2023 ARP Catalog

23 Recognizing Common Failures 4. Cyclic fatigue failure originated by hydrogen embrittlement. L-19, H-11, 300M, Aeromet 100 and other similar high strength “quench and temper” steel alloys, popular in drag racing, are particularly susceptible to “hydrogen embrittlement.” Extreme care must be exercised when handling these materials, and kept well oiled at all times to prevent hydrogen gas and moisture from accumulating and attacking the metal. This type of failure is easily mistaken with Stress Corrosion. The spot on the first photo is the origin of the crack and the original stress riser. The second photo is a SEM photo at 30X magnification. 5. Cyclic fatigue cracks propagated from a rust pit (stress corrosion) Again, L-19, H-11, 300M and Aeromet 100, are particularly susceptible to stress corrosion, while 8740 and ARP2000 alloys are less susceptible to stress corrosion. These materials must be kept well oiled at all times and never exposed to moisture including sweat. The photos illustrate such a failure. The first picture is a digital photo with an arrow pointing to the double origin of the fatigue cracks. The second photograph at 30X magnification shows a third arrow pointing to the juncture of the cracks propagating from the rust pits. Inconel 718, ARP 3.5 and Custom Age 625+ are immune to both hydrogen embrittlement and stress corrosion. 6. Cyclic fatigue cracks initiated by improper installation clamp load Many connecting rod bolt failures are caused by insufficient clamp load. When a fastener is insufficiently preloaded during installation the dynamic load may exceed the clamping load resulting in cyclic tensile stress and eventual failure. The first picture is a digital photo of such a failure with the bolt still in the rod. The white arrows indicate the location of a cut made to free the bolt and the black arrow shows the origin of the fatigue crack. In the second picture – an SEM photo at 30X magnification clearly shows (1). The origin of the failure and the telltale “thumbprint” or “beach mark” (2). Tracks of the outwardly propagating fatigue cracks and (3). The point where the bolt (unable to carry any further load) breaks-away. 6. 5. 4. FASTENER TECH