Showing papers on "Tensile testing published in 2022"

Dynamic precipitation and enhanced mechanical properties of ZK60 magnesium alloy achieved by low temperature extrusion

Abstract: In this study, dynamic precipitation was observed in ZK60 (Mg-6.0Zn-0.5Zr) alloy during low temperature extrusion. The microstructure and mechanical properties of the alloy were investigated for various extrusion temperatures (400 oC, 350 oC and 300 oC). Decreasing extrusion temperature resulted in an increased dynamic precipitation as well as grain refinement. With decreasing extrusion temperature from 400 oC to 300 oC, the grain size was refined from 9.6 μm to 1.4 μm, and the dynamic recrystallization was changed from a discontinuous mechanism to a continuous one. The low temperature extrusion (300oC) resulted in an increased defect density and a supersaturated Zn in Mg matrix, which promoted the nucleation and potential for precipitation during extrusion process. The dynamic precipitates accelerated the nucleation of new grains during the dynamic recrystallization process. Besides the grain size, the texture intensity was also reduced. Due to the grain refinement and texture weakening during low temperature extrusion, both the mechanical strength and tensile ductility were improved with decreasing extrusion temperature. When extruded at 300 oC, the ZK60 alloy achieved a relatively high tensile yield strength (TYS) of 295 MPa and a remarkably high tensile elongation of 27.7%.

Hydrogen embrittlement fracture mechanism of 430 ferritic stainless steel: The significant role of carbides and dislocations

Abstract: The effect of electrochemical hydrogen charging on the mechanical properties and the fracture mechanism of AISI430 ferritic stainless steel were studied by tensile test. Electrochemical hydrogenation experiments of 2, 4, 8 and 12 h on the tensile specimen with a current density of 50 mA/cm2. Hydrogen has a significant impact on the mechanical properties of the material: the elongation of the material is reduced from 27.8% to 13.0%, the yield strength is increased from 325 MPa to 380 MPa, and the tensile strength is increased from 500 MPa to 575 MPa. The obvious increase in tensile strength is attributed to two aspects: (1) the hydrogen-induced dislocations pinning; (2) the influence of the second phase carbide particles on the critical shear stress of dislocation glide. As the hydrogen outgassing time increases, the hardness measurement proves the softening effect of hydrogen. Since the reduction of the binding energy of the carbide and ferrite matrix interface is attributed to the HEDE mechanism, the crack nucleation in the vicinity of the carbide leads to the generation of the cleavage (C) region. Under low diffusible hydrogen content, the samples are mainly C mode and F-MVC mode guided by the synergistic effect of hydrogen-enhanced decohesion (HEDE), hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced strain-induced vacancies (HESIV) mechanisms and under high diffusible hydrogen content, the transgranular (TG) mode replaces the F-MVC mode, and the HEDE mechanism is more dominant than the HELP mechanism.

Experimental investigation on the effect of open fire on the tensile properties and damage evolution behavior of granite

Abstract: The stability of rock mass after fire is a concern of many engineering projects. In this paper, the effects of open fire and different cooling methods on granites were investigated. The static Brazilian test, dynamic Brazilian test, and P-wave velocity test were carried out to evaluate the mechanical properties and damage evolution behavior. A high-speed camera is employed to monitor the failure process of rock. The microstructures of the treated granites were observed by scanning electron microscope (SEM). The results showed that the fire duration and cooling treatment have a significant effect on the P-wave velocity, static nominal tensile strength, and dynamic nominal tensile strength of granite. These characteristics decrease rapidly during 0 to 10 min, and slowly decrease after 20 min. Compared to the air-cooling treatment, the water-cooling treatment has greater damage to the heated granite. To better understand the results, the rapid heating process of open fire heating was simulated using Abaqus software. The results reveal that there is a thinner compressive stress zone at the bottom of the specimen, and there is a large zone in the middle of the sample with higher tensile stress. The crack extension would be expanded due to high tensile stress, leading to the reduction of the tensile strength of the rock. This paper aims to better predict the degree of damage of rock materials after actual fire, as well as preliminarily explore the effect of the fire exposure duration and fire extinguishing method on the tensile properties of rock.

High-temperature mechanical properties of FeCoCrNi high-entropy alloys fabricated via selective laser melting

Abstract: The high-temperature application of high-entropy alloys (HEAs) fabricated via selective laser melting (SLM) relies on an in-depth understanding of the mechanical properties and deformation mechanisms involved. This study conducted tensile testing of FeCoCrNi HEA, at various temperatures and strain rates, where the microstructure was systematically characterized before and after deformation. The FeCoCrNi HEAs fabricated via SLM exhibited a greatly enhanced tensile strength at room temperature compared to those produced by traditional processing, but the strength at high temperature was significantly compromised. Experimental data were used to calculate the parameters of a constitutive model based on three classical mathematical models to predict the flow behavior at elevated temperatures. The softening mechanism was attributed to the evolution of the dislocation network, and a structure–mechanism–property relationship at elevated temperatures was established. Further, cracks initiated at the grain boundaries at elevated temperatures owing to nano-clustering. These results are expected to contribute to the development and improvement of SLM-HEAs for use at high temperatures.

Influence of Non-Metallic Inclusions on Local Deformation and Damage Behavior of Modified 16MnCrS5 Steel

Abstract: This work investigates a ferrite matrix with multiple non-metallic inclusions to evaluate their influence on the global and local deformation and damage behavior of modified 16MnCrS5 steel. For this purpose, appropriate specimens are prepared and polished. The EBSD technique is used to record local phase and orientation data, then analyze and identify the size and type of inclusions present in the material. The EBSD data are then used to run full phase crystal plasticity simulations using DAMASK-calibrated material model parameters. The qualitative and quantitative analysis of these full phase simulations provides a detailed insight into how the distribution of non-metallic inclusions within the ferrite matrix affects the local stress, strain, and damage behavior. In situ tensile tests are carried out on specially prepared miniature dog-bone-shaped notched specimens in ZEISS Gemini 450 scanning electron microscope with a Kammrath and Weiss tensile test stage. By adopting an optimized scheme, tensile tests are carried out, and local images around one large and several small MnS inclusions are taken at incremental strain values. These images are then processed using VEDDAC, a digital image correlation-based microstrain measurement tool. The damage initiation around several inclusions is recorded during the in situ tensile tests, and damage initiation, propagation, and strain localization are analyzed. The experimental results validate the simulation outcomes, providing deeper insight into the experimentally observed trends.

Laser directed energy deposition of AISI 316L stainless steel: The effect of build direction on mechanical properties in as-built and heat-treated conditions

Abstract: The purpose of this study was to evaluate the effect of build direction on the mechanical properties of AISI 316L stainless steel using the Laser Directed Energy Deposition (L-DED) process, in the as-built and heat-treated conditions. The test specimens were manufactured in vertical and horizontal directions for tensile and impact tests. In addition, analysis test specimens cube-shaped were manufactured for microstructural and microhardness evaluation. The microstructure in the as-built condition showed a combination of cellular, equiaxial dendritic, cellular dendritic and columnar dendritic, while the microstructure in the heat-treated condition showed a homogeneous characteristic, composed by differently sized grains. The microhardness evaluation in the heat-treated condition presented a reduction of 26.52% compared to the as-build condition. The ultimate tensile strength of horizontal test specimens in the as-built condition was 4.86% higher than the heat-treated condition, whereas the ultimate tensile strength of vertical test specimens in the as-built condition was 5.55% higher than the heat-treated condition. The impact resistance of horizontal test specimens in the heat-treated condition was 12.36% higher than the as-built condition, whereas the impact resistance of vertical test specimens in the heat-treated condition was 18.92% higher than the as-built condition. Briefly, the build direction directly affects the mechanical properties of components manufactured through the L-DED process, and it is possible to improve certain mechanical properties, such as ductility and toughness, through heat treatment.

Secondary phases strengthening-toughening effects in the Mo–TiC–La2O3 alloys

Abstract: Mo–TiC–La2O3 molybdenum alloys were strengthened and toughened by the synergistic action of nano-carbide particles and rare earth oxides. In this paper, the Mo–TiC–La2O3 alloy system was prepared by powder metallurgy. The microstructure was characterized by optical, scanning, and transmission electron microscopy. The mechanical properties were tested using the hardness tester and universal tensile testing machine. The grain size of the Mo–TiC–La2O3 alloy is smaller than the Mo–TiC and Mo–La2O3 alloys. The strength and elongation of annealed Mo–TiC–La2O3 alloy are 1291 MPa and 6.6%, respectively. The strength and ductility of the annealed Mo–TiC–La2O3 alloy are higher than the Mo–TiC and Mo–La2O3 alloys. According to the interfacial mismatch between the secondary phases and the matrix, along with oxygen impurities interactions, the mechanisms of strengthening and toughening of the secondary phases in the Mo–TiC–La2O3 alloy were revealed.

An Investigation into the Characterization of the Hardening Response of Sheet Metals using Tensile and Shear Tests with Surface Strain Measurement

Abstract: The adoption of full-field digital image correlation (DIC) strain measurement in recent years has fundamentally transformed conventional analysis procedures for the mechanical characterization of sheet metals. The wealth of local strain data that is now available until fracture has enabled experimentalists to propose new analysis techniques for tensile and shear tests to obtain the hardening response to large strain levels. Although the tests show great potential, significant gaps remain surrounding their applicability. The choice of test geometry and suitability of the analysis across a broad range of sheet metals with diverse hardening and anisotropic behaviour remain open questions. Critically, the assumptions within the tensile and shear methodologies in applying surface strain measurements to large strains while omitting through-thickness gradients have not been verified. The objective of the present study is to perform a critical evaluation of constitutive characterization techniques for sheet metals under quasi-static, ambient conditions. A series of virtual experiments were developed to numerically replicate the testing and analysis procedures followed in lab-scale experiments to extract the hardening response. A simple shear and two standard tensile test geometries were evaluated across a broad range of plasticity characteristics including different hardening rates and anisotropic behaviour. For each case, the input hardening curve in the virtual experiment is treated as the exact solution and compared to the hardening response obtained from each analysis method. The so-called area reduction method (ARM) used for tensile specimens was found to be relatively insensitive to the specimen geometry, yield exponent, and plastic anisotropy but overestimated the flow stress. The performance of the ARM method hinges upon the aspect ratio of the cross-section to avoid shear band formation and requires empirical correlations to improve its predictions at large strains. Despite their simplicity, tensile analysis methods that use small extensometers or local DIC point data are shown to systematically overestimate the hardening behaviour. Simple shear tests can provide accurate estimations of the hardening response, but the choice of geometry is material dependant. The study concludes by considering a strongly anisotropic mild steel to evaluate the tensile and shear characterization strategies with the results of a hydraulic bulge analysis.

Additive manufacturing of cobalt-based alloy on tool steel by directed energy deposition

Abstract: Cladding hard-surfacing alloys on tool steel is an effective approach to enhance the surface properties of tool steel. In this study, a Co-based alloy was deposited on tool steel by Directed Energy Deposition (DED) following a three-factor three-level design of experiment matrix with varied laser power, scan speed, and powder flow rate. The microstructure of the deposits was characterized using scanning electron microscopy (SEM). The residual stress on the surface of the samples was measured by the X-ray diffraction (XRD) sin2ψ technique. The parameters that produced promising deposits were used to fabricate samples for tensile test, four-point bending test, Charpy impact test, and hardness measurement. The result reveals that the processing parameters have a significant role in the residual stress of the coatings. Residual stress reduces with the increase of laser energy density. Cracks were found at samples with energy density below a threshold. Tensile testing of the coating/substrate combined structure reveals fracture at the coatings with an ultimate tensile strength of 633.9 ± 54.7 MPa. The bi-material interface survived the tensile test, indicating a strong interfacial bond. The four-point bending test of coating/substrate laminates shows an ultimate flexure strength of 860.6 ± 36.9 MPa. Cracks initiated from the coatings ignored the interface and penetrated the substrate, suggesting a solid bi-material bond. Charpy impact test shows the absorbed energy of coating/substrate laminates is more than doubled that of the substrate.

Tensile properties of TIG welded 2219-T8 aluminum alloy joints in consideration of residual stress releasing and specimen size

Abstract: Different dimensions of tensile test specimen can be used for qualification of welding procedure according to the existing standard. However, they are found to be dissimilar in tensile properties. In this study, the tensile properties of TIG welded 2219-T8 aluminum alloy joints were tested with different tensile specimen sizes. The welding residual stress releasing, specimen size effect, as well as fracture criteria were taken into account by using a sequential “welding - cutting - tensile” finite element simulating method. Results indicated that the degree of welding residual stress releasing decreased with the increasing of specimen width. For specimens with 250 mm width, over 80% of the residual stresses were kept. Tensile testing result showed that the tensile strength and elongation of joint decreased when the specimen width increased. The differences in tensile properties were proved to result from the size distinctions, rather than residual stress releasing, because the stress triaxialities were different in the specimens with different sizes. This phenomenon was precisely simulated by using the Johnson–Cook fracture criterion. Eventually, the constitutive equations of Johnson–Cook model were established based on the experimental tensile strengths to describe the tensile behaviors of 2219 aluminum alloy joints.