A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 2 1 9 Profilometry-based indentation plas- tometry (PIP) was developed and commercialized by Plastometrex in the U.K. This unique testing method offers the materials science, engineering, and processing community a paradigm shift in the mechanical characterization of metal systems. Rooted in indentation, PIP testing gives metallurgical and mechanical engineers a simple way to determine nominal stress-strain curves up to the ultimate tensile strength (the point at which the onset of plastic instability arises), as well as true stress-strain curves, Voce plasticity parameters, yield strength, and simulated Brinell hardness values in less than five minutes. Another notable feature is that PIP testing only requires a sample thickness of roughly 2 mm, and length and width dimensions of just 6 mm. This can lead to significant cost savings compared with sample requirements for uniaxial tensile testing, specimen machining, and component manufacturing. For example, assuming a rectangular specimen geometry, the volume of material required for PIP testing per stress-strain curve is 72 mm3, whereas an ASTM E8 sub-size specimen (prior to machining to test coupon geometry) requires a volume of 2100 mm3 per stress-strain curve. The result is a reduction in material costs of approximately 97% when using PIP testing and plasticity analysis. PIP THEORY AND PRACTICE PIP is performed using an indentation plastometer (Fig. 1). As noted by Tang et al., PIP-obtained tensile stressstrain curves are derived by loading a hard spherical tip into a given specimen until reaching a known force[1]. This is followed by measuring the resultant indent profile and performing an iterative finite element model (FEM) simulation of the same test until best-fit plasticity parameters are achieved. More details regarding the process of obtaining tensile stress-strain curves via PIP analysis will be discussed shortly; the true stress- strain relationship (metallic plasticity behavior) and deformation response of thematerial are formulated via the Voce plasticity model, as far as an analytical framework for PIP testing of strain hardening material systems is concerned[2]. Numerous constitutive plasticity laws and analytical expressions are found in the literature, such as those presented by Hollomon, Swift, Ludwik, Harley and Srinivasan, Ludwigson, and Baragar[3]. However, the implementation of a Voce plasticity framework within PIP testing centers on the observation that the Voce model was capable of characterizing the plasticity response of metallic material systems with true strain hardening rates that approach zero. Further, the Voce model was also the most adept plasticity model for effectively capturing accurate, true stressstrain behaviors across a range of alloys and metals. In contrast with the Ludwik-Hollomon plasticity model, which is expressed as: σ = σy + Kεn wherein σ represents the von Mises applied stress, σy represents the von Mises yield stress, ε represents the von Mises plastic strain, K represents the strain PROFILOMETRY-BASED INDENTATION PLASTOMETRY BRINGS SPEED AND ACCURACY TO METALLURGICAL R&D This unique testing method offers economic benefits, sustainability advantages, and the potential to slash the time required to develop new alloys and processing recipes. Bryer C. Sousa* and Danielle L. Cote* Worcester Polytechnic Institute, Massachusetts Matthew A. Priddy Mississippi State University Victor K. Champagne, Jr.* U.S. Army Research Laboratory, Maryland *Member of ASM International
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