Investigative Summary

1. Based upon the performed the tests and examinations performed by our metallurgical testing laboratory, it is our opinion that the failure of the submitted steel bracket occurred by reverse bending fatigue failure mechanism followed by a rapid ductile overload fracture. Fatigue cracking initiated inside a 90° bend, where stresses are inherently high.

2. The fatigue cracks propagated under the cyclic test loads through ~90% of the load-bearing cross-section. The final rapid overload fracture occurred through the remaining narrow metal ligament adjacent to the apex of the bend.

3. Fatigue is a progressive (time-dependent) failure mechanism that leads to initiation of small cracks when a part is subjected to repeated or fluctuating stresses, which exceed the fatigue strength of the material. The maximum applied stresses are below the tensile strength of the material and, therefore, the failures occur gradually, over an extended period of time.

4. An appreciable area on the fracture surface was subjected to extensive post-failure wear damage; however, some better-preserved areas on the fracture were suitable for SEM examination. The examination revealed coarse, evenly-spaced crack progression marks, known as fatigue striations. Such features positively identified the failure mechanism as reverse bending fatigue. The coarse striation appearance implied a low-cycle, high-stress fatigue mechanism, with the loading stresses extending into the plastic zone of the stress-strain curve. Such striation appearance are indicative of excessive test loading conditions that required a metal failure analysis investigation to identify the root cause.

5. Metallographic examination of the bracket material revealed a uniform microstructure consisting of ferrite and pearlite, typical of a hot-rolled ASTM A 1011 carbon steel. No evidence was observed of pre-existing steel defects, excessive nonmetallic inclusions, or any other detrimental material conditions that could have contributed to the failure.

6. Hardness testing results met the requirements of ASTM A1011, Sheet and Strip Designations CS Types A and B. Chemical testing confirmed the bracket material as ASTM A 1011 carbon steel.

7. We respectfully recommend reviewing the fatigue test conditions, to identify the likely source of the excessive cyclic loading applied to the submitted bracket.

1. Visual and stereoscopic examination of the bracket revealed that the failure occurred at one of the two 90° bends (see Photo 1, arrow).

2. The fracture surface showed two distinct regions. The first region occupied ~90% of the total fracture. It had a thumbnail-like shape, exhibited a relatively flat appearance with evidence of wear damage, and contained semi-circular crack progression lines (beach marks) emerging from a site on the inner surface of the bend. Beach marks are associated with propagation of fatigue cracks under cyclic bending and/or tensile loading The site of the beach marks emergence was identified as the crack origin. The second region was dark-colored and fibrous, and occupied the remaining ~10% of the surface. Fibrous appearance is commonly associated with ductile overload failures (see Photo 2, arrows).

3. The observed surface features are commonly associated with propagation of fatigue cracks under cyclic bending loading, followed by a rapid ductile overload fracture, when the reduced load-bearing cross-section could not carry the applied stresses.

1. An appreciable area on the fracture surface was subjected to extensive post-failure wear damage; however, some better-preserved areas on the fracture were suitable for SEM examination. The examination revealed coarse, evenly-spaced crack progression marks, known as fatigue striations (see Photo 3 on the following page). Such features positively identified the failure mechanism as reversed-bending fatigue.

2. The coarse striation appearance implied a low-cycle, high-stress fatigue mechanism, with the loading stresses extending into the plastic zone of the stress-strain curve. Such striation appearance may be indicative of excessive test loading condition.