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TOOLS AND TECHNIQUES

Visualizing Residual Stresses in Plastic and Glass Joseph Maciejewski

Submitted: 10 November 2016 / Published online: 7 December 2016 Ó ASM International 2016

Abstract Polariscopes can be used to visualize the residual stress patterns in transparent or translucent materials. Residual stresses contribute to the fracture stress of failed components. This article describes the construction and usage of a lab polariscope made for less than US$150. Keywords Polariscope
Residual stress
Stress pattern
Plastic fracture
Glass fracture

Residual stress is often mentioned in failure analysis as a contributing factor in fracture. Residual stress is the result of one area of the part pulling or pushing against another due to differential thermal contraction and/or plastic deformation and spring-back during forming [1, 2]. Residual stresses are known to contribute to material cracking and resistance to cracking, especially in welds [2, 3] and surface-modified layers such as case-hardening or shot-peening [4]. These stresses are particularly important to the failure analyst when considering brittle failure modes such as fatigue, environmental stress cracking, or stress corrosion cracking (SCC). Unfortunately, residual stresses in metals cannot be visualized due to the opaque nature of the material. However, these stresses are known to be present via their influence on fracture (e.g., change in fatigue strength of shot-peened parts) [4] and by specialized X-ray diffraction techniques [5]. On the other hand, clear and translucent materials such as glass, amorphous polymers, and semi-crystalline J. Maciejewski (&) Applied Technical Services, Inc. - Materials Testing, 1049 Triad Court, Marietta, GA 30062, USA e-mail: [email protected]

polymers provide the failure analyst with a unique opportunity to visualize for the customer, qualitatively, the residual stress patterns present in the samples. This can significantly affect the understanding of the overall situation. In addition, there is the potential to use demonstration pieces of these materials to help communicate the concepts of residual stress in metal components. The concept of the polariscope has been known for quite some time, particularly in custom glass manufacturing and failure analysis [6]. A traditional polariscope uses (in order) a white light source, a linear polarization screen, the part under examination, a second linear polarization screen, and the observer (or camera). Residual stresses in the transparent part affect the polarized light extinction patterns, and the second polarizing screen is rotated to most distinctly highlight these birefringence patterns. An example is shown in Fig. 1, which shows the residual stress patterns in a plastic watch stand. Higher concentration of the birefringence patterns is an indication of higher residual stress or disruption of a residual stress pattern [7]. A stressfree or uniform stress field will show up as a uniform color [8]. Such an instance is demonstrated in Fig. 2, which shows a custom labora