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Q2 (16 Marks) Materials & Testing 🔥 Repeated 3x in exams
MEKM • Written Exam

(a) Explain fatigue cracking, stating its causes and propagation. (8)

(b) Explain, how poor maintenance and engine overload may contribute to the risk of fatigue cracking of cylinder head holding studs. (8)

Appeared In: Jul 2026Jun 2025Feb 2025

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Part (a)

Fatigue is associated with the effects that a fluctuating or alternating load may have on a component. If the component is subjected to loads which are repeated a large number of times, it may fail without any permanent deformation to give warning of impending fracture. The stress levels causing failure will be lower than ultimate tensile stress of the material and may be below the yield stress limit. The fatigue crack normally originates at some form of stress raiser such as a corrosion pit or sharp corner. The crack progress until finally failing. The surface of a fatigue failure normally shows two zones, one a glossy smooth surface the other a crystalline surface. The burnished surface often shows lines called ‘beach markings’ caused by periods of stress separated by periods of rest. The crystalline structure shows the final rapid failure.

Part (b)

When the cylinder head is tightened down, the cylinder head studs are in tension. When the engine is operating, the tensile stress in the studs increases as the gas pressure in the cylinder rises. The design of the engine ensures that as long as the engine is operated within the correct parameters, then the material is loaded below the fatigue limit and will not fail regardless of the number of stress cycles. If the engine unit is overloaded, due to early injection or because too much fuel is injected, then the maximum stress in the studs is increased so that it is above the stress limiting curve and will fail after a number of cycles. If the studs are overtightened by increasing the jacking pressure above that set by the engine builder, then the initial tensile stress will be too high and when the engine is operated, even under correct parameters, again the maximum stress will be too high and failure will occur after a set number of cycles. Stress raising points such as corrosion or mechanical damage can lead to crack propagation even though the studs have been correctly tightened and the engine operated correctly.

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