Q6. (a) Explain fatigue cracking, stating its causes and propagation. (8)
(b) Explain how the risk of fatigue cracking of cylinder head holding studs is liable to be increased by poor maintenance and engine overload. (8)
2018/JUL? Most candidates had little understanding of fatigue which is the most common cause of engineering failures and should be basic knowledge for any engineering student. Descriptions are available in any basic mechanical engineering textbook.
>> Most mentioned cyclical stress and some even mentioned tensile and compressive stresses. However, fatigue only occurs during varying Tensile stress, compressive stress plays no part. The use of the Stress amplitude against log No. of cycles graph would have demonstrated the action of fatigue but very few used this.
>> For part b) there were many mixed and confusing answers. If the Stress amplitude against log No. of cycles graph had been used in part a) the candidate could have referred to that to explain the addition of static bolt stress to the varying tensile stress of cylinder operation. Some candidates even mentioned bolts going into compression which is not possible and shows deficiency of engineering knowledge.
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