Experimental Mechanics最新文献

Background

Dynamic mechanical parameters are significant to study the dynamic evolution, but multiple constitutive parameters are difficult to be measured or calculated directly in various dynamic tests. This paper develops a dynamic integrated digital image correlation (DIC) inversion method to characterize multiple parameters of 2A12-T4 aluminum alloy for the split Hopkinson pressure bar (SHPB) test.

Methods

Affine transformation is used to construct virtual deformed images based on time-varying images of dynamic impacts and deformation responses of finite element method (FEM). Through imitating the principle of DIC, the image correlation function embedded the Johnson–Cook constitutive parameters is constructed. The optimal constitutive parameters are obtained by the gradient optimization algorithm when the images are most correlated.

Results

For dynamic tests of same strain rate, optimal parameters are obtained by substituting multiple experimental images to co-invert. By substituting inversion results into the FEM simulation, the simulated stress–strain curve is obtained. Meanwhile by extracting the stress–strain signals of multiple experiments, the real mean stress–strain curve is obtained. Through the coincidence of two stress–strain curves, the feasibility of dynamic integrated DIC inversion method and the accuracy of the inversion results are verified.

Conclusion

This dynamic inversion method is universal to obtain multiple parameters in various dynamic experiments, which considers the strain rate and time-varying. Besides this method simplifies the inversion process and obtains multiple dynamic parameters more easily.

Background

The last two decades have seen a growing trend toward the use of inflatable membranes for spaceborne structures. The spaceborne inflatable membrane structures are the promising solution for the compact and lightweight reflector antenna. The inflation technique is used for pressurizing the inflatable membrane structure once the satellite reaches to its predefined orbit.

Objective

The objective of the study is to demonstrate the use of the residue gas inflation technique for the complete deployment of the inflatable thin membrane boom with different folding patterns. The study also aims to find out generalized relation to calculate the safe mass of residue gases to be kept inside spaceborne membrane boom.

Method

The novel analytical relation for the safe mass of residue gases that can be carried for any size of the inflatable boom is established. A comparative study is performed to investigate the effect of variation in a folding pattern on the proposed inflation technique. Experimental, numerical, and analytical approaches were employed for the proposed study.

Result

The results show that the total inflation time is inversely proportional to the mass of the residue gases. Through the comparative study, it has been observed that the change in the inflation time is negligible for different folding patterns with the same mass of residue gas. The result confirms that the safe mass of residue gas is successfully deploying the inflatable boom in the vacuum environmental condition keeping the stresses in the boom in the tolerance limit.

Conclusions

The findings of this research provide insights into a simple and cost-effective design solution for the inflation system along with safe mass of the residue gases which can be used for any size of spaceborne inflatable boom.

Background

The behavior of shape memory alloys that admit large reversible deformations in response to thermal excitation has been extensively studied in recent years. Yet, the number of works dealing with springs made from these alloys is rather limited in spite of their attractiveness in various applications.

Objective

To bridge this gap we designed and constructed an experimental system for characterizing the behavior of the springs. It enables precise control of the three state variables: temperature, elongation, and force.

Methods

Control of the sample temperature is achieved by immersing it in a water-filled thermal bath, where the water temperature is adjusted using a thermoelectric Peltier device. A tension-compression motorized unit sets the spring elongation and a force gauge is used for measuring the force exerted on the spring. The data is continuously monitored and acquired with a self-coded LabVIEW program. An important aspect is the calibration procedure developed for identifying the spring load-free state and ensuring the repetitiveness of the measurements.

Results

Experiments in which the elongation or the force were measured as a function of the temperature demonstrate the role of the phase transformations. Isothermal experiments enabled to characterize the variations of the force versus the elongation at different temperatures.

Conclusions

The proposed system facilitates the execution of highly accurate experiments through which the complex history-dependent behavior of shape memory springs can be revealed and studied.

Background

The vulnerability of polymeric composite sandwich structures in marine applications to air explosions highlights a significant gap in our understanding of the dynamic behavior of the curved sandwich structures, which is essential for design improvements.

Objective

This study aims to explore the dynamic response and failure mechanisms of curved sandwich composite panels subjected to air-blast loading, providing insights into their structural integrity under such conditions.

Methods

Experiments were performed using laboratory-simulated air shocks generated by a shock tube, employing high-speed photography and digital image correlation to measure deflections on the back surface of the panels. The panels, made with PVC closed-cell foam cores of two densities (H45 and H130), were tested across three curved geometries (radii of 112 mm, 305 mm, and infinity) under various boundary conditions.

Results

Findings indicate an increase in deformation with a decreased radius of curvature under simple support conditions, a trend that reverses under arrested displacement conditions. Moreover, a reduced radius significantly enhances panel strength and resistance to interfacial damage, with the primary failure mode transitioning from core shear cracking to interfacial debonding as core density increases.

Conclusions

The study reveals that the radius of curvature, boundary conditions, and core density significantly affect curved sandwich panels’ dynamic response and performance. Panels with smaller radii and higher core densities exhibit increased strength, though boundary conditions introduce variable effects on deformation behavior.

Background

Polyurethane foams have many uses ranging from comfort fitting seats and shoes to protective inserts in helmets and sports equipment. Current military helmet designs employ foam pads of varying densities and bulk material properties to help absorb energy from impacts ranging from quasi-static to ballistic level strain-rates.

Objective

This study aims to analyze the thermomechanical uniaxial compression behavior of a high density liner foam pad and a low density liner foam pad used in the Advanced Combat Helmet. These experiments were conducted under strain-rates of (10^2) s(^{-1}) and under temperature conditions ranging from -20 to 40 °C. This temperature range was chosen to simulate desert and arctic conditions, with a strain-rate regime chosen to represent loads that would occur often throughout the life of the helmet, such as drops, bumps from riding in a vehicle, or heavy collisions from falling.

Method

Multiple experimental apparatuses were used in this study, including a Shimadzu TCE-N300 thermostatic chamber (used to create the varying temperature environments) and a custom-built drop-test system (used to induce intermediate strain-rates). Every experiment was paired with two accelerometers and a high speed camera used for Digital Image Correlation (DIC) to analyze sample deformation and resultant acceleration. The foam’s mechanical response and energy absorption properties were investigated from the measured stress-strain curves. Additionally, each foam composition was analyzed with X-ray computed micro-tomography (XCT) to investigate microstructure properties pre and post-mortem.

Results

Results show that temperature decreased the energy absorption of the low density composition by 48% ± 5% as temperature changed from -20 °C to 40 °C, while energy absorption increased by 53% ± 16% for the high density composition over the same temperature.

Conclusion

A comparison between the loading response and the material’s density characteristics revealed that the foam’s mechanical properties are heavily dependent on strain-rate applications, as well as environmental factors including temperature. Several important characteristics surrounding each foam composition’s deformation mechanics and damage tolerance as a result of temperature are discussed.

Background

Digital Image Correlation (DIC) is a widely employed full-field measurement technique in the realm of experimental mechanics. Nevertheless, mitigating measurement errors, particularly in fields with large strain gradients, remains a challenge.

Objective

The Gaussian window is employed to weight the correlation criterion in order to enhance measurement accuracy, and this method is called Gaussian window weighted DIC (GW-DIC). However, the optimization of the weighted correlation criterion does not guarantee that the displacement vector iterates to its optimal solution as the Gaussian window parameter changes during the iteration.

Methods

A new power window and the power window weighted DIC (PW-DIC) are proposed. The parameters of this power window keep constant during the iteration, and can be selected by given self-adaptive strategy for accuracy or preset according to the presumed deformation of the region of interest (ROI) for efficiency.

Results

The calculation example of synthetic images with imposed homogeneous deformation indicates that, the proposed power window is more effective than the Gaussian window when weighting the correlation criterion. For multi-directional deformation fields, both the displacement and strain accuracy of PW-DIC with self-adaptive parameters are at least 18% superior to those of conventional DIC. The tensile experimental dataset indicates that PW-DIC is more accurate and stable than GW-DIC.

Conclusions

PW-DIC with self-adaptive parameters is better suited for strain measurement in fields with large strain gradients. The weighted correlation criterion with preset parameters can potentially serve as a substitute for conventional correlation criterion.

Background

The dynamic mechanical properties and permeability evolution of deep rocks under coupled osmotic-mechanical conditions are vital for evaluating the stability of surrounding rock in deep rock engineering and further improving deep mining efficiency. However, there is currently no valid experimental system to measure both the dynamic mechanical response and the permeability evolution of deep rocks.

Objective

In this study, a novel experimental system is developed for determining dynamic compressive properties and permeability evolution of deep rocks subjected to coupled differential pore pressure and confinement.

Methods

The experimental system is composed of a dynamic loading system, an in-situ stress system, a differential pore pressure system, and a data acquisition system. The differential pore pressure system is introduced in the dynamic loading system, and the validation of the proposed system is verified by checking the stress wave propagation in the bars and the dynamic force balance on the two loading ends of specimens. It indicates that the differential pore pressure device added to the dynamic loading system barely influences the measurement of the dynamic behaviors of rocks. A homogenous green sandstone (GS) is employed to verify the feasibility and reliability of the proposed system. Dynamic compressive strength, permeability evolution, and failure mode of GS under cyclic dynamic impact loading in combination with coupled osmotic-confining pressure are explored using the proposed system.

Results

The stress–strain curves change with the increase of impact number, and the cyclic impacts deteriorate the dynamic compressive strength of GS. The permeability of GS first increases and then decreases with the impact number. The differential pore pressure enhanced the permeability of GS under the same impact cycle. The main fracture mode of the GS specimen is mainly compressive-shear fracture in combination with a tensile fracture in the middle of the specimen due to the coupling effect of the reflected stress wave and the osmotic-confining pressure.

Conclusions

The proposed experimental system is valid and effective to measure and observe the dynamic compressive behaviors and permeability evolution of rocks under coupled osmotic-mechanical conditions.

Background

In a previous work, the problem of identifying residual stresses through relaxation methods was demonstrated to be mathematically ill-posed. In practice, it means that the solution process is affected by a bias-variance tradeoff, where some theoretically uncomputable bias has to be introduced in order to obtain a solution with a manageable signal-to-noise ratio.

Objective

As a consequence, an important question arises: how can the solution uncertainty be quantified if a part of it is inaccessible? Additional physical knowledge could—in theory—provide a characterization of bias, but this process is practically impossible with presently available techniques.

Methods

A brief review of biases in established methods is provided, showing that ruling them out would require a piece of knowledge that is never available in practice. Then, the concept of average stresses over a distance is introduced, and it is shown that finding them generates a well-posed problem. A numerical example illustrates the theoretical discussion

Results

Since finding average stresses is a well-posed problem, the bias-variance tradeoff disappears. The uncertainties of the results can be estimated with the usual methods, and exact confidence intervals can be obtained.

Conclusions

On a broader scope, we argue that residual stresses and relaxation methods expose the limits of the concept of point-wise stress values, which instead works almost flawlessly when a natural unstressed state can be assumed, as in classical continuum mechanics (for instance, in the theory of elasticity). As a consequence, we are forced to focus on the effects of stress rather than on its point-wise evaluation.