Experimental Mechanics最新文献

Background

Digital Image Correlation (DIC) accuracy is dependent on both image discretization strategies and speckle pattern quality; however, a quantitative understanding of their combined influence within the Finite Element-based DIC (FE-DIC) framework remains limited.

Objective

This study aims to systematically evaluate how discretization parameters (element type and size) and speckle pattern characteristics affect the metrological performance of FE-DIC and to introduce a quantitative metric for assessing speckle quality.

Methods

Benchmark numerical datasets from the DIC Challenge organized by the International DIC Society (2D DIC Challenges 1.0 and 2.0), a synthetic beam displacement model, and experimental Mode I crack mouth opening displacement (CMOD) data from basalt rock tests were analyzed using FE-DIC. Four finite element types 3-noded triangle, 4-noded quadrilateral, 6-noded triangle, and 9-noded quadrilateral were tested for element sizes ranging from 10 to 100 pixels. A novel Gradient Ratio Index (GRI) is developed to quantify speckle pattern quality by coupling global intensity gradient information with an element size dependent local black-to-white pixel distribution, thereby serving as a bridge to interpret the combined influence of speckle characteristics and discretization.

Results

Lower-order elements demonstrated superior robustness for fine discretizations and low-contrast speckles, whereas higher-order elements provided enhanced spatial resolution for complex displacement fields. The GRI exhibited a strong inverse correlation with FE-DIC measurement uncertainty and revealed a plateau region that objectively identifies optimal element sizes.

Conclusions

The findings establish a quantitative link between speckle characteristics and discretization strategy, offering practical guidelines for optimizing FE-DIC configuration in experimental mechanics.

Background

Wire and arc additive manufacturing (WAAM) enables rapid production of large-scale metal components, but inherent anisotropy caused by directional solidification remains a critical challenge. The piezomagnetic-mechanical coupling characteristics in WAAM-produced metals and their directional dependencies are poorly understood.

Objective

This research aims to investigate the correlation between piezomagnetic anisotropy and mechanical properties in low-carbon steel produced by WAAM under tensile stress.

Methods

ER70S-6 steel specimens were fabricated via robotic WAAM and extracted at five angles relative to the deposition direction. Monotonic tensile tests with synchronized piezomagnetic measurements were conducted using the universal testing machine and fluxgate magnetometer. Microstructural characterization was performed via optical microscopy and electron backscatter diffraction (EBSD) to analyze grain morphology, phase distribution, and crystallographic orientation.

Results

The results show that as the angle between the tensile loading orientation and the deposition orientation increases, the elastic modulus, ultimate tensile strength, and ductility decrease progressively. The piezomagnetic behavior mirrors these mechanical properties, with more significant magnetic induction changes observed in the direction parallel to the deposition layers. Three characteristic parameters were defined to describe the piezomagnetic properties of the WAAM material, and a linear relationship between these parameters and the printing direction of the specimen was established.

Conclusions

The directional alignment of deposition layers governs both mechanical and magnetic responses through microstructural anisotropy. Piezomagnetic analysis provides enhanced sensitivity for quantifying WAAM material anisotropy, offering a non-destructive diagnostic method to evaluate directional performance in additively manufactured components.

Background

Although considerable progress has been made in understanding the hypervelocity impact behaviours of metallic targets, further quantitative investigation is still needed into crater morphology and rear-face damage, as well as the predictive capability of constitutive models under extreme loading conditions.

Objective

This study aims to experimentally characterize the damage features of Q345 steel targets subjected to hypervelocity impacts and to establish a validated numerical framework for predicting impact-induced damage and deformation.

Methods

Hypervelocity penetration tests are conducted using tungsten alloy projectiles with spherical, ogival, and hemispherical shapes at velocities from 3.0 to 5.5 km/s. Three-dimensional laser scanning and digital image processing techniques are utilized to analyse the morphology, dimensions, and spallation patterns of the craters. A modified Johnson–Cook (MJC) constitutive model, incorporating enhanced damage evolution and a temperature correction term, is implemented into Smoothed Particle Hydrodynamics (SPH) algorithm through secondary development in LS-DYNA, thus forming MJC-SPH method.

Results

Both experimental and numerical simulations indicate that tungsten alloy projectiles undergo melting and fragmentation, accompanied by the ejection of powder and intense light. The steel targets are completely penetrated, and the resulting impact craters exhibit an approximately circular shape, featuring a concave impact surface and lip-like cracks on the rear surface. Increasing projectile velocity significantly enlarges crater dimensions. Ogival and hemispherical projectiles produce substantially larger rear-surface craters than spherical projectiles, indicating enhanced penetration efficiency. Oblique impact results in elliptical crater morphology and severe lip damage.

Conclusions

These advancements enhance the predictive capability for steel structural behaviours under hypervelocity impacts and contribute to the design of impact-resistant protective structures.

Background

Dragonfly wings exhibit exceptional aerodynamic performance, largely due to their passive inertial deformations during flapping, which are crucial for lift generation and attitude control. Understanding and replicating these deformations is key to advancing the design of flapping-wing micro air vehicles (FWMAVs). However, directly measuring the inertial deformations of natural wings in situ remains a challenge.

Objective

This study aims to characterize the full-field inertial deformations of entire dragonfly wings while preserving their natural constraints and posture.

Method

Here we present a novel method combining base excitation with common-path electronic speckle pattern interferometry. By exciting the wing base at its first two natural frequencies, we isolate and amplify the inertial bending and torsional deformations.

Results

Our results reveal that the torsional axis of the wing is coincident with the crest line of its bending deformation. Furthermore, by correlating higher-order mode shapes with the venation architecture, we find that the forewing functions analogously to a 'sail with a spar', whereas the hindwing resembles a sector of an 'umbrella'. This tandem configuration with distinct mechanical roles likely underpins the dragonfly's superior maneuverability.

Conclusions

The proposed method has achieves the measurement of the inertial deformation of the dragonfly wings. The findings provide quantitative insights into the passive deformation mechanisms of dragonfly wings and offer a bio-inspired design rationale for the next generation of high-performance FWMAVs.

Background

Ultra High Toughness Cementitious Composites (UHTCC) and Reactive Powder Concrete (RPC) are advanced cementitious materials ideal for explosion/impact-resistant scenarios, yet conventional triaxial tests (≤ 200 MPa) leave their 1 GPa-level high-pressure mechanical behavior uncharacterized.

Objective

This study aims to address the gap in characterizing the mechanical behavior of UHTCC and RPC under high mean pressures at the 1 GPa level, which exceed the capacity of conventional testing methods.

Methods

An oedometric compression device capable of applying up to 1 GPa of mean pressure was designed and fabricated using 60SiCrVA spring steel. Oedometric compression tests were then conducted to obtain the mechanical behaviors of UHTCC and RPC under 1 GPa of mean pressure. Additionally, based on the data from the oedometric compression tests, the strength surfaces of the HJC model and K&C model, as well as the equation of state (EOS) parameters of the HJC model, for the two materials were calibrated, and a systematic solution process was provided.

Results

The test results show that RPC has higher strength under high mean pressure, while UHTCC exhibits better plastic deformation capacity. The validity of the oedometric compression tests and the calibrated parameters was verified through numerical simulations, as well as comparisons between the triaxial data from the literature and the experimental data in this study.

Conclusions

The oedometric compression device designed in this study can apply a mean pressure of up to 1 GPa to specimens, thus complementing the research on the mechanical properties of UHTCC and RPC under 1 GPa-level high mean pressure.

Background

Deep learning-based digital image correlation (DL-DIC) has emerged as a new paradigm for image deformation measurement, offering a promising and powerful solution to circumvent the limitations of traditional DIC. However, all data-driven deep neural networks used in existing DL-DIC lack a metric to quantify the matching quality at each pixel point, and therefore cannot guarantee reliable or provably correct results. How to assess the image matching quality of DL-DIC and the metrological performance of various DL-DIC networks is therefore a critical and fundamental problem towards its practical use.

Objective

In this work, we propose a simple and effective metric called squared intensity residual (SIR) to assess the pixel matching quality of DL-DIC.

Methods

SIR is defined as the squared intensity difference (or squared gray level residual) of matched pixel points in the reference and deformed images. Also, to mitigate the adverse influence of inevitable image noise and ambient illumination variations on SIR calculation, the reference and target images are first denoised using Gaussian low-pass filtering, followed by the application of an illumination coefficient correction scheme.

Results

Experimental results demonstrate that SIR reliably assesses matching quality and effectively quantifies spatial error distribution of DL-DIC outputs.

Conclusions

The proposed SIR can serve as an objective, fair and practical metric for evaluating the metrological performance of various DL-DIC methods, thereby laying a solid foundation for practical DL-DIC applications.

Background

Hypervelocity impacts of small objects, such as space debris, pose significant challenges to the structural reliability of glass due to the extreme stresses and crack propagation they induce. While conventional optical techniques can qualitatively observe stress wave propagation, quantitative evaluation of stress fields and their association with fracture mechanisms remains a critical gap.

Objective

This study aims to develop and validate a novel in situ photoelastic imaging method for quantitatively analyzing stress wave propagation behavior in glass under hypervelocity impact conditions.

Methods

A new combined experimental and analytical system was developed using a two-stage light-gas gun to conduct hypervelocity impact experiments on silica glass plates (dimensions: 60 × 60 × 15 mm³). Both linear- and circular-polarized photoelastic images were obtained during the experiments using synchronized high-speed cameras. Time-dependent stress fields were calculated via an integrated photoelasticity approach using the onion-peeling method, which was reformulated into an integral representation to accommodate general axisymmetric conditions.

Results

The proposed analysis method successfully determined time-dependent stress-fields in impacted glass plates. Longitudinal stress waves exhibited wavelength elongation due to mechanical responses near the elastic limit under hypervelocity conditions. Observations also revealed dynamic interactions between stress wave propagation and crack initiation, highlighting the critical role of back-surface-reflected stress waves in fracture initiation.

Conclusions

This study demonstrates the utility of in situ photoelastic observation combined with quantitative analysis for exploring stress wave behavior and subsequent fracture mechanisms in glass. The proposed method has the potential to advance the understanding of dynamic material behavior and improve the design and reliability of glasses in high-energy applications, such as space debris protection.

Graphical Abstract

Background

With advancing research in flexible robotics, human biomechanics and material failure mechanisms, higher requirements are imposed on measurement system adaptability. Traditional three-dimensional digital image correlation (3D-DIC) method employs fixed measurement position, limiting measurement range and position without adaptive adjustment capability.

Objective

This study aims to develop an adaptive 3D-DIC method capable of continuously tracking and measuring deformation of targets undergoing large-range both in-plane and out-of-plane motion, and pose variations.

Methods

The 3D-DIC system is affixed to the end effector of a robotic arm, which facilitates camera movement. This method achieves real-time tracking and observation of speckle targets through YOLOv11-based detection combined with position-based visual servoing. It also performs accurate deformation measurements by mapping the reconstructed 3D coordinates to a fixed world coordinate system to eliminate the effects of camera motion.

Results

The proposed system achieved sub-millimeter displacement accuracy, with maximum errors of 0.0558 mm over 190 mm in-plane motion and 0.0699 mm over 165 mm out-of-plane motion. Compared with fixed 3D-DIC, it produced more stable and accurate principal strain fields across large-range motions. Continuous observation was further validated under pose variations (20 poses, up to 105.2°). In finger–glass contact sliding experiments (197 mm lateral and 136 mm longitudinal), the method captured strain-field evolution from onset to steady sliding and localized stick–slip instants via velocity-spike detection.

Conclusions

The proposed robotic-assisted adaptive 3D-DIC method effectively addresses measurement challenges under complex motion conditions. This method effectively facilitates finger-glass contact measurements, providing more authentic experimental data for contact mechanics and haptics.

Background

This work evaluates the efficacy of diffraction based residual stress determination for materials with monoclinic and triclinic crystal symmetry. Rietveld refinements of full diffraction patterns have been successfully applied to low symmetry materials to determine lattice parameter strains, which then can be used to determine residual stresses. However, the monoclinic and triclinic crystal structures with non-fixed lattice angles make it difficult to even conceptualize how to determine an average strain based upon the changes to the four (a, b, c, γ, for monoclinic) or six (a, b, c, α, β, γ, for triclinic) lattice parameters.

Objective

In our approach, we leverage the refinement of an anisotropy strain parameter with lattice parameters to recalculate lattice strains for individual reflections and use those strains as the basis for determining the overall average strain.

Methods

This novel Hybrid Elastic Strain Average, HESA, approach enables the inclusion of the multiplicity and texture information at the single grain family level in the averaging without reconstructing texture. Results: We show the accuracy and uncertainty of the stresses determined during uniaxial testing of monoclinic DU-6Nb using the HESA scheme is essentially equivalent to what can be determined for materials with higher crystal symmetry, e.g. cubic, hexagonal and orthorhombic, which have been reported in the literature for decades.

Conclusions

Quantitative comparison of the stresses determined using HESA and traditional strain averaging techniques demonstrates consistently improved fidelity with respect to the known ground truth stress state.

Background

Room-temperature mechanical behavior of bulk quasicrystal alloys remains insufficiently understood because extreme brittleness and fabrication difficulties have limited systematic studies of strain-rate-dependent failure mechanisms.

Objective

This study aims to clarify how strain rate governs the compressive strength and failure mechanisms of a thermodynamically stable, single-phase Al₆₃Cu₂₅Fe₁₂ quasicrystal alloy and to establish a mechanism-informed failure model valid across loading rates.

Methods

Bulk single-phase Al₆₃Cu₂₅Fe₁₂ was fabricated using rapid hot pressing sintering to overcome processing constraints associated with quasicrystals. Uniaxial compression tests were performed over a range of strain rates, and the associated failure characteristics were analyzed by linking macroscopic responses to micro-mechanistic signatures. A physically based failure model was developed to incorporate the strain-rate-dependent transition in dominant failure mode.

Results

The compressive strength exhibits a nonmonotonic dependence on strain rate, indicating a switch in controlling damage mechanisms. Under quasi-static loading, grain-boundary defect initiation and propagation dominate, reducing strength. Under dynamic loading, dislocation-mediated slipping becomes prevalent, leading to an abrupt increase in compressive strength. The proposed model captures these rate-dependent trends and the underlying mechanism transition.

Conclusions

Strain rate dictates a clear shift from grain-boundary-controlled degradation to dislocation-dominated failure in Al₆₃Cu₂₅Fe₁₂, enabling improved interpretation and prediction of quasicrystal alloy mechanical performance.