Soil Mechanics in Relation to Structural Engineering Written Discussion on Mr. P.L. Capper's Paper

Appearance of Fatigue Failure The structural engineer has abandoned cast iron as a constructional material largely because of its lack of ductility in the tensile test and demands nowadays that all metals used for load carrying parts should possess appreciable ductility. Ductility is of course an essential requirement for fabrication but with the use of ductile metals has grown up the idea that a structural part will exhibit appreciable deformation and so give warning before it fails. This is true enough if failure is due to an overload once accidentally applied but large numbers of failures in service occur every year which give no such warning, are not preceded by any visible deformation even in materials with 20 per cent. or more percentage elongation, and are not caused by loads in excess of those for which the part has been designed. The fracture appears as a crack, that is a clean break, and its surface is generally of smooth, velvety appearance. An example of two such cracks in one welded beam is shown in Fig. I. All such failures are caused by a very large number of load applications, often all of them in the range of permissible stresses. The phenomenon is known as “Failure from Fatigue.” R. Weck

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