Experimental Mechanics最新文献

Background

Laser powder bed fusion (L-PBF) additive manufacturing (AM) is used for building metallic parts layer-by-layer and often generates non-uniform thermal gradients between layers during fabrication, resulting in the development of residual stresses when parts are cooled down.

Objective

The impact of modulated laser used during the L-PBF process on residual stresses in Inconel 718 (IN718) material was investigated. The impact of build directions on residual stress is also determined.

Methods

The contour method is employed to measure the full-field residual stress component on the cross-section of samples. A complementary residual stress measurement method, incremental hole drilling, was employed for obtaining in-plane residual stress components.

Results

The results show that the residual stress distribution is sensitive to the build direction, with a higher magnitude of residual stress in the direction of build than that in the transverse direction. Multiple measurements with the same manufacturing parameters show good repeatability.

Conclusion

Residual stresses in the as-built parts are significant and hence a further consideration regarding relieving residual stresses is required when post-thermal treatments are developed.

Background

Building S-N curves for materials traditionally involves conducting numerous fatigue tests, resulting in a time-consuming and expensive experimental procedure that can span several weeks. Thus, there is a need for a more efficient approach to extract the S-N curves.

Objective

The primary purpose of this research is to propose a reliable approach in the framework of thermodynamics for the rapid prediction of fatigue failure at different stress levels. The proposed method aims to offer a simple and efficient means of extracting the S-N curve of a material.

Methods

In this paper, a method is introduced based on the principles of thermodynamics. It uses the fracture fatigue entropy (FFE) threshold to estimate the fatigue life by conducting a limited number of cycles at each stress level and measuring the temperature rise during the steady-state stage of fatigue.

Results

An extensive set of experimental results with carbon steel 1018 and SS 316 are conducted to illustrate the utility of the approach. Also, the efficacy of the approach in characterizing the fatigue in axial and bending loadings of SAE 1045 and SS304 specimens is presented. It successfully predicts fatigue life and creates the S-N curves.

Conclusion

The effectiveness of the approach is evaluated successfully for different materials under different loading types. The results show that the temperature rise is an indicator of the severity of fatigue and can be used to predict life.

Background

This study investigates the effects of pores on the mechanical properties of metals produced by additive manufacturing, which can limit strength and ductility.

Objective

This research aims to both measure and model the rate of crack growth emanating from these pores in additively manufactured Ti-6Al-4 V fabricated with laser powder bed fusion.

Methods

Uniaxial tensile samples containing intentionally embedded penny-shaped pores were mechanically tested to failure, and loading was interrupted by a series of unload steps to measure the stiffness degradation with load. The factors contributing to reduction in stiffness, namely (1) elastic and plastic changes to geometry, (2) the effect of plastic deformation on modulus, and (3) crack growth, were deconvoluted through finite element modeling, and the crack size was estimated at each unloading step.

Results

The stiffness-based method was able to detect stable crack growth in samples with large pores (1.6% to 11% of the cross-sectional area). Crack growth as a function of strain was fit to a model where the crack driving force was based on equivalent strain and a model where the crack driving force was based on energy release rate.

Conclusions

Significant crack growth occurred only after the onset of necking in samples containing small pores, while samples containing large pores experienced continuous crack growth with strain.

Graphical Abstract

Background

Strain on soft materials plays a fundamental role in understanding the mechanical behavior of engineering soft machines and devices. However, it is challenging to noninvasively measure strain distribution on soft materials in situ.

Objective

In this study, we investigate the mechanism for the strain modulation of spectral reflectance in soft materials like silica gels. This mechanism is about the effect of strain-induced roughness variance on the spectral reflectance.

Methods

Based on a simplified Torrance-Sparrow model, we established a theoretical relationship related the reflectance variance to the strain undergone by the specimen. By HSI, we measure the reflectance changes caused the strain; by CLSM experiment, we determine the surface roughness under different strains.

Results

The theoretical model is verified by the reflectance analysis based HSI and the roughness analysis based on CLSM. The calibrated model parameters respectively from HSI and CLSM obtain a good agreement.

Conclusions

For soft materials similar to silicone rubber, within the strain range covered in this paper, the surface roughness of the material has a linear relationship with strain, while the reflectance has an inverse quadratic relationship with roughness. Our proposed mechanism provides a possibility for non-contact strain measurement based on HSI.

Background

Extensive research was conducted to analyze the ultrasonic fatigue behavior of ASGU-T64 cast aluminum alloy under different humidity environments. The study placed particular emphasis on investigating the factors influencing crack initiation, as well as the propagation of both short and long cracks. By examining the alloy's performance in various moisture conditions, a comprehensive understanding of its fatigue behavior was achieved.

Objective

The primary objective is to elucidate the mechanism underlying crack initiation and accurately predict the lifespan of short and long cracks. The ultimate goal is to determine how crack initiation size affects the percentage of crack initiation life in relation to the overall fatigue life.

Method

Scanning Electron Microscope (SEM) and Electron Back Scatter Diffraction (EBSD) were employed and provided valuable insights into the characteristics of the facets. Furthermore, computational methods were utilized, employing the Paris crack growth law, to accurately determine the growth lives of both short and long cracks. By combining experimental and computational approaches, a comprehensive understanding of the fracture behavior and crack growth mechanisms was achieved, contributing to the advancement of knowledge in this field.

Results

Through this study, it was discovered that fatigue cracks in the AS7GU-T64 alloy consistently initiated on the surface of the sample, primarily due to the presence of persistent slip bands (PSBs). Each facet observed on the fracture surface corresponded to an entire grain within the short crack area. While the stress intensity factor fell within the range of 3.5 to 10 MPa·√m for all three environments, it was found that the stress intensity factor in dry air exceeded that of saturated air and distilled water conditions. Importantly, the percentage of fatigue life attributed to crack initiation was found to be heavily dependent on the humidity of the testing environment and the applied stress amplitude. These insights highlight the intricate relationship between environmental conditions, stress intensity factor, crack initiation, and the overall fatigue life of the AS7GU-T64 alloy.

Conclusion

Humidity negatively affects the ultrasonic fatigue life of the AS7GU-T64 alloy. Furthermore, the size of crack initiation was identified as a significant factor influencing the percentage of crack initiation life in relation to the overall fatigue life.

Background

The simple shear experiment is widely used for the calibration of plasticity models due to straightforward post processing. The specimen can be as simple as a rectangular strip of sheet metal, but the maximum strain is limited by early initiation of fractures from the free edges. Avoiding this drawback has been a major motivation for the development of new specimens with optimized edge geometries or the in-plane torsion test, but at the cost of a more complex analysis of the test and often a reduction of the gauge section.

Objective

The objective of the present work is to overcome the initiation of fracture from the free edges during simple shear experiments. Our goal is to double the achievable maximum strain, while keeping the size of the specimen and the post processing simplicity of a standard simple shear test.

Methods

A sequential single shear test is proposed, consisting of several two steps sequences on a notched geometry. First, an interrupted shear test is performed up to a specified displacement value. Then, the damaged free edges of the specimen are removed through milling. The specimen is then ready for the following sequence of shear and re-machining.

Results

Experiments are performed on three engineering materials, with up to five loading-machining sequences. The maximum attained effective strain is up to two times the one reached during a monotonic experiment. Numerical simulations are used to validate the shear stress and strain calculations from experimental measurements. Practical recommendations are derived for the choice of the displacement step size and Digital Image Correlation analysis.

Conclusion

It is found that the maximum strain attained before the undesired failure of the specimen during simple shear test can be substantially extended through repeated re-machining of the specimen boundaries, enabling behavior identification at larger strains.