Page 261 - Failure Analysis Case Studies II

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3.2. Stress acting between assembly hole and bolt root
It is known from Fig. 5 that I# and 2# bolts fractured at the location between the assembly holes
and the bolt root. Assume that the fracture surfaces were circular cones and were acted on by a
uniform tensile stress Q (Fig. 8).
On this basis, a,, is comparable to the value obtained above for However, the load-
bearing area is reduced by the presence of the assembly holes, and there are appreciable stress
concentration factors at the various changes in cross-section. However, a tensile test to fracture of
the bolt showed that the threaded portion was still the weakest element with respect to static loading
(Fig. 9).

4. REMEDIAL MEASURES

(I) Increase the distance between the shoulder and the assembly holes (dimension h in Fig. 5).
(2) Increase the radius of the filler between the head and shank of the bolt.
(3) Use thread rolling instead of threading for manufacturing the thread form.
(4) Consider the use of forging for producing the basic bolt shape.