Experimental Mechanics最新文献

Background

Residual stress development in precipitation strengthened aluminium foundry alloys has seen little attention, despite the prevalence of their use over a wide array of applications.

Objective

This study aims at the evaluation of the residual stress in a cast aluminium benchmark that develops during precipitation heat treatment and determines the preferable stress relaxing techniques for such applications.

Methods

The stress states in the as-cast, T4 and T6 tempers of the same AlSi7Cu0.5Mg (A356 with 0.5 wt% Cu) sample were determined through a novel application of the contour method, standard hole drilling, deep hole drilling and incremental deep hole drilling.

Results

The results of all measurement techniques lie within approximately 40 MPa for all regions available for comparison, with the greatest differences occurring between the contour method and deep hole drilling for the T6 component. It is shown that the peak tensile residual stresses are almost identical between the heat-treated components (75 MPa), but the distribution and magnitude of compressive residual stress are found to be significantly different.

Conclusions

Among the measurement techniques evaluated, the contour method and incremental hole drilling are found to be more suitable for T6 temper, while all techniques perform equally well for T4 temper due to its relatively low strength. It is hypothesised that the difference between the as-cast and heat-treated samples is due to solution heat treatment and quenching, while the difference in T4 and T6 tempers is attributed to the response to ageing.

Background

Comprehensive datasets quantifying the coupled thermo-mechanical and electrical properties of shape memory alloys (SMAs) are lacking, as are standardized techniques for robust characterization. This hampers accurate modeling and design of SMA-based components. Objective: This work develops an automated experimental system to enable simultaneous measurement of stress-strain-temperature behavior and electrical resistivity evolution in NiTi SMA wires under controlled stress conditions. Methods: Customized test frames apply precise mechanical stresses while allowing for in situ electrical measurements and infrared imaging during complete thermal cycling protocols. Specialized instrumentation including a Keithley 2002 multimeter, Agilent E3631A programmable power supply, and FLIR A615 thermal camera are integrated with LabVIEW-based software routines for complete automation of the characterization process. Rigorous metrology principles are implemented throughout the measurement procedure to improve accuracy, repeatability, and consistency compared to prior manual techniques. Results: Extensive datasets are generated which reveal pronounced stress-dependencies in key SMA material parameters including transformation temperatures, recoverable strain, and electrical resistivity. A 3D regression model describes the comprehensive relationship between resistivity, temperature, and applied stress across the entire characterization domain. Conclusions: The automated measurement framework and methodology establishes a foundation for high-fidelity, reliable acquisition of coupled SMA property data. This will enable more accurate modeling and design of components and systems incorporating SMA actuation or sensing functions.

Background

X-ray imaging addresses many challenges with visible light imaging in extreme environments, where optical diagnostics such as digital image correlation (DIC) and particle image velocimetry (PIV) suffer biases from index of refraction changes and/or cannot penetrate visibly occluded objects. However, conservation of intensity—the fundamental principle of optical image correlation algorithms—may be violated if ancillary components in the X-ray path besides the surface or fluid of interest move during the test.

Objective

The test series treated in this work sought to characterize the safe use of fiber-epoxy composites in aerospace and aviation industries during fire accident scenarios. Stereo X-ray DIC was employed to measure test unit deformation in an extreme thermal environment involving a visibly occluded test unit, incident surface heating to temperatures above 600^oC, and flames and soot from combusting epoxy decomposition gasses. The objective of the current work is to evaluate two solutions to resolve the violation of conservation of intensity that resulted from both the test unit and the thermal chamber deforming during the test.

Methods

The first solution recovered conservation of intensity by pre-processing the path-integrated X-ray images to isolate the DIC pattern of the test unit from the thermal chamber components. These images were then correlated with standard, optical DIC software. The second solution, called Path-Integrated Digital Image Correlation (PI-DIC), modified the fundamental matching criterion of DIC to account for multiple, independently-moving components contributing to the final image intensity. The PI-DIC algorithm was extended from a 2D framework to a stereo framework and implemented in a custom DIC software.

Results

Both solutions accurately measured the cylindrical shape of the undeformed test unit, recovering radii values of (R = 76.20 pm 0.12) mm compared to the theoretical radius of (R_{theor}=76.20) mm. During the test, the test unit bulged asymmetrically as decomposition gasses pressurized the interior and eventually burned in a localized jet. Both solutions measured the heterogeneous radius of this bulge, which reached a maximum of approximately (R=91) mm, with a discrepancy of 2–3% between the two solutions.

Conclusions

Two solutions that resolve the violation of conservation of intensity for path-integrated X-ray images were developed. Both were successfully employed in a stereo X-ray DIC configuration to measure deformation of an aluminum-skinned, fiber-epoxy composite test unit in a fire accident scenario.

Background

One of the biggest challenges in soft robotics is the variability and controllability of stiffness. Jamming-based approaches have been of interest to change stiffness dramatically by increasing friction between grains, layers, or fibers. Besides, magnetorheological elastomers (MREs) that exhibit magnetic field-dependent viscoelasticity have significant potential as a stiffness variation material. This study investigates the unique mechanics of magnetic jamming of MRE sheets exploring stiffness change both due to jamming and variable viscosity.

Methods

Sample MREs and flexible neodymium-iron-boron (NdFeB) magnets are manufactured. Uniaxial tensile tests supported with digital image correlation are performed to characterize the materials. Multi-layer jamming structures comprised of MREs and NdFeB magnets are developed and validated through 3-point bending experiments and finite element simulations.

Results

Results show that the stiffness of the multi-layer structure is higher under magnetic field. Furthermore, the stiffness change is increased when MREs are used instead of PDMS as layers.

Conclusion

This study proves the concept of magnetic jamming of MRE layers. The results are crucial for the possible soft robotic implementation of the proposed hybrid stiffening approach combining jamming with viscoelasticity modification.

Background

Characterizing material properties of thin sheets for design or manufacturing purposes is an essential concern in many engineering applications. This task is particularly challenging for materials with a pronounced anisotropic and nonlinear mechanical behavior.

Objective

A hybrid, experimental-numerical approach for the characterization of the mechanical, nonlinear response of thin, anisotropic, deformable materials is proposed. In contrast to classical approaches where various biaxial tension tests are analyzed, the main goal here is the complete characterization based on one single experiment.

Methods

The proposed approach is based on a novel non-standard experimental setup which is on the one hand easy to install and use, and which on the other hand intentionally induces a strongly inhomogeneous strain field in the specimen capturing as many deformation modes and intensities as possible. The resulting displacement field can be measured using e.g., digital image correlation, and is then accessible to the parameter identification as full-field data. To allow for an efficient identification, an extended equilibrium gap method is presented, where unknown boundary force distributions applied in the experiment are computed iteratively. The approach’s feasibility is assessed through virtual full-field data obtained by numerical simulation of the proposed experimental setup using predefined parameter values and applying realistic noise. That way, a quantitative assessment of the method’s performance regarding two specifically chosen material models is enabled.

Results

Provided that the stiffness-related material parameters are indeed linear in the stress equations, a quadratic optimization problem can be constructed to allow for a unique identification of the parameter values. Analysis show that reference parameter values for calendered rubber as well as coated textile fabric can be identified, even when realistic noise is applied to the virtual test data.

Conclusion

Based on the presented investigations, the proposed method has been found to be feasible for the accurate identification of stiffness-related parameters of anisotropic, nonlinear thin sheets using a single experiment.

Background

The elastic modulus of polyethylene terephthalate (PET) sheets is typically measured through destructive tests that require specific sample preparation and time-consuming testing procedures.

Objective

To improve the efficiency of measuring the elastic modulus of PET sheets, research on a non-destructive measurement approach using guided Lamb waves was conducted.

Methods

In this approach, the group velocity of the zero-order symmetric Lamb wave mode (S0 mode) at a single frequency is first measured from PET sheets. The semi-analytical finite element method (SAFEM) is used as the forward model to calculate the corresponding numerical group velocity. Particle swarm optimisation (PSO) is used to update the elastic modulus in the SAFEM model until the numerical group velocity from the model matches the experimental results.

Results

The results show that measuring the group velocity data at a single frequency is sufficient for elastic modulus measurement while the material thickness can be assumed as a constant, which improves the efficiency of the measurement. The identified modulus differs from the tensile modulus of the material due to the frequency dependence of the elastic modulus. However, this discrepancy could be eliminated by using a linear regression model.

Conclusions

The method mentioned above can achieve non-destructive and efficient measurement of the elastic modulus of PET sheets, which can potentially be applied for in-line quality inspection in PET bottle production processes.

Background

Shear forces related to friction are one of the risk factors shown to increase the likelihood of wheelchair users developing pressure injuries (PIs), especially in the buttocks region. Thus, reducing the coefficient of friction between the seat and the person’s pants could be a means toward reducing the incidence of PIs.

Objective

This study aimed to: (1) determine the coefficients of friction of seven commonly worn pant fabrics and two seat cover fabrics and (2) investigate the effects of a deformable seat cushion on the measurement of the coefficients of friction.

Methods

A mechanical system (termed sled) had a pants fabric secured to its bottom and was placed on top of a custom seat pan with seat cover. The seat pan was tilted from horizontal until the sled system started to slide on the seat pan. The coefficient of friction between the two fabrics was calculated using kinematics data obtained from a motion capture system.

Results

The office fabric seat cover produced smaller coefficient of friction than the vinyl seat cover for all of the pant fabrics. Women’s khakis demonstrated one of the smallest coefficients of friction, and denim demonstrated one of the largest coefficients of friction consistently across both seats covers.

Conclusion

Replacing traditional vinyl seat covers in wheelchairs with a fabric closer to an office seat fabric is one approach to reducing frictional forces at the seat interface. Although optimal pant fabrics can be identified for shear force reduction, the results also depend on the seat cover fabric.

Background

The unit pulse integral method is used extensively with the incremental hole-drilling residual stress measurement technique. The ASTM E837 standard, which applies only to isotropic materials, recommends the use of Tikhonov regularization to reduce instability when many depth increments are used. In its current formulation, Tikhonov regularization requires the decoupling of stress, as is possible for isotropic materials. The fully coupled integral method is needed for residual stress determination in layered composite laminates and is currently employed without Tikhonov regularization. This causes greater sensitivity to measurement errors and consequently large stress uncertainties. An approximate method of applying Tikhonov regularization exists for biaxial composites, but is not applicable to more complex laminates.

Objective

Extend Tikhonov regularization to the fully coupled integral method to improve residual stress determination in composite laminates.

Methods

This work investigates the use of the approximate and fully coupled regularization approaches in an angle ply composite laminate of [+45/-45/0/90]_s construction. Experimental validation in a [0/+45/90/-45]_s laminate is also presented where the regularized fully coupled integral method is compared to the series expansion method that includes all in-plane stress and strain directions simultaneously in a least-squares solution.

Results

The regularized integral method produces comparable results to those of series expansion while requiring twelve times less FE computation to calculate the compliances. The optimal degree of regularization is also more convenient to determine than the optimal combination of series order required by series expansion.

Conclusions

The new method is easily applied and should find wide application in the measurement of residual stresses in composite laminates.

Background

Advanced ceramic matrix composites (CMC) are aimed at highly demanding thermo-mechanical environments, such as space or hypersonic applications. Their design and manufacturing require a reliable, cost-effective method for estimating and quantifying CMC mechanical properties.

Objective

CMC composite materials present orthotropic mechanical properties. Different tests are often conducted to measure all independent elastic properties. These variety of tests are expensive and demand significant time and effort. Therefore, it is highly desirable to have one mechanical test that can generate all or most of the elastic properties.

Method

This study proposes a single Brazilian disc (BD) test setup. It investigates an efficient inverse problem to evaluate the Young’s and shear moduli of an orthotropic Liquid Silicon Infiltrated (LSI) C/C–SiC CMC. A combined BD test and Digital Image Correlation (DIC) technique were used for this purpose. The BD test employs a circular disc compressed diametrically in varying orientations of the tested multi-layered CMC 0°/90 ^o woven carbon fibers. The BD introduces a multi-axial stress and strain field highly affected by these primary elastic moduli. DIC enables full-field multi-axial strain calculations in large spatial variations.

Results

A numerical study resulted in a unique iterative inverse-mechanics test procedure based on BD tests conducted on orthotropic materials at two material orientations. This approach was applied to measure C/C-SiC material properties, showing an efficient and reliable appraisal of major elastic moduli.

Conclusions

The presented approach may drive down both the duration and cost of the mechanical evaluation, thereby potentially improving future process designs.