Monotonic Tension Tests Research Articles

An experimental and theoretical investigation on the hyper-viscoelasticity of polyamide 12 produced by selective laser sintering.

Polyamide 12 (PA12) is vastly utilized in many additive manufacturing methods, such as Selective Laser Sintering (SLS), and a better understanding of its mechanical behaviors promotes available knowledge on the behaviors of 3D-printed parts made from this polymer. In this paper, SLS-produced standard tensile specimens are studied under monotonic and cyclic tension tests, as well as stress relaxation experiments, and the obtained force-displacement responses are shown to be consistent with a hyper-viscoelastic material model. This finding is also observed in typical pantographic structures produced by the same manufacturing parameters. To propose a constitutive model for predicting these behaviors, the convolution integral of a strain-dependent function and a time-dependent function is developed where the material parameters are determined with the use of both short-term and long-term responses of the specimens. Numerical results of the presented model for standard test specimens are shown to be in good agreements with the experimental ones under various loading conditions. To prove the capabilities of the proposed model in studying any SLS-produced part, finite element implementation of the constitutive equations is shown to provide numerical results in agreement with the empirical findings for tensile loading of the 3D-printed pantographic structure.

Mechanical grouting sleeve of rebars: experimental investigations

To ensure the reliability of prefabricated concrete structures and reduce the cost of sleeve, a new type of mechanical grouting sleeve of rebars was designed. According to the specification of the rebar joint, monotonic axial tension test and cyclic load test of the new rebar sleeve were performed. The working mechanism and connection performance of sleeve were studied. The results show that the unidirectional tensile strength of the sleeve joint can meet the requirement of 1.1 times of the tensile strength of the rebar. The deformation of mechanical grouting meets the requirements of standard under cyclic load. The strength and deformation properties of the rebar joint meet the requirements of standard of JGJ107-2010 for I level joint when the grouting length is large. The working mechanism of the mechanical grouting sleeve is discussed, which provides a new design method for the connection of rebar with different diameters. The research results are of great significance for promoting the development of prefabricated concrete structures.

Effect of Post-Deposition Heat Treatment on the Mechanical Behavior and Deformation Mechanisms of a Solid-State Additively Manufactured Al–Mg–Si Alloy

Abstract The effects of post-deposition heat treatment on the fatigue behavior of AA6061 processed by additive friction stir deposition (AFSD) were investigated for the first time in this work. A heat treatment to recover the T6 temper was performed on AFSD AA6061 is then subjected to strain-controlled fatigue and monotonic tension testing. Microstructural analysis revealed abnormal grain growth resulting in bimodal grain size distribution. Mechanical testing indicated a full recovery of the strength of the AA6061-T6 temper with comparable fatigue performance to the as-deposited AFSD AA6061. Fractography revealed deformation mechanisms in the post-deposition heat treatment not observed in the as-deposited samples, however, the fatigue resistance remained unchanged. A microstructure-sensitive fatigue model was implemented to capture the effects of the heat treatment process on the fatigue performance of the post-deposition heat-treated AFSD AA6061.

Analysis of the Mechanical Performance of Sleeve Considering the Different Distributions of Grouting Defects

To study the influence of grouting defects on the mechanical properties of grouting sleeves, 49 groups of specimens with different specifications were made considering the length and location of defects, monotonic axial tension tests were carried out to study the influence of grouting defects on its failure process, failure mode, load-displacement curve, bearing capacity and other mechanical properties, and the influence law of different distribution defects on the grouting sleeve was analyzed. This research shows that there are two forms of sleeve failure: steel bar fracture failure and steel bar pullout failure. The bearing capacity of specimens with a defect length of 2D and 3D varies with the defect type. The ultimate displacement of specimens with a defect length of 2D varies with the defect type. The ultimate displacement of specimens with a defect length of 3D increases with the increase in bearing capacity. By moving defects, it is found that if there is a vertical overlap of defects in the upper or middle part of the specimen, the bearing capacity of the specimen will be greatly affected.

Appropriateness of hydrostatic pressure-based modeling for strength differential effect in advanced high strength steel

Although numerous models have been proposed to describe the strength differential (SD) effect, their appropriateness and efficiency in the case of advanced high strength steel (AHSSs) have not been investigated. To this end, this study investigates the SD effect in AHSS using a hydrostatic-pressure-dependent plasticity theory. Although pressure model can effectively predict the SD effect in AHSSs with only one coefficient, the original formulation is experimentally inconsistent in the first quadrant of a yield surface. Hence, this study developed a new formulation to remove the inconsistency in a simple manner and validated the pressure model using new experimental data reported for two recently developed AHSSs: DP980 and MART1500Y. The SD effect is characterized based on monotonic compression and tension tests in three different directions with respect to the rolling direction to obtain the stress–strain curves and r-values. Next, the pressure model is used to predict the plastic behavior of both steel sheets. The parallel transition of flow stress and similarity in r-values between the tension and compression states supports the appropriateness of the proposed pressure-based modeling. Furthermore, these insights substantiate the fact that deformation mechanisms cannot explain the SD effect in both steel sheets. Therefore, complex yield functions that describe the SD effect in general materials are not necessary in the case of AHSS. Additionally, the pressure model is incorporated into a distortional plasticity model for applying it to springback in the bending process. The distortional plasticity model with and without the pressure model is calibrated using tension–compression curves, thereby revealing that the SD significantly affects the calibration. Finally, a U-draw bending experiment and the corresponding finite element simulation are conducted to compare the springback; the results show that considering the SD effect improves the springback simulation accuracy.

Ultra-lightweight low-carbon LC3 cement composites: Uniaxial mechanical behaviour and constitutive models

The present study aims to characterize the constitutive models of a novel ultra-lightweight low-carbon LC3 cement composite (ULCC-LC3) under monotonic uniaxial compression and tension tests. The compressive stress–strain curves of ULCC-LC3 were obtained based on axial compressive tests. The curve characteristics were analyzed and the data was fitted with the existing concrete compression constitutive model. Modified axial compression constitutive model for ULCC-LC3 was proposed and verified using the independent test data and related data of ULCC in other literatures. The characteristics of tensile stress–strain curves of ULCC-LC3 were also analyzed and studied. Based on existing experimental data, the applicability of the Tetsushi et. al.’s model to this composite was verified. The experimental results show that use of PE fibers can improve the ductility of ULCC-LC3. As compared with normal concrete, ULCC-LC3 exhibits a higher compressive peak strain (over 0.3% for fiber reinforced specimens and over 0.5% for non-fiber reinforced specimens). These findings contribute to the understanding of the mechanical behavior of ULCC-LC3 and provide constitutive models that can be used in numerical simulations and engineering applications to promote the use of this novel ultra-lightweight low-carbon material.

Tensile behaviors of stainless steel anchor channel systems embedded in concrete: Experimental and numerical analyses

Stainless steel anchor channels with channel bolts (SSAC-CBs) are extensively used in reinforced concrete structures subjected to corrosive environments owing to the excellent heat and corrosion resistance, appreciable inelastic deformation capacity, and high ductility of stainless steel. However, limited tests and numerical simulations have been conducted describing the tensile behaviors of SSAC-CB assemblies. We therefore conducted static monotonic tension tests to investigate the structural behaviors of SSAC-CBs embedded in concrete in terms of failure mode, tensile load-bearing capacity, ductility, and plastic development response. The test variables included the anchor/channel profile type, stainless steel grade, and tensile load position. The SSAC-CB specimens exhibited three dominant failure modes in tension: combined yielding of the channel lips and webs, flexural deformation of the channel, and concrete splitting. The ductility ratios of the test specimens ranged from 4.56 to 8.45, indicating a significant inelastic deformation capacity. Based on the maximum tensile loads obtained in the experiments, the ultimate limit state principle was employed to recommend tensile design resistances according to anchor channel type. In addition, nonlinear finite element analysis models were constructed in ABAQUS using three-dimensional solid elements to facilitate detailed investigations of the tensile behaviors of SSAC-CBs embedded in concrete; the numerically obtained failure modes and loads agreed well with the experimental responses. Finally, the experimental data were applied to propose and validate analytical expressions for the tensile load-bearing capacities of SSAC-CBs embedded in concrete corresponding to their different failure modes under tensile load.

Physico-mechanical properties and brittle to plastic transition of a thermally cracked marble under uniaxial tension

Thermal stress induces severe changes in rock microstructures, resulting in the variations of physical and mechanical properties. To study thermal effects on rock physical properties and tensile behaviors, monotonic and cyclic tension tests were conducted on marble samples heated at different temperatures. The results show that marble samples are weakened by thermal damage, showing a decrease in tensile strength and P-wave velocity, and an increase in porosity. The changing trends of Young's modulus and failure strain with temperature appear to be scattering and complicated probably depending on the deformation types (i.e., nonlinear elastic deformation and plastic deformation). Nonlinearity is a fundamental feature of tensile stress versus strain curves, which is enhanced when the sample is heated at higher temperature. Even under low tensile stress levels, permanent damages are always observed, highlighting a hysteresis in the stress-strain curves obtained from cyclic tension tests. A transition from the nonlinear elastic deformation to the plastic deformation occurs at approximately 300 - 400°C. A crack density based constitutive model is introduced for the nonlinear elastic deformation (25 - 300°C). A modified Voce exponential function is recommended to capture the tensile stress and strain relationships for the plastic deformation (400 - 700°C).

A structure metric for quantitative assessment of fracture surfaces in 3D conceived based on confocal laser scanning microscopy data

Assessment of fracture surfaces typically involves the identification of qualitative features such as dimples, grain facets, and river patterns to determine type of fracture. The present study formulates a measure termed Real-to-Projected Area (RPA) ratio/metric to quantitatively assess fracture surfaces. Fractures surfaces of several metallic alloys tested monotonically and in fatigue to fracture including three Mg alloys, a Ti alloy in two conditions, and a Ni-based superalloy were observed using confocal laser scanning microscopy in 3D and quantitatively assessed using the metric. The metric successfully separated fracture surfaces obtained after fatigue testing from those obtained after monotonic tension testing. Specifically, the values of the metric were found to be in the range of 40–80 pct for fatigue fracture, while 90–140 pct for monotonic tension fracture. As a result, the metric can be used to distinguish between fracture in fatigue and fracture in monotonic tension when fracture types / loading modes are unknown. Moreover, the metric is sensitive to ductility of tested alloys as well as any presence of critical defects in the microstructure responsible for rapid fracture such as intermetallic inclusions. An added utility of the metric is also to identify outliers in the data such as invalid mechanical tests. • Real-to-Projected Area (RPA) ratio/metric is defined to quantitatively assess fracture surfaces. • Fracture surfaces of several alloys are observed using confocal laser scanning microscopy and quantified using the metric. • The metric can successfully separate fatigue fracture surfaces from monotonic tension fracture surfaces. • The metric is sensitive to ductility of tested alloys and defects in the microstructure such as intermetallic inclusions. • The metric can identify outliers in testing to fracture data such as invalid mechanical tests.

Analysis of ductile damage and fracture under reverse loading

This paper investigates the influence of reverse and cyclic loading on the damage and fracture behavior of the aluminum alloy EN-AW 6082-T6. A series of uniaxial tension–compression and shear tests with newly designed shear specimen has been performed to analyze the development of plastic deformations and damage under different stress conditions. In addition, several monotonic uniaxial tension and compression tests are conducted to detect the strength-differential (SD) effect. Hence, combined hardening law is incorporated into the hydrostatic-stress-sensitive yield function to predict irreversible deformations under cyclic loading. A newly defined strain triaxiality is proposed to describe the strain state and to reveal the damage evolution processes. To accurately model cyclic plasticity, strain triaxiality is incorporated into the extended Voce strain hardening, and the angle parameter between effective back stress and reduced stress is added to Chaboche’s kinematic hardening. Furthermore, a modified anisotropic stress-state-dependent continuum damage model is considered to evaluate the damage and fracture evolution processes. Additionally, the digital image correlation (DIC) technique is applied to measure the strain fields during the experiment. Numerical simulations are used to predict the distribution of stresses and strains. Finally, the distribution and amount of damage are used to explain the fracture behavior detected by scanning electron microscopy (SEM) pictures taken in the critical parts of the specimens. SEM images and numerically predicted damage stains confirm that the different failure modes depend on the loading history.

Impact of loading rate during hybrid simulation on seismic response of steel structures

AbstractThis study investigates the influence of loading rates on the seismic response of steel structures. A series of coupon tests under typical earthquake‐induced loading rates are carried out. In addition to the conventional monotonic tension test, the specimens are also tested under a discontinuous loading protocol that is analogous to the ramp‐and‐hold scheme employed in the pseudo‐dynamic hybrid simulations. The test results are used to develop a uniaxial steel material model to account for the strain rate effect and stress relaxation. The material model facilitates the reuse of existing steel hysteresis models and can be used to model different structural components, such as columns, beams, braces, etc. The material model is then used for a parametric study to evaluate the impact of various loading rates on the global seismic response of a single degree of freedom structural system with different structural periods, and types and intensities of ground motions. It is found that the impact of loading rates is more significant for structures with a period of 0.5 s or less and it thus requires a fast control of the physical specimen in pseudo‐dynamic hybrid testing. In addition, the material model is applied to a five‐storey buckling restrained braced frame (BRBF) building with a structural period of 1.2 s. It is found that the earthquake‐induced loading rates have a minor impact on the global response of the structure.

Characterizing the Fatigue Behavior of Wrought Fe–Co–2V Using Experimental Techniques

Abstract Fe–Co–2V (Hiperco®2 equivalent) is a soft ferromagnetic material that is commonly used for electrical components that require robust magnetic performance. Despite the excellent magnetic properties of Fe–Co–2V, it often exhibits low strength, ductility, and workability due to an ordered B2 microstructure. The mechanical properties exhibit considerable dependency on grain size and degree of order, which are influenced by processing methods. A thorough understanding of Fe–Co–2V’s fatigue performance is required to predict mechanical reliability under operating loads; however, limited fatigue data currently exist for Fe–Co alloys. This work characterizes the fatigue properties of wrought Fe–Co–2V through strain-controlled fatigue testing and fractography. Young’s modulus, ultimate strength, and yield stress were determined through monotonic tension tests. The fatigue behavior was quantified using fully reversed, strain-controlled fatigue testing for applied strain amplitudes ranging from 0.10% to 1.00%. Subsequently, the Coffin–Manson strain-life curve was fit to the experimental data. Failure mechanisms were investigated through fractography with a scanning electron microscope. Inspection of the failure surfaces revealed that crack initiation occurred at defects located on or near the specimen surface with a localized region of crack propagation prior to the transgranular cleavage fracture. Additionally, two material models were calibrated from the experimental static and cyclic experimental testing. The characterization of the fatigue behavior of wrought Fe–Co–2V presented herein will aid in the fatigue analysis of Fe–Co–2V components and the development of analytical fatigue modeling methodologies.

Mechanical properties of bridge-steel weldments at elevated temperatures

The fire-resistance design of welded steel bridges depends on the temperature-dependent mechanical properties at elevated temperatures. This study investigates experimentally the temperature-induced degradation of the mechanical properties of a bridge-steel weldment. A butt-welded joint, containing the base metal (BM) Q345qD steel and weld metal (WM), was welded via submerged arc welding. Monotonic tension tests of BM and WM cylindrical specimens were carried out under various temperatures in the range of 20–700 °C, and their stress-strain relationships, failure modes, yield strength, elastic modulus, and ultimate strength at elevated temperatures were obtained and analyzed. The reduction factors of yield strength, elastic modulus and ultimate strength are compared with those recommended in current design codes. It is shown that elevated temperatures can decrease significantly the material performance of the Q345qD weldment, and the degradation of the mechanical properties of the BM and WM differs with increasing temperature. The test data indicate that the reductions in yield strength, elastic modulus, and ultimate strength at 700 °C from those at 20 °C are 78%, 55%, and 84%, respectively, for the BM and 75%, 53%, and 79%, respectively, for WM. The predictive equations for characterizing the material properties and stress-strain relationships of the BM and WM at elevated temperatures are proposed, thereby providing essential data for evaluating the fire response of steel bridges.

Experimental investigations of glued-in rod connections in CLT

The widespread availability of cross-laminated timber (CLT) provides opportunities to extend the use of wood structures beyond low-rise residential construction. Glued-in rods (GiR) are an interesting technical solution for numerous timber structural applications, amongst them, the not yet exploited potential in conjunction with CLT. This paper summarises experimental investigation on the performance of GiR in CLT. Two 5-ply CLT panel thicknesses (139 and 175 mm), two steel rod diameters (dR = 12.7 and 19.1 mm), and five anchorage lengths (la = 6dR, 10dR, 12dR, 14dR and 18dR) were considered in applications parallel and perpendicular to the CLT major strength axis. A total of 220 uni-axial quasi-static monotonic tension tests were conducted. The results, assessed in terms of load-carrying capacity (ranging from 14 kN to 154 kN) and failure modes (plug-shear failure and lamination tear-out), demonstrated the impact of the connection’s geometric design parameters and that steel rods glued into CLT offer an alternative high capacity connection. The results will serve as the benchmark for subsequent numerical modelling, including the determination of joint capacity.

Tensile and flexural behaviors of connections between steel beam and concrete-encased-and-filled steel tube column

Concrete-encased-and-filled steel tube (CEFT) columns with a circular tube section can be potentially used for hollow precast concrete (PC) constructions, in which thin PC encasement is used to reduce the lifting weight of a large PC column. In the present study, connection methods were studied for steel beam-CEFT column joints with the thin concrete encasement. Two sets of testing were performed focusing on the local behavior of the connection: 1) monotonic tension test for beam flange-column connections, and 2) cyclic loading test for beam-column joints. In the first test, the local out-of-plane behavior of the flange plate-tube connections and the associated damage mode of the concrete encasement were identified. The load-deformation relationships of the test specimens were evaluated by nonlinear finite element analysis. Based on the results, exterior beam-column joint specimens were designed, and the flexural behavior of the connections was verified under cyclic loading. The joint specimens were strengthened with connection methods such as tension bars, thick steel tube, and vertical continuity plate, inducing the beam failure. In particular, the thick tube and vertical continuity plate were effective in restraining the concrete damage. The tensile and flexural strengths of the test specimens were evaluated using design equations.

The recovery stress of hot drawn and annealed NiTiNb SMA considering effect of temperature and cyclic loads: Experimental and comparative study

Shape memory alloy (SMA) has good memory effect and hyperelasticity, which can be applied in civil engineering for seismic shock absorption and active strengthening. The Nickel-titanium-niobium (NiTiNb) SMA is widely adopted as reinforcement to increase the capacity of bridge piers and/or girders. However, the NiTiNb SMA is inconvenient to apply for two reasons. First, the NiTiNb SMA is generally processed by annealing the row wires at a high temperature (850℃ ± 10℃), meaning that a lot of energy is required. Second, the annealed SMA usually needs to be pre-tensed at a low temperature (about −60℃) to obtain a sufficient recovery stress, which requires a specific device and environment. Comparatively, the NiTiNb SMA treated with hot drawn process doesn’t require high temperature in the fabrication, thus consumes less energy. Meanwhile, the hot drawn SMA can be pre-tensed at room temperature to obtain an adequate recovery stress (around 700 MPa), which overcomes the inconvenience of the annealed SMA. So far, however, the mechanical properties of the hot drawn SMA have not been fully studied, which limits its practical application. In this study, therefore, the mechanical properties of hot drawn NiTiNb SMA wires were investigated through the monotonic tension, thermal excitation and cyclic loading tests. The mechanical properties of the hot drawn SMA were compared with those of the annealed ones. The results showed that, under the monotonic loading, the yield and ultimate strength of the hot drawn SMA is greater than the annealed one by 12.7% and 43.0%. Compared to the annealed SMA, the maximum and remained recovery stress of the hot drawn SMA increases by 70.5% and 18.2%, respectively. Moreover, the hot drawn SMA is better than the annealed SMA in resisting the degradation of effective stress under the incremental cycle amplitude test.

A molecular dynamics based cohesive zone model for interface failure under monotonic tension of 3D four direction SiCf/SiC composites

Aiming at a mechanics-based prediction on the progressive damage in SiC fiber reinforced SiC matrix (SiCf/SiC) composites, multiscale simulation coupling molecular dynamics (MD) and finite element method (FEM) is established with specific concern on the silicon carbide/pyrocarbon (SiC/PyC) interface separation behavior. The MD simulation is employed to evaluate the interface damage with preset crack. Interfacial mechanical response at microscale between SiC and PyC under pure normal and tangential stresses is captured. Accordingly, a cohesive zone model (CZM) for the SiC/PyC interface separation is established, generalized to mixed loading conditions and validated by FEM fiber push-out simulation. Integrating proposed CZM to FEM to construct 3D four direction SiCf/SiC composites, the multiscale damage model is eventually consistent with the data from monotonic tension test. This research provides new physical insights on SiC/PyC interface damage under complex stress state, and contributes to a promising tool for mechanical performance prediction on 3D four direction SiCf/SiC composites.

Mechanical properties and microscopic analysis of sintered rhenium subjected to monotonic tension and uniaxial fatigue

Some reports on the mechanical testing of pure rhenium can be found in the literature. However, very few studies have been published regarding the fatigue properties of rhenium. To gain new knowledge on this subject, the present research aimed to determine mechanical properties in monotonic tension, hardness, and stress-controlled fatigue tests of sintered rhenium. The results of the basic mechanical tests are in agreement with data in the literature. The determined exploratory S–N fatigue curve for pure, sintered rhenium is a unique result, and it cannot be compared with others. Based on the monotonic tests data, estimated S–N curves were also determined. The comparison of the experimental curve and the estimated curves showed the usefulness of approximation methods to estimate the fatigue properties of rhenium. Considering the high cost of conducting research on rhenium, this is especially important for engineering design. Subsequently, microscopic fracture analysis and microstructure evolution observation were performed. Microscopic observations were made using a light optical microscope and a scanning electron microscope. Based on the performed metallographic analyses, it was found that under fatigue and monotonic loadings, cracks propagated along the grain boundaries, and the pores present in the material did not affect this process. It was confirmed that the basic deformation mechanism in sintered rhenium is twinning. To sum up, the present work provides new data on the strength properties of rhenium and helps to better understand the deformation and damage processes of this metal.

Uncovering the coupled impact of defect morphology and microstructure on the tensile behavior of Ti-6Al-4V fabricated via laser powder bed fusion

The preferred defect domain space for enhanced mechanical properties is a critical unknown in the selective laser melting (SLM) additive manufacturing community. This paper presents a systematic approach to relate mechanical properties and the microstructure in the keyhole and the lack of fusion domains for Ti-6Al-4V parts. SLM laser processing conditions were modified to generate three similar amounts of defects in the keyhole and lack of fusion domains. Material from each processing condition was scanned and quantified using x-ray computed tomography to map the defect size and distribution. Samples containing these resultant defects were compared in monotonic tension testing to determine the preferred defect domain for a given amount of defects. While it is known that high amounts of each type of defect is unfavorable, it is shown here that it is more favorable to be in the keyhole domain—at a similar amount of defects—than in the lack of fusion domain. This is attributed to not only the higher sphericity of the keyhole defects but also to the larger amount of crystallographic texture in the keyhole samples which has not been previously well established. Keyhole samples up to 6% porosity maintain yield and UTS values when compared to the stock samples with only a slight loss in elongation. In all cases, when the defect density increases the elongation is reduced. These findings illustrate the preferred location in the processing window for SLM Ti-6Al-4V parts that will be critical for ensuring optimal part performance.

Experimental investigation on the effects of fabric architectures on mechanical and damage behaviors of carbon/epoxy woven composites

The mechanical behaviors and damage evolutions of carbon/epoxy woven fabric composites with three different geometries, i.e., one plain weave and two twill weave patterns with different areal densities, are studied under tensile loading. The effects of weave patterns on mechanical properties are investigated by monotonic and cyclic tension tests. Remarkable variations in stress–strain curve, Poisson’s ratio, residual strain and strain map exist in the three composites. Crimp ratio is found to be a critical factor to govern the mechanical properties. With smaller crimp ratio, a quasi-linear stress–strain curve with higher elastic modulus and strength is observed. The stress–strain curves of composites with higher crimp ratio contain transition stages with significant tangent modulus degradation. Elastic modulus, strength and damage initiation are all correlated with the crimp ratio linearly regardless of the fabric pattern. Dramatic nonlinear evolution in Poisson’s ratio occurs in the composite with higher crimp ratio. Cyclic tension results indicate that the residual strain is a more appropriate damage indicator than the unloading elastic modulus. Microstructure examination shows that damage developments are essentially related to the fabric geometry, and result in various mechanical behaviors. This study provides important insights into the geometry-deformation mechanism-mechanical property relationship of the woven composites.