ABSTRACT In this paper, we chart a path to a method that enables us to extract temporal and spatially varying pressure loading effects on the transient response of steel plates under near‐field blast loading, employing ultra‐high‐speed cameras and DIC to measure the transient deformation field. The study addresses the challenges of obtaining full‐field, high‐fidelity DIC measurements in extreme blast environments by conducting small‐scale detonations in close proximity to steel target plates using two ultra‐high‐speed camera systems. A comprehensive error analysis of these systems is reported and challenges the historic norms of reported accuracy and repeatability in such testing, showing that errors as low as 0.01 mm can be achieved in transient measurements. Pressure measurements obtained via non‐contact DIC are compared with data from Hopkinson pressure bar measurements, providing cross‐validation of the methods. This research highlights the critical influence of several factors on the reliability of the results, including the chosen camera system, the geometry of the target plate and the errors introduced during DIC processing. The study demonstrates that, with careful attention to experimental design, short exposure times, thorough error evaluation for each camera system and consideration of the structural response, blast test results from DIC can be independent of the camera system used. Furthermore, the study finds that the design of the plate for obtaining accurate impulse distributions is more critical than the inherent camera system uncertainties. Within these limitations, the spatial distribution and temporal development of the impulse loading inferred from the DIC velocity data shows excellent correlation with direct measurements of impulse applied to a nominally rigid target by an identical explosive detonation. This offers a path to a method that could achieve the hitherto impossible task of extracting accurate data on the blast load applied in the extreme nearfield to deforming targets.

ABSTRACTThis research proposes a sensitivity‐based framework for selecting the optimal prescribed loading path for a biaxial cruciform specimen. Optimality here is determined by the direction and magnitude of the prescribed displacement that minimizes the influence of random noise on the material model parameter identification. Using simulated experimental data based on finite element simulation, in this work, we identify the material model parameters of a Ludwik hardening model and plane stress implementation of the Hill‐48 yield criterion using finite element model updating (FEMU). Our analysis reveals that the identification (or estimator) uncertainty of model parameters depends on the displacement boundary conditions (i.e., loading sequence) and the ground‐truth value of the individual parameters. Optimal experimental design (OED) criteria based on the Fisher information matrix were investigated to mitigate indecision in the choice of optimal load path when the identification uncertainty of different material model parameters optimized at different load paths. The determinant of the Fisher information matrix was chosen here as the more useful metric due to its ability to capture uncertainty of the most influential material model parameters. The proposed framework demonstrates potential for real‐time automated load step selection using scalar criteria derived prior to mechanical loading. The framework can be generalized to other geometries, boundary conditions and material models, allowing this procedure to be utilized for different experimental configurations and materials.

ABSTRACTPolylactic acid (PLA) is a bio‐based material that can potentially replace environmentally harmful petroleum‐based polymers in engineering applications due to its high strength. The fused filament fabrication (FFF) additive manufacturing (AM) technique has recently been widely used to manufacture PLA‐based components. However, the build orientation in the FFF technique influences the fracture properties in different fracture modes due to the stacking of the material layers. Although there are studies done on the mode I and II fracture behaviour of the PLA manufactured by FFF, no study is available to investigate the effect of build orientation on all three fracture modes, including Modes I, II and III. This study aims to experimentally investigate the effect of build orientation on fracture behaviour (e.g., fracture toughness and crack propagation) of PLA fabricated by FFF for three pure fracture modes (Modes I, II and III). Mode I and II fracture toughness values were extracted using the symmetric and asymmetric four‐point bending tests, respectively. A Mode III testing procedure is developed to apply appropriate loading in a tensile testing machine. Specimens were fabricated by FFF in three build orientations and tested. Fracture toughness and corresponding fracture energy values were calculated for different build orientations. Crack propagation in the samples after fracture was investigated through the images taken with a camera and scanning electron microscope. Based on fracture surfaces, it was observed that the failure could be of interlayer, intralayer and combined types, considering the build orientations. The differences in the fracture surfaces govern the fracture behaviour of PLA materials fabricated in different build orientations. Build orientation was observed to be a dominant factor in all three fracture modes of FFF‐printed PLA. The testing procedures and the fracture properties provided in this study can be used to develop FFF‐printed PLA‐based engineering applications.

ABSTRACTTantalum–tungsten alloy liners manifest intricate non‐uniform deformation attributes during the formation of explosively formed projectile (EFP) with a significant long rod, eliciting heterogeneous deformation gradients at different positions. This phenomenon culminates in localized necking phenomena, precipitating structural failure and subsequent fracture occurrences. To address the elevated temperature, severe strain rates and intense strain conditions intrinsic to the fabrication of tantalum–tungsten alloy EFPs during formation, dynamic high‐temperature and high‐strain‐rate split Hopkinson pressure bar (SHPB) tests were conducted. Furthermore, the integration of the Zener–Hollomon parameter facilitated the establishment of a nuanced correlation between dynamic mechanical parameters (including strain rate, strain and temperature T) and the activation energy Q of deformation for tantalum–tungsten alloy liner materials under the ambit of thermal–mechanical coupling. Leveraging the outcomes derived from LS‐DYNA, a theoretical framework was devised to delineate a critical fracture characterization model for the Zener–Hollomon parameter in terms of strain, thereby facilitating the prediction of fracture occurrences. Notably, experimental validations underscored the efficacy of this model, thereby facilitating precise prognostication of fracture events. The findings of this study can guide the design of tantalum–tungsten alloy liners for long‐rod EFP formation under explosive loading.

ABSTRACTThis paper deals with a method for strain determination around boreholes in plastic specimens in the context of residual stress analyses. In contrast to the frequently used hole drilling method with strain gauges, the strains are determined using digital image correlation (DIC). In the investigations presented, load stresses occur instead of residual stresses, which is caused by defined loading conditions. Furthermore, an adaption of the hole drilling method is developed, called staggered machining, which includes the successive drilling of a larger and a smaller borehole to obtain deformation data in larger depths compared to the conventional approach. For strain evaluation, a special method is used with parametrisation of an annulus around the borehole. In this region, B‐spline approximation of the DIC displacements is carried out using an optimisation procedure of the smoothing parameter. The tests of the entire method show very plausible and precise results. Furthermore, a comparison with simulation demonstrates good correlation of the strain distributions.

ABSTRACTConstitutive behaviour of fusion‐relevant steels in the post‐necking regime is necessary for detailed design of future fusion device components. A finite element model updating (FEMU) procedure using digital image correlation (DIC), particle swarm optimisation (PSO) and a novel post‐necking constitutive law was developed. A DIC‐like strain filter was introduced within the optimisation loop to reduce cost function errors associated with spatial filtering of the DIC processing. The effects of the DIC process and experimental noise on the uncertainty of optimised parameters were quantified using synthetic image deformation techniques. For large plasticity problems, the effect of measurement system noise was found to be negligible. The FEMU method was applied to small‐scale specimens of Eurofer97 ferritic‐martensitic steel of two thicknesses. Strain rate sensitivity was found to be necessary to explain observed post‐UTS uniform strain due to delayed localisation onset. Local strain data from DIC measurements have been demonstrated to improve the accuracy of the identified constitutive model compared to global data alone. Additionally, DIC strain data can provide information to support the inclusion of additional model physics, such as rate sensitivity.

Abstract The mechanical performance of protective coatings is crucial in daily industrial environments, with transition metal nitrides being among the most commonly used hard coatings for machining tool protection. However, determining their elastoplastic properties via conventional methods can be challenging due to the thickness‐dependent response of film/substrate systems. In this study, we utilised two sputtering Ti‐Al‐N films as a model hard thin film/soft substrate system to showcase an alternative methodology to the Oliver and Pharr method. This alternative approach involves determining Young's modulus, yield stress and hardness through indentation stress–strain curves obtained from nanoindentation tests, effectively decreasing the substrate's contribution. This decrease was corroborated by finite element simulations conducted on films with thickness below 1.0 μm. The elastoplastic properties determined using our methodology fell within the range reported for typical Ti‐Al‐N films. Furthermore, by applying our methodology, we were able to correlate and discuss the observed differences in mechanical behaviour between the two films based solely on their microstructural, compositional and morphological properties. Thus, we have demonstrated a viable alternative methodology to address substrate contribution challenges in the mechanical characterisation of thin film/substrate systems when employing an indenter with a large radius of curvature (~650 nm). This research holds potential implications for the design of protective submicrometric films with industrial applications.