Post by The Madison Group | Plastic Consulting

TMG Thinks Plastics | with Jeffrey A. Jansen Sometimes, one of the most important fractures in a failure analysis is the one created intentionally in the laboratory. That may sound odd, but laboratory fractures can be extremely useful reference points. They allow the analyst to compare an unknown service fracture against fractures produced under known conditions. In this case, the failed part was a polycarbonate structural medical housing. Early in the investigation, there was a reasonable question raised by the client: did the part crack because of overload or impact? The actual failure mode, identified through the failure analysis, was environmental stress cracking (ESC). But it was important to do more than simply classify the failure as ESC. It was also important to show why overload and impact were not supported by the fracture evidence. That is where the laboratory fractures became valuable. The tensile overload fracture showed clear evidence of ductile deformation. Visually and microscopically, there was stress whitening and permanent deformation, consistent with macro-scale ductility. In the SEM images, the fracture surface showed a feathered morphology, parabolic markings, and stretching along the edges of the features. The open ends of the parabolas pointed back toward the fracture origin. The impact fracture looked different. Near the origin, the fracture surface was smoother and more brittle in appearance. Some increased deformation was observed farther away from the origin, but the overall macro-scale appearance was much less ductile. In the SEM images, river markings extended away from the smooth origin zone. The service fracture from the failed component did not match either the tensile overload or impact laboratory fractures. Instead, the fracture features, combined with the broader analytical findings, supported environmental stress cracking. That is the value of controlled laboratory fractures. They are not just interesting images. They provide comparison standards that help separate possible failure modes from the failure mode actually supported by the evidence. Fractography is not just about looking at a broken surface. It is about asking what the fracture surface is trying to tell us and then evaluating that interpretation against known references. If you want to discuss failure analysis or fractography, feel free to reach out: [email protected].