Experimental Mechanics最新文献

Background

Spent AGR (advanced gas-cooled reactor) fuel cladding may suffer from stress corrosion cracking (SCC) during the interim storage period in cooling ponds and compromise the structural integrity of fuel storage.

Objective

To better understand the effect of SCC, a new small punch test (SPT) setup was developed in this study that can use a small volume of sample to limit the safety concerns about irradiated materials.

Methods

The SPT setup accelerated SCC in a surrogate material 304 stainless steel by introducing a circulation of a heated corrosive solution. Preliminary tests were performed to find the loading and environmental conditions that can develop SCC in the surrogate material. A finite element model was used to estimate the mechanical behaviour of the material during the test.

Results

Several samples were tested under different conditions, and SCC and other forms of corrosion behaviours were observed on the samples. The effects of different corrosive environments were obtained by further characterisation including scanning electron microscopy (SEM) and optical profilometry.

Conclusions

The experiment demonstrated the new setup can develop SCC from a small volume of sample in a short period of time. Several improvements are listed including extra procedures to enable the experiments on the irradiated fuel materials.

Background

Pressure-sensitive adhesives (PSAs) are integral to various industrial applications, yet a significant gap remains in accurately assessing their impact properties under dynamic conditions. This limitation hampers the optimization of PSAs for specific uses where impact resistance is critical.

Objective

This study aims to develop an experimental method to evaluate the impact properties of PSAs, providing a reliable and reproducible technique to assess their performance.

Method

We designed an experimental setup to simulate real-world impact conditions, incorporating high-speed cameras and an image analysis algorithm to capture the adhesive's behavior under sudden loads. The method's novelty lies in its ability to quantify maximum failure load and adhesion failure mechanisms in the dynamic loading of PSAs.

Results

The experimental results reveal critical insights into the impact resistance of various PSA formulations, highlighting significant differences in energy dissipation and failure patterns.

Conclusion

These findings offer new data not previously available in the literature, enabling a more precise evaluation of PSA performance. The developed method provides a robust framework for assessing the impact properties of PSAs, offering valuable guidance for the design and selection of adhesives in applications requiring enhanced impact resistance. This work bridges the gap between quasi-static testing and realistic dynamic performance, contributing to the advancement of PSA technology.

Background

Plates are essential structural elements in many practical applications and commonly undergo buckling failures because of compressive types of loadings. An accurate prediction of buckling loads without destructing the plate has always been an important factor for researchers and designers.

Objective

This study investigates the buckling load of axially compressed plates under rotational restraints through a combined approach of experimental testing and the vibration correlation technique (VCT).

Methods

The experimental setup contains equipment to apply elastic rotational restraints to simulate practical structural conditions. Buckling of the plates with diverse length-to-width ratios (a/b) and the stiffness of rotational restraint K were examined through a specially designed fixture. Additionally, mode shapes through the buckling tests were extracted and the influence of rotational restraints on the post-buckling behavior was discussed.

Results

It is noted that the elastic boundary conditions significantly affected the post-buckling behavior, resulting in notable variations in the load-carrying capacity of the plates. An exponential relationship between the load-carrying capacity and the a/b ratios, exhibiting a systematic decrease as a/b increased from 1.5 to 3. The effect of K on limit loads showed a maximum change of 6% within the scope of the study and it goes to 2% at a/b = 3. However, K is observed to have a significant impact on the post-buckling behavior and the load-carrying capacity in the post-buckling region is almost maintained at higher K values.

Conclusion

The influence of rotational restraints on the prediction capability of the VCT approach is highlighted. Probabilistic error distribution analysis indicates an average error of 12%, with a 99% confidence interval. The outcomes of this investigation contribute to the refinement of predictive models and methodologies for evaluating buckling loads under realistic conditions.

Background

The development of high-speed camera technologies allows phenomena that occur at high speeds to be visualized and analyzed with precision. To ensure the quality of the results and provide metrological traceability, it is important that the camera is calibrated in the time parameter. This work proposes a traceable calibration methodology for high-speed cameras.

Methods

The calibration was conducted using an indirect method in which a laser emits controlled pulses in a 2500 Hz square wave. These pulses are monitored by an oscilloscope while simultaneously being captured by a camera. By comparing the pulses recorded by the oscilloscope with those captured by the camera, the calibration results are determined, and an uncertainty analysis is developed.

Results

With the proposed method, the calibration range was from 5400 fps to 400.000 fps, with an uncertainty of 0.02% at maximum frame rate and the main source of uncertainty comes from the calibration of the oscilloscope.

Conclusion

A detailed uncertainty assessment shows traceability and the quality of the results and the presented calibration method allows for the calibration of cameras up to 400,000 fps, making it suitable for a wide range of dynamic testing applications.

Background

The adiabatic temperature increase at high strain rates can affect the martensitic phase transformation, but the strain rate itself may also play an important role in determining the rate of phase transformation. To date, no systematic work has been carried out to investigate and isolate the effects of strain rate and adiabatic heating on the deformation-induced α′-martensite transformation.

Objective

Uncoupling the effects of high strain rate and adiabatic heating on strain induced martensitic phase transformations in a metastable austenitic steel.

Methods

Strain incremental experiments were carried out with a designed strain control fixture to assess the effect of strain rate effects on phase transitions. The effect of adiabatic heating of the specimens on the phase transformation is assessed by comparing interrupted and incremental tests.

Results

The results of the strain increment experiments indicate that the increase in strain rate has an inhibitory effect on the phase transformation. Comparing the interrupted and incremental tests, the results show that the adiabatic temperature rise inhibits the phase transformation of martensite.

Conclusion

The decoupling of the strain rate and adiabatic temperature increase on α′-martensite transformation was successfully realized by effectively reducing the adiabatic temperature rise of the samples by adopting the strain increment test method during the high strain rate application process.

Background

The shear strength of an engineering material is a critical mechanical parameter, however, its measurement often encounters challenges especially for new materials. Moreover, little research was conducted on the size effect of the shear strengths.

Objective

This study is to determine the specimen thickness effect on the interlayer shear strengths of two types of additively manufactured polymers.

Methods

A combined experimental and numerical investigation of the interlayer shear strength measurement was conducted, and its application targeted polylactic acid and polyamide using fused filament fabrication and selective laser sintering, respectively. A necking-shaped shear specimen was proposed to measure the interlayer shear strengths with the aid of both 3D finite element analysis and 3D digital image correlation.

Results

All specimens showed a consistent pure shear fracture pattern, and the shear strengths increased as the specimen thickness increased.

Conclusions

Future interlayer shear strength measurements should specify a fixed specimen thickness for fair comparisons.

Background

Creating structural materials with mesoscale architectures and functional gradients facilitates the synergistic achievement of outstanding strength, stiffness, and damping, which is essential for effectively mitigating extreme mechanical waves and vibrations. In contrast to conventional stochastic foams, deterministic architected materials fabricated by three-dimensional (3D) printing, such as minimal surface-based gyroid lattices, offer a broad design space to achieve exceptional mechanical performance with efficient material utilization.

Objective

Using 3D printed elastomeric gyroid lattices as a model cellular material system, this work focuses on studying the quasi-static and dynamic mechanical behavior of soft gyroid lattices made from viscoelastic elastomeric materials, as well as the effects of incorporating pre-compressive strain as a strategy to tailor the dynamic performance of gradient gyroid lattices.

Methods

Soft gyroid structures based on viscoelastic elastomeric polymer were 3D printed by stereolithography (SLA). We performed quasi-static compression up to 70% strain to study the mechanical behavior and energy absorption performance of the 3D printed gyroid lattices. Dynamic mechanical analyses in compression mode at different applied static precompressions were conducted to understand the effects of structural gradients on dynamic material properties.

Results

We show that the integration of viscoelastic elastomeric material with gradient architecting—compared with uniform periodic lattices—leads to superior independent control over dynamic material properties. Under harmonic excitations, by leveraging the structural gradient of the gyroid lattice with applied static precompression, we demonstrate a greater tunability of dynamic stiffness in graded-gyroids compared with the uniform gyroid structure. In graded-gyroids, we achieve a substantial enhancement in dynamic stiffness (over 600%) while maintaining the inherent damping capabilities, thus overcoming the common trade-off between stiffness and damping seen in engineering materials.

Conclusion

Our study shows the potential of 3D printed architected cellular structures with tailored structural gradients as advanced lightweight structural materials for extreme damping, shock-absorbing, and robust robotic material applications.

Background

Over-deterministic least-squares methods of extracting SIFs from measured full-field quantities in conjunction with asymptotic fields has been the mainstay of experimental fracture mechanics. The vision-based methods of Digital Image Correlation (DIC) to determine displacements and Digital Gradient Sensing (DGS) to determine stress gradients have played an important role in this regard.

Objectives

In DIC and DGS, two or more orthogonal fields are measured simultaneously. Yet, while extracting SIFs, often only one of the components is picked based on intuition/legacy. This could result in erroneous SIF values under mixed-mode conditions.

Methods

Robustness of SIF extraction by utilizing all components in tandem is demonstrated over a wide range of pure- and mixed-mode conditions. An edge-notched semi-circular specimen geometry is used to create different mode-mixities. The data from DIC and DGS are processed using both the combined fields and legacy approaches. The accuracy and robustness of the former relative to the latter is demonstrated for (a) different number of higher order terms in the asymptotic series, (b) crack tip location uncertainty, and (c) different regions of data extraction.

Results

An order of magnitude reduction in standard deviation and root-mean-squared error in mixed and pure mode SIFs are seen for DIC and the combined fields method. Marginal improvements are seen when the crack tip position or the region of interest are varied in DGS.

Conclusions

Robustness of extracting mixed-mode SIFs accurately by employing all measured fields concurrently in an over-deterministic least-squares approach is superior to using a single component based on intuition/legacy.

Background

Numerical simulation is becoming essential in the mechanical design of sheet metal components, requiring advanced material models, composed of many unknown parameters, to accurately describe complex material behavior. Traditionally, these parameters are identified through multiple quasi-homogeneous tests, each providing specific mechanical data on a particular strain state. The emergence of heterogeneous mechanical tests has revolutionized this process by enabling the capture of a wide range of strain states in a single experiment.

Objective

This study focuses on the experimental analysis of three heterogeneous mechanical tests, previously studied numerically. The main objective is to confirm the quality and relevance of the mechanical deformation observed when using real data and evaluate the sensitivity of these tests to different high-strength steels.

Methods

Uniaxial loading tests were conducted on three different specimen designs, using Stereo Digital Image Correlation to capture the mechanical fields on the surface. Multi-DIC systems were used to measure the out-of-plane behavior observed for a specimen design to increase the strain richness provided by the test. The repeatability of these tests is checked due to their complex designs.

Results

The results show that the potential of heterogeneous mechanical tests remains unchanged when tested in real-world experimental settings.

Conclusions

When combined with full-field measurement techniques, these can provide a wide range of mechanical behavior data from a single test, reducing the number of tests needed for advanced material characterization.

Background

Debonding between a cementitious material and a reinforcement is a mechanical phenomenon of great interest. It cannot be quantified directly through standard tests since it occurs within the material bulk.

Objective

The goal is to develop an experimental method for quantifying debonding during in-situ pull-out tests that also induce damage in the mortar matrix.

Method

A 1/50 scale foundation model is subjected to a pull-out test in an X-ray tomograph. A finite-element-based Digital Volume Correlation analysis with mechanical regularization is conducted based on a three-dimensional mesh constructed to reproduce the geometry of the foundation and reinforcement.

Results

Heterogeneous regularization with a single-node mesh has little effect on the correlation residuals. Using split nodes to describe the interface drastically reduces the correlation residuals in the reinforcement. If cracking occurs in addition to debonding, introducing a heterogeneous regularization based on damaged elements improves the quantification of debonding.

Conclusion

By splitting the nodes at the interface and localizing regularization in damaged elements, the reinforcement and mortar kinematics is better captured and thus debonding as well.