page_title: Forensic materials engineering
section_title: Metals and alloys
hierarchical_section_title: Forensic materials engineering / Metals and alloys
caption_attribution_description: English: image of fatigue of material
context_page_description: Forensic materials engineering, a branch of forensic engineering, focuses on the material evidence from crime or accident scenes, seeking defects in those materials which might explain why an accident occurred, or the source of a specific material to identify a criminal. Many analytical methods used for material identification may be used in investigations, the exact set being determined by the nature of the material in question, be it metal, glass, ceramic, polymer or composite. An important aspect is the analysis of trace evidence such as skid marks on exposed surfaces, where contact between dissimilar materials leaves material traces of one left on the other. Provided the traces can be analysed successfully, then an accident or crime can often be reconstructed. Another aim will be to determine the cause of a broken component using the technique of fractography.
context_section_description: Metal surfaces can be analyzed in a number of ways, including by spectroscopy and EDX used during scanning electron microscopy. The nature and composition of the metal can normally be established by sectioning and polishing the bulk, and examining the flat section using optical microscopy after etching solutions have been used to provide contrast in the section between alloy constituents. Such solutions (often an acid) attack the surface preferentially, so isolating features or inclusions of one composition, enabling them to be seen much more clearly than in the polished but untreated surface. Metallography is a routine technique for examining the microstructure of metals, but can also be applied to ceramics, glasses and polymers. SEM can often be critical in  determining failures modes by examining fracture surfaces. The origin of a crack can be found and the way it grew assessed, to distinguish, for example, overload failure from fatigue. Often however, fatigue fractures are easy to distinguish from overload failures by the lack of ductility, and the existence of a fast crack growth region and the slow crack growth area on the fracture surface. Crankshaft fatigue for example is a common failure mode for engine parts. The example shows just two such zones, the slow crack at the base, the fast at the top.
caption: Crankshaft fatigue fracture
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