Metallurgical Technologies, Inc.
Case Study: Analysis of a fractured retention stud from a CNC tool holder collet
A fractured retention stud from a CNC auto change tool holding collet and a comparison retention stud from the same manufacturer were analyzed to determine the cause of failure. One of the retention studs was fractured through the shank in a spiral fashion from the region of the first thread. A second stud was intact but cracked in the same manner.
A close-up view of the fractured retention stud. A brittle torsional spiral fracture is observed.
A detailed SEM image of the brittle intergranular fracture found along the outer surface in the area of initiation.
View of cracking in the root of the first thread in a cross-section through the cracked stud. The wide opening of the crack indicates a high stress load and the likely initiation site. A high magnification view of the crack tip is detailed in the following image.
The crack appears to follow prior austenitic grain boundaries indicating brittle fracture.
CONCLUSIONS:
Results indicate that the retention stud fractured due to instantaneous torsional overload. The case-hardened steel is in a brittle condition and does not give, but cracks when overtorqued. A torque wrench adaptable to the stud was devised so that the proper torque loading could be maintained. No further failures have been observed.
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