Today's KNOWLEDGE Share:Fatigue Striations (PC HEADLIGHT LENS)

Today's KNOWLEDGE Share:

Fatigue Striations

I completed a failure analysis on a polycarbonate automobile headlight lens. Cracking was identified within the lens during inspection conducted after completion of performance testing. This testing included exposure of the lens to vibratory stress.

The external macro examination indicated that the cracking lacked characteristics associated with micro ductility, and displayed features associated with brittle fracture. The crack was completed and the fracture surface was examined with the aid of a scanning electron microscope (SEM). The SEM examination revealed a single crack origin, positioned immediately adjacent to a threaded boss design feature. The origin corresponds to a design corner, which acted as a point of stress concentration, multiplying the applied vibratory load.

A characteristic feature present on the fracture surface was the presence of radiating band features. The bands presented features indicative of arrest markings associated with dynamic crack propagation. At high magnification, the bands displayed characteristics of fatigue striations, corresponding to crack propagation through alternating cycles of cracking and arrest. This was consistent with the stated stress loading which precipitated the failure.

The fatigue striations on this project were textbooks for plastic materials. Fatigue cracking generally initiates at inhomogeneities within the microstructure, particularly at points of stress concentration, as was the case in this instance. The imposed stresses typically produce a complex process of both interactions of the defects, resulting in the initiation of microscopic cracks. The presence of the crack under load creates a further condition of stress concentration around the crack tip. When the stress maximum within this region exceeds the yield strain, a zone of damage is formed immediately in front of the crack tip, resulting in craze formation. Continued cyclic stresses lead to disentanglement of polymer molecules chains through cumulative rupture of the craze fibrils and coalescence of micro voids. Ruptured crazes are evident in the images below representing the headlight lens project. This process represents crack propagation and corresponds to the formation of bands of fatigue striations, as indicated below.

Once again, fractography clearly tells the story of how the component failed.

Source:The Madison group

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