Experimental Mechanics最新文献

Background

Modifying the mechanical properties of the solid phase of a porous material, in this study calcium-silicate-hydrate, is frequently possible by changing synthesis conditions, but changes in these conditions can also influence porosity, which in turn may affect the mechanical properties of the porous material. Experimental methods to decouple porosity from the viscoelastic properties of the porous material will aid in optimization of the structure of the solid phase to achieve the desired mechanical properties.

Objective

Explore different nanoindentation techniques in order to determine the viscoelastic properties of the solid phase (without the affect of porosity) of a stiff porous material via experimental methods alone.

Methods

Compacted pellets of calcium-silicate-hydrate were prepared with different porosity and subjected to three nanoindentation techniques to determine viscoelastic behavior and the influence of porosity: dynamic, stress relaxation, and creep. Results of the porosity and of the viscoelastic behavior measurements were analyzed with a reverse-micromechanics model to determine viscoelastic properties of the solid phase, which has not been achieved previously for calcium-silicate-hydrate. These methods can be used in development and refinement of materials to determine how changes in the solid phase (e.g. molecular structure) influence viscoelastic behavior while considering the effect of porosity.

Results

Dynamic nanoindentation was found to be unreliable for the stiff material studied in this work. Normalized stress relaxation and creep data were found to be independent of porosity. Reverse micro-mechanics modeling allowed for characterization of the creep modulus that is consistent with other studies that used computational or synchrotron x-ray methods to characterize mechanical properties of the solid calcium-silicate-hydrate phase.

Conclusion

Creep experiments provide more reliable data than dynamic or stress relaxation experiments. When the porosity is known, reverse-micromechanics modeling can be used determine the creep modulus of the solid phase and thus be used to predict creep modulus of a composite with an arbitrary porosity. If the porosity is not known, the viscoelastic properties of the solid phase can still be compared to each other using a normalized creep modulus that is independent of porosity.

Background

Powder bed fusion (PBF) offers enhanced opportunities to manufacture complex components with a high degree of geometric freedom. However, understanding and designing for mechanical properties remains challenging due to numerous factors, such as processing parameters, building direction, and heat treatments.

Objective

In this study, we revealed that the As-built and heat-treated mechanical properties differ from those achieved through traditional manufacturing methods, even when using the same alloy and heat treatments. This phenomenon arises from the intricated microstructures and porosity caused by the repetitive, rapid heating/cooling process involved.

Results

To quantitatively investigate the properties, the conventional heat treatments combining a hot isostatic pressing (HIP), solution, and aging treatment, were conducted on 17-4 PH stainless steel printed in both horizontal and vertical directions. Our findings demonstrate that HIP, coupled with aging treatment, was the most effective method for reducing porosity, and enhancing hardness and yield strength by (56%) and (118%), respectively, while there was a slight decrease in elongation by (5.6%). The high temperature and pressure during HIP enabled the recrystallization of As-built microstructure into lath martensite, and the aging treatment facilitated the production of precipitates to enhance the strength. The solution treatment, however, resulted in poor elongation to (9.3%) while the yield and tensile strength showed similar levels to As-built parts due to insufficient time to recrystallize the As-built microstructure.

Conclusions

We believe these results will offer valuable insights into the manufacturing and post processing not only of PBF 17-4PH stainless steel but also of other alloys.

Background

Though proposed by Lake and Yeoh in 1978 for vulcanized rubber characterization and possessing unique advantages with respect to traditional fracture characterization approaches, the Y-shaped cutting technique has been applied to a limited number of materials.

Objective

This limited implementation may be due to researchers’ unfamiliarity with the effects of Y-shaped cutting conditions and technique limitations, as well as a lack of standards. This review and best practices guide aims to provide a detailed road-map of the capabilities of Y-shaped cutting, with guidance for designing, executing, and interpreting its results.

Method

By performing Y-shaped cutting at a constant blade propagation rate, fracture initiation effects encountered in many ‘tearing’ tests are bypassed. Meanwhile, unlike other contact-driven fracture conditions (needle insertion or cutting), the ‘leg’ separation renders the cutting nearly ‘frictionless’ under a variety of conditions.

Results

Y-shaped cutting possesses two unique attributes. Under certain conditions (Zhang and Hutchens in Soft Matter 17(28):6728–6741, 2021), it can yield a fracture energy independent of sample and cutting implement geometry. In contrast to soft solid crack blunting, cutting reduces both the finite stretch and failure process zones to within a field of view readily imaged on a microscope, useful for microstructural studeis. To facilitate access to the above advantages, we summarize experimental variables and their role the fracture response and/or successful cutting and establish a pseudo-standard using silicone.

Conclusion

This overview and recommendations empowers researchers to implement this highly-tunable cutting method in order to provide insights into other classes of materials in the future.

Background

Composite materials have been extensively used in various industry fields due to their distinguishing characteristics. However, low-velocity impact loads would undermine the mechanical properties of composite structures significantly.

Objective

To improve the integrity and safety of composite structures, it is imperative to unearth the accurate locations of low-velocity impact loads efficiently.

Methods

In this research, a novel approach hybridising response similarity search and optimisation strategy is developed. The innovation of the approach comes from the adoption of a “divide-and-conquer” strategy to alleviate extensive computations for time history reconstruction during the impact load localisation process so as to optimise computational efficiency and accuracy. In more detail, the approach is comprised of two localisation processes: (i) a coarse process to quickly identify several potential positions for an impact load via response similarity measurements based on time-domain and frequency-domain signals; (ii) a precise process to fine-tune the exact location of the impact load by minimising the nominal residual between the reconstructed and actual responses from the above potential positions.

Results

Experiments are conducted on a carbon fibre composite sandwich panel to validate and demonstrate the effectiveness and superiority of the approach in terms of localisation efficiency and accuracy. It indicates that the approach achieves 100% accuracy in impact load localisation. It also shows that the approach only takes approximately 4.0 s to localise 20 impact load cases, which is only about one-eighth of the time taken by the traditional optimisation strategy approach to fulfil the same function.

Conclusions

The hybrid approach designed based on response similarity search and optimisation strategy can greatly improve localisation efficiency and localisation accuracy.

Background

Digital image correlation (DIC) is widely used as a noncontact optical deformation measurement method. However, optical DIC encounters difficulties when measuring displacement and strain at high temperatures, including false deformation caused by heat haze and image overexposure caused by intense thermal radiation. X-ray imaging is not affected by these factors, so the combination of X-ray imaging and the DIC algorithm (X-DIC) holds the potential for measuring deformation during high-temperature tests.

Objective

This study investigated the ability of X-DIC to measure deformation in high-temperature experiments, expand the applicable temperature range of X-DIC, and provide a reliable method for obtaining deformation measurements in high-temperature experiments.

Methods

A combination of X-ray digital radiography (DR) images and the DIC algorithm was used to measure deformation. Numerical experiments based on synthetic images were used to evaluate the measurement accuracy of X-DIC, and the influence of different DIC parameters on the measurement error was discussed. Ductile iron and C/SiC composites were subjected to tensile tests at different temperatures from ambient temperature to 1000 °C, and different deformation measurement methods were used to simultaneously measure the deformation of the samples to verify the accuracy of the X-DIC results.

Results

In the numerical experiments, the displacement measurement error of X-DIC is less than 0.02 px. The relative error between the X-DIC and blue-light DIC measurements of the tensile deformation of ductile iron at 500 °C is 0.65%. When the deformation of the C/SiC composite materials was measured at 1000 °C, the root mean square error (RMSE) of the strain data obtained by X-DIC and optical DIC was 1.12 × 10^–4.

Conclusions

These results prove that X-DIC has high measurement accuracy. Compared with optical DIC, X-DIC is insensitive to high-temperature environments and provides alternative experimental methods for high-temperature deformation measurements.

Background

The experimental detection of small and large strains requires special approaches of full-field measurement techniques and their evaluation on 3D curved surfaces of components.

Objectives

Since classical digital image correlation methods have difficulties with the application of paints in some applications, one aim is to use a method in which the surface roughness is used to apply the strain calculation.

Methods

In this paper, 2D digital image correlation is applied to 2D intensity maps extracted from a coherence scanning interferometer together with height information. Height information are used to reconstruct the 3D motion of tracked material points. Surface interpolation and strain calculation are performed using globally formulated radial basis functions.

Results

The entire procedure leads to an appropriate technique for determining the in-plane strains in curved surfaces of parts, whereas the expected accuracy for various levels of the radial basis functions are discussed in detail.

Conclusions

Particularly, coherence scanning interferometry yields highly accurate height information. To smooth the surface motion, it turns out that in particular a regression analysis is required, where we apply radial basis functions with various approximation levels. This is an alternative procedure for surface strain determination.

Background

Understanding biaxial loading response at the microstructural level is crucial in helping better design sheet manufacturing processes and calibrate/validate material deformation models.

Objective

The objective of this work was to develop a low-cost testing apparatus to probe, with sufficient spatial resolution, the micro-mechanical response of a sheet material in-situ under biaxial loading conditions.

Methods

The testing apparatus fabricated as a part of this study operates in a similar fashion to a standard bulge test and uses oil pressure to generate biaxial loading conditions. This biaxial testing apparatus was operated within a synchrotron beamline to characterize the mechanical response of a flash-processed steel sheet using in-situ high-energy X-ray diffraction (XRD) measurements. The GSAS-II package was utilized to develop a workflow for the analysis of the large volume of diffraction data acquired. The workflow was then used to extract the peak position, width, and integrated intensity of the XRD peaks corresponding to the major body-centered cubic phase.

Results

The equi-biaxial nature of the loading in the measured area was independently corroborated using experimental (XRD) and simulation (finite element analysis) methods. Furthermore, we discuss the evolution of elastic strain in the major body-centered cubic phase as a function of applied oil pressure and location on the steel sheet.

Conclusions

A key advantage of the biaxial apparatus fabricated in this synchrotron study is demonstrated using the results obtained for the flash-processed steel sheet – i.e., mapping the lattice plane-dependent response to biaxial loading for a relatively large sample area in a spatially resolved manner.

Background

Residual stresses exist in many manufactured materials and must be measured and taken into account for safe structural design. Established residual stress measurement methods are either destructive or require substantial material-dependent calibration.

Objective

The present work is aimed at developing an indentation-based method for measuring residual stress that causes minimal specimen damage, does not require a stress-free reference specimen, and has the capability to identify both the size and direction of the surface residual stresses. In this initial study, the simpler case of equi-biaxial stresses is addressed in preparation for subsequent general stress evaluations.

Methods

The surface displacements around an indentation made by a conical indenter are measured using digital image correlation. The residual stresses are then identified by comparison to the results of a finite model of the indentation process.

Results

The proposed method is shown to 2–5 times more sensitive to the presence of residual stresses than other commonly used indentation methods, particularly for materials with low Hollomon exponent n. In example measurements, axi-symmetric residual stresses were determined within 8% of the material yield stress.

Conclusions

The initial study presented here successfully considered the equal-biaxial stress case. The proposed method is attractive for future development because it gives directional information and therefore can be extended to the general non-equal-biaxial case.

Background

The gradation of thermal expansion coefficient was analyzed in the earlier study. The analytical formulation derived here, which is quite different, should be validated to understand the thermal stress distribution in a laminated composite and functionally graded material. Besides this solution, a validated numerical model can also be used to optimize the material gradation of plates in terms of sustainability.

Objective

To validate the analytical formulation derived here, an experimental model is presented to understand the thermal stress concentration for functionally graded and laminated composite plates. A numerical model is also validated to extend to understand the effects of the number of layers, the thickness of a layer, the gradation function, the ratio of elastic moduli, and the coating.

Methods

The experimental problems in the production of the experimental models with layers of different elastic moduli are discussed here. In the experimental analysis, a three-dimensional photoelastic stress analysis of two- and four-layer composite plate was used to mechanically model the thermal expansion. The analytical solution for the thermal stress in a free plate was derived by the strain suppression method based on the principle of superposition. The numerical models were analyzed using finite element software. The step variation in the experiment was used as a reference point for a continuous or multi-layer (> 2) step variation of material coefficients in the models.

Results

The variation of stress concentration is shown for various cases of laminated and continuous gradations of elastic modulus. The four-layer experimental model provides the difference in thermal stress distribution as a result of a layered coating. The validated analytical and numerical models provide reasonable results. An empirical formula to optimize the material gradation in terms of elastic modulus is derived.

Conclusions

The experimental model can be used to analyze thermal stress in functionally graded materials. The gradations of the material in the plate or the coating of the plates can be optimized by the validated analytical and numerical models. The empirical formula can be used to determine the elastic modulus of the coating to minimize the stress concentration.

Background

Being an image-based optical technique for full-field deformation measurements, the ultimate purpose of digital image correlation (DIC) is to realize accurate, precise and pixel-wise displacement/strain measurements in a full-automatic manner without users’ inputs.

Objective

In this work, we propose a task-optimized neural network, called RAFT-DIC, to achieve user-independent, accurate and pixel-wise displacement field measurements.

Methods

RAFT-DIC is based on the state-of-the-art optical flow architecture: Recurrent All-Pairs Field Transforms (RAFT). We make two targeted improvements that fundamentally enhanced its measurement accuracy and generalization performance. Firstly, we remove all the down-sampling operations in the encode module to improve the perception of spatial information, and reduce the number of pyramid levels of the correlation layer to increase the small displacement accuracy. By building the correlation layer to compute the similarity of pixel pairs, and iteratively updating the displacement field through a recurrent unit, RAFT-DIC introduces the prior information of DIC measurement to guide the displacement estimation with high accuracy. Secondly, we develop a novel dataset generation method to synthesize customized speckle patterns and diverse displacement fields, which facilitate the construction of a robust and adaptable dataset to improve the network generalization.

Results

Both simulated and real experimental results demonstrate that the accuracy of the proposed method is approximately an order of magnitude higher than pervious deep learning-based DIC (DL-DIC).

Conclusions

The proposed RAFT-DIC shows higher accuracy as well as stronger practicality and cross-dataset generalization performance over existing DL-DIC methods, and is expected to be a new standard architecture for DL-DIC.