Читать книгу Root Cause Failure Analysis - Trinath Sahoo - Страница 102

Every structure has a load limit beyond which it is considered unsafe. An applied load that exceeds this limit is known as overload. When a component fails because due to a single application of a load greater than the strength of the component, it is termed as overload failure. The nature of fracture arising due to overload failure could either be ductile or brittle or a combination of the two.

In general, ductile fractures are associated with metal flow at failure zone due to plastic deformation and fibrous‐surface appearance. In brittle fractures, plastic deformation is almost absent and the surface shows irregular bright facets of a cleavage type. Establishing the origin of a fracture is essential in failure analysis, and the location of the origin determines which measures should be taken to prevent a repetition of the fracture. The fracture‐surface characteristics that show the direction of crack propagation (and conversely, the direction toward the origin) include features such as chevron marks, crack branching, and river patterns. Features that help identify the crack origin include concentric fibrous marks, radial marks, and beach marks. By a study of these features, crack progress can be traced back to the point of origin, and then, it can be ascertained whether the crack was initiated by an inclusion, a porous region, a segregated phase, a corrosion pit, a machined notch, a forging lap, a nick, a mar, or another type of discontinuity, or was simply the result of overloading.

Some of the questions that should be raised concerning the nature, history, functions, and properties of the fractured part, and the manner in which it interacts with other parts, to find out root cause of failure are‐

 Loading. Where the nature, rate, and magnitude of the applied load correctly anticipated in the design of the part? Were repeated or cyclic loadings involved? What was the direction of the principal stress relative to the shape of the part? Where residual stresses present to an undesirable degree?

 Material. Was the recommended alloy used? Where its mechanical properties at the level expected? Where surface or internal discontinuities present that could have contributed to failure? Did the microstructure conform to that prescribed?

 Shape. Did the part comply with all pertinent dimensional requirements of the specification? Did the part have sufficient section thickness to prevent local overloading? Where fillets formed with sufficiently large radii? Where there adequate clearances between interacting parts? Where any of the contours deformed during service? Was there evidence of mechanical surface damage?

 Environment. Was the part exposed to a corrosive environment or to excessively high or low temperatures? Was the surface of the part suitably protected? Where the properties of the part altered by the exposure? Was there interaction (for example, galvanic) between the material of the part and that of adjacent components?