The Quantitative Analysis of Fatigue Cracking Under Programmed Loading.

Premature failure of an undercarriage cylinder occurred during a fatigue test in which programmes of severe and standard loading cycles had been applied representing severe and standard ground handling conditions. Fatigue crack growth rates were predicted using a fracture mechanics rationale based upon a modified version of the Forman law for fatigue crack growth. The predicted rates agreed, to within or - 30, with the rates measured as the spacings of fatigue striations on the fracture surface and a predicted total life agreed with the actual fatigue life to within 10. This indicated that the undercarriage loadings had been applied correctly and that errors in the loading were not the cause of the premature failure. It was found that the fatigue cracks initiated at a change in section with a poorly machined finish. A fracture mechanics analysis of the striation spacing growth rates close to the change in section indicated that the rough machining effectively increased the kt of the change in section from 1.7 to nearly 5, producing premature crack initiation and a short fatigue life. Author