Showing papers in "Strain in 2021"

Towards Material Testing 2.0. A review of test design for identification of constitutive parameters from full‐field measurements

Abstract: Full-field optical measurements like Digital Image Correlation or the Grid Method have brought a paradigm shift in the experimental mechanics community. While inverse identification techniques like Finite Element Model Updating (FEMU) or the Virtual Fields Method (VFM) have been the object of significant developments, current test methods, inherited from the age of strain gauges or LVDTs, are generally not well adapted to the rich information provided by these new measurement tools. This paper provides a review of the research dealing with the design and optimization of heterogeneous mechanical tests for identification of material parameters from full-field measurements, christened here Material Testing 2.0 (MT2.0).

Crack detection via strain measurements in fatigue testing

Abstract: Fatigue cracks have appeared as a significant issue for joints and connections in existing steel structures in the last decades. Therefore, those are a major inspection and maintenance matter for any steel structure's operator. This emphasises the importance of using a reliable detection method to determine the crack size and assessing the severity of such a crack on the structural integrity of a structure. In this article, the effectiveness of strain measurement in detecting fatigue cracks in transversal non‐load carrying welded attachment subjected to out of plane axial loading is studied. Numerical analysis and experimental investigations allowed to correlate the decrease in strain measured by attached gauges to the crack depth at the weld toe. In addition, different strain evolution patterns were found during fatigue testing, and the fracture surfaces of the specimens were observed to interpret these patterns. Moreover, the crack position with respect to the weld toe surface was predicted via strain measurements.

High strain rate elasto‐plasticity identification using the image‐based inertial impact (IBII) test part 1: Error quantification

Abstract: Current high strain rate testing procedures generally rely on the split Hopkinson bar (SHB). In order to gain accurate material data with this technique it is necessary to assume the test sample is in a state of quasi-static equilibrium so that inertial effects can be neglected. During the early portion of an SHB test it is difficult to satisfy this assumption making it challenging to investigate the elastic-plastic transition for metals. With the development of ultra-high speed imaging technology the image-based inertial impact (IBII) test has emerged as an alternative to the SHB. This technique uses full-field measurements coupled with the virtual fields method to identify material properties without requiring the assumption of quasi-static equilibrium. The purpose of this work is to develop the IBII method for the identification of elastoplasticity in metals. In this paper (part 1) the focus is on using synthetic image deformation simulations to analyse identification errors for two plasticity models, a simple linear hardening model and a modified Voce model. Additionally, two types of virtual fields are investigated, a simple rigid body virtual field and the recently developed sensitivity-based virtual fields. The results of these simulations are then used to select optimal processing parameters for the experimental data analysed in part 2.

High strain rate elasto‐plasticity identification using the image‐based inertial impact (IBII) test part 2: Experimental validation

Abstract: Current high strain rate testing techniques typically rely on the split-Hopkinson bar (SHB). The early response in an SHB test is corrupted by inertia making it difficult to accurately characterise the transition from elasticity to plasticity for metals. Therefore, a new test method is required. This article is the second in a two part series which aims at developing a new high strain rate test for elasto-plasticity identification using the ImageBased Inertial Impact (IBII) method. The goal of this article is to validate the new method experimentally using IBII tests on aluminium 6082-T6 (minimal rate sensitivity) and stainless steel 316L (rate sensitive). Comparison of the quasi-static and dynamic stress-strain curves for the aluminium case showed minimal difference providing experimental validation of the method. The same comparison for the steel showed that the method was able to detect rate sensitivity.

Experimental investigation of the strain-rate effects on the failure of composite materials with off-axis tensile tests on unidirectional plies

Abstract: The present study experimentally investigates the mechanical response of T700/M21 Carbon Fibre Reinforced Polymers subjected to coupled shear and transverse tension at intermediate strain rates ranging from 10−3 to 15 s−1. Off-axis tension specimens with transverse and oblique tabs are used for the various tests. Moreover, the specimen aspect ratio is quite low, since dynamic tests are performed in this study. The axial stress–strain curves are successfully simulated using finite element simulations with a viscoelastic model. Based on these numerical simulations, specific stress correction factors are computed to take the effect of end constraints into account for such low aspect ratio specimens. Finally, an increase in the material strength with the strain rate is observed.

Investigation of the 2D assumption in the image‐based inertial impact test

Abstract: The image-based inertial impact (IBII) test has shown promise for measuring properties of composites at strain rates where existing test methods become unreliable due to inertial effects (> 10 2 s −1). Typically, the IBII tests are performed with a single camera, and therefore, to use surface measurements for material property identification, it is necessary to assume that the test is two-dimensional. In this work, synchronised ultra-high-speed cameras are used to quantify the relevance of this assumption when nonuniform, through-the-thickness loading is applied to interlaminar samples. Initial experiments revealed that an angular misalignment of approximately 1° between the impact faces of the waveguide and projectile created a bending wave that propagated along the sample behind the axial pulse. Even under these conditions, consistent measurements of stiffness were made by assuming a linear distribution of the behaviour through-the-thickness. When the misalignment was reduced to 0.2°, the effects on single-sided measurements were significantly reduced. The two alignment cases were compared to show that three-dimensional loading had a small effect on stiffness identification (approximately 5% bias) relative to failure stress (approximately 30% bias). This study highlights the importance of impact alignment for reliable characterisation of the interlaminar failure stress and was used to establish guidelines for diagnosing loading issues from single-sided measurements.

Speckle pattern creation methods for two-dimensional digital image correlation strain measurements applied to mechanical tensile tests up to 700°C

Abstract: The purpose of this study is to develop novel speckle pattern techniques for digital image correlation (DIC) kinematic measurements of mechanical tests at high temperatures, typically from 400 to 700°C. In this context, the speckle pattern should not only meet morphological criteria (size, density, distance) in order to improve spatial resolution, but it should also present a high contrast and resist high temperature and strain levels. To find a speckle pattern matching these specifications, a comparison was performed on six types of speckle made using different techniques. First, a computer-generated speckle pattern that meets DIC criteria was numerically designed to produce six types of speckle pattern. Next, the speckle patterns produced using these six techniques were compared in terms of speckle morphology, image quality and adherence to titanium alloy TA6V material at high temperatures. From 25 to 600°C, the speckle pattern made by the technique combining anodisation and laser engraving named M5 technique gave the best contrast (highest value of mean intensity gradient [MIG] and Shannon entropy value) and the adherence of 200% of strain measurements to the TA6V material. At 700°C, speckle image quality is considerably reduced due to oxidation of the titanium alloy, and this may not be suitable for DIC measurements. Only the speckles produced by painting in which the paint plays a protective role provide with a better speckle contrast compared with other techniques. However, these speckle patterns enable only a strain measurement of 22% by the DIC method. This article concludes with guidelines for producing a speckle pattern suitable for high-temperature mechanical tests.

On the use of stereo‐digital image correlation for the alignment of a fatigue testing machine in accordance with international standards: A feasibility study

Abstract: Stereo-digital image correlation (SDIC) is used for the analysis and quantification of parasite bending stresses in a fatigue specimen due to its mounting in a misaligned load frame. The aim of this work is to provide an easy way to estimate the induced parasite bending stresses due to the load frame misalignment, using the out-of-plane displacement measurements obtained by SDIC and beam theory. Analytical solutions of parasite stresses as a function of out-of-plane misalignments are provided for flat fatigue specimens having (a) tangentially blending fillets between the uniform test section and the ends and (b) continuous radius between the ends. Simple procedures for aligning a commercial load frame in accordance with international standards are also provided. The bending stresses profile along the specimen main axis obtained by numerical simulations have shown a very good agreement with the results obtained by the analytical models. The proposed methodology could complement standard alignment techniques based on samples instrumented with strain gauges or, in their absence, even replace established procedures.

Creep deformation measurement of ex-service 12% Cr steel over nonuniform stress fields using digital image correlation

Abstract: Deterioration assessment of materials is essential to the continued effective operation of critical components in thermal power plants. Establishing the degree of creep exhaustion of power engineering alloys operating at high temperatures and stresses guides maintenance strategies to ensure reliable plant operation. Within progressive inspection philosophies, traditional laboratory-based creep testing is often difficult to conduct on ex-service steel due to the limited material availability from which to machine standard specimen geometries. This work investigates a technique for the measurement of creep strain curves at several stresses and at 600°C from a single test using a nontraditional specimen geometry together with full-field strain measurement through digital image correlation (DIC). Of interest is ex-service X20CrMoV12-1 (X20) which is widely used in older, subcritical thermal power plants. The paper aims to show that multiple creep curves can be resolved over a spatially varying stress field using DIC whilst preserving material economy. Differences in creep behaviour between ex-service X20 with varying levels of service exposure are evident from quantitative comparisons of the creep strain and rate curves through threshold stress computation which agrees with hardness measurements and microstructural observation of subgrains and precipitates using electron microscopy. These single-specimen tests yield high densities of creep data which can be used in the calibration of creep damage models for characterisation of ex-service X20.