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Several unique analytical techniques to comprehensively analyze plastic parts can provide the information needed to determine the nature and cause of plastic component failures.
Although procedures for failure analysis of metals are well documented, those for plastic parts are not. The final goal of both metallurgical and polymeric failure investigations remains the same, namely the determination of the mode and cause of failure. Therefore, the general steps involved in evaluating a failed plastic part parallel those applicable to a metallic component. The first step should include a thorough inspection, initially with an optical stereomicroscope, and subsequently with a scanning electron microscope.
For both metals and plastics, the purpose of this initial inspection is to characterize the fracture surface, and to determine the failure mode and crack origin location. However, because of inherent physical differences, the fracture surface morphology varies significantly between metals and plastics.
In subsequent steps, mechanical testing provides evaluation of tensile, impact, and hardness properties of both types of materials. While procedures vary, the primary purpose of comparing the measured results to a specification, or to data generated by a known good sample, remains constant. Cross sections of both types are prepared and inspected, but two slightly different aspects of the materials are evaluated. In the investigation of a metal, the microstructure is analyzed. With plastics, the degree of fusion and the orientation and dispersion of the filler materials are determined.
While many aspects of the investigation are similar, it is important to recognize the distinct differences that necessitate unique testing programs. The principal difference is based on composition and structure. Unlike metals, polymers have a molecular structure, which includes characteristics such as molecular weight, crystallinity, and orientation, which has a significant impact on the properties of the molded article. Additionally, plastic resins usually contain additives such as reinforcing fillers, plasticizers, colorants, antidegradants, and process aids. It is this combination of molecular structure and complex formulation that requires specialized testing. This leads to the most significant variation in the failure investigations, which is the evaluation of chemical composition.
Normally, the chemical composition of metals is analyzed by one of several elemental spectroscopic techniques, such as Inductively Coupled Plasma Spectroscopy (ICP). The analysis is relatively straightforward, with the results detailing the elemental...