Experimental Mechanics最新文献

Background

Due to their flexible configuration and lightweight characteristics, film structures have gained significant attention in the field of aerospace engineering. The scales of film structures typically range from several meters to over ten meters. Stereo-digital image correlation (stereo-DIC) methods offer distinct advantages for obtaining full-field measurement results. However, challenges persist in fabricating high-quality speckle patterns and addressing the problem of imaging reflections, particularly for large-scale transparent or semi-transparent film structures.

Objective

This paper presents an experimental measurement method for large-scale, transparent thin-film structures. The method focuses on fabricating high-quality digital speckle patterns without altering the vibration characteristics of thin film, as well as addressing the problem of imaging reflections.

Methods

A combined large-scale backlighting system and transmission imaging are introduced to solve the problem of reflections. To avoid altering the characteristics of the thin film, a single-particle transfer printing technique is developed. A large umbrella thin-film structure with a diameter of 6 meters is selected to validate the effectiveness of the proposed method. The structure is composed of multiple steel trusses and fan-shaped films.

Results

With high-quality speckle patterns and solving the problem of reflections, the full-field displacement results of the umbrella thin-film structure are measured. The first-order and second-order natural frequencies along with corresponding mode shapes are further obtained.

Conclusion

The effectiveness of the experimental method is demonstrated through rotational and vibration tests conducted on the large umbrella thin-film structure. This method provides a powerful means for studying the mechanical behavior and vibration characteristics of large-scale thin-film structures.

Background

There are a limited number of commercially available sensors for monitoring the deformation of materials in-situ during harsh environment applications, such as those found in the nuclear and aerospace industries. Such sensing devices, including weldable strain gauges, extensometers, and linear variable differential transformers, can be destructive to material surfaces being investigated and typically require relatively large surface areas to attach (> 10 mm in length). Digital image correlation (DIC) is a viable, non-contact alternative to in-situ strain deformation. However, it often requires implementing artificial patterns using splattering techniques, which are difficult to reproduce.

Objective

Additive manufacturing capabilities offer consistent patterns using programmable fabrication methods.

Methods

In this work, a variety of small-scale periodic patterns with different geometries were printed directly on structural nuclear materials (i.e., stainless steel and aluminum tensile specimens) using an aerosol jet printer (AJP). Unlike other additive manufacturing techniques, AJP offers the advantage of materials selection. DIC was used to track and correlate strain to alternative measurement methods during cyclic loading, and tensile tests (up to 1100 µɛ) at room temperature.

Results

The results confirmed AJP has better control of pattern parameters for small fields of view and facilitate the ability of DIC algorithms to adequately process patterns with periodicity. More specifically, the printed 100 μm spaced dot and 150 μm spaced line patterns provided accurate measurements with a maximum error of less than 2% and 4% on aluminum samples when compared to an extensometer and commercially available strain gauges.

Conclusion

Our results highlight a new pattern fabrication technique that is form factor friendly for digital image correlation in nuclear applications.

Background

The damage and failure behavior of ductile metals depends on the stress state as well as on the loading history. Biaxial experiments with suitable specimens can be used to targeted generate different loading conditions, thus allowing the investigation of a wide variety of load cycles with different stress states.

Objective

In the biaxial experiments with the newly presented HC-specimen cyclic shear loads are superimposed by various constant compressive and tensile loads. Buckling during compressive loading in both axes is avoided by an additional newly introduced downholder.

Methods

The strain fields at the surfaces of the biaxial specimens are evaluated by digital image correlation (DIC), and after failure the corresponding fracture surfaces are analyzed by scanning electron microscopy (SEM). Associated numerical simulations employing the presented material model provide information on the current stress states.

Results

The introduced downholder successfully prevents buckling during compressive loading. The strain fields detect a clear influence of the shear direction and a tensile superposition of the cyclic shear load leads to more brittle and a compressive superposition to more ductile behavior. The accompanying numerical calculations reveal the associated, different stress states.

Conclusions

The new experimental program with biaxially loaded specimens for the investigation of damage and failure behavior under cyclic loading enables the targeted examination of a wide variety of load cycles and is thus suitable for the comprehensive analysis of these phenomena.

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.