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A newsletter dedicated to keeping attorneys informed of the technical side of product liability cases.
Issue 61: June/July/Aug. 2014
Connection Failure Part 1: Welds
By John L. Ryan, P.E.
© 2014 M.A.S.E. LLC
Connection failures can result in devastating results if the failure is part of a structural failure or unintentional release of stored energy. There are two components to correctly connecting components—design of the connection, and construction/implementation of the connection. Examples of connection failures that have led to serious injury or death include tie rods, chair bolts, screws, and welds, steel truss bolts/rivets, pressurized vessel welds/bolts, ladder structural component connections, wheel studs, aircraft engine connections, control arms, and many more.
Porosity occurs when air bubbles get trapped in welds during the solidification process. For porosity to occur, air or other gases have to enter the weld, and be prevented from escaping. The presence of gases in the first place can be due to hydrocarbons on the base material, moisture in the welding process, or wind at the weld point. As the rate of solidification of the weld material increases, escape of trapped gases becomes more difficult. Faster welds may result in faster cooling rates and increased rates of solidification and increased amounts of porosity. Welds made on a vertical surface tend to result in greater porosity due to a decreased ability of the gases to escape. Welding on a horizontal surface greatly improves the ability of the trapped gases to escape. Preventing gases from forming is best accomplished through effective cleaning of the surfaces to be joined.
Porosity weakens the strength of the weld due to a reduction in the cross sectional area of the weld which directly results in a reduction of the weld strength. While some argue that the size of the individual porosities are important, others argue that the overall cross sectional area of the air pockets is the most critical element of weld porosity. The fatigue life of a weld is decreased as weld porosity increases.
Weld discontinuities can result in catastrophic failure
Dross / Oxide
This discontinuity occurs due to inadequate surface preparation, and is typically caused by metal burrs or sharp corners. Dross essentially refers to any foreign material less dense than the parent metals. It is also linked to weld porosity, and can negatively impact the strength of the weld.
Incomplete fusion occurs when the weld metal and base materials do not properly fuse together. If the temperature of the base metal is not elevated to its melting point, incomplete fusion can occur. This can be due to welding at too far of a distance from the metals being joined. It can also be due to a non-uniform heating of both sides of a welded joint, which can occur due to an incorrect weld angle. Other causes of incomplete weld fusion include oxides or other foreign material present in the materials or on the surface of the materials, and inadequate amounts of weld energy either due to the welder not being at the correct setting, or due to joining materials which require a welder with greater energy to properly fuse thicker materials. The amount of current as well as the voltage must be adequate for proper fusion of metals and weld material. Improper joint preparation can also result in this discontinuity. Detecting lack of fusion can be difficult to perform by typical nondestructive methods that can identify other defects. The strength of an inadequately fused weld is greatly reduced, and may result in failure. Not only are the materials not joined properly, they are now notched which increases the stress concentration at the area making failure more likely. Prevention of this discontinuity includes ensuring that the weldment pool is not allowed to get ahead of the weld arc. Careful attention on the part of the welder is important to identify if the weld is not developing properly.
Incomplete Joint Penetration
Incomplete joint penetration occurs when the weld does not extend adequately into the base materials. The failure of penetrating the base materials can be due to an angle of a groove weld that is too tight, or if the weld or base material does not reach an adequate temperature. Solutions to incomplete joint penetration include increasing the temperature of the weld process by increasing voltage, increasing wire feed rate, and decreasing rate of weld. Weld cracking and failure can occur due to incomplete joint penetration.
Cracking is typically the result of stress concentrations that occur near weld discontinuities. High stresses and reduced ductility can result in weld cracking. Cracking can occur due to rapid cooling of melted weld metal. Cracking can lead to catastrophic failure of a weld. Attention to appropriate temperatures, and component preparation can minimize the chance of crack occurrence.
Weld Profiles Defects
This category of defects includes a variety of defects such as undercut, overlap, excessive convexity, and insufficient throat. These defects involve unfilled weld pockets or weld shape and size defects. Undercut welds occur when grooves formed from the weld process are not filled due to excessive heat.
Welds must be designed like other connections such as bolts to ensure that the weld can adequately withstand forces that the welded connection will be subjected to, along with an appropriate factor of safety. Simple engineering calculations can show if a weld meets engineering design criteria.
Detecting Weld Defects
Many of the discontinuities discussed in this newsletter can be detected using radiography, liquid penetration, magnetic particle inspection, and ultrasonic inspection methods. If there has already been a failure of a weld, greater information is often available due to the fracture occurring at a weak point of the weld. Having access to the cross-section of a failed weld can provide information on failure modes that are harder to detect such as incomplete fusion.
How We Can Help
At MASE, we can determine the cause of an accident, whether it is due to a design defect, manufacturing error, material defect, or human error. We can help you determine if a weld failure is the result of a weld defect or a design defect. Call us at (855) 627-6273 or email us at firstname.lastname@example.org
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© 2014 Mechanical and Safety Engineering