A structural component was made from high strength steel which was heat treated

Question

A structural component was made from high strength steel which was heat treated to yield strength 120, 000 psi and fracture toughness 60, 000 psi-in1 / 2. It was ensured that a crack of more than 0. 5 inch can be detected by NDT. However, it was reported from the field that the component failed after several months of service. What could be the probable causes of fracture and explain? From the fractured surface, an edge crack length was found to be 1. 5 inches. What was the applied stress at the time of fracture. If the result of SEM fracture surface examination revealed that it was an intergranular fracture and was caused by SCC. The threshold stress intensity ( K iscc) for this material was found to be 22. 5 ksi-in1 / 2. What stress level would be to prevent further stress corrosion failure? In case (c ), if K app. > K iscc under what condition does the final fracture occur?

Explanation / Answer

Causes are : 1. Material Toughness —Material toughness can be defined as the resistance to unstable crack propagation in the presence of a notch. For linear-elastic behavior the material toughness is measured in terms of a static critical stress-intensity factor under conditions of plane stress (Kc), of plane strain (KIc), or for dynamic loading (KId). For elastic-plastic fracture behavior, the material toughness may be measured in terms of ductility related parameters such as JIc, R-curve, COD 2. Flaw Size—Brittle fractures initiate from flaws or discontinuities of various kinds. These discontinuities can vary from extremely small cracks, for example, from within a weld arc strike (as was the case in the brittle fracture of a T-2 tanker during World War II), to much larger weld or fatigue cracks. Even though only small flaws may be present initially, repeated loading (fatigue), or stress corrosion can cause them to enlarge, possibly to a critical size where brittle fracture can occur. 3. Stress Level—Tensile stresses (applied, residual, or both) are necessary for brittle fractures to occur.

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