ABSTRACT The shear stress difference method (SDM) plays a central role in photoelastic stress separation. Direct compatibility with standard experimental setups enables straightforward full‐field recovery of stress components. The practical application of SDM is constrained by pronounced fringe streak artefacts and non‐physical discontinuities in recovered normal‐stress maps. These shortcomings arise from three interrelated challenges: (i) irregular fluctuations of the shear‐stress component perpendicular to numerical integration paths that accumulate during integration, producing path‐to‐path discrepancies; (ii) boundary residual stresses that introduce bias in initial‐value determination; and (iii) cumulative errors when integration paths traverse low‐quality measurement zones. To overcome these limitations, we developed a mechanism‐driven framework: it combined (a) a directional smoothing operator targeting perpendicular shear fluctuations while preserving legitimate stress gradients; (b) an informed initial‐value selection rule and a low‐temperature annealing protocol to mitigate boundary bias; and (c) a partitioned reconstruction workflow that prioritized high‐quality regions and propagated interface results into lower quality zones to limit error growth. This framework was validated against an analytical solution for a radially compressed disk and a numerical simulation of a radially compressed ring. The results demonstrated substantial suppression of streak artefacts, restored continuity and strong quantitative agreement with reference fields. This approach was robust and practically implementable, and it provided a reliable foundation for broader photoelastic stress separation tasks.
The CANDU (CANadian Deuterium Uranium) nuclear reactors are pressure tube reactors. The fuel channels of the reactor comprise a calandria tube and a smaller diameter pressure tube (pressure boundary) concentric within the calandria tube. The pressure tube, made from a zirconium alloy, Zr-2.5wt.%Nb, contains the nuclear fuel and pressurized coolant. In some scenarios, such as a postulated loss of coolant accident, the horizontally oriented pressure tube gets heated to high temperatures (above 650 degrees C) and deforms into contact with the calandria tube either through primarily downward deformation under its weight (sagging) or primarily radial deformation under internal pressure (ballooning). This deformation establishes a more direct path to remove heat from the fuel to the heavy water moderator that surrounds the calandria tube. A method was developed to apply stereo digital image correlation paired with thermography images to measure deformation of prototypical biaxial stress tests and Zr-2.5wt.%Nb pressure tubes demonstrating ballooning behaviour and compare experimental measurements to predictive deformation models developed from tests on uniaxially stressed specimens. The experiments were performed at an internal pressure of 3.5 MPa and a heating rate of 7.5 degrees C/s representing low heating rates of a pressure tube in a CANDU accident scenario and provided transient surface deformation and temperature for comparison against models. The model predictions were in good agreement with the experimental measurements especially when the uncertainty in the temperature measurements is considered.
At present, a multibeam optical stress sensor (MOSS) is the preferred instrument for measuring thin-film stress in situ and in real time. The thin-film stress-induced curvature is measured using the reflection from the substrate of an array of parallel laser beams, while the curvature is converted to the thin-film stress using Stoney's equation. To accurately track the displacements on the detector, each laser spot is fitted with a Gaussian intensity profile, a method that requires a few, narrow, isolated laser spots and, consequently, most of the detector area remains unused. To increase the accuracy, this work proposes an alternative method named the speckle stress sensor (SSS). It is based on (i) integrated digital image correlation (IDIC), known for its high displacement field accuracy for non-optimal brightness patterns if an accurate description of the actual kinematics is incorporated, and (ii) a suitable pattern of a large number of overlapping spots, created with a simple LED and perforated plate. First, it is shown that IDIC is a more robust and more accurate method, even for analysing images with isolated laser spots. Next, a simple SSS LED set-up has been developed that generates high-quality images with overlapping spots, resulting in at least a twofold increase in stress sensor accuracy. Suggestions for further improvements are also proposed.
Digital Image Correlation (DIC) is a popular camera-based, non-contacting method to obtain full-field displacement and strain measurements. In a vibration-based fatigue test, DIC is desirable because DIC can continue to monitor strain after a strain gage would have failed. When applied to vibration-based testing, one of the major challenges of DIC is maintaining sufficient lighting. Large out-of-plane displacements necessitate small apertures for improved depth of field while high frequency oscillation requires short exposure times to reduce motion blur. In this work, the tradeoffs in aperture, exposure time, gain, and external lighting are examined. First, a Design of Experiment (DOE) response surface test matrix is generated for each light source and aperture size. Second, strains are estimated using a laser vibrometer for which there is a pre-determined relationship between its measured displacement and the strains measured by a strain gauge at lower amplitudes. Third, strains are additionally measured by fitting an analytical solution to full-field deformation obtained using DIC. Further, a Monte Carlo Method uncertainty propagation was performed to determine the uncertainty between the two strain measurements. The strobe lights provided the most accurate and stable strain measurements while the ring lights provided the smallest tolerance range. In most cases, the use of Gain was found to be relatively harmless, which runs counter to the common wisdom that it should be generally avoided in DIC. In some extreme cases, Gain even produced less error than leaving the Gain as zero, but there are also counter-examples for which increased Gain produced worse outcomes. Thus, although in some situations Gain may be the least-worst option based on the needs of the experiment, DIC users should be cautioned not to over-rely on Gain without careful consideration of other lighting methods.
ABSTRACT The assessment of ductile damage at high strain rates poses challenges both at an experimental and numerical modelling levels. Indeed, testing requires special facilities, such as the split‐Hopkinson bar (SHB), through which it is not trivial to reproduce the multiaxial stress states that are mandatory for a robust material characterisation and tuning of numerical damage models. The paper proposes the use of a minimal but effective set of cylindrical, notched and shear‐tension specimens which can be tested using a conventional SHB only to investigate material strains to fracture at different (low) triaxialities and Lode angles. The materials investigated are a 17‐4PH steel and a Ti6Al4V titanium alloy. Experimental ultimate fracture strains corresponding to the different stress states are shown, along with the results of damage model calibration. It is proved that the approach can lead to an accurate dynamic characterisation, and that two widely used damage models can provide effective predictions even when high strain rates are involved. Finally, a comparison between dynamic and quasi‐static ductile fracture behaviour for the two materials is presented and discussed.
