nuts & bolts

aircraft building

Inspecting Welds Discovering discontinuities and defects Ron Alexander & Scott Helzer

E

valuating a weld for its integrity is one of the most important parts of welding. During the evaluation phase, the builder is looking for irregularities, which are often called discontinuities. A discontinuity is any interruption in the uniform nature of an item. A bump in a runway is a discontinuity because it interrupts the pavement’s smooth surface. In welding, discontinuities are such things as cracks, porosity, undercut, incomplete fusion, underfill, and overlap. Because each of them affects the serviceability of a weld, builders must be able to visually detect

Figure 1. Throat crack in a fillet weld.

102

JULY 2004

them—and describe their nature, location, and extent—to determine whether the discontinuity requires repair or can be left intact. A discontinuity is not a defect. A discontinuity is some feature that introduces an irregularity in an otherwise uniform structure. A defect is a specific discontinuity that can weaken the structure and make it unsuitable for its intended service. A discontinuity becomes a defect when its size or concentration exceed the standards that define the acceptable limits. So, if we refer to a defect, we are implying that it requires treatment to bring it into

acceptable limits. This introduction to weld discontinuities will address their characteristics, causes, and cures without specific reference to their acceptability. To help builders understand why certain discontinuities are unacceptable regardless of their size or extent, we’ll talk in general terms about the critical effects of certain discontinuities. Discontinuities fall into two general groups: linear and nonlinear. Linear discontinuities are at least three times longer than their widths. Nonlinear discontinuities have length and width dimensions

that are less than three times their widths. When it’s aligned perpendicular to the applied stress, a linear discontinuity is usually more critical than a nonlinear one because it’s more likely to propagate and cause a failure. Another measure of how critical a discontinuity is to structural integrity is its end condition. In general, the sharper the end the more critical it is because a sharper discontinuity is more likely to propagate. (Cracks grow from sharp corners because they are the focus of the concentrated stress, but drilling a hole in the metal or plastic at the end of the crack spreads out the stress and keeps the crack from propagating.) Again, propagation also depends on the orientation to the applied stress. Sharp ends are most often found on linear discontinuities, and if it lies transverse to the applied stress it seriously affects that member’s ability to carry an applied load. What loads a part of the structure bears is the final element in judging how critical a discontinuity is to airframe integrity. For example, if the component is under pressure, discontinuities that constitute a large percentage of a weld’s wall thickness will usually be most damaging. If the structure will be subject to cyclic loading, discontinuities forming sharp notches on the surface will generally lead to failure more readily than those beneath the surface. Surface notches act as stress risers, which tend to concentrate the stresses at that notch point. Such a stress concentration can result in a localized overload condition, even though the stress on the full cross section may be low. As an example, you can break a piece of welding wire two ways. You can bend it back and forth (seemingly forever) until it breaks. Or you can notch the wire’s surface by smacking it with something sharp, and then bending the wire a couple of times at the notch (a significant EAA Sport Aviation

103

DOES YOUR SAVINGS ACCOUNT GROW FAST ENOUGH TO PAY FOR A BLOWN ENGINE?

NEW Larger Display!

NEW Larger Display!

NEW Larger Display!

All Instruments STC’d/PMA’d, many TSO’d as Primary Replacements. You’ll be money ahead to invest in EI instruments, which will protect your engine and save you money! Here’s how:

Head off major problems.

Find minor problems (clogged injectors, worn rings, fouled plugs, sticky valves, etc.), before they become major repair bills or safety issues!

Gain significant fuel savings.

It’s vital to lean your engine properly. If you’re leaning “blind,” you could be taking a chance of causing preignition, detonation, excessive buildups on valves and cylinder walls, or overtemping the valves and heads.

Reduce Maintenance Costs! Running your engine at proper temperatures and pressures WILL keep your engine running healthier and longer. Stop adding extra legs to your flights.

How much time have you wasted with unnecessary fuel stops because of a lack of accurate fuel information?

Stop harming your engine with inaccurate RPM readings. You could be cruising at redline and not even know it! Mechanical RPM gauges are notoriously inaccurate. Pre-diagnose your engine problems.

This will substantially minimize the troubleshooting time and costs of your mechanic. Just imagine being able to tell your mechanic to check cylinder #3’s injector for a clog or a fouled plug!

When you think about the cost of a cracked cylinder, top overhaul or even the cost of fuel, our instruments can yield a much better return on your investment than a savings account! They can pay for themselves many times over, and even make your flight safer by helping you make crucial decisions at a glance.

Your engine and your safety are worth investing in! Over 40 models available. Contact us today!

EI

Electronics International Inc. Phone: (541) 318-6060 Fax: (541) 318-7575 www.Buy-EI.com

aircraft building concentration of stress). In short, structures free of discontinuities that create sharp notches are the most sound, and visual inspection is one of the most effective ways to discover potential problems. With this background, let’s look at some of the more common weld and base metal discontinuities.

Cracks Cracks are the most critical discontinuity because their ends are extremely sharp, which means they’ll grow larger when stress is applied. Cracks are born when the load applied to a member exceeds its tensile strength. In other words, the component was over stressed. This can occur during welding, immediately after, or when a load is applied. Cracks fall into two broad categories—hot and cold—depending on the temperature of the metal when they occurred, and certain types of cracks fall into each group. Hot cracks usually occur as the metal solidifies, and their propagation cracks are intergranular, meaning the cracks occur between individual grains. On the fracture surfaces of a hot crack you may see var-

ious colors, or “temper,” on the fracture faces that indicate the presence of that crack at an elevated temperature. Cold cracks occur after the metal has cooled to ambient temperature. Usually, they result from service conditions that would be considered cold cracks. But delayed, or underbead, cracks resulting from trapped hydrogen are also cold cracks. The propagation of cold cracks can be intergranular or transgranular (through the individual grains). Cracks are also defined by their direction relative to the weld’s longitudinal axis. Those that parallel the longitudinal axis are longitudinal cracks, and those perpendicular to the longitudinal axis are transverse cracks. These directional references apply to cracks occurring in either the weld or base metals. Longitudinal cracks can result from transverse shrinkage stresses of welding or stresses associated with service conditions. Transverse cracks are generally caused by the longitudinal shrinkage stresses of welding acting on welds or base metals of low ductility. Figure 1 illustrates throat cracks in a fillet weld.

Figure 2. Cracks are identified by their location and direction.

Finally, cracks are differentiated by their location relative to the various parts of the weld such as throat, root, toe, center, underbead, heataffected zone, and base metal crack (figure 2).

Key Terms & Definitions Arc Strike—a discontinuity resulting from an arc consisting of any localized remelted metal, heataffected metal, or change in the surface profile. Convexity—the maximum distance from the face of a convex fillet weld perpendicular to a line joining the weld toes. Defect—a discontinuity that exceeds the permissible limits and requires repair or replacement. Discontinuity—any irregularity in the normal pattern of a material or interruption of the uniform nature of an item. I n c l u s i o n—entrapped foreign solid material, such as slag, flux, tungsten, or oxide. I n c o m p l e t e F u s i o n—a discontinuity where fusion did not occur between weld metal and fusion faces or adjoining weld beads. Incomplete Joint Penetration—a joint root condition in a groove weld where weld metal does not extend through the joint thickness. 104

JULY 2004

Intergranular—conditions that occur at or follow the metal’s grain boundaries. An intergranular crack would initiate and propagate along a metal’s grain boundaries. Overlap—in fusion welding, the protrusion of weld metal beyond the weld toe or weld root. Porosity—cavity-type discontinuities formed by gas trapped during solidification of a weld bead. Safe Ending—(or stop drilling) drilling a small hole at each end of a crack to keep it from growing. Stress Risers—conditions such as notches, cracks, or geometry that increase the applied stress. Transgranular—conditions that cross or pass through the metal’s grains. A transgranular crack runs across the grains, as opposed to an intergranular crack, which runs along the grain boundaries. Undercut—a groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal.

Throat cracks extend through the weld along the weld throat, or the shortest path through the weld’s cross section. These longitudinal cracks are generally considered to be a hot crack. Because you can see them on the weld face, they are also known as a centerline crack. Joints exhibiting high restraint transverse to the weld axis are susceptible to throat cracking, especially where the weld cross section is small. Such things as thin root passes and concave fillet welds could result in a throat crack because their reduced cross sections may not be sufficient to withstand the transverse weld shrinkage stresses. Crater cracks occur where an individual weld pass terminates. If the technique used by the welder to terminate the arc does not completely fill the molten weld puddle, the result could be a shallow region, or crater. This thinned area, combined with the shrinkage stresses from welding, may cause individual or a network of crater cracks to radiate from the center of the crater. When there is a radial array of crater cracks, they are commonly referred to as star cracks. An underbead crack is related to welding, but it is located in the heat-affected zone instead of the weld metal. Underbead cracks are caused by the presence of hydrogen in the weld zone, with it coming from the filler metal, base metal, surrounding atmosphere, or surface contamination. If there is some source of hydrogen present during the actual welding operation, it may be absorbed by the heat-affected zone. Typically occurring below the surface, underbead cracks are difficult to detect. However, they may propagate to the surface, and you’ll find them directly adjacent to the weld fusion line in the heataffected zone. When cross-sectioned, underbead cracks often EAA Sport Aviation

105

An Old Friend Is Back Just Like in the Good Old Days All the Randolph products, all the Randolph colors, all the Randolph quality. An aviation icon is back on the market again... to stay.

800-362-3490 Or e-mail us at info@ randolphaircraft . com

Quality, Service, Experience = The NEW DUATS Golden Eagle FlightPrep® Software

CSC DUATS along with Stenbock and Everson FlightPrep® bring you a powerful new flight planning, filing and weather briefing tool. The new DUATS Golden Eagle FlightPrep® software has these features: • Flight planning using Relief or IFR Charts • NEXRAD Overlay on both Relief and IFR Chart • Graphical TFRs on both Relief and IFR Chart • 8 Zoom levels • Navigational overlay information selectable by Zoom Level. This includes: – VORS – AIRPORTS – SUA – Obstructions – CLASS B, C, D, and E Airspace – INTERSECTIONS\AIRWAYS • Lat/Long Easy locator on charts • Radial/DME Pointer • DUATS Weather Briefings – Standard – Outlook – Abbreviated • DUATS Flight Planner and Filing • Golden Eagle FlightPrep® Offline Flight Planner DUATS Technical Support and Order Line: 1.800.345.3828 by e-mail [email protected] www.duats.com

Computer Sciences Corporation Federal Sector 15000 Conference Center Drive Chantilly,VA 20151-3808

aircraft building appear to run directly parallel to the fusion line of a weld bead. Because underbead cracking may not propagate until many hours after welding, they are also called delayed cracks. High strength steels are particularly susceptible to this type of cracking, and when working with these materials you should visually inspect the weld areas 48 to 72 hours after they have cooled to the ambient temperature.

Incomplete Fusion

Elegant …… Affordable

Esqual VM-1 A New Concept in Sport Aviation Cruise 165 mph - Land at 40mph Light Sport model available Builds in two weeks at our Builder Seminar Call or email for details and free info CD Visit Esqual at Airventure booth 407 - North Display www.esqualna.com [email protected] 920-836-2096 3270 Breezewood Ln Neenah WI 54956

Incomplete fusion is a weld discontinuity in which fusion did not occur between weld metal and fusion faces or adjoining weld beads. In other words, the fusion is less than that specified for a particular weld. Because of its linearity and relatively sharp end condition, incomplete fusion is a significant weld discontinuity. We most often think of incomplete fusion as an internal flaw, but it can occur at the surface of the weld. Quite often, incomplete fusion also has slag inclusions, and the presence of slag due to insufficient cleaning may prevent the fusion from occurring. A number of things can cause incomplete fusion, but the most common cause is improper manipulation of the welding electrode. Some processes are more prone to this problem because there is not enough concentrated heat to adequately melt and fuse the metals. In other situations, the actual configuration of the weld joint may limit the amount of fusion that can be attained. Finally, extreme contamination, including mill scale and tenacious oxide layers, could also prevent the attainment of complete fusion. We will conclude the discussion of weld inspections next month by discussing inclusions, porosity, undercut, underfill, overlap, convexity, and arc strikes. 106

JULY 2004