Experimental Mechanics最新文献

Background

The accuracy of Digital Image Correlation is considerably influenced by the quality of images taken from the specimen surface. While previous have examined the impact of camera parameters on DIC results, the relationship between camera characteristics and DIC errors remains unclear.

Objective

In this study, a new theoretical model is introduced to estimate the DIC errors sourced from the camera.

Methods

The model is derived from the EMVA 1288 standard and contains camera gain, bit depth, and noise error. To validate the model, its results were compared with the real errors calculated from DIC results, and to determine the accurate error, various noise and gains effects were applied to digital images and then displacement and strain were numerically applied to these manipulated images and original images. The error calculated from the DIC successfully matched the error estimated by the model, proving the applicability of the models.

Results

The individual effects of noise, camera gain, and bit depth are analyzed separately, exploring their respective influences on the DIC. Subsequently, a simple formula is proposed to express camera performance in DIC.

Conclusions

Results showed that the DIC accuracy is considerably influenced by the camera gain, and temporal dark noise has a notable impact on DIC accuracy, particularly in scenarios with low-contrast speckle patterns. However, the influence of bit depth is negligible.

Background

Additive Manufacturing offers the opportunity to build lattice structures with benefits in manufacturing efficiency and weight. For the determination of the fatigue properties of lattice structures, it lacks a method to determine the deformation under mechanic stress.

Objective

A digital image correlation (DIC) algorithm was implemented. The algorithm determines strains within a subset in an uncommon way by physically interpreting the subset shape function and does not need neighboring subsets, therefore.

Method

With a monochrome background this shape function-based strain determination is able to determine the deformation of a whole lattice unit cell, even if the background is visible in sectors of the subset. The implementation is validated by comparing the results in quasi-static tests on bulk material specimens to the results tactile sensors and a conventional DIC program. Then the deformation of lattice unit cells in fatigue tests is evaluated.

Results

The shape function-based strain determination performs well in quasi-static tests even for large deformations. The deformation of lattice unit cells is determined successfully, whereby conventional DIC algorithms can be challenged if the lattice’s strut diameter becomes close to the image resolution. The determined strains are appropriate for lifetime prediction and fractures can be detected.

Conclusion

The shape function-based strain determination is a suitable tool for determination of large local strains as well as strains in lattice structures, which do partially not cover the background in the whole region of interest due to periodic empty spaces between the lattice struts. For determination of strain fields, conventional DIC algorithms will still be more efficient in this state of development.

Background

The characterization of the moment–curvature relationship for thin components within the elastic–plastic regime yields crucial insights not readily ascertainable through conventional tensile testing. However, most conventional bending testers only measure the force–displacement data of specimens without providing the bending moment and curvature information directly.

Objective

We aim to develop a pure-bending tester based on the cochleoid theory that can directly measure the bending moment–curvature response of thin components.

Methods

The bending moment is determined by employing a flexural pivot with a known spring constant paired with dual laser displacement sensors. By approximating the cochleoid as an eccentric arc trajectory, we move and rotate one end of the specimen to increase the curvature gradually. Finally, the moment–curvature relationship of the specimens can be obtained.

Results

The practical capability of the bending tester is demonstrated by measuring moment–curvature data from various specimens, including PET sheets, aluminum sheets, and Nylon 6 monofilaments. Cyclic bending and relaxation tests are performed on these typical specimens. The measurement results agree well with the theoretical predictions.

Conclusions

The instrument serves as a valuable tool for characterizing the bending properties of diverse small-scale components. Its versatility facilitates comprehensive assessments of the bending behavior of various materials and structures.

Background

Investigations into the fatigue failures mechanism of Grey Cast Iron (GCI) water pipes are inhibited by the lack of a lab-based method to conduct extensive high-cycle biaxial fatigue test programmes.

Objective

The work presented in this paper developed and tested a novel experiment capable of causing controlled fatigue failures of GCI pipe specimens in the high-cycle fatigue regime using bending and internal water pressure fatigue loading.

Methods

A novel four-point bending and internal water pressure fatigue testing system was developed to apply constant amplitude out-of-phase biaxial loading to 58 mm diameter GCI pipes at 1.7 Hz. To verify the ability of this equipment to apply known stresses and repeatable loads to pipe specimens a series of tests were conducted. A finite element model of the pipe specimen was used to estimate the strains and displacements applied by the equipment.

Results

Experimental strains and displacements were mainly within ± 10% of the estimated values and the pressure amplitudes measured over 10³ cycles were within ± 3% of the average. Dynamic load effects occurred at higher bending loads, but these were quantified and accounted for. Trial destructive tests revealed that the lifespan of leaking fatigue cracks in GCI pipes with uniform wall-loss subject to combined internal pressure and bending fatigue loads is less than 1% of the total cycles-to-burst.

Conclusions

The experimental method developed was able to apply combined, out-of-phase internal pressure and bending fatigue loads accurately and consistently to small-dimeter GCI pipes, and cause these pipes to develop high-cycle fatigue regime failures.

Background

Full-field, quantitative visualization techniques, such as digital image correlation (DIC), have unlocked vast opportunities for experimental mechanics. However, DIC has traditionally been a surface measurement technique, and has not been extended to perform measurements on the interior of specimens for dynamic, full-scale laboratory experiments. This limitation restricts the scope of physics which can be investigated through DIC measurements, especially in the context of heterogeneous materials.

Objective

The focus of this study is to develop a method for performing internal DIC measurements in dynamic experiments. The aim is to demonstrate its feasibility and accuracy across a range of stresses (up to (650,)MPa), strain rates ((10^{3})-(10^6,)s(^{-1})), and high-strain rate loading conditions (e.g., ramped and shock wave loading).

Methods

Internal DIC is developed based on the concept of applying a speckle pattern at an inner-plane of a transparent specimen. The high-speed imaging configuration is coupled to the traditional dynamic experimental setups, and is focused on the internal speckle pattern. During the experiment, while the sample deforms dynamically, in-plane, two-dimensional deformations are measured via correlation of the internal speckle pattern. In this study, the viability and accuracy of the internal DIC technique is demonstrated for split-Hopkinson (Kolsky) pressure bar (SHPB) and plate impact experiments.

Results

The internal DIC experimental technique is successfully demonstrated in both the SHPB and plate impact experiments. In the SHPB setting, the accuracy of the technique is excellent throughout the deformation regime, with measurement noise of approximately (0.2%) strain. In the case of plate impact experiments, the technique performs well, with error and measurement noise of (1%) strain.

Conclusion

The internal DIC technique has been developed and demonstrated to work well for full-scale dynamic high-strain rate and shock laboratory experiments, and the accuracy is quantified. The technique can aid in investigating the physics and mechanics of the dynamic behavior of materials, including local deformation fields around dynamically loaded material heterogeneities.

Background

To ensure reliability of additively manufactured components in structural applications, an understanding of the combined behavior of pores and stress state on failure behavior is required.

Objective

This research aims to identify the capabilities and limitations of stress- and strain-based fracture models in describing failure in complex additively manufactured structures.

Methods

SS316L brackets with a three-dimensional truss-based geometry, in which stress state and pore size varied among struts, were fabricated with laser powder bed fusion. Fracture models considering both stress state and pore size, formulated in terms of stress (pore-size dependent Mohr–Coulomb, or P-MC) and strain (pore-size dependent Modified Mohr–Coulomb, or P-MMC), were calibrated and used to predict the fracture behavior of the brackets.

Results

The P-MMC fracture model correctly predicted the experimentally observed fracture locations for 11 out of 12 samples, while the P-MC fracture model correctly predicted 10 out of 12 samples. Below a critical pore size, stress state effects dominated the fracture behavior, and above this, pore size was the critical factor, where capturing both factors was crucial at intermediate pore sizes.

Conclusions

The P-MC fracture model was appropriate for predicting the maximum load-bearing capacity for all samples in this study, while the P-MMC fracture model was shown to be only applicable for samples containing small pores. The importance of incorporating both stress state and the presence of pores in a fracture model is necessary to ensure confidence in the load carrying capacity of additively manufactured structures.

Graphical Abstract

Background

Closed cell polymer bead foams are widely used in industrial applications due to their extraordinary damping and insulation properties. To understand the structure-property-relations at different deformation states the volumetric structure of polymer walls, cells and beads should be statistically analysed.

Objective

The presented work is focused on the statistical analysis of the changing cell structure of expanded polypropylene bead foams of different density at distinct compression states.

Methods

Cylindrical bead foam specimens are scanned by x-ray computed tomography at 3-5 different compression states. The reconstructed volume information is segmented and statistically analysed.

Results

It could be shown that, among others, the cell sphericity and their orientation relative to the plane normal to the loading direction are sensitive parameters to the deformation state. With regard to the material symmetry level, a shift of the isotropic foam to transversal isotropic structure was observed. No sudden, stability related, deformation or failure could be observed.

Conclusions

Good metrics for the deformation analysis of expanded polypropylene bead foams from in-situ computed tomography tests are the cell sphericity and orientation. Compression deformation leads to a gradually change of material symmetry level from isotropy to anisotropy.

Background

Accurate prediction of the plastic behavior of AA2024‒T4 requires a deep understanding of the mechanical response of the material under different loading conditions. For alloy sheets, the material orientation and deformation are two important factors whose effects should be clarified.

Objective

This work focuses on the complex relationships among the material orientation, deformation, and tension‒compression asymmetry of AA2024‒T4.

Methods

The tension, compression, and shear responses of materials at different orientations are experimentally investigated through dog bone, cuboid, and butterfly specimen, respectively. In addition, the tension‒compression asymmetry is embedded in the anisotropic parameters rather than an additional independent parameter.

Results

Tension‒compression asymmetry is sensitive to orientation and degree of deformation. The tension‒compression asymmetry tends to be stable with increasing degree of deformation. But the evolution law of tension–compression asymmetry can be affected by orientation.

Conclusions

An additional parameter describing the asymmetry is required for isotropic plastic modeling. This parameter can be ignored when the anisotropic situation is considered because such an effect will be implied in the anisotropic parameters. In addition, the influence of degree of deformation on tension–compression asymmetry and plastic anisotropy can be reflected by the evolutions of anisotropic parameters.

Background

This study reports on the performance estimation of Digital Volume Correlation (DVC) for tomographic applications. The performance of DVC can be evaluated in terms of two distinct errors: the random error, directly linked to image quality, and the interpolation error, which is the one of the most significant systematic error generated by DVC algorithms. However, the existing methods provide only a limited estimate of the interpolation error, or allow only the random error to be assessed.

Objective

A new method is proposed to evaluate the interpolation error coupled with the random error in a simple and fast way to assess the overall performance of DVC for any tomographic application.

Methods

This new method proposes to apply a rotation to the sample (instead of the usual translation) to evaluate the interpolation error. This rotational movement generates linearly varying displacement fields, and each point of a displacement field describes a distinct non-integer voxel position. As this rotation is a rigid body motion, the random error associated with tomographic noise is also taken into account.

Results

This new method can generate several thousand interpolation error measurement points in only two acquisitions, allowing a very detailed and local assessment of this error. Additionally, and compared to existing methods in the literature (repeat scan), this method does not underestimate the random error, essential for assessing the overall performance of the DVC.

Conclusions

The proposed method efficiently evaluates DVC performance by accurately assessing both interpolation and random errors through rotational sample movement, improving the reliability in DVC measurements.