The Problem of Brittle Failure

A number of accidental failures of ships during Second World War drew attention of engineers and metallurgist on the problem of brittle fracture. The said ships had all failed showing brittle fracture wherein the crack propagated suddenly without any plastic deformation and producing loud report due to sudden release of strain energy. As a result fracture tendency in material like steel develops due to

A.    a triaxial state of stress,
B.    low temperature, and
C.    high strain rate or rapid rate of loading.

It has been recognised that most brittle failures in service conditions take place due to combined effects of triaxial state of stress, such as existing at a notch and low temperature. These effects are, however, accentuated, by high strain rate, hence the laboratory tests combining all these three conditions are performed to determine tendency of a material to fail in brittle manner. Such tests known as impact tests are performed in impact testing machine – one such machine is shown in Figure

                                                 Figure: Charpy Impact Testing Machine