Experimental Mechanics最新文献

Background

Many aspects of ductile failure through microvoid coalescence remain elusive due to the challenging spatial and temporal scales it operates on. Experimentally resolving all aspects of the process remains a significant goal of researchers. Much of the current understanding has been derived from post-mortem metallography, leaving key aspects of its evolution undocumented.

Objective

This work builds on efforts using X-ray computed tomography (XCT) characterize voids and their evolution under loading.

Methods

It employs in-situ XCT tensile testing on 316L Stainless Steel samples that were constructed by laser powder bed fusion that contain tailored, pre-existing voids with a spatial scale relevant to the growth and evolution stages of microvoid coalescence. Pre-existing voids extended the observation window for monitoring void growth and interaction under loading. They also enhanced fiducial correlation of voids during deformation.

Results

Void populations were found to increase under loading as their deformed dimensions rendered them detectable by the XCT algorithm. Neighboring voids underwent interconnection events by a cleavage process when stress concentrations between them exceeded the macroscopic yield stress. Pores that did not undergo interconnection events were found to revert to their initial size and population after unloading. Finally, the porosity structure before failure was correlated to features on the fracture surface with high fidelity.

Conclusions

This unique combination of in-situ XCT tensile testing on samples with tailored void structure enabled new visualization and quantification of void evolution under load as well as strong correlation to the observed stress–strain behavior and post-mortem fracture characteristics.

Background

The Two-Dimensional Digital Image Correlation (2D-DIC) method is widely used as a non-contact full-field kinematic measurement, but it presents significant errors related to temperature effects including the image con- trast and heat waves. Consequently, results of mechanical displacement or strain measured by the 2D-DIC method, especially strains in the elastic domain of materials, is significantly dispersed.

Objective

The aim of this study is to propose a very simple 2D-DIC method, using commercial DIC software with no need of additional storage to accurately measure strain and displacements at high temperatures, typically at the hot metal forming temperatures, from 400 ^◦C to 750 ^◦C.

Methods

This study demonstrates the influence of temperature effects (radiation and heat waves) on strain measurements obtained with the 2D-DIC method in the elastic regime (ε < 0.05) of the TA6V titanium alloy material at high temperatures. Furthermore, the strain measurement errors at different temperatures were characterized by the Background Oriented Schlieren technique (BOS). Correction methods using temperature-dependent low-pass filters for strain measurement errors are suggested.

Results

The correction methods allow separating mechanical strain fields and strain measurement errors caused by temperature effects. The efficiency of the correction methods is demonstrated by identifying the Young’s modulus (E) and the Thermal Expansion Coefficient (TEC) of the TA6V. After corrections, E and the TEC of the TA6V are close to the reference values found in the literature. Conclusion: The coefficient R² from the linear regression method to determine the Young’s modulus from tensile test at 600 ^◦C increases from 0.783 to 0.989, revealing the great potential of using the improved-2D-DIC method for full-field kinematic measurements of mechanical tests at high temperatures.

Background

Finite element analysis (FEA), digital image correlation (DIC), and strain gauge are usually used for test monitoring. Reconstructed by FEA strain field and strain gauges, the fusion field (DT-SFRM) has higher accuracy than FEA, and DT-SFRM accuracy is acceptable for small structural deformation. However, large deformation of thin-walled structures is accompanied by complex strain field, and the difference in strain distribution between FEA and strain gauges data increases sharply. The low-accuracy FEA strain field and small amount of strain gauges lead to insufficient accuracy of DT-SFRM.

Objective

This study aims to construct a high-accuracy strain field for test monitoring.

Methods

A strain field reconstruction method based on digital twin (DT) by multi-level fusion of multi-type data (DT-MFMD) is proposed. Large number of relatively high-accuracy DIC data is used to improve global FEA strain field accuracy. The high-accuracy strain gauges are used to further improve fusion field accuracy of FEA and DIC. A tensile test of large opening plate shell (LOPS) is performed to verify the advantage of DT-MFMD.

Results

The AvgErr (6.6%) and MaxErr (23.5%) of DT-MFMD are 16.0% and 58.8% lower than those of DT-SFRM, and the DT-MFMD is less affected by large deformation. In addition, the strain gauges number of DT-MFMD reduces by more than 62.5% under the same accuracy goal compared with that of DT-SFRM.

Conclusions

The DT-MFMD is validated to have a better application prospect for large deformation and complex strain field distribution.

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.