Experimental Mechanics最新文献

Background

Main desired features of biaxial tests are: uniformity of stresses and strains; high strain levels in gauge areas; reliable constitutive parameters identification. Despite cruciform specimen suitability to modern tensile devices, standard testing techniques are still debated because of difficulties in matching these demands.

Objective

This work aims at providing rational performance objectives and efficient cruciform specimens shapes in view of constitutive characterization.

Methods

Objective performance is evaluated along particular lines lying on principal directions in equibiaxial tensile tests. A rich specimen profile geometry is purposely optimized via finite elements analysis by varying cost function and material compressibility. Experimental tests, monitored via digital image correlation, are carried out for validation.

Results

New shapes are designed and tested in a biaxial tensile apparatus and show to perform better than existing ones. Elastic parameter identification is efficiently performed by only exploiting full field strain measurements along statically significant lines.

Conclusions

Small gauge areas and small fillet radii cruciform specimens approach the ideal deformation behaviour. For the constitutive parameters identification in planar tensile experiments, it suffices to monitor strains along the gauge lines.

Background

For the static loading test in the aerospace field, conventional strain field reconstruction methods relying on finite element analysis (FEA) or test data are difficult to meet the accuracy requirements of test monitoring.

Objective

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

Methods

An improved strain field reconstruction method based on digital twin (DT) named as DT-SFRM is proposed. The DT is built by data fusion of FEA results and test data, which combines the benefits of these data. The FEA conducted before formal test provides approximate strain field distribution, and the strain gauges data with high accuracy are used to modify FEA strain fields in real time. After that, the real-time DT is used to determine the possible risk regions of test articles. Finally, a large opening cylindrical shell (LOCS) buckling test is conducted to validate the advantages of DT-SFRM.

Results

Results show that the accuracy of DT-SFRM is much higher and less affected by the nonlinearity of test data than that of conventional methods. Compared with the time cost by conventional real-time FEA (about 50 min), the DT method only takes 9s to reconstruct strain field, and the possible risk regions predicted by DT-SFRM are more consistent with test buckling regions of LOCS than conventional methods.

Conclusions

The DT-SFRM is validated to have a higher accuracy and better monitoring effect, and it is more suitable for test monitoring of complex structures.

Abstract

Abstract

Background

Hydrogels are one of the most ubiquitous polymeric materials. Among them gelatin, agarose and polyacrylamide-based formulations have been effectively utilized in a variety of biomedical and defense-related applications including ultrasound-based therapies and soft tissue injury investigations stemming from ballistic and blast exposures. Interestingly, while in most cases accurate prediction of the mechanical response of these surrogate gels requires knowledge of the underlying finite deformation, high-strain rate material properties, it is these properties that have remained scarce in the literature.

Objective

Building on our prior works using Inertial Microcavitation Rheometry (IMR), here we present a comprehensive list of the high-strain rate (> 10(^3) 1/s) mechanical properties of these three popular classes of hydrogel materials characterized via laser-based IMR, further showing that the choice in finite-deformation, rate-dependent constitutive model can be informed directly by the type of crosslinking mechanism and resultant network structure of the hydrogel, thus providing a chemophysical basis of the the choice of phenomenological constitutive model.

Methods

We analyze existing experimental gelatin IMR datasets and compare the results with prior data on polyacrylamide.

Results

We show that a Neo-Hookean Kelvin-Voigt (NHKV) model can suitably simulate the high-rate material response of dynamic, physically crosslinked hydrogels like gelatin, while the introduction of a strain-stiffening parameter through the use of the quadratic Kelvin-Voigt (qKV) model was necessary to appropriately model chemically crosslinked hydrogels such as polyacrylamide due to the nature of the static,covalent bonds that comprise their structure.

Conclusions

In this brief we show that knowledge of the type of underlying polymer structure, including its bond mobility, can directly inform the appropriate finite deformation, time-dependent viscoelastic material model for commonly employed tissue surrogate hydrogels undergoing high strain rate loading within the ballistic and blast regimes.

Background

Nanoindentation experiments require the calibration of the tip area function, which involves up to 9 fitting coefficients following classical method. These coefficients are determined from indentation tests on a reference material. However, their values may vary from one test batch to another. Consequently, these coefficients cannot describe the amplitude of the indenter tip defect.

Objective

The main objective of this study is to propose a contact area function that uses only one fitting coefficient to represent the indenter tip defect. This coefficient corresponds to the distance between the blunt and ideal indenter tip.

Methodology

To demonstrate the efficiency of the proposed contact area function, we reanalyzed nearly 40 calibration procedures, while keeping the same experimental protocol, performed between 2014 and today. A novel two-step calibration methodology is advanced. We compared the results of the proposed method to those obtained with the classic methodology.

Results

This two-step methodology was applied to a fused silica calibration sample. The values of the Young's modulus and instrumented hardness are equals to 71 and 10 GPa, respectively. The length of the indenter tip defect increases gradually from 5 to 30 nm accordingly to the frequency of use of the indenter. The values of the mechanical properties calculated by this methodology are in good agreement with those obtained using the classical contact area function.

Conclusion

The methodology presented in this paper demonstrates its ability to accurately calibrate the tip area function. This new calibration procedure considers both the Young’s modulus and the tip defect parameter as free parameters. Furthermore, the calibration parameters have a clear physical meaning and their values remain stables from one batch to another.

Background

Rolling resistance and aerodynamic losses cause a significant part of a truck’s energy consumption. Therefore there is an interest from both vehicle manufacturers and regulators to measure these losses to understand, quantify and reduce the energy consumption of vehicles. On-road measurements are particularly interesting because it enables testing in various ambient conditions and road surfaces with vehicles in service.

Objective

Common driving loss measurement devices require unique instrumented measurement wheels, which hinders effective measurements of multiple tyre sets or measurements of vehicles in service. For this purpose, the objective is to develop a novel load-sensing device for measuring braking or driving torque.

Methods

The strength of the measurement device is calculated using finite element methods, and the output signal is simulated using virtual strain gauge simulations. In addition to the signal simulation, the device is calibrated using a torsional test rig.

Results

The simulation results confirm that the device fulfils the strength requirements and is able to resolve low torque levels. The output signal is simulated for the novel cascaded multi-Wheatstone bridge using the strains extracted from the finite element analysis. The simulations and measurements show that the measurement signal is linear and not sensitive to other load directions. The device is tested on a truck, and the effort of mounting the device is comparable to a regular tyre change.

Conclusions

A novel driving loss measurement device design is presented with an innovative positioning of strain gauges decoupling the parasitic loads from the driving loss measurements. The design allows on-road testing using conventional wheels without requiring special measurement wheels or instrumentation of drive shafts, enabling more affordable and accurate measurements.

Abstract

Abstract