Experimental Mechanics最新文献

Background:

The recent development of Physics-Augmented Neural Networks (PANN) opens new opportunities for modeling material behaviors. These approaches have demonstrated their efficiency when trained on synthetic cases.

Objective:

This study aims to demonstrate the effectiveness of training PANN using real experimental data for modeling hyperelastic behavior.

Methods:

The approach involved two uni-axial experiments equipped with digital image correlation and force sensors. The tests achieved axial deformations exceeding 200% and presented non-linear responses. Twenty loading steps extracted from one experiment were used to train the PANN. The model architecture was optimized based on results from a validation dataset, utilizing equilibrium gap loss computed on six loading steps. Finally, 544 loading steps from the first experiment and 80 steps from a second independent experiment were used for testing purposes.

Results:

The PANN model effectively captured the hyperelastic behavior across and beyond the training loads, showing superior performance compared to the standard Neo-Hookean model when assessed using various evaluation metrics.

Conclusions:

Training PANN with experimental mechanical data shows promising results, outperforming traditional modeling approaches.

Background

Polymers in practical applications often face diverse torsional loads, such as polymeric gears, couplings, scaffolds, etc. Meanwhile, additive manufacturing enables the creation of intricate geometries for specific needs and its application to fabricate various component parts has grown exponentially. Nevertheless, research on cyclic and reversed cyclic torsional loading of additively-manufactured polymers is very limited.

Objective

Mechanical characterization of monotonic, cyclic, and reversed cyclic torsion in polylactic acid (PLA), PLA Premium, and PLA Tough materials.

Methods

Specimens were 3D-printed with a 0° build orientation using an extrusion technique and two infill orientation angles (± 45° and 0°/90°). Specimens were subjected to underwent monotonic, cyclic, and reversed cyclic torsion until failure.

Results

Regardless of material type, ductile fracture governed the behavior under monotonic loading and brittle failure under cyclic and reversed cyclic loadings. Specimens with a ± 45° infill orientation outperformed their 0°/90° counterparts across all materials, with PLA Premium exhibiting superior performance compared to PLA and PLA Tough. Importantly, it was demonstrated that the previously-proposed multilinear idealized shear stress-shear strain curve, developed for monotonic loading of 15 different polymers, also applies to the envelope curves of cyclic and reversed cyclic loading in PLA-based polymers. Thus, it is useful as material model input for numerical simulation purposes.

Background

Compression experiments are widely used to study the mechanical properties of materials at micro- and nanoscale. However, the conventional engineering stress measurement method used in these experiments neglects to account for the alterations in the material’s shape during loading. This can lead to inaccurate stress values and potentially misleading conclusions about the material’s mechanical behavior, especially in the case of localized deformation.

Objective

Our goal is to calculate true stress in cases of localized plastic deformation from standard experimental data (displacement-force curve, aspect ratio, shear band angle and elastic strain limit).

Methods

We use a simple mechanical-geometrical approach based on reasonable physical assumptions to get analytic formulas of true stress and eliminating the need for finite element computations. Furthermore, in numerical simulations of pillar compression, the formula-based true stress demonstrates strong alignment with the theoretical true stress.

Results

We propose analytic formulas for calculating true stress in cases of localized plastic deformation commonly encountered in experimental settings for a single band oriented in arbitrary directions with respect to the vertical axis of the pillar.

Conclusions

The true stress computed with the proposed formulas provides a more precise interpretation of experimental results and can serve as a valuable and simple tool in material design and characterization.

Background

The unique non-uniform polar nanoregions and complex phase structure near morphotropic phase boundaries (MPBs) in relaxor ferroelectric materials lead to rich microstructure changes (domain transition, phase transition) under external field stimulation. This not only results in the material with extremely high electromechanical properties, but also greatly affects their mechanical properties and stability.

Objective

This study investigated the fundamental mechanical properties of the rhombohedral phase (R-phase) and tetragonal phase (T-phase) structures of the relaxor ferroelectric single crystal PMN-PT material using the nanoindentation with different shapes of indenters.

Methods

The basic mechanical properties of the material were measured by nanoindentation, and the fracture caused by indentation was analyzed by scanning electron microscopy.

Results

The elastic modulus of R-phase relaxed ferroelectric materials showed a significant dependence on the indentation depth, and the hardness of different phases (R, T-phase) materials all show obvious indentation size effects (ISE). Under the loading of the spherical indenter, both R and T phase materials exhibited a pop-in phenomenon caused by the transition from elastic to inelastic. Under the loading of the Berkovich indenter, the R and T phase materials showed different fracture characteristics of crack propagation response with the increase of the indentation depth.

Conclusions

The result demonstrate that the mechanical properties of relaxor ferroelectric materials are significantly related to their phase structure, providing guidance for the design of load bearing and material selection in the practical application of related ferroelectric devices.

Background

Stereo-digital image correlation (DIC) measurements can be challenging when working with small specimens. Achieving high-precision data requires careful selection of hardware, stereo-rig design, illumination, speckling, calibration, and minimization of noise levels.

Objective

This study presents an optimized stereo-DIC setup based on parfocal zoom lenses for full-field measurements at magnifications ranging from 0.5× (14.3 × 16.9 mm²) to 2× (3.5 × 4.2 mm²).

Methods

The advantages of using parfocal zoom lenses over fixed-focal-length lenses (and extension tubes) for full-field measurements at small fields of view (FOVs) are demonstrated through quantitative comparisons of the temporal evolution of pseudo-strains and null strain analysis from rigid body translation experiments. The optimal speckling parameters for each magnification are determined by analyzing gray-level histograms, mean intensity gradient ((overline{nabla text{G}})), and subset size. The challenges of calibration at high magnifications are discussed, along with strategies for obtaining acceptable results.

Results

The accuracy of the presented stereo-DIC setup is evaluated through the study of localized phase transformation on a 1 mm diameter superelastic NiTi wire under tension, column buckling, and compression deformations. The presented setup provides highly consistent full-field data over the 0.5–2× magnification range.

Conclusion

The results highlight the benefits of using parfocal zoom lenses for stereo-DIC measurements over a range of small FOVs.

Background

Polymeric foam materials can show a strongly non linear compressible elastic response. For certain applications, it is necessary to know the volumetric behavior of the material under hydrostatic compression. Existing devices for hydrostatic compression testing use a multiaxial testing machine or a fluid to transmit pressure to the foam. They are either complex to set up, or do not allow for hydrostatic pressures of several MPa to be applied or for volume variations of several tens of percent to be achieved. Besides, when pressure is applied to the sample via a fluid, it is difficult to prevent penetration of the fluid into the foam, particularly when it is open-cell.

Objective

This paper presents a hydrostatic compression test for polymeric foams that does not present these limitations.

Methods

A cylinder of a nearly incompressible material (silicone) is molded around a spherical sample of the polymeric foam of interest. The whole set is subjected to confined compression in a rigid chamber. Post-processing is developed, based on finite element analysis, to determine the hydrostatic stress in the foam and its volume ratio from the axial load and displacement data.

Results

Finite element simulations show that the foam sample is subjected to a state close to hydrostatic compression. The test was applied to several samples of elastomeric microcellular polyurethane foams of different densities. The results are in line with expectations, with limited scattering.

Conclusions

The Sphere Foam In Rubber (SFIR) test allows to reach volume reductions of several tens of percents and hydrostatic stress levels of several MPa, on any kind of polymeric foams, provided that its bulk modulus is at least 100 times lower than that of the surrounding nearly incompressible material used. It can be easily implemented with very standard equipment.

Background

Three-dimensional digital image correlation (3D-DIC) is a non-contact monitoring technique that is able to provide accurate three-dimensional strain and displacement measurements. Previous research has shown that 3D-DIC can detect micron-scale cracks in structures as they emerge; however, because 3D-DIC is an optical sensing technique, unfavorable visual conditions due to high heat, large deformations, or a significant distance between the structure and the 3D-DIC cameras can make crack detection difficult or impossible.

Objective

This research aims to develop machine learning algorithms capable of detecting characteristic crack signals in these scenarios.

Methods

Localized point velocities obtained via 3D-DIC were transformed into 2D color images for machine learning segmentation. A novel dataset processing technique was utilized to produce the training dataset, which overlayed simplistic crack analogs on top of the first 50 images from the test. Different parameters from this technique were investigated to determine their effect on the model’s accuracy and sensitivity.

Results

The resulting model detected the onset of significant cracking with an accuracy comparable to acoustic emissions sensors. Varying the processing parameters yielded models that could detect evidence of cracking earlier, at the cost of potentially higher false positive rates. The model also performed well on structures imaged in similar testing setups that were not included in the training dataset.

Conclusion

This data processing technique enables crack detection in scenarios where acoustic emissions and other sensors cannot be used. It additionally allows processes already utilizing 3D-DIC to obtain additional information about material performance during testing or operation.

Background

In the mechanical testing of high-temperature structural materials, ultra-high temperature deformation measurement is very necessary and very challenging.

Objective

To overcome the challenge of using single-camera stereo-digital image correlation (stereo-DIC) for ultra-high-temperature measurement.

Methods

An ultraviolet single-camera stereo-DIC system combining active UV illuminations, an ultraviolet camera, a single UV narrow bandpass filter, a reflective prism and two reflectors was established. In addition, two types of high temperature speckle patterns were prepared A tensile test of C/C composites at 2600 °C was conducted to verify the effectiveness and accuracy of the developed technology.

Results

The ultraviolet single-camera stereo-DIC system has excellent resistance to thermal radiation. As well, the two types of speckle patterns are available at 2600 °C. And the values of elastic modulus calculated by the developed technology and high-temperature extensometer are very close to each other, and the relative errors are less than 7%.

Conclusions

The well matched strain results with high-temperature extensometer data demonstrates that the ultraviolet single-camera stereo-DIC is an effective ultra-high temperature deformation measurement technology and has great potential in characterizing the deformation response of materials at ultra-high temperatures.