AbstractCarotid pulse signal (CPS) is one of the most important biomarkers for evaluating cardiovascular risk. To realize easy‐to‐operate and accurate CPS detection, a single‐camera telecentric imaging stereo‐digital image correlation (stereo‐DIC) system is established. The system uses a specially designed colour separation device and a single colour camera attached with a telecentric lens to record images of blue and red colours from different optical paths. By processing the recorded two sub‐channel colour images using regular stereo‐DIC algorithm, full‐field 3D displacements of the skin on the neck directly over the carotid artery are obtained. The maximum fluctuations in the out‐of‐plane displacement direction are extracted as CPS, based on which velocity, acceleration and frequency can be further extracted. The performance of the established system was demonstrated by detecting the pulse signal of a volunteer before and after exercise.

AbstractThe mechanical properties of fibre‐reinforced thermoplastics and their dependencies on the manufacturing process, fibre properties, fibre concentration and strain rate have been researched intensively for years in order to predict their macroscopic behaviour by numerical simulations as precisely as possible. Including the microstructure in both real and virtual experiments has improved prediction precision for injection‐moulded glass fibre‐reinforced thermoplastics significantly. In this work, we apply three established methods for characterisation and modelling to an injection‐moulded and to a 3D printed material. The geometric properties of the fibre component as fibre orientation, fibre length and fibre diameter distributions are identified by analysing reconstructed tomographic images. For comparing the fibre lengths, a recently suggested new method is applied. Based on segmentations of the tomographic images, we calculate the elastic stiffness of both composites numerically on the microscale. Finally, the mechanical behaviour of both materials is experimentally characterised by micro tensile tests. The simulation results agree well with the measured stiffness in case of the injection‐moulded material. However, for the 3D printed material, measurement and simulation differ strongly. The prediction from the simulation agrees with the values expected from the image analytic findings on the microstructure. Therefore, the differences in the measured behaviour have to be contributed to the matrix material. This proves demand for further research for 3D printed materials for predictable prototypes, preproduction series and possible serial application.

AbstractThe failure strain of a tube is a function of the biaxial strain ratio (axial strain/hoop strain) to which it is subjected. The relationship between failure strain and the strain ratio can be determined experimentally using expansion due to compression tests with a tensile load (EDCT), in which a ductile pellet placed inside the tube is compressed axially so it expands in diameter and imposes a hoop strain on the tube. At the same time, a tensile load on the ends of the tube creates an axial strain. This study investigates the capabilities and limitations of EDCT tests using two devices that allow experiments to be performed on a standard tensile testing machine. The first device applies an axial force on the ends of the sample, and the second device applies an axial displacement. Tests on zirconium alloy tubes confirmed that the failure strain is dependent on the strain ratio and the metallurgical state of the material. EDCT tests can produce a range of strain ratios, but there is an upper limit on the strain ratio that can be obtained, and it is dependent on the plastic behaviour of the sample and the friction conditions between the components.

AbstractA temperature gradient was induced in 316 stable austenitic stainless‐steel tension specimens, and the strain and temperature evolution during tensile deformation was monitored using optical and infrared cameras. The combination of global load with full‐field strain and temperature provided local information on the thermomechanical state of the investigated material. The deformation did not fully concentrate on the hotter portion of the specimen, but instead, the hottest portion strain hardened enough so that the colder portions of the specimen also experienced plastic deformation. Evidently, heat release occurred with plastic deformation and altered the initial temperature gradient as deformation progressed. The Taylor–Quinney coefficient was computed in integral and differential forms, and both are presented as a function of temperature and strain. The Johnson–Cook plasticity model was calibrated through an inverse method procedure in which only five tests were used, and the obtained temperature and strain rate dependencies of the model were comparable to those found in the literature for the same material. A local analysis was done to quantify the impact of adiabatic heating on the mechanical behaviour of the material.

AbstractIn order to improve the thread‐forming quality of diesel engine piston connecting rod, the internal thread cold extrusion process was adopted for the through‐hole structure of 42CrMo4 high‐strength steel. The influence of different structural parameters of the tap on the residual stress distribution of the cold extrusion internal threads and self‐wear were discussed with the help of three‐dimensional elastic–plastic finite element model. The wear of tap was observed by an electron microscope, coordinated with the residual stress distribution of thread was measured by X‐ray diffraction to explore the relationship between residual stress distribution of the cold extrusion internal thread, tool wear and tap structure. The optimal combination of tap structure parameters of residual stress and tool wear was obtained through grey relation method. The residual compressive stress and the depth of stress layer near the thread surface increased and the wear condition of the extrusion tap was improved when the tap with optimised structural parameters was used for processing, while maintaining the original thread connection strength. The results show that the internal thread with better‐forming quality can be obtained by using the optimised extrusion tap, while the service life of the tap was prolonged.

AbstractThe implementation of digital image correlation (DIC) involves several different problems, such as image prefiltering, image intensity gradient calculation, image intensity interpolation at subpixel positions, shape function construction and strain calculation. This paper offers a unified insight into the nature of several key problems in DIC technique. We treat all the problems involved in the former mentioned key steps as fundamentally the same problem, that is, reconstructing an analytical description from a discrete and noisy sampling of a signal, such as discrete image intensity and displacement at some scattered nodes. From the reconstructed analytical description, the gradient and physical value at subpixel or integral pixel position can be analytically calculated without extra error. Here, we solve all these problems using the same mathematical tool, the meshfree method, leading to a unified DIC (U‐DIC) method. This method introduces errors solely during the steps involving the determination of the continuous description. However, it effectively avoids errors in the remaining steps. It holds the best balance between spatial resolution and measurement resolution for both displacement and strain measurements compared to 28 state‐of‐the‐art DIC algorithms based on the benchmark tests on DIC Challenge 2.0. It also holds all the unique advantages of the advanced meshfree DIC (MF‐DIC) compared to conventional local DIC and global DIC. The novel concept and excellent balance between spatial resolution and measurement resolution make U‐DIC an attractive replacement for conventional DIC methods. Additionally, the consistency of the implementation procedures in U‐DIC can simplify the parameter selection in DIC, which is of potential for the standardization of DIC technique.

AbstractAccurate determination of thermo‐mechanical properties in precipitation hardenable materials using an electro‐thermal mechanical testing (ETMT) system is a well‐established challenge. The non‐uniform distribution of temperature resulting from heating based on the joule effect (i.e., resistivity heating) leads to heterogeneous deformation along the gauge length, owing to the temperature dependency of mechanical properties, which makes their direct measurements complicated. This study presents an evaluation of four different miniaturised sample geometries that were tested to achieve an optimised sample with acceptable uniform strain and temperature distributions in the gauge volume. In‐situ displacement mapping, using digital image correlation (DIC), was utilised to calibrate the optimised sample dimensions with the aim of forcing the deformation to the hottest region of the gauge lengths during the tests. Tests were carried out on Inconel 718 (IN718) at 720°C, an optimal temperature for the precipitation of γ″, the primary strengthening particle in this alloy. The results showed that only in the case of the geometry proposed in this study (i.e., a sample with a short gauge length [~2 mm]) did the deformation acceptably localise at the centre, compared to other geometries. A correction methodology is developed that equates the strain measured using DIC over the 2 mm gauge length of the modified sample geometry with the strain measured using the linear variable differential transformer (LVDT) integrated to the ETMT, making future tests on IN718, and other precipitation hardenable materials, possible without the need for the use of a DIC system.

AbstractPerforming in situ scanning electron microscope (SEM) tests is an interesting way to visualise strain heterogeneities under mechanical loading. An essential step before performing the tests is to define the acquisition conditions. The aim of this paper is to propose a classification of the acquisition conditions that are most important for the accuracy of strain measurements using digital image correlation (DIC) in in situ SEM tests. More than 200 image pairs were acquired using a field emission gun SEM. The influence of different acquisition conditions was investigated: acceleration voltage, probe current, working distance, magnification, number of integrated images, image resolution, integration and number of integrated images, scan speed, contrast, brightness and exposure time of the sample in a given area. The methodology implemented in this work is an interesting tool for detecting scan line shift, drift distortion, spatial distortion and rastering artefacts. It allows the optimization of SEM acquisition conditions for strain measurements. Finally, optimal acquisition conditions for in situ testing are proposed and used to perform a tensile test on pure copper. The main factors highlighted include the size of the subset used in the DIC, the beam stabilisation time before image acquisition and the size of the images, which play a significant role in the results. It is recommended to apply the methodology to each device to optimise the acquisition conditions.

AbstractTungsten carbide particles reinforced metal matrix composite (MMC) coatings can significantly improve surface wear resistance owing to their increased surface hardness. However, the presence of macro‐ and micro‐residual stresses in MMC coatings can have detrimental effects, such as reducing service life. In this study, neutron diffraction was used to determine the residual stresses in spherical fused tungsten carbide (sFTC) reinforced Cu matrix composite surface deposits after laser melt injection. We also developed a thermo‐mechanical coupled finite element model to predict residual stresses. Our findings reveal that sFTC/Cu composite deposits produced with a preheating temperature of 400°C have low residual stresses, with a maximum tensile residual stress of 98 MPa in the Cu matrix on the top surface. In contrast, the sFTC/bronze (CuAl10Ni5Fe4) composite deposit exhibits very high residual stresses, with a maximum tensile residual stress in the Cu matrix on the top surface reaching 651 MPa. These results provide a better understanding of the magnitudes and distributions of residual stresses in sFTC‐reinforced Cu matrix composite surface deposits manufactured via laser melt injection.

AbstractPrecise determination of the remaining service life of technical components requires sufficient knowledge of fatigue crack growth behaviour and the growth rate of defects. Cracks in real components often experience multiaxial far field stresses due to their complex geometry and composite loadings acting on it. Digital image correlation (DIC) is well established for crack length and displacement measurements, but it usually requires sample preparation with speckle paint and interferes with mechanical extensometers. To overcome these limitations, we use a novel 2D DIC system combining a graphics processing unit (GPU) with a CoaXPress 2.0 camera, acquiring up to 3 GB/s of image data. It enables real‐time evaluation of both integral strain like an extensometer and full‐field DIC on images selected automatically in real‐time. This combination enables the use of one single sensor for strain‐controlled testing and fatigue crack growth characterisation. The full‐field displacement is compared to a finite‐element model (FEM) simulating the actual crack contour measured by the DIC system. The results show that high‐performance DIC has the potential to simultaneously simplify crack‐growth experiments and provide comprehensive fracture mechanical information.