Conventional Tensile Tests Research Articles

Determination of localized stress–strain behavior of fused silica: Inverse numerical analysis from nanoindentation test

Because the localized mechanical properties of fused silica are unlikely to be obtained via conventional tensile testing, an inverse numerical analysis has been applied to deduce these properties using the load–displacement curve from nanoindentation testing. The mechanical properties were initially assumed, and the load–displacement curve was numerically simulated using three-dimensional elastic–plastic finite element analysis. The mechanical properties were adjusted until the numerical curve corresponded to the experimental curve, and then the localized mechanical properties in the vicinity of an indentation could be estimated. Unfortunately, the inverse numerical analysis requires time-consuming numerical calculation, involving many repetitions, by experienced researchers. In the present work, the influence of mechanical properties on the nanoindentation parameters of fused silica was evaluated, and the systematical adjustment of mechanical properties to obtain a satisfactory load–displacement curve has been proposed. It is considered that this procedure can be applied for the evaluation of localized stress–strain behavior of fused silica.

Characterization of Anisotropy of Strength in API-X80 Line Pipe Welds Through Instrumented Indentation

In this work, an instrumented indentation technique with a nearly flat tip indenter was used to measure the yield strength of API-X80 line pipe weld. Using this technique, the yield strength can be estimated directly from the indentation load–displacement response. The yield strength of the weld metal was measured in the transverse (hoop) and longitudinal (long) directions of the weld, and the results indicated that the weld metal exhibits anisotropy in terms of the yield strength by as much as 100 MPa, which could lead to a non-conservative estimate when strength mismatch is considered. Conventional tensile testing obtained strength values to within 4.6% of the indentation measurements and confirmed the results. Hardness maps also show a 20 HV0.5 difference in the two directions of the weld, which supports anisotropy of yield strength. The microstructure analysis using SEM reveals a slight difference in the grain size, which partially explains the differences in properties.

Mechanical Characterisation of Single-Walled Carbon Nanotube Heterojunctions: Numerical Simulation Study.

The elastic properties of single-walled carbon nanotube heterojunctions were investigated using conventional tensile, bending and torsion tests. A three-dimensional finite element model was built in order to describe the elastic behaviour of cone heterojunctions (armchair–armchair and zigzag–zigzag). This comprehensive systematic study, to evaluate the tensile, bending and torsional rigidities of heterojunctions, enabled the formulation analytical methods for easy assessment of the elastic properties of heterojunctions using a wide range of their geometrical parameters.

Construction of True Stress-Strain Curve of Metallic Material by Artificial Neural Network and Small Punch Test

Small punch test (SPT) is used to evaluate mechanical properties of metallic materials by a miniature specimen. A method combining SPT and artificial backpropagation neural network for determining the true stress-strain curve of metallic materials is proposed. The load-displacement curves of different hypothetical materials were obtained by the finite element model of SPT with considering Gurson-Tvergaard-Needleman (GTN) damage parameters and used to train a backpropagation neural network. The relationship between the load-displacement curve of SPT and the true stress-strain curve of the conventional uniaxial tensile test was established based on the trained neural network, which is validated by the experimental results of X80 pipeline steel. The results demonstrate that the established relationship can be used to predict the true stress-strain curve of the metallic materials and then to determine their elastoplastic properties by SPT.

Effect of Electric Current Pulse Type on Springback, Microstructure, Texture, and Mechanical Properties During V-Bending of AA2024 Aluminum Alloy

AbstractThe present work focused on the effect of the electric current pulse type on the springback, microstructure, texture, and mechanical properties during the V-bending process of AA2024 aluminum alloy. In order to investigate this effect, three different forming conditions including conventional V-bending and electrically assisted V-bending with square and sinusoidal pulses were considered. The results indicated that the amount of springback significantly decreased from 45.5 deg (for the sample formed via conventional V-bending) to 24 deg by applying the sinusoidal pulse. Microstructural observations revealed lower stored energy in the samples formed by electric current pulses which resulted in larger grain size compared with the samples formed without electric pulses. In addition, the result showed that the intensity of the 〈111〉||BLD (bend line direction) fiber texture reduced after applying electric current pulses, whereas it was very strong in the sample formed without electric pulses. It was suggested that the electric current pulses led to change the slip plane of the dislocations from {111} to {110} through cross slip. The applying electric current pulses decrease the ultimate tensile strength (UTS) from 471.1 MPa (for the conventional tensile test) to 448.0 and 426.7 MPa for the square and sinusoidal pulses, respectively. On the other hand, the electric pulses improved the formability of the AA2024 alloy owing to the activation of more slip systems, inhibition of dislocation pinning, the promotion of dislocation movement, and the acceleration of restoration mechanisms.

A coupled stress-triaxiality-dependent damage viscoplasticity model on crack initiation and propagation in high-strength rail steel

In this paper, a continuum damage enhanced formulation of effective (undamaged) material properties is proposed to predict fracture behaviour of various notched tensile specimens and a compact tension (CT) specimen for a high-strength rail steel that is commonly known to be insensitive to change in hydrostatic stress. The presented constitutive model considers a thermodynamically nonlinear, isotropic continuum damage, and stress-triaxiality-dependent effective stress using a viscoplastic regularization method to reduce strain and damage localization at the sharp notch tip. Conventional loading-unloading tensile test for damage measurement and monotonic tensile test on a notched specimen are used to calibrate the constitutive equation which consists of a set of eight model parameters. Further validation is performed on the geometry transferability of the model parameters to three different notched tensile specimens in the high stress triaxiality regime. The calibrated constitutive model is then applied to simulate crack initiation and propagation in the mode I plane-strain condition, where the computed result of load versus crack mouth opening displacement shows good agreement with those determined from CT specimens. The study concludes that the critical damage parameters for fracture are relatively constant among different notched specimens of high-strength rail steel, and thus as a proper fracture criterion for crack initiation at and crack propagation from the sharp notch tip. The study also found that the complementing mechanisms for the stress-triaxiality-dependent damage evolution and strain-hardening-induced enhancement of effective matrix material properties, respectively, result in the overall hydrostatic-stress insensitivity, and thus is often regarded as a material that belongs to the classical J2 plasticity.

Optimising the strength-ductility-toughness combination in ultra-high strength quenching and partitioning steels by tailoring martensite matrix and retained austenite

Undesirable fracture resistance has led to critical safety concerns in ultra-high strength quenching and partitioning (Q&P) steels. The present work proposes a new heat treatment method of tailoring simultaneously martensite matrix and retained austenite to enhance the fracture resistance of ultra-high strength Q&P steels, while keeping the good strength-ductility combination. To this end, both conventional and pre-cracked tensile tests are conducted on different Q&P steels with various microstructures consisting of different martensite matrix and retained austenite. Microstructure is analysed by neutron diffraction, atom probe tomography, transmission electron microscopy and dilatometry. For the optimum microstructure, the volume fraction of retained austenite play an important role in enhancing the strength-ductility combination, with an ultimate tensile strength reaching 1500 MPa and uniform elongation over 10%. In addition, the dislocation mobility in the martensite matrix of the optimum microstructure is enhanced due to the dislocation recovery, solute carbon depletion, and the transformation of transition carbides into cementite. The enhanced dislocation mobility reduces the flow stress and results in an improvement of intrinsic toughness of the martensite matrix. Furthermore, the size of cementite at grain boundaries of the optimum microstructure is so small (tens of nanometres) that brittle intergranular fracture is prevented. In summary, the new heat treatment method suggests that the optimal treatment should tailor simultaneously the martensite matrix and the retained austenite to optimise the strength, ductility and fracture resistance combination of ultra-high strength Q&P steels.

Novel 2D Dynamic Elasticity Maps for Inspection of Anisotropic Properties in Fused Deposition Modeling Objects

In this study, a novel ultrasonic non-destructive and non-invasive elastography method was introduced and demonstrated to evaluate the mechanical properties of fused deposition modeling 3D printed objects using two-dimensional dynamical elasticity mapping. Based on the recently investigated dynamic bulk modulus and effective density imaging technique, an angle-dependent dynamic shear modulus measurement was performed to extract the dynamic Young’s modulus distribution of the FDM structures. The elastographic image analysis demonstrated the presence of anisotropic dynamic shear modulus and dynamic Young’s modulus existing in the fused deposition modeling 3D printed objects. The non-destructive method also differentiated samples with high contrast property zones from that of low contrast property regions. The angle-dependent elasticity contrast behavior from the ultrasonic method was compared with conventional and static tensile tests characterization. A good correlation between the nondestructive technique and the tensile test measurements was observed.

On the Constraint Factor Relating Uniaxial and Indentation Yield Strength of Polycrystalline Materials Using Spherical Microindentation

Abstract High-throughput (HT) material property characterization has become increasingly important for the rapid and cost-effective development of new material systems. In recent years, spherical indentation using the Pathak-Kalidindi (P-K) protocol has been shown to be a reliable and HT method for capturing the elastic-plastic properties of several different types of materials. However, because of the higher hydrostatic pressure under the indenter tip and the constraint of the surrounding elastically deforming material, the indentation yield strength is higher than the uniaxial yield strength and they can be related by a scaling factor, referred to as the constraint factor. For fully plastic indentation, the constraint factor was found by Tabor to be ~2.8. However, the constraint factor is not well defined for elastic-plastic indentation and depends on the strain induced. In this work, the application of the P-K indentation protocol is demonstrated on a set of aluminum 7050 samples that have undergone different solution and aging treatments. Conventional uniaxial tensile and compression tests were also conducted on the same material. The constraint factor at the 0.2 % offset yield point was determined from the indentation and uniaxial measurements to be ~1.4. Experimental observations from our work and a similar work on aluminum 6061 showed a dependence of the constraint factor at the 0.2 % offset yield point on the uniaxial plastic properties. To explain these experimental observations, further investigation was carried out using finite element simulations of spherical indentation on materials with different yield strengths and hardening parameters. New insights are provided on the constraint factor at the 0.2 % yield point, which show that it increases with the ratio of the plastic to elastic strain. It was also found that the effect of the hardening is much less than that of the yield strength on the constraint factor at the 0.2 % yield point.

Cyclic deformation characteristics of S355 and S690 steels under different loading protocols

Despite of excellent high strength to self-weight ratios of the S690 steels, when compared with the S355 steels, there is a widespread concern regarding the ductility of the S690 steels. It is generally considered that the ductility of the S690 steels is significantly lower than that of the S355 steels – this is the general understandings the authors attempt to investigate.This paper presents an experimental investigation into cyclic deformation characteristics of both S355 and S690 steels through low-cycle high-strain cyclic tests with two different loading protocols. A detailed account of the results of 32 cyclic tests on both the S355 and the S690 funnel-shaped coupons is presented. Effects of four different target strains and two different loading frequencies are also examined in details. For the ranges of loading protocols, strain amplitudes, and frequencies considered, the hysteretic responses of these coupons of the two steels are compared directly in terms of engineering stress–strain curves based on their nominal diameters. Microstructures of the fractured coupons of the two steels are also identified for comparison.Contrary to the general understandings, it is demonstrated that the high strength S690 steels do have a good ductility under both monotonic and cyclic actions. Moreover, depending on specific loading protocols and target strains, the cyclic deformation characteristics of the S690 steels are demonstrated to be superior to those of the S355 steels in terms of the number of cycles completed prior to failure and their corresponding energy dissipation characteristic under various target strains up to ±10.0%.The findings of this experimental investigation highlight the importance of establishing ductility requirements and cyclic deformation characteristics for the high strength S690 steels in accordance with specifically designed cyclic tests rather than relying solely on conventional monotonic tensile tests.

Analytical and finite element modelling of hydraulic bulge test on extra deep drawing steel with fractography study

ABSTRACT In this paper, analytical, finite element modelling (FEM) and experimental studies are conducted on the biaxial stretching of extra deep drawing steel sheet using hydraulic bulge test, where the true plastic strain exceeds 70%, in contrast to the 20–30% strain achieved in the conventional uniaxial tensile test. The analytical modelling includes four different approaches, namely, Hill, Panknin, Chakrabarty and Kruglov. FEM is done using three yield criteria, Hill-48, Barlat-89 and Barlat-2000. Experimental results show that the stress-strain relationship using bulge radius based on Panknin approach and thickness based on Kruglov approach in the analytical study and using Barlat-2000 yield model in FEM study are in close agreement with the experimental curve. Fractography study, conducted on specimens from tensile and bulge test, reveals/confirms the presence of more deep elongated shear dimples in bulge test compared to equiaxed dimples in the tensile test. This corroborates with the experimental results of achieving the high level of plastic deformation till fracture in the hydraulic bulge test.

Scanning acoustic microscopy analysis of the mechanical properties of polymeric components in photovoltaic modules

AbstractThe characterization of mechanical properties of polymeric backsheet and encapsulation foils of photovoltaic (PV) modules usually involves destructive techniques, such as dynamic mechanical analysis and tensile testing. In this article, we validate the scanning acoustic microscopy (SAM) as a nondestructive characterization method to estimate the mechanical properties of polymeric multi‐layers in PV modules. The acoustic speed inside each individual material is measured and its mechanical properties calculated. For this validation, other techniques are employed to determine the values of density, thickness, and elastic modulus of the foils. The viscoelastic properties of the polymeric foils are temperature dependent. The longitudinal modulus measured acoustically at 15 MHz and the Young's modulus from conventional tensile testing were compared at different temperatures. Both techniques show good agreement regarding the change in mechanical properties caused by temperature variation. The disagreement in the absolute values of these parameters is explained by the different frequency magnitudes at which both measurements are performed. This confirms that SAM is a reliable technique to measure changes in the viscoelastic properties of the foils in PV modules nondestructively, and thus to obtain more information about aging‐induced material changes and degradation.

Application of Pre-Strained Steels in Empirical Correlation Between Small Punch Test and Uniaxial Tensile Test

In order to research the method of strength empirical correlation between conventional small punch test (SPT) and uniaxial tensile test, a series of austenitic stainless steel including pre-strained SUS304 have been tested in this study. The conventional SPT is conducted on a small disc-shaped specimen whose edge is firmly gripped by a die, and the specimen is deformed by a punch. The method of empirical correlation between SPT and uniaxial tensile test is a direct way to obtain the mechanical properties of materials. Through establishing the strength empirical correlation, it can achieve to calculate the strength of material by SPT which is nondestructive to equipments. However, the per-strained steels have never been tested in this method. This study is to fill that gap and to obtain the empirical correlation between SPT and uniaxial tensile test with pre-strained steel. In this study, a series of austenitic stainless steel including SUS304 after different levels of pre-strain were tested successively by uniaxial tensile test and SPT. It is found that the tensile strength obtained from uniaxial tensile test increases with the increasing levels of pre-strain. However, the maximum load obtained from prestrained SPT specimen does not increase with the increasing levels of pre-strain. It is contradictory to the linear relation between maximum load and tensile strength. According to the analysis of conventional discshaped SPT specimen, the directions of maximum load obtained from SPT and tensile strength from tensile test are not uniform. It results in the non-linearity between the maximum load and the tensile strength with pre-strained steel, and it indicates the pre-strained steel cannot be applied to the conventional disc-shaped SPT specimen. Furthermore, the prestrained steel is a typical kind of anisotropic material. Therefore, extending to anisotropic material, the conventional disc-shaped SPT specimen is not suitable for the method of strength empirical correlation.

Examining Failure Behaviour of Commercially Pure Titanium During Tensile Deformation and Hole Expansion Test

Hole expansion ratio is a material parameter which defines the extent to which sheet metals are formed. Research has shown that, the stress state observed at the hole edge after hole expansion test is similar to those observed during conventional uniaxial tensile test. However, conventional tensile test methods are not efficient in evaluating material edge formability. This work utilised optical non-contact measuring techniques to examine failure behaviour during tensile test and hole expansion test of commercially pure titanium sheet, fabricated with either abrasive water jet cutting or electric discharge machining. The work found that, the deformation mode in conventional tensile testing are governed by localised necking and subsequently diffused necking prior to failure. Deformation mode observed in hole expansion test is characterised by localised necking with no visible occurrence of diffused necking prior to failure. The highest strains are concentrated at the hole edge during hole expansion test due to their sensitivity to the hole preparation method with accompanying multiple localised necking sites resulting in non-uniform deformation. Strains become concentrated in the bulk material microstructure rather than the machined edge during tensile testing resulting in single localised deformation site and a more homogenous deformation.

Interfacial characteristics and mechanical properties of duplex stainless steel bimetal composite by heat treatment

The interfacial morphologies play a crucial role in mechanical properties of laminated/bimetal composite. This study investigates the effects of heat treatments on the interfacial characteristics, mechanical properties and fracture behaviour of a 2205 duplex stainless steel/AH36 carbon steel bimetal composite with varying interface situations fabricated by the hot-rolling and subsequent heat treatments. The bimetal composite was annealed from 850 °C to 1150 °C in steps of 100 °C with soaking 1 h. Heterogeneities of grain size and element concentration exist in the microstructural evolution of bonding zone adjacent to interface after annealing treatments, which were observed by the optical microscope and scanning electron microscope (SEM) coupled with the energy dispersive X-ray spectroscopy (EDS). It was found that the diffusion transition zone of Cr and Fe alloy elements between component 2205 and AH36 steel layers present an increasing trend with the rise of annealing temperature. The results of conventional, shear and compact tensile test indicate that the tensile strength of 2205/AH36 bimetal composite gradually decreases but the fracture elongation increases with the rising annealing temperature, the thicker alloy element diffusion zone results in the stronger interfacial shear strength and better ductility of bimetal composite, but the thick and strong interface has less positive effect than the mechanical properties of bimetal composite itself on preventing the fatigue crack growth across the interface.

Evaluation of Sheet-Coated Composite Waterproofing Joint Types by Analysis of Tensile Strength Change and Concrete Displacement Resistance Testing under Environmental Degradation.

Sheet-coated composite waterproofing (SCCW) have been developed to overcome the natural weakness of singly-ply coating or sheet waterproofing systems for roofing, but there are currently multiple types of SCCW joints. Conventional standard tensile strength testing results show that all SCCW joint types seem to pass the minimum requirement and current selection of SCCW type is dictated on the principle of ‘higher tensile strength is better’, but it has not been experimentally studied as to which type is the optimal to respond to environmental degradation while under the effect of zero-span tensile stress occurring during concrete joint displacement. In this study, five types of SCCW joints were tested: Overlap Bond (OB) type, Overlap Heated-Air Welding (OH) type, Butt Joint I Type (BI), Butt Joint T Type (BT), and Butt Joint Separation Movement Type (BS). These types of joint designs were subjected to Alkali, NaCl, and H2SO4 exposure, and temperature change (60 °C and −20 °C) for determining changes to tensile strength in the joint section. Tensile strength change results are compared to joint displacement resistance test results of specimens that were treated with chemical and temperature degradation. With the exception of chemical exposure conditioning, the Overlap type joints generally had higher tensile strength compared to the butt joint types, but joint displacement test results showed the opposite results, suggesting that complex joints found in SCCW require new evaluation method for quality assessment.

Plastic deformation of pure copper in ultrasonic assisted micro-tensile test

The softening effect of ultrasonic vibration on pure copper is studied from a new perspective with micro-tensile tests, where the gauge length of the specimen is one order of magnitude smaller than the ultrasonic wavelength. With this configuration, the amount of flow stress reduction increases linearly with vibration amplitude whereas the flow stress reduction is insensitive to the studied strain rate ranging from 0.06/s to 1/s. Temperature rise associated with ultrasonic vibration is minimal from infrared thermal imaging. In situ digital image correlation (DIC) analysis shows strain localization near ultrasonic source whereas uniform strain distribution was observed during conventional tensile test. Optical microstructure characterization shows that area fraction of annealing twins in the deformed copper reduced from 3.3% to 1.8% with ultrasonic vibration. This is possibly attributed to enhanced interaction of dislocation between twin boundaries which act as non-regenerative dislocation source. Electron backscatter diffraction (EBSD) results show that ultrasonic vibration promotes preferential grain re-orientation and reduces the misorientation within grains.

A methodology for obtaining primary and secondary creep characteristics from indentation experiments, using a recess

A procedure is described for Indentation Creep Plastometery (using a spherical indenter), which is analogous to that developed previously for Indentation Plastometry. As in that case, it is based on iterative numerical simulation of the indentation process, with repeated comparison between an experimental outcome and the corresponding model prediction, systematically varying the values of parameters in a constitutive law until optimal agreement is achieved. The constitutive law used here is the Miller–Norton relationship, which covers both primary and secondary creep regimes (although the transition between them is not well-defined). The experimental outcome is the penetration depth as a function of time, under a constant applied load. An important feature of the procedure is the prior creation of a spherical recess in the sample, having a pre-selected depth and a curvature radius equal to that of the indenter. This allows control over the stress levels created during the indentation creep testing and can be used to ensure that no (time-independent) plastic deformation is stimulated during the test. In the absence of such a recess, this is virtually unavoidable, since the stress levels created during initial contact between a spherical indenter and a flat surface tend to be very high. Such plasticity introduces unwanted complications into creep testing. Confirmation of the viability of the procedure is provided via comparisons between the creep characteristics of pure nickel samples at 750 °C, obtained in this way and via conventional uniaxial tensile testing.

GPGPU-parallelised hybrid finite-discrete element modelling of rock chipping and fragmentation process in mechanical cutting

Mechanical cutting provides one of the most flexible and environmentally friendly excavation methods. It has attracted numerous efforts to model the rock chipping and fragmentation process, especially using the explicit finite element method (FEM) and bonded particle model (BPM), in order to improve cutting efficiency. This study investigates the application of a general-purpose graphic-processing-unit parallelised hybrid finite-discrete element method (FDEM) which enjoys the advantages of both explicit FEM and BPM, in modelling the rock chipping and fragmentation process in the rock scratch test of mechanical rock cutting. The input parameters of FDEM are determined through a calibration procedure of modelling conventional Brazilian tensile and uniaxial compressive tests of limestone. A series of scratch tests with various cutting velocities, cutter rake angles and cutting depths is then modelled using FDEM with calibrated input parameters. A few cycles of cutter/rock interactions, including their engagement and detachment process, are modelled for each case, which is conducted for the first time to the best knowledge of the authors, thanks to the general purpose graphic processing units (GPGPU) parallelisation. The failure mechanism, cutting force, chipping morphology and effect of various factors on them are discussed on the basis of the modelled results. Finally, it is concluded that GPGPU-parallelised FDEM provides a powerful tool to further study rock cutting and improve cutting efficiencies since it can explicitly capture different fracture mechanisms contributing to the rock chipping as well as chip formation and the separation process in mechanical cutting. Moreover, it is concluded that chipping is mostly owed to the mix-mode I-II fracture in all cases although mode II cracks and mode I cracks are the dominant failures in rock cutting with shallow and deep cutting depths, respectively. The chip morphology is found to be a function of cutter velocity, cutting depth and cutter rake angle.

Short-Term Characterization of Spherical 100Cr6 Steel Samples Using Micro Compression Test.

For the establishment of a novel development process of new structural materials, short-term characterization methods capable of testing hundreds of spherical micro samples are needed. This paper introduces a compression test on spherical micro samples as a short-term characterization method to investigate the elastic-plastic deformation behavior. To demonstrate the potential of this newly developed method, the micro compression test is performed with a maximum loading of 300 N on 100Cr6 (AISI 52100 bearing steel) samples, with a diameter of 0.8 mm, in 15 different heat treatment conditions. The austenitizing temperature is varied between 800 and 1150 °C. Tempering of the samples is carried out in a differential scanning calorimetry process with temperatures of 180, 230 and 300 °C. Out of force-displacement curves and stress-strain relations, so-called descriptors (characteristic values) which are sensitive to the applied heat treatment can be extracted. The change of mechanical properties due to heat treatment and the resulting microstructure is presented by the trend of a stress descriptor in dependence of austenitizing and annealing temperature, which can be compared to the trend of the tensile strength as a material property obtained by conventional tensile tests. The trend of the descriptor determined in the compression test on spherical samples indicates the validity of this approach as a short-term characterization method.