Experimental Mechanics最新文献

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.

Background

One potential application of additively fabricated lattice structures is in the blade containment rings of gas turbine engines. The blade containment rings are expected to be able to absorb the kinetic energy of a released blade (broken blade) in order to protect the engine parts from damaging. Metallic lattice-cored sandwich plates provide a gap (free space) between two face sheets, which helps to arrest the released blade and increases the energy absorption capability of containment rings.

Objective

The objective was to investigate numerically the projectile impact response of Body-Centered-Cubic (BCC) Electron-Beam-Melt (EBM) lattice-cored/Ti64 face sheet sandwich plates as compared with that of an equal-mass monolithic EBM-Ti64 plate.

Methods

The projectile impact simulations were implemented in LS-DYNA using the previously determined flow stress and damage models and a spherical steel impactor at the velocities ranging from 150 to 500 m s⁻¹. The experimental projectile impact tests on the monolithic plate were performed at two different impact velocities and the results were used to confirm the validity of the used flow stress and damage models for the monolithic plate models.

Results

Lower impact stresses were found numerically in the sandwich plate as compared with the monolithic plate at the same impact velocity. The bending and multi-cracking of the struts over a wide area in the sandwich plate increased the energy absorption and resulted in the arrest of the projectile at relatively high velocities. While monolithic plate exhibited a local bent area, resulting in the development of high tensile stresses and the projectile perforations at lower velocities.

Conclusions

The numerical impact stresses in the sandwich plate were distributed over a wider area around the projectile, leading to the fracture and bending of many individual struts which significantly increased the resistance to the perforation. Hence, the investigated lattice cell topology and cell, strut, and face sheet sizes and the lattice-cored sandwich plate was shown potentially more successful in stopping the projectiles than the equal-mass monolithic plates.

Background

Carbon fiber-reinforced polyetheretherketone (CF-PEEK) composites, produced via material extrusion (ME) 3D printing, offer excellent physical and mechanical properties for aerospace and biomedical applications. However, their layered microstructure from the additive manufacturing process makes them susceptible to interlaminar shear failure.

Objectives

This study investigates the interlaminar shear strength (ILSS) of additively manufactured CF-PEEK composites and unreinforced PEEK. It focuses on the relationship between microstructure, influenced by the mismatch angle between adjacent layers and layer height, and interlaminar shear behavior of CF-PEEK composites.

Method

Short beam shear (SBS) tests are used to evaluate ILSS, with digital image correlation (DIC) capturing in-situ full-field strain fields to observe interlaminar failure mechanisms. Fractographic examinations are also performed to confirm the observed trends.

Results

The experimental findings unveil three key points: (1) An increase in the mismatch angle enhances ILSS, shifting the failure mode from interlaminar shear to bending stress. For pure PEEK, this enhancement can reach 60–70%, while CF-PEEK shows a 30–40% increase. (2) Layer height has contrasting effects: it does not significantly impact ILSS in pure PEEK, but in CF-PEEK composites, a shorter layer height increases ILSS by more than two to three times compared to thicker layers. (3) CF-PEEK composites outperform pure PEEK in ILSS by 40–50% at a layer height of 200 µm. However, this trend reverses at a layer height of 400 µm.

Conclusions

These outcomes suggest the potential for producing PEEK and CF-PEEK composites via ME technique with enhanced ILSS, thereby offering improved structural reliability in their applications.

Background

Additively-manufactured parts contain residual stresses induced by manufacturing. These residual stresses can be relaxed or redistributed by thermal loading. The presence of internal stress influences the dynamic response of parts, and this is of particular interest in thin plates subject to thermoacoustic loading in hypersonic vehicles and fusion reactors.

Objective

To measure the changes in shape and modal frequencies caused by thermal loading of geometrically-reinforced thin plates that were additively manufactured in Inconel 625.

Methods

Plates were additively-manufactured in landscape and portrait orientations using laser powder bed fusion. The plates were heated to a nominal temperature of 820 ̊C, which was expected to alleviate the residual stress from the build process. Pre- and post-heating, their modal frequencies were found experimentally and pulsed-laser stereo (3D) digital image correlation was used to evaluate their modal shapes. The resultant modal frequencies and shapes were compared with those from a subtractively-manufactured plate.

Results

It was found that the heat cycle changed the shape of the plates relative to their as-manufactured state in addition to changing their natural frequencies and modal shapes.

Conclusions

The change in shape induced by heating caused shifts in the natural frequencies and changes in the corresponding modal shapes. The results show quantitatively for the first time the important role that residual stresses can play in the dynamic response of geometrically-reinforced thin plates manufactured by additive and subtractive processes.

Background

Laser welding has a faster processing speed than other welding techniques. However, defects can occur under various welding conditions, and high safety and reliability are required for applying laser welding to actual mechanical structures.

Objective

This study focused on estimating the fatigue limit by dissipated energy which is the energy loss resulting in fatigue damage owing to localized plastic deformation. This study was conducted to determine whether the fatigue limit of aluminum alloy laser welds can be rapidly estimated using the dissipated energy.

Methods

In a test with a stepwise increase in stress amplitude, the dissipated energy and the strain were measured by infrared thermography and digital image correlation from displacement measurements with a visible camera, respectively. In the fatigue limit estimation using dissipated energy, the fatigue limit is determined by the empirical rule that the stress amplitude with increasing the dissipated energy is the estimated fatigue limit.

Results

Laser welds exhibited the highest dissipated energy at the fracture origin of the joint. Therefore, the crack initiation point of welded joints can be visualized by measuring the dissipated energy. If the boundary value of both groups in the domain decomposition method using the least-squares approximation is the estimated fatigue limit, the estimated fatigue limits for the aluminum alloy laser welds and those base material specimens are almost consistent with the actual fatigue limits.

Conclusions

The fatigue limit estimation using the dissipated energy can be applied to aluminum alloy laser welds.