Monotonic Tensile Tests Research Articles

Fatigue and fracture of ultrasonically welded aluminum alloys to carbon fiber reinforced thermoplastics

AbstractThis work evaluates the fatigue behavior of ultrasonically welded aluminum AA5024) to fiber reinforced thermoplastic (PEEK) joints at a stress ratio of R > 0. Initial load increase tests were carried out and, based on a detected material reaction caused by fatigue damage, the load horizon for subsequent constant amplitude tests was derived. After identifying characteristic stress level, microscopic investigations of hybrid fracture surfaces were carried out. Damage behavior and failure mechanisms in comparison to monotonic tensile tests were investigated and discussed in detail. For a fatigue limit of 2·106 cycles, the applied maximum force corresponds to 35% of the maximum lap shear force for monotonic testing. A drop in stiffness to 88% close to failure was observed.

Experimental and numerical study on creep behaviors of 2D twill woven quartz fiber/silica matrix composites

This study investigates the creep deformation, damage, and rupture behaviors of 2D woven SiO2/SiO2 composites via experimental and numerical methods. In situ monotonic tensile tests and creep tests were conducted at 900 °C using a self-designed experimental system and digital image correlation. The tested specimens were characterized by X-ray computed tomography and scanning electron microscopy to conduct quantitative analyses and fracture observations. The obtained creep strain–time curves consist of primary and secondary stages, similar to the creep strain–time curves of most ceramic matrix composites. The matrix at the intersection of fiber bundles cracked under tensile loading. During subsequent creep loading, the propagation of matrix cracks, interfacial debonding, and fiber breakage in longitudinal fiber bundles were observed. At the mesoscale, the creep rupture entails a mechanism analogous to that observed in the monotonic tensile tests. Overall, the SiO2/SiO2 composites employed in this study exhibit excellent potential for long-term operation under mechanical loads at high temperatures. Next, a micromechanics-based creep model was proposed to simulate the creep behavior of the composites. In this model, the primary creep law and rule of mixtures were combined to describe the stress redistribution of various constituents and predict the deformation of the composites. In addition, the rupture life was predicted based on the global load-sharing model, two-parameter Weibull model, and shear lag model. The degradation of the matrix modulus and fiber strength was also considered to improve the accuracy of the simulation. The predicted results were in good agreement with the experimental data.

Multiscale investigation on fatigue properties and damage of a 3D braided SiC/SiC + PyC/SiC composites in the full stress range at 1300 °C

Monotonic tensile and fatigue tests of a SiC/SiC composites were conducted at 1300 °C in the full stress range. The macroscopic behaviors were studied based on the strain data. The mesoscopic morphology was observed by X-ray computed tomography, and the microanalysis was conducted using SEM, EDS and XRD. Besides, the interfacial debonding strength (IDS) were measured by nano-indenter. The results reveal that the fatigue behaviors can be divided into three zones. The inelastic strains accumulation and stiffness reduction can be observed in all three zones due to matrix cracking, interface damage, and failure of fibers. The fatigue life is long in the run-out zone because the maximum stress is lower than the proportional limit stress (PLS). In the stress-insensitive zone, the fracture depends on high-temperature and oxidation effects. The failure in the stress-sensitive zone is dominated by the fiber strength. The interface behaviors greatly affect the fatigue life above the PLS.

Effect of grain size on dynamic strain aging behavior of C-bearing high Mn twinning-induced plasticity steel

In this work, the effects of grain size on the tensile properties, serrated flow, Portevin-Le Châtelier (PLC) band evolution, dislocation density, and microstructure evolution of Fe–22Mn-0.6C steel were investigated using digital image correlation (DIC), X-ray diffraction (XRD), transmission electron microscope (TEM) and monotonic tensile tests. Results show that the yield strength, tensile strength, and work hardening value of Fe–22Mn-0.6C steel increase whereas its elongation decreases with decreasing grain size. The critical strain of serration decreases whereas the amplitude of serration increases with decreasing grain size. Moreover, the plateau of serrations is larger and that the strain concentration within PLC bands is more severe in fine-grained steel than in coarse-grained steel. The dynamic strain aging (DSA) behavior of the steel is enhanced by grain refinement. The speed of PLC band movement decreases faster in fine-grained steel than in coarse-grained steel, which results in a decrease in plasticity. XRD and TEM analyses show that the high dislocation density and unique twin substructure may be the main reasons for the strong DSA phenomenon observed in fine-grained steel.

Damage comparative analysis of needled C/SiC composite under vibration fatigue loading and monotonic tensile loading

Needled C/SiC composite is composed of fiber and matrix, witch fatigue limit is related to the vibration loading frequency. A kind of plate specimen made of needled C/SiC composite was tested on a shaking table, and the vibration fatigue life of each specimen was obtained. In addition, the monotonic tensile test of needled C/SiC composite specimen was also carried out. Furthermore, the fracture morphologies from vibration test and monotonic tensile test were compared by scanning electron microscope. And the mechanism causing these differences was also explained.

Investigation of self-heating and damage progression in woven carbon fibre composite materials, following the fibres direction, under static and cyclic loading

Infrared thermography is commonly used as a non-destructive testing technique for damage monitoring of composite materials under mechanical loadings. Self-heating tests consist in monitoring the stabilized heating of a material submitted to cyclic loading for increasing values of stress level. It appears that the load threshold from which the thermal behaviour changes can be related to the fatigue limit of the tested material. In this paper, this stress threshold is compared to the heating of a woven thermoplastic composite material submitted to a monotonic tensile test. Indeed, during a quasi-static tensile test, the material temperature cools down, due to thermoelastic effect, before warming up again, due to both viscous effects and first damage evolutions. The comparison, which is made for the warp direction only, is also based on microscopic optical scanning of the specimen edge and passive acoustic monitoring. It is shown that thermal changes detected in the composite samples are associated with damage occurring under both static and cyclic loading, for similar stress levels. This result indicates that the static tests make it possible to estimate a damage threshold, therefore a potential fatigue limit, even faster than with self-heating tests, which opens very promising prospects as for the determination of the fatigue limit of woven composite materials reinforced by carbon fibre yarns.

The Low-Cycle Fatigue Behavior, Failure Mechanism and Prediction of SLM Ti-6Al-4V Alloy with Different Heat Treatment Methods.

Selective laser melting (SLM) is a promising additive manufacturing (AM) process for high-strength or high-manufacturing-cost metals such as Ti-6Al-4V widely applied in aeronautical industry components with high material waste or complex geometry. However, one of the main challenges of AM parts is the variability in fatigue properties. In this study, standard cyclic fatigue and monotonic tensile testing specimens were fabricated by SLM and subsequently heat treated using the standard heat treatment (HT) or hot isostatic pressing (HIP) methods. All the specimens were post-treated to relieve the residual stress and subsequently machined to the same surface finishing. These specimens were tested in the low-cycle fatigue (LCF) regime. The effects of post-process methods on the failure mechanisms were observed using scanning electron microscopy (SEM) and optical microscopy (OM) characterization methods. While the tensile test results showed that specimens with different post-process treatment methods have similar tensile strength, the LCF test revealed that no significant difference exists between HT and HIP specimens. Based on the results, critical factors influencing the LCF properties are discussed. Furthermore, a microstructure-based multistage fatigue model was employed to predict the LCF life. The results show good agreement with the experiment.

Experimental study of hysteretic behavior of corroded steel with multi-angle welding joints

To rationally characterize the macroscopic behavior of in-service offshore structures affected by the coupled welding-corrosion action in steel components, the hysteretic behavior of corroded D36 marine structural steel with multi-angle welding joints is experimentally studied. To simulate possible corrosion-welding scenarios, the tested specimens are prepared with the gas arc welding (MAG) method for three welding angles and processed with the accelerated electrochemical corrosion for five corrosion degrees. The elasticity modulus, tensile strength, and elongation of welded-corroded steel is observed in the monotonic tensile test conducted before the cyclic test where the hysteretic performance and energy dissipation ability is mostly concerned. On the basis of these test results, a generalized hysteretic material model for welded-corroded steel which consists of a skeleton curve, an unloading curve and a reloading curve is proposed. All the parameters used to describe these curves are related to the degree of corrosion and welding angle, determined by the regression on the test data. The effectiveness of the proposed model is verified with the tested data, including those from the specimens additionally and specifically tested.

Investigations on the influence of cold‐forming and associated residual stresses on the fatigue strength of thin‐walled details

AbstractA code for the design of cold‐formed thin‐walled structures in accordance with fatigue requirements currently does not exist in Europe. Part 1993‐1‐3 of the Eurocode [1] only standardizes the static design. In part 1993‐1‐9 [2] for design and construction under fatigue loads cold‐formed thin‐walled profiles are explicitly excluded. Corresponding elements have, however, become established in the construction of racking systems, as they represent an efficient and economical way of creating storage capacities. In contrast to the former use of storage systems, manual retrieval and storage (S/R), increasing automation through S/R‐machines leads to cyclical loads in uprights and struts. In addition to purely static loads, this results in a dynamic component that cannot be neglected.Experimental investigations, such as monotonic and cyclic tensile tests, are used to determine changes in material properties after cold‐forming by bending. Special focus is put on the influence of bending radii in combination with the thickness on the plastic deformation and associated internal residual stresses. These parameters can have a significant impact on the fatigue behavior. The aim of the tests with a new invented testing device is to provide a basis for the development of adequate fatigue design rules for storage systems.

Experimental investigation into NiTi shape memory alloy panels under cyclic shear loading

Shape memory alloys (SMAs) are widely employed in buildings and bridges as energy dissipation devices because of their superelasticity, shape memory, fatigue resistance, and high damping performance. In this study, SMA plates were fabricated into a shear device as a seismic structure. To understand the seismic performance of the SMA plate, an experimental study was conducted on the tension and shear behaviours of SMA plates under cyclic loading. Monotonic and cyclic tensile tests were carried out on 12 SMA plate coupons, and the important influencing factors, including the annealing temperature, annealing duration, and loading protocol, were evaluated. Based on the principle of maximum elastic modulus of austenite, maximum energy dissipation, and minimum residual strain, an optimal heat treatment scheme for a Ti–55.8 wt%Ni SMA plate with a thickness of 1.6 mm was suggested: annealing at 500 °C for 5 min. Further, the optimised SMA plate was installed in a shear device to conduct cyclic shear loading tests. Three pure shear tests with the SMA plate, structural steel, and stainless steel were designed and performed. The test results showed that the elastic modulus of the SMA plate is the lowest, which facilitates elastic shear buckling, and the tension field effect could significantly enhance the ultimate bearing capacity after buckling. In particular, when the applied shear is unloaded to zero, the SMA plate does not produce any residual deformation. Compared with the structural and stainless steel, the SMA plate exhibits more ideal self-centring behaviours, perfect fatigue fracture resistance, slow stiffness degradation, and greater yield deformation capacity; however, the initial stiffness, displacement ductility ratio, and energy dissipation capacity are slightly lower than those of the other test specimens. Although the SMA plate develops a large deformation, it produces a fracture failure mode without any evident sign.

A visco-hyperelastic model with Mullins effect for polyurethane elastomers combining a phenomenological approach with macromolecular information

The classical generalized formulations for nonlinear viscoelastic behaviour do not represent the state of the art of modelling elastomeric materials and significant effects observed on polyurethane elastomers are not accurately predicted such as the loss of stiffness and the presence of residual strain by the Mullins effect. In this context, the present work proposes a model building strategy for visco-hyperelastic materials with Mullins effect merging a phenomenological approach with a macromolecular one. Polyurethane elastomers are modelled as a composite material consisting of crystalline and flexible phases. The dissipative effects observed in the viscoelastic behaviour are then related to the reorganization of the microstructural crystalline domain during the deformation process. Adding a softening parameter to the viscous strain rate predicted by Bergstrom–Boyce viscoelastic model led to more accurate predictions when describing the mechanical behaviour of these materials. The proposed model strategy is assessed based on measured data obtained from Polyurethane elastomers of different hardness under monotonic tensile tests and cyclic tensile tests. The model built with the proposed approach provided predictions closer to measured data than the predictions provided by the model solely based on phenomenological aspects for the analysed cases.

Experimental testing and finite element method analysis of the anterior cruciate ligament primary repair with internal brace augmentation

The internal brace (IB) technique is a promising treatment option for repairing the proximal rupture of the anterior cruciate ligament (ACL). This paper presents a biomechanical evaluation of the IB technique. Sixteen cadaveric sheep knees underwent monotonic tensile tests, cyclic loading, and passive flexion-extension testing. Data were compared in a series of eight control specimens with an intact ACL and eight repaired specimens where the ACL was cut and repaired using the IB. In parallel with the mechanical testing, finite element analysis (FEA) was performed to investigate the influence of IB loading on the femur-ACL-tibia complex (FATC). The 3D geometry of the FATC was reconstructed from CT scans of the sheep. The IB 3D model was integrated with the 3D FATC for FEA to obtain the femur-repaired ACL with IB - tibia complex (FRA-IB-TC) group. For the intact specimens, the mean (±SD) failure load in the tensile testing was 937 N (±192 N), while for the FRA-IB-TC specimens, it was 519 N (±52 N). The FRA-IB-TC remained biomechanically stable during the cyclic loading testing. The FEA demonstrated an increase in ACL stress to 24.59 MPa and displacement values of 0.391 mm. The IB construct exhibited shear and notch effects at the button-suture-bone fixation site. Testing on this sheep model allowed us a parametric analysis of the impact of the IB repair technique. However, the results will need to be confirmed in a human model. In conclusion, although the IB technique has biomechanical drawbacks, the mechanical properties of the technique are satisfactory.

Quantitative damage evaluation of LY225 steel under monotonic tensile loading based on acoustic emission entropy

The purpose of this paper is to establish a quantitative relationship between acoustic emission (AE) signal acquisition data and the void growth model (VGM) of low yield point steel LY225 under monotonic tensile loading, to realize the real-time evaluation of plastic damage and the prediction of ductile fracture of materials. A series of material tests, finite element analysis and scanning electron microscope (SEM) tests were carried out to calibrate the VGM fracture parameters of LY225 steel, including the characteristic length l∗ and the critical value of VGI. It is found that the AE signals under monotonic tensile loading have a prominent two-stage characteristic and the cumulative AE entropy of AE signals in LY225 steel is proportional to the volume of material involved with plastic deformation. Then a bilinear function model between cumulative AE entropy and void growth index VGI is proposed and calibrated with monotonic tensile test results from a set of standard rectangular section specimens, and the effectiveness of the proposed model in more complex stress and strain state is verified by tests of a set of variable cross-section flat specimens.

Experimental evaluation and numerical simulation of low-yield-point steel shear panel dampers

Shear panel dampers are considered as a type of reliable passive energy dissipation device for seismic resistant structures with a damage-controlling concept. It has been revealed that the shear bulking of core plates and the premature failure of boundary plates are the main concerns for a shear panel damper that result in insufficient energy dissipation and undesirable ultra-low cycle fatigue performance. This study utilizes square steel tubes serving as out-of-plane stiffeners for the core plates made of a low-yield-point steel with a nominal yield stress of 225 MPa (named LYP225 steel). Meanwhile, a reduced flange plate section is adopted to mitigate the end fracture. An experimental study is carried out on three full-scale damper specimens with different flange plates and loading protocols. The test results show that the square tubes well prevent the bulking of the core plates and all the specimens exhibit plump hysteretic response and satisfactory energy dissipation capacity. The reduced plate sections reduce the likelihood of crack propagation occurring at the flange end, and thereby improve the ultra-low cycle fatigue performance. In order to develop the numerical model of shear panel dampers, a combined-hardening constitutive model with a memory surface is applied to represent the plasticity of LYP225 steel. Based on a plasticity analysis, a simplified calibration procedure is proposed associated with the three-dimensional constitutive model, where the plasticity parameters of LYP225 steel are determined using only the monotonic tensile test data. Eventually, the numerical results of uniaxial specimens and damper specimens are in good agreement with the test results, which validates the effectiveness of the constitutive model and the proposed calibration method.

Study on Finite Element Model of the Prefabricated Reinforced Concrete Beam-Column Joints with Grouted Sleeves

Based on the monotonic tensile test of grouted sleeve specimens conducted, this paper uses multifactor regression analysis to construct the equivalent constitutive relationship of grouted sleeve specimens under uniaxial tension. The study based on this constitutive relationship of grouted sleeves and the effect of bond and slip between steel and concrete were considered. The prefabricated reinforced concreted beam-column joints with grouted sleeves were presented with finite element software ABAQUS. The seismic behavior of prefabricated reinforced concreted beam-column joints with grouted sleeves under low-frequency cyclic loading was then investigated. In addition, parametric studies via finite element analysis were performed to examine the influence of various parameters on the strength and energy dissipation capacity of the specimens. The simulation results show that plastic deformation was mainly observed near the beam-column interface; the hysteretic curve of this joint was plump. The test results showed that good energy dissipation and displacement ductility capacities can be achieved. The error of yield load between the numerical simulated and experimental result was 7.11%, the error of peak load was 6.88%, the error of ultimate load was 3.76%, and the error of displacement ductility was 7.84%. Results showed that the calculated results obtained in the paper agree well with test results from the references. The finite element model adopted in this paper can reflect the seismic behavior of the prefabricated reinforced concrete beam-column joints with grouted sleeves by using equivalent constitutive relation.

Surface morphology characteristics and mechanical properties of corroded cold-formed steel channel sections

The effects of corrosion on the surface morphology characteristics and mechanical properties of corroded cold-formed steel were investigated. 26 coupon specimens accelerated corrosion by the outdoor periodic spray test were subjected to monotonic tensile test to obtain the mechanical properties. The surface topography of 10 coupon specimens were scanned to obtain the three-dimensional coordinate data. The relationship between the corrosion characteristic parameters, the mechanical properties of corroded cold-formed steel and the corrosion rate was proposed. The corrosion morphology and mechanical properties of corroded hot-rolled steel and cold-formed steel were compared and analyzed. Finally, the finite element model based on the corrosion morphology was established. Results indicated that for the specimens with the same mean of corrosion depth, the standard deviation of corrosion depth and maximum corrosion depth of the hot-rolled steels were larger than those of the cold-formed steels. The reduction in the elastic modulus, the yield strength and the ultimate strength of the hot-rolled steels was obviously lower than that of the cold-formed steels. The yield ratio did not change significantly with the increaseing corrosion rate for two types of steel. The finite element model can accurately predict the failure modes, fracture position and stress-strain curves of the corroded coupon specimens. The cold-worked operation and small thickness can accelerate the degradation of mechanical properties of corroded specimen, so more attention should be paid to the corrosion problem of cold-formed steel.

Experimental investigation on fatigue behavior of butt-welded high-strength steel plates

This paper presents an experimental investigation into mechanical properties and fatigue performance of butt-welded high strength steel plates. Four typical high-strength steels (HSSs) were selected, with steel grades of 460, 550, 690, and 960 MPa. Mechanical properties were measured through monotonic tensile tests, cold bending tests, Charpy V-notch impact tests and hardness tests. A total of 114 fatigue tests were conducted on butt-welded high-strength steel plates. Comparisons of mechanical properties and fatigue performance are made among HSSs tested and conventional strength steels (CSSs) with a yield strength around 235 to 355 MPa. The natural slope of S–N curves for HSS butt-welded connections varied from 3 to 5. Two fatigue design approaches are recommended with one based on natural slope and one based on a fixed slope of 3 as for CSSs.

On the estimation of tensile yield stress for polymer materials based on punch tests

The Small Punch Test (SPT) has been widely used for the high throughput evaluation of mechanical properties of metallic materials. However, all the correlations between characteristic punch loads and mechanical properties measured from tensile tests thus far have been developed based on the variation of specimen thickness. This paper introduce a new approach to the estimation of tensile yield stress from punch test measured load-displacement curves based on the variation of punch indenter diameter. Mechanical properties of high density polyethylene (HDPE) are first characterized using both monotonic tensile and punch tests at various loading speeds. Linear correlations between the maximum punch load and the square indenter diameter or the indenter diameter are established, based on which the tensile yield stress could be expressed as a function of maximum punch load and the square indenter diameter or indenter diameter. The yield stress estimated using the proposed correlations is compared with those measured from tensile tests and the results indicate that the correlation based on the variation of indenter diameter yields better prediction results than the correlation based on the variation of square indenter diameter. Good agreement has been achieved between the predicted results using correlation based on the variation of indenter diameter and the yield stress measured from tensile tests, suggesting the proposed approach is applicable for the estimation of mechanical properties for HDPE materials based on punch test.

Effect of parallel deposition path and interface material flow on resulting microstructure and tensile behavior of Al-Mg-Si alloy fabricated by additive friction stir deposition

In this paper, the effect of overlapping parallel additive depositions on microstructure and mechanical properties in Additive Friction Stir Deposition (AFS-D) was examined. In particular, the AFS-D process was employed to make parallel depositions of AA6061 with a 6.35 mm overlap to effectively bond the two parallel layers. The AFS-D process draws on similar physics to friction stir welding in that frictional heat and plastic deformation is exploited to deposit metallic materials from the center of a hollow rotating tool as it traverses across the build table to produce consecutive metallurgically bonded layers. In this work, the microstructural aspects of the 6.35 mm overlapping raster interface were characterized using optical and scanning electron microscopy. Aluminum oxide particles were observed at the raster interface, which were located at layer boundaries. Additional grain refinement was also apparent as a direct result of multiple stirring passes within the overlapping deposition region. Mechanical characterization via microindentation and monotonic tensile testing observed a hardness gradient in the overlapping region, but the parallel deposition layers exhibited comparable tensile strength to a single row of deposited AA6061. The influence of existing oxides on the mechanical results was observed to have limited effect on the properties longitudinally across the raster. This study determined that parallel layers of AA6061 can be successfully deposited via the AFS-D technique. The resulting deposit exhibited a strong metallurgical bond across the parallel layers despite the presence of surface oxidation on the unprepared feedstock and substrate.

Tensile behavior of stainless steel clad plates with different cladding ratios

The stainless steel clad plate is a new type of composite material with high performance, which combines the advantages of the substrate and cladding material, showing good mechanical properties. Nevertheless, previous studies are still insufficient to support the design of it as structural members. In this paper, the monotonic tensile tests of 24 stainless steel clad plate coupons were carried out. The stress-strain curves of the stainless steel clad plate with different cladding ratios were obtained, as well as that of the substrate and cladding materials. Besides, the layered method was used to modeling the stainless steel clad plate in finite element analysis (FEA), which was proved to be valid. Based on the verified model, four sets of finite element models (FEM) were added to cover the cladding ratio that was not involved in the test. Finally, the relationship between the mechanical properties of the stainless steel clad plate and its cladding ratio was discussed according to the experimental and FEA results. Finally, it was found that the tensile mechanical properties of stainless steel clad plate are between the substrate and the cladding, and changed linearly with the cladding ratio. Besides, it was also found that the stainless steel clad plate shows obvious anisotropy in the small deformation stage, while the anisotropy gradually weakens with the strain increased.