Failure Analysis of a Striking Bar
This paper describes the failure investigation of a tubular shaft that is part of a hammer drill assembly. The failure investigation was particularly challenging as the fracture surfaces were completely damaged during service subsequent to the failure process. However, careful examination of the component and its assembly revealed many clues that pointed to the root causes of failure. The basic failure mode was identified as torsional fatigue failure in combination with high bending stresses. The failure was determined to be originated on the inside diameter of the tubular shaft. The root causes were determined to be operational overload in the presence of rough machining marks on the bore surface and higher than necessary operating torque required to overcome the dry adhesive friction in the system. The preventative measures recommended were manyfold including improving surface finish on the bore diameter, reducing dry sliding friction, decreasing the overall level of dynamic loads by appropriate design changes, and adding a surface strengthening heat treatment.
KeywordsTorsional fatigue Bending stresses Stress concentration Hammer drill
The author wishes to thank the management of Modern Industries Inc., Erie for permission to publish the photographs. The work was carried out at Modern’s Pittsburgh Labs while the author worked there.
- 1.G.J. Roe, B.L. Bramfitt: Notch Toughness of Steels, 10th edn, vol. 1, irons, steels and high-performance alloys, (ASM International, Geauga, 1990), pp. 737–754Google Scholar
- 2.T.V. Philip, T.J. McCaffrey, in Ultrahigh-Strength Steels, ASM Handbook, 10th edn., vol. 1, Irons, Steels and High-Performance Alloys, (ASM International, Metals Park, OH, 1990), pp. 432–434Google Scholar
- 3.D.J. Wulpi, Understanding How Components Fail, 2nd edn. (ASM International, Metals Park, 2000)Google Scholar
- 4.B. Boardman, in Fatigue Resistance of Steels, ASM Handbook, 10th edn., vol. 1, Irons, Steels and High-Performance Alloys, ASM International, Metals Park, OH, 1990), pp. 673–688Google Scholar