Experimental Mechanics最新文献

Background

Reliably measuring sharp details in displacement and strain maps returned by full-field measurement techniques remains an open question in the photomechanics community.

Objective

The primary objective of this study is to improve and fine-tune a deconvolution algorithm in order to limit the blur that obscures the details in displacement and strain maps.

Methods

Checkerboard patterns are used and processed with a spectral method, namely the Localized Spectrum Analysis (LSA), and the raw maps returned by this technique are deconvolved. The influence of various settings on the quality of the results is studied by using synthetic images deformed through a well-vetted reference displacement field.

Results

It is shown that linking the size of the analysis window used in LSA on the one hand, and the size of the second derivative kernel employed in the deconvolution algorithm on the other hand, ensures the convergence of the deconvolution algorithm in all cases. This was not the case with the initial version. The ratio between these sizes, which optimizes the metrological performance of LSA followed by deconvolution, is identified. The influence of the sampling density of the checkerboard pattern in the images is also examined. The efficiency of the deconvolution algorithm employed with optimized settings is illustrated with strain maps obtained on two specimens, one in shape memory alloy, and the other in wood.

Conclusions

It is shown in this study that deconvolution with optimized settings is an effective tool to enhance small and sharp details in strain maps obtained with LSA.

Background

The accurate measurement of residual stresses (RS) is crucial for predicting the performance of mechanical components, as RS can significantly impact fatigue life, fracture, corrosion, and wear resistance. Different experimental methods were developed to measure RS, including non-destructive techniques. Among these methods, instrumented nanoindentation has emerged as a promising approach to assess equi- or non-equi-biaxial RS states. This technique analyzes variations in the mechanical response of indentation on a stressed or stress-free component to estimate residual stresses. Previous studies proposed different approaches to establish a relationship between RS and indentation parameters, such as contact area, peak load, mean contact pressure, indentation work, etc. However, the correlation between RS and peak load variation, commonly assumed to be linear, showed limitations, particularly when dealing with compressive RS.

Objective

The aim of this work is to develop a hybrid procedure, based on finite element (FEM) simulations and experimental analyses, to measure the equi-biaxial residual stresses. In particular, it is based on the analysis of the nanoindentation peak load variation generated by the presence of residual stresses on a component.

Methods

To overcome the limitations of the linear assumption, nanoindentation experiments were combined with finite element analyses (FEA). FEA simulations were used to estimate the correlation between RS and peak load variation, providing a better understanding of the non-linear relationship. A proper experimental setup, consisting in a stress generating jig, was designed and manufactured to perform nanoindentations on a sample, made by aluminium alloy AA 7050 T451, subjected to external mechanical stress with the aim to validate the FEA model. FEA and the digital image correlation (DIC) technique were also used to verify that the induced stress field was the expected one.

Results

Obtained results revealed that the proposed method is a valid way to measure residual stresses. In fact, it offers an improved correlation between RS and peak load variation. In addition, by integrating nanoindentation experiments and FEA, a more accurate assessment of RS can be also achieved.

Conclusions

This research contributes to the development of a consistent methodology for RS measurement using instrumented nanoindentation.

Background

Rubber mounts are widely used to isolate vibrating components. Their complex stiffness characteristics, including dynamic stiffness and loss factors, are highly concerning in terms of vibration analysis and optimization. Rubber mounts show non-linear behavior with preload, leading to difficulty to predict their complex stiffness. Dynamic testing is generally necessary.

Objective

An approach to identify the complex stiffness of preloaded rubber mounts in both vertical and horizontal directions simultaneously is developed.

Methods

Tested Frequency Response Functions (FRF) of a mass suspended by rubber mounts are transformed to an FRF matrix of the mass center to decouple the Z Degrees of Freedom (DOF) and RZ DOF from other DOFs, which allows complex stiffness to be identified from the two decoupled DOFs. A software tool to implement automatically the FRF transformation and parameter identification is developed. An EPDM rubber mount is tested using the device and its complex stiffness is identified using the software to validate the proposed approach.

Results

The driving-point FRFs of the mass center calculated from the identified complex stiffness are very close to the corresponding FRFs determined from the test data. The comparison between the Finite-Element Analysis (FEA) results of the surficial FRFs and the test results shows good consistency as well. Therefore, the proposed approach and its supporting algorithm are validated.

Conclusion

the proposed approach allows for swift identification of high-accuracy complex stiffness of preloaded rubber mounts in both vertical and horizontal directions simultaneously.

Objective

Validate and assess the limitations of the Shear Compression 0 Specimen (SCS0) as a simple shear specimen for quasi-static and dynamic large strain loading conditions. Propose a simple data reduction procedure, using a simple, back of the envelope method, as a first approximation for the strain, as opposed to cumbersome numerical simulations and avoid the use of ad-hoc data reduction factors.

Methods

Static and dynamic finite elements simulations were performed in which the large deformation options was turned on and off. Assessment of the Lode parameter in each case and evaluation of the accuracy of the specimen’s strains and stresses as determined through simple data reduction and full numerical simulations.

Results

The SCS0 was shown to undergo simple shear, both statically and dynamically, as evidenced from the very low values of the Lode parameter. The calculated stress is in excellent agreement with the measured one, determined using simple strength of materials definitions. When assuming the corresponding kinematics, it is observed that the calculated and the measured strain diverge to an extent of about 25%. This discrepancy is shown to result from the assumption of large geometrical deformations in the numerical model as opposed to the simple analytical kinematics.

Conclusion

The conclusion is that the SCS0 is now fully validated, and the experimentalist will decide which strain approximation is suitable, between analytical and numerical.

Background

Intermediate-strain-rate mechanical testing of soft and biological materials is important when designing, measuring, predicting, or manipulating an object or system’s response to common impact scenarios. Open source micro-mechanical test instruments that provide high spatial and temporal resolution volumetric strain field measurements, non-destructive testing and gripping of soft materials with low elastic moduli, programmable strain rates spanning from (10^{-6}) s(^{-1}) to (10^{2}) s(^{-1}), and biocompatibility for living cell cultures and tissues in one instrument are lacking in the current literature.

Methods

We introduce a micro-tensile testing device developed to meet all these criteria while being straightforwardly accessible to the end user. This device sits atop an inverted microscope stage, granting the researcher access to 3D spatial resolutions as low as 100 nm and frame rates only limited by the camera speed and availability of recordable photons. The micro-tensile specimen is attached to the test device by a specially designed fixture. This enables a material to be cast into the mold assembly and tested without being manually manipulated before or after testing. The tensile deformation is controlled by two voice-coil linear actuators synchronized to pull a specimen in opposing directions. A field of view focused centrally on the specimen experiences a highly-controllable uniform tensile strain with minimal rigid body motion.

Results

We validate the resulting in-plane strain fields on a 2D poly-dimethylsiloxane (PDMS) substrate and a heterogeneous polyurethane foam using Digital Image Correlation (DIC) and volumetrically on 3D polyacrylamide (PA) hydrogels using Digital Volume Correlation (DVC). High-Rate Volumetric Particle Tracking Microscopy (HR-VPTM) is used to quantify and validate the 3D volumetric strain fields at impact-relevant rates. The device can apply up to 200% engineering strain with peak strain rate up to approximately 240 s(^{-1}) to a 7 mm long dogbone specimen. Proof-of-concept biocompatibility was tested on 2D and 3D in vitro neural cell cultures, demonstrating the versatility and applicability for both soft materials and living biomaterials.

Conclusion

We demonstrate and validate a versatile micro-tensile impact device for soft materials and in vitro cellular biomechanics investigations. The achievable strain rates for such a design are some of the highest we have found reported to date and enable experiments that replicate the full range of observable large material deformations seen during real-world blunt impacts.

Background

Reliable numerical predictive tools are instrumental in the high-end and robust design of encapsulated electronic assemblies. Process optimization and residual stress calculations require a rigorous cure simulation, which considers the transient chemical, thermal and mechanical constitutive behavior of the curing resin. Though this subject has been widely studied for epoxy-based composite materials, fewer studies have been presented on a non-reinforced bulk of low glass-transition temperature (Tg) resin.

Objective

This research aims to numerically and experimentally study the cure behavior and the development of residual stresses and strains in such epoxy based encapsulants.

Methods

The computational study is performed using a commercially available finite element cure process analysis software, and the experimental study is performed by a specially designed test specimen, employing various strain sensing techniques.

Results

The results show good compatibility between experimental and numerical predictions of the thermal behavior and cure-induced residual stresses, which validates the use of the simulative tool for process design. Process induced stress relaxation in the resin is numerically and experimentally demonstrated, which enables a mapping of the process stages at which full viscoelastic modeling is required. The substantial effect of chain mobility on cure shrinkage and residual stress development in this type of materials is numerically demonstrated.

Conclusion

The extensive numerical and experimental investigation of the cure process performed in this study provided insights to both process modeling and design.

Background

Understanding and predicting the behavior of additively manufactured (AM) parts in real-case scenarios is essential for optimizing the design process. Little literature presents a throughout investigation on the influence of the stress state on the anisotropic response of AM materials, and there has not been a great effort to validate the applicability of conventional material models for AM components.

Objective

This work aims to assess the effect of building orientation and the stress state on the mechanical response of as-built laser powder bed fusion (L-PBF) AlSi10Mg and to propose, based on the experimental results, a material model able to represent its mechanical response thoroughly.

Methods

Several mechanical characterization tests, including uniaxial tensile and compressive tests, tensile tests on round-notched bars, and shear tests, were carried out for each investigated building direction (0°, 45°, 90°). The Cazacu-Barlat yield surface was selected to describe the mechanical behavior of the material. Material parameters were identified by inverse calibration and verified using finite element simulation of performed experimental tests.

Results

The results showed a more consistent effect of the building direction on ductility and maximum stress value, while the effect on yield stress was less significant. Under multiaxial stress states, the anisotropic behavior became less noticeable yet present. No anisotropic behavior was observed under shear conditions. In tension and compression, a slight asymmetry in response was noted. Computational results were found in agreement with the experimental data.

Conclusion

The influence of both stress state and of the building direction has been systematically investigated by performing several characterization tests on different sample geometries. In combination with mechanical testing, a material model has been proposed and validated to show the applicability of conventional modeling techniques to AM material.