Experimental Mechanics最新文献

Background

Cyclic loading is a leading cause of material failure and accurate testing methods are required to predict the life of components.

Objective

This work investigated the challenges of performing reversed three-point bending experiments for characterising the fatigue life of polymers and composites, and sought to develop guidelines for improving the testing method.

Methods

Testing was performed with polypropylene (PP) and glass-fibre reinforced PP on a test fixture that featured double-sided supports. Monotonic and fully reversed (i.e. stress ratio R = - 1) cyclic bending experiments were conducted to characterise the effects that the test conditions, including the type of supports and level of contact between the specimens and anvils, have on the measurements.

Results

Stresses generated during reversed bending are highly dependent on the type of fixture configuration used. Most configurations that were tested led to mechanical interlocking that altered the bending moments and generated membrane stresses. In this work, the purest form of reversed bending was achieved using double-sided pivoting supports, and a 0.35 mm gap between specimens and anvils to allow sliding in the supports.

Conclusions

This configuration provided the most similar stress state to that obtained from the standardised simply-supported beam experiments that are used for characterising the quasi-static flexural properties of materials, and was ultimately the configuration that provided the most reliable fatigue data.

Background

Additive manufacturing (AM) is rapidly growing, with new AM methods continually in development. Alloys processed with novel methods require systematic characterization to understand and validate the materials, especially for demanding fields.

Objective

This study characterized the mechanical properties and failure mechanisms of a Ti-6Al-4V alloy manufactured with bound metal deposition (BMD), a form of metal extrusion (MEX) AM.

Methods

Specimens made of Ti64 were printed via Desktop Metal’s Studio System 2 through a printing, debinding, and sintering process. The microstructure was analyzed with optical metallography and a newly developed open-source porosity analysis tool. Scanning electron microscopy (SEM), optical microscopy, and compositional analysis of green, brown, and sintered parts were conducted to study the material and its failure modes. Sintered specimens were tensile and hardness tested.

Results

As-sintered specimens exhibited ductility more than 10 times lower than wrought Ti64, partially due to contamination/impurity that formed brittle (alpha )-case titanium. Sources of contamination may have included the sacrificial Al(_2)O(_3) interlayer, the wax/polymer binder, and/or impurity introduction from the furnace. Fractography imaging found quasi-cleavage fracture initiating at areas of high surface roughness along the ceramic interlayer surface of the parts and transitioning into dimple rupture and intergranular decohesion.

Conclusions

Elevated contamination levels, high surface roughness, and internal porosity led to low elongation and ultimate strength in the Ti64 BMD alloy. With the processing route presented here, BMD for Ti64 may not be suitable for applications that demand high ductility and strength with minimal impurities, although with process refinement, the method may be promising for certain applications

Background

Many aspects of ductile failure through microvoid coalescence remain elusive due to the challenging spatial and temporal scales it operates on. Experimentally resolving all aspects of the process remains a significant goal of researchers. Much of the current understanding has been derived from post-mortem metallography, leaving key aspects of its evolution undocumented.

Objective

This work builds on efforts using X-ray computed tomography (XCT) characterize voids and their evolution under loading.

Methods

It employs in-situ XCT tensile testing on 316L Stainless Steel samples that were constructed by laser powder bed fusion that contain tailored, pre-existing voids with a spatial scale relevant to the growth and evolution stages of microvoid coalescence. Pre-existing voids extended the observation window for monitoring void growth and interaction under loading. They also enhanced fiducial correlation of voids during deformation.

Results

Void populations were found to increase under loading as their deformed dimensions rendered them detectable by the XCT algorithm. Neighboring voids underwent interconnection events by a cleavage process when stress concentrations between them exceeded the macroscopic yield stress. Pores that did not undergo interconnection events were found to revert to their initial size and population after unloading. Finally, the porosity structure before failure was correlated to features on the fracture surface with high fidelity.

Conclusions

This unique combination of in-situ XCT tensile testing on samples with tailored void structure enabled new visualization and quantification of void evolution under load as well as strong correlation to the observed stress–strain behavior and post-mortem fracture characteristics.

Background

The Two-Dimensional Digital Image Correlation (2D-DIC) method is widely used as a non-contact full-field kinematic measurement, but it presents significant errors related to temperature effects including the image con- trast and heat waves. Consequently, results of mechanical displacement or strain measured by the 2D-DIC method, especially strains in the elastic domain of materials, is significantly dispersed.

Objective

The aim of this study is to propose a very simple 2D-DIC method, using commercial DIC software with no need of additional storage to accurately measure strain and displacements at high temperatures, typically at the hot metal forming temperatures, from 400 ^◦C to 750 ^◦C.

Methods

This study demonstrates the influence of temperature effects (radiation and heat waves) on strain measurements obtained with the 2D-DIC method in the elastic regime (ε < 0.05) of the TA6V titanium alloy material at high temperatures. Furthermore, the strain measurement errors at different temperatures were characterized by the Background Oriented Schlieren technique (BOS). Correction methods using temperature-dependent low-pass filters for strain measurement errors are suggested.

Results

The correction methods allow separating mechanical strain fields and strain measurement errors caused by temperature effects. The efficiency of the correction methods is demonstrated by identifying the Young’s modulus (E) and the Thermal Expansion Coefficient (TEC) of the TA6V. After corrections, E and the TEC of the TA6V are close to the reference values found in the literature. Conclusion: The coefficient R² from the linear regression method to determine the Young’s modulus from tensile test at 600 ^◦C increases from 0.783 to 0.989, revealing the great potential of using the improved-2D-DIC method for full-field kinematic measurements of mechanical tests at high temperatures.

Background

Finite element analysis (FEA), digital image correlation (DIC), and strain gauge are usually used for test monitoring. Reconstructed by FEA strain field and strain gauges, the fusion field (DT-SFRM) has higher accuracy than FEA, and DT-SFRM accuracy is acceptable for small structural deformation. However, large deformation of thin-walled structures is accompanied by complex strain field, and the difference in strain distribution between FEA and strain gauges data increases sharply. The low-accuracy FEA strain field and small amount of strain gauges lead to insufficient accuracy of DT-SFRM.

Objective

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

Methods

A strain field reconstruction method based on digital twin (DT) by multi-level fusion of multi-type data (DT-MFMD) is proposed. Large number of relatively high-accuracy DIC data is used to improve global FEA strain field accuracy. The high-accuracy strain gauges are used to further improve fusion field accuracy of FEA and DIC. A tensile test of large opening plate shell (LOPS) is performed to verify the advantage of DT-MFMD.

Results

The AvgErr (6.6%) and MaxErr (23.5%) of DT-MFMD are 16.0% and 58.8% lower than those of DT-SFRM, and the DT-MFMD is less affected by large deformation. In addition, the strain gauges number of DT-MFMD reduces by more than 62.5% under the same accuracy goal compared with that of DT-SFRM.

Conclusions

The DT-MFMD is validated to have a better application prospect for large deformation and complex strain field distribution.