A “Toolbox” for Forensic Engineers

Materials in Distress 31
1. Flowing to shape in the liquid, powder or semiviscous state — sand
casting, continuous casting, injection molding, extrusion, rotomolding,
etc.
2. Deforming to shape in the hot solid state — forging, rolling, extrusion,
drawing, thermoforming, etc.
3. Cutting to shape in the solid state — machining processes such as
turning, milling, sawing, drilling, etc.
4. Joining together using a liquid phase — fusion welding, soldering,
brazing, adhesives, sintering, etc.
5. Joining together in the solid state — riveting, bolting, mechanical
fasteners, etc.
In addition, one or more of several post-shaping processes may be
applied to a formed product to control its mechanical properties, resist
environmental degradation or improve service performance, including:
1. Bulk heat treatment throughout the component section — hardening
and tempering of steels, controlled transformations, precipitation
hardening treatments, annealing
2. Surface heat treatments — case hardening, induction hardening,
thermal toughening (e.g., glass)
3. Coatings — diffusion, plasma coatings, spraying, dipping, electroplating,
anodizing
4. Mechanical treatment of the surface — peening, polishing
Details of and variations within these themes are beyond the scope of
this book. Suffice it to remark that each of these shaping processes and
secondary treatments presents the opportunity to incorporate characteristic
defects, and it may be essential when investigating the cause of a particular
failure to have knowledge of the potential defects that may have been introduced
during manufacture.
2.2.1 Defects by Omission
One of the simplest errors is the omission of a key stage of the manufacturing
process, not so much in shaping the product where a mistake would be
obvious as in treatments used to achieve specific microstructural conditions
or surface properties. Using the “wrong” material is another error; most steels
and aluminum alloys look the same regardless of their chemical composition,
but fractional or small percentages of alloying elements are absolutely vital
in determining the level of mechanical properties. Such faults are often difficult
to spot because the physical appearance of the defective product may