Direct detection of plasticity onset through total-strain profile evolution

Abstract

Plasticity in solids is dependent on microstructural history, temperature, and loading rate, and sampledependent knowledge of yield points in structural materials adds reliability to mechanical behavior. Yielding is commonly measured through controlled mechanical testing, in ways that either distinguish elastic (stress) from total deformation measurements or identify plastic slip contributions. In this paper, we show that yielding can be unraveled through statistical analysis of total-strain fluctuations during the evolution sequence of profiles, measured in situ, through digital image correlation. We demonstrate two distinct ways of quantifying yield locations in widely applicable crystal plasticity models for polycrystalline solids, using either principal component analysis or discrete wavelet transforms. We test and compare these approaches for synthetic data of polycrystals and a variety of yielding responses through changes in applied loading rates and strain-rate sensitivity exponents.

Detection of the onset of yielding and creep failure from digital image correlation

“…A schematic of the method can be seen in Fig. 1(b), and the method has previously been shown to work well with simulated strain fields [8] mimicking the fields obtained using DIC. In this paper, the method is used in detecting the yielding of two commercial metal alloys during monotonic tensile loading of the sample.…”

“…These observations on the locations of the peaks in the second PCA component motivate the following definition [8] for the PCA-based prediction of yielding. We define the yield strain prediction as the position of the peak PCA y = arg maxPCA 2 , (9) and this clearly is close to the yield strain defined from the stress-strain curves using Eq.…”

“…We study [8,[40][41][42] tensile loading in the x-direction for 3D polycrystalline samples with sample dimensions in (x, y, z): (64,64,64) (3D), promoting the perspective of investigating (0.13 mm) 3 sub-mm cubic 3D samples. We assume that the linear size of each cubic pixel is 2 μm, and the chosen Voronoi tesselation [see Fig.…”

“…The model [40,42] utilizes the phenomenological crystal plasticity theory, capturing slip-based macroscale plasticity, with standard constitutive laws for metals [41]. The model captures in a self-consistent manner the basic physical mechanisms of crystal plasticity, as they take place in most metals, and it captures finite deformations in a cubic grid [8]. The model is solved by using an FFT-based spectral method [40].…”

“…The important question then is as follows: Are these engineering definitions of yielding good enough? To at least avoid the problem of predetermined thresholds and constant points, we define here the yield strain to be the maximum of the second derivative of * Corresponding author: tero.j.makinen@aalto.fi stress with respect to strain [8],…”