 123–160
34–36 133–187
15–34
3–11
64
a Ref.
6. convert J/m to ft·lbf /in., divide by 53.38. c Gardner or falling dart. d To convert N·m (J) to in.·lb, divide by 0.113. e Unless otherwise stated f At −29◦ C g Dry as molded. hPoly(butylene terephthalate). i LCP indicates liquid crystalline polymer. b To
a triblock copolymer such as poly(styrene–butadiene–styrene) (SBS). The toughening is believed to be due to the nucleation and controlled growth of extensive crazing at the rubber–PS interface (29). The appearance of stress-whitening is due to the high density of crazes generated by deformation. The stress–strain behavior of ABS, PS, and a typical HIPS are shown in Figure 20. It is readily apparent that HIPS has much lower modulus and tensile strength, but much higher elongation
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Fig. 19. Electron micrograph of an osmium tetroxide stained ultrathin section of HIPS. This micrograph shows the typical morphology of the dispersed rubber particles. The thin lines connecting the particles are crazes formed after the material has undergone deformation. PS 40
ABS
Stress, MPa
30 MIPS
HIPS
20
10
0
0.25
0.5 Elongation, cm
0.75
1.0
Fig. 20. Typical uniaxial tensile stress–strain behavior of PS, medium-impact PS (MIPS), high-impact PS (HIPS), and ABS (30). To convert MPa to psi, multiply by 145. Courtesy of Springer-Verlag.
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than PS. At the cost of a decrease in modulus and strength, ductility and impact toughness are greatly enhanced because of the higher volume fractions of rubber particles in the tougher materials. Acrylonitrile–butadiene–styrene polymers (qv), the impact-modified version of SAN (styrene–acrylonitrile copolymer), are produced in the same manner as HIPS. Block copolymers such as SBS may also be used as the second phase. The morphologies of ABS and HIPS are very similar except that for optimal impact toughness the rubber particles in ABS are smaller than those in HIPS, ca 1–5 µm in diameter. The difference between ABS and HIPS, as far as mechanical behavior is concerned, is due to the fact that SAN is more ductile than PS. Consequently, the impact toughness of ABS is higher than that of HIPS. Because of compositional and morphological differences, these materials exhibit a range of mechanical properties. For a given volume fraction of rubber particles in ABS, higher AN (acrylonitrile) content enhances toughness. The toughening mechanism is believed to consist of the formation of numerous shear bands in addition to massive crazing (29); both are due to the presence of the rubber particles. In certain ABS grades deformation in tension causes the formation of cavities inside the rubber particles, leading to the formation of shear bands from these cavitated particles (31). The toughest member in this category of plastic materials is a blend of poly(xylenol ether) [PXE, poly(2,6-dimethylphenylene oxide)] and PS and a rubbery impact modifier; PXE is also known as PPO [poly(phenylene oxide)] (Table 6). However, PPO, a registered trademark, can be confused with poly(propylene oxide), and therefore PXE is the preferred abbreviation. PXE is completely miscible with PS and forms a true solid solution (32); T g is high and the material is ductile. Melt viscosity is also very high. Processability is improved by blending with PS; the viscosity of the mixture is approximately the weighted average of the two components, whereas T g varies with the composition (32). Compared with the weighted average, moduli and yield strengths are higher but the fracture toughness is lower (32,33). Commercial grades are usually impact-modified by a dispersion of rubbery particles, which are sometimes obtained by directly blending PXE with HIPS instead of PS. The stress–strain behavior of HIPS/PS/PXE blends is shown in Figure 21. The PXE/HIPS blends exhibit a good overall combination of mechanical properties. The toughening mechanism is believed to be a combination of shear banding and massive crazing (29). The PXE/HIPS blends are much more expensive than HIPS or ABS. Furthermore, they require precise
Table 6. Composition of HIPS/PS/PXE Blends (wt%) Investigated Blend A B C D E
HIPS
PS
PXE
50 50 50 50 50
50 37.5 25 12.5 0
0 12.5 25 37.5 50
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50 E
40
D
C
Stress, MN/m2
B 30 A 20
10
0
0
10
20
30
40
50
60
70
80
Strain, %
Fig. 21. Uniaxial tensile stress–strain behavior of HIPS/PS/PXE blends at 20◦ C showing the effects of matrix composition. Strain rate 4 × 10 − 4 s − 1 (29). To convert MN/m2 to psi, multiply by 145. Courtesy of Applied Science Publisher Ltd.
processing control in injection molding because of the relatively high melt viscosity of commercial PXE. The terms HIPS, ABS, and PXE refer to families of polymers, within which there are variations in composition, morphology, and properties. This is especially true of PXE blends, which exhibit behaviors in between those of HIPS and PXE. Processing and applications must be specific to each grade. As in other materials, pigments, flame retardants, and especially reinforcing fillers can have significant effects on impact toughness. The relevant properties must be determined for each composition. Since the toughness of these materials depends on the presence of rubber particles, the destruction or oxidation of these particles reduces impact toughness. Excessively high temperatures or shear rate during processing and prolonged exposure to oxidative environments destroy or cross-link the rubber and must be avoided. Prolonged exposure to ultraviolet radiation degrades these materials because of the decomposition of the PBD rubber and sometimes PXE. Saturated elastomers resist oxidative degradation better than unsaturated PBD. In such instances, the low temperature impact toughness of the material might be impaired because the T g of the saturated rubber is usually higher than that of PBD. UV stabilizers may improve the resistance of PXE to sunlight. The rubber particles act as effective sites for the nucleation of crazes in cyclic fatigue.
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Therefore fatigue initiation occurs more readily in the impact-modified versions than in the unmodified versions; crack propagation is slower in the former (30).
Polycarbonates, Other Ductile Glassy Polymers, and Their ImpactModified Versions. A number of glassy polymers possess some degree of ductil-
ity at or near room temperature at moderate rates of strain (
