Experimental Mechanics最新文献

Background

The mechanical properties of biological tissues change over time and with disease progression. Quantifying these mechanical properties can thus be instrumental for medical diagnosis and for evaluation of tissue viability for transplant. However, soft and biological materials are exceptionally challenging to mechanically characterize using conventional testing methods, which are hindered by limitations of sample size, fixturing capabilities, and sample preparation.

Objective

We hypothesize that Volume Controlled Cavity Expansion (VCCE) is well-suited to capture subtle mechanical differences in biological tissue. The objective of this work is therefore twofold: first, we seek to quantify how stiffness of liver and gelatin evolve with age. In achieving this understanding, we aim to demonstrate the precision of VCCE in measuring subtle changes in the mechanical properties of biological tissues.

Methods

Performing VCCE tests over 15 days in samples of gelatin and liver (porcine and bovine), we track the evolving pressure-volume response and deformation limits of the materials.

Results

In both materials, we observed time-dependent variation of the stiffness and fracture thresholds. In gelatin VCCE repeatably captured stiffening over time, which was correlated with a higher fracture stress. This was in contrast to observations in bovine liver, where stiffening corresponded to a lower fracture stress. Porcine liver initially stiffened, then reversed this trend and relaxed.

Conclusion

Through this work we show that liver and gelatin stiffen with age, and that this trend is measurable via VCCE. These results highlight the utility of VCCE and call attention to the need for a new class of mechanism based constitutive models that are capable of capturing variations in material over time with a minimal number of parameters.

Background

Improving the understanding of how a refractory material responds to thermal shocks and allowing the validation of finite element models require a valuable tool for experimental data collection.

Objective

This paper introduces an innovative, sophisticated, and highly reliable experimental device designed to apply a controlled cyclic thermal gradient in a disk-shaped ceramic refractory sample and to simultaneously monitor thermomechanical response and potential damage.

Methods

This device, named Advanced measurements for in-situ Thermomechanical monitORing of large sample uNder thermal grAdient, is based on a CO₂ laser beam to generate a calibrated thermal flux sequence at the top face while accurately measuring temperature field at the bottom face by an infrared camera. The displacement field of the bottom face is also continuously monitored by a stereo-vision system, enabling a precise measurement of 3D displacements and, thus, of the local strains. An accurate monitoring of the crack extension is performed thanks to the Two-Part Digital Image Correlation technique.

Results

Throughout the thermal cycling sequence applied to an exemplar sample, the device has proved to be a robust and reliable system able to provide very accurate experiment data in terms of displacement, strain, temperature fields and crack length/opening.

Conclusions

This device represents a significant advancement in in-situ monitoring of a refractory sample and contributes to the comprehensive characterization of materials under thermal gradients. More investigations and comparison with thermomechanical Finite Element modelling are shown in a second part of this paper.

Background

Transparent armor systems are traditionally designed following a trial-and-error approach, which involves high development costs associated with ballistic testing. This research article presents a novel methodology, termed quasi-static multi-punch shear testing, within the domain of transparent armor systems.

Objective

The primary aim is to establish a correlation between multi-hit ballistic tests at Level III-A according to the NIJ 0108.01 standard, achieved through an adaptation of the single-shot ballistic limit methodology, and the quasi-static multi-punch shear testing. The objective is to utilize a simple experimental methodology that provides insights into the multi-hit ballistic behavior of transparent armors.

Methods

Parameters such as absorbed energy and observed damage mechanisms were utilized to assess the potential relationship between these tests. Transparent armor samples that underwent testing using the quasi-static multi-punch shear test were subsequently cross-sectioned using a water jet cutting machine to facilitate visualization of material damage. In addition, drawing on insights from quasi-static multi-punch shear testing results, the K-means clustering algorithm was employed to predict the likelihood of a specific transparent armor system passing a multi-hit ballistic test.

Results

Various damage mechanisms were observed as a function of the punch displacement, and correlations were made with the load–displacement curves. Furthermore, the implementation of the K-means clustering algorithm successfully classified transparent armor into two groups: those that passed the ballistic test and those that did not.

Conclusions

This research significantly advances understanding of transparent armor system behavior under multi-hit conditions and offers a promising predictive tool for evaluating their performance through straightforward and cost-effective experimentation.

Background

Since residual stresses can affect the mechanical performance of in-service engineering structures, accurate evaluation of biaxial residual stresses is of great significance to service safety. Previous indentation methods for evaluating biaxial residual stresses usually require a stress-free sample as a reference, which is difficult to obtain in engineering.

Objective

In present work, a spherical indentation method for evaluating biaxial residual stresses without using stress-free sample was established.

Methods

To avoid using stress-free sample, a method for deriving the loading work in unstressed state from the load-depth curve in stressed state, was established. Through dimensional analysis and finite simulations, biaxial residual stresses were quantitatively correlated to the fractional change in loading work between stressed and unstressed states, and the flattening factor of residual imprint. Based on such correlations, biaxial residual stresses can be evaluated without using stress-free sample. A biaxial stress-generating jig was used to validate the method experimentally.

Results

Finite element analyses and experimental results demonstrate that the proposed method could evaluate biaxial residual stresses with reasonable accuracy.

Conclusions

Combined with portable micro-indentation device, the proposed method has broad application prospects in evaluating biaxial residual stress of in-service engineering structures.

Background

The anisotropic behavior of sheet metal has a significant influence on the plastic forming process, especially the accurate description of the plastic flow (r-value).

Objective

The purpose of this paper is to reveal the influence of different evolutionary r-value solution methods on yield model prediction.

Methods

Uniaxial and biaxial tensile experiments were carried out for DP590. The principles and results of r-value calculation based on slope method and polynomial fitting method are compared and analyzed. On this basis, the inverse solution method based on exponential function is established. The prediction results of Hill48, Hill48-non and Yld2000-2D yield models based on different r-value solution methods were compared. Furthermore, anisotropic yield models calibrated by different r-value solution methods are used as user material subroutine (VUMAT) to achieve cup-drawing simulation in ABAQUS.

Results

In the theoretical prediction of anisotropic yield model, the prediction accuracy of the three yield models is the best when the new inverse exponential function method is used. The earing height obtained by the three yield models based on inverse exponential function method calibration is more consistent with the physical experiment, while the prediction accuracy of the traditional slope method is the worst.

Conclusions

The appropriate r-value solution method can improve the predictive accuracy of anisotropic yield model. The research results provide a method for the anisotropic parameter calibration strategy of yield model considering anisotropic evolution, and provide an effective reference scheme for improving the prediction accuracy of deformation behavior in sheet metal stamping.

Graphical Abstract

Background

the development of additive manufacturing technologies (3D printing) has made it possible to manufacture complex structures such as architected materials. However, traditional inspection methods are not suited to these materials, which require volume inspection to examine their internal structure.

Objective

the aim is to provide a 3D shape measurement method based on the initial computer-aided design (CAD) model used for 3D printing and X-ray radiographs.

Method

the CAD model is deformed until its virtual radiographs obtained by simulating the absorption of X-rays through the solid register with experimental radiographs. This registration is achieved by minimising a cost function with respect to the position of control points using radial basis function interpolation.

Results

the method’s performance is first evaluated using synthetic data. Its robustness is assessed with respect to image resolution, number of radiographs and noise level. Subsequently, the geometry of a solid with a tetrahedral architecture was quantified by means of a mere five radiographs. Global variation in shape and local defects in lattice structure can be detected.

Conclusions

the method enables the in-volume shape of architected materials to be checked without reconstructing the 3D computed tomography volume, but from just a few radiographs. It is robust and can detect local defects.

Background

In situ Computed Tomography is a valuable tool to investigate failure mechanics of materials in 3D. For brittle materials with sudden fracture like concrete however, state-of-the-art methods such as Digital Volume Correlation fail to produce displacement fields that display the discontinuous behavior of load induced cracking correctly.

Objective

The main objective is to develop an algorithm that calculates displacement fields for large-scale in situ experiments on concrete.

Methods

The algorithm presented is based on a 3D Optical Flow method solved by a primal-dual procedure and equipped with a coarse-to-fine scheme based on morphological wavelets. The algorithm is publicly available. Our evaluation focuses on the beneficial use of morphological wavelets over classical ones, and on the ability to produce reliable results with limited data. Applying the primal-dual scheme to in situ tests and using morphological wavelets are novel contributions.

Results

The results show that our algorithm cannot only cope with large volume images, but also produces discontinuous displacement fields that yield high strain in fractured regions. It does not only perform better than state-of-the-art methods, but also achieves sufficient results on reduced data. The morphological wavelets play a key role in this finding - they even allow to deduce cracks of widths less than a voxel.

Conclusion

Displacement calculation for in situ tests of brittle materials requires voxel-accurate displacement fields that allow for discontinuities. The presented algorithm fulfills these requirements and therefore is a powerful tool for future understanding of failure mechanics in concrete.

Background

Nickel-based superalloys are key materials for aero-engine hot-end components, and fatigue is one of their most typical failure forms. In the field of fatigue research, in-situ characterization of crack growth behavior is crucial, and more intuitive and accurate characterization methods need to be developed.

Objective

In this work, to better understand their fatigue crack growth behavior, we have developed new methods for in-situ characterization of crack growth behavior using laser thermography detection technique.

Methods

According to the thermal images of sample surfaces captured during the fatigue process, a method for positioning crack tip based on Prewitt edge detection is proposed, and a novel parameter, i.e., the crack opening temperature gradient (COTG), is defined to evaluate the crack closure effect.

Results

Based on the variation characteristics of COTG with load rate, the crack initial opening load rate (CIOLR) and crack opening load ratio (COLR) can be determined under different fatigue cycles. The results show that CIOTG and COTG tend to decrease with increasing fatigue cycles.

Conclusion

This work provides a visual and quantitative in-situ method for crack detection and characterization of the crack closure effect in fatigue testing.