Standard Tension Test Research Articles

Effects of fracture area measurement method and tension test specimen type on fracture strain values of 980 class AHSS

With increasing focus on “local formability” and fracture behavior of advanced high strength steels (AHSS), the effects of tension test specimen type and fracture area measurement method on fracture strain values were examined. Three 980 class AHSS and three standard tension test specimens with various widths were evaluated. Fractures types were grouped into three categories according to appearance and thickness profile “shape”—Type 1: Perpendicular to the tensile axis (across specimen width) with a V-shape thickness profile; Type 2: Irregular transition from Type 1 to Type 3 with a W-shape thickness profile; and Type 3: Angled across the specimen width with a U-shape thickness profile. All materials exhibited Type 1 fractures when tested with Subsize specimens. However, as the specimen width-to-thickness ratio increased, the fracture type changed from Type 1 to Type 2 or from Type 1 to Type 3. In contrasting the effects of specimen type and fracture area measurement method, specimen type (width) has a far greater impact on the consequent fracture strain value. For tension testing no clear universal relationship exists between the width-to-thickness ratio and the fracture strain. These observations suggest that, when reporting fracture strain values, the specimen type, the material thickness, and the fracture area measurement method must be indicated.

Tensile behavior of hybrid multi-bolted/bonded joints in composite laminates

In composite structures, the strength of a standard single-lap joint with multiple bolts at best matches the strength predicted by the standard open-hole tension (OHT) test, which is about 50% of the tensile strength of the unnotched material. Although bonded joints do not have such limitation, they carry other drawbacks. The advantages of bolting and bonding may be combined in hybrid/bonded-bolted (HBB) joints. This study investigates HBB joints using carbon and glass-fiber reinforced composites with up to three bolts. It is found that multi-bolt specimens with or without adhesive fail in net-tension at the outer bolts like in OHT tests. However, HBB joint is not anymore limited by the OHT strength. The addition of the adhesive increases the strength of a three bolts joints by 70% for cross-ply laminates and 30% for quasi-isotropic laminates. The synergy between the bolts and the adhesive in the HBB system is interpreted by the fact that the outer bolts limit peel stresses and concurrently, the adhesive reduces the stress concentration around the bolts. This is particularly important for the cross-ply configuration where the stress concentrations around the holes are high. Other features observed suggest that for multi-bolted HBB joint, only external bolts are needed. Such joint configuration combines the safety provided by the bolts and the efficient load transfer provided by the adhesive.

Microstructural evolution, mechanical properties and fracture toughness of near β titanium alloy during different solution plus aging heat treatments

Ti-55531 (Ti-5Al-5Mo-5V-3Cr-1Zr) is near β titanium alloy, which plays a significant role in manufacturing landing gears and flap tracks in the aerospace industry. This study aims to find out an optimal heat treatment to obtain an excellent balance of strength, ductility and fracture toughness. For this purpose, the Ti-55531 alloy was subjected to solution plus aging treatments and mechanical properties tests. Optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) were adopted to observe the microstructural evolution including morphology, distribution and size. Standard tensile and compact tension (CT) tests were carried out to obtain the yielding/tensile strength (YS, TS), elongation (EL) and fracture toughness (KIC) of Ti-55531 alloy. The effects of solution temperature, aging temperature and time on the microstructural evolution and fractography were analyzed. The relationships between the mechanical properties and microstructures were qualitatively described. The influencing mechanism of plastic deformation and crack propagation path on the KIC were discussed. Simultaneously, the values of YS and KIC were predicted based on mechanical property models, the predicted values were compared with tested data. Finally, an optimal heat treatment was proposed for the Ti-55531 alloy, to obtain a good balance of strength, ductility and fracture toughness.

Friction stir processing of dual phase steel: Microstructural evolution and mechanical properties

The influence of friction stir processing (FSP) on the microstructure and mechanical properties of a DP 600 steel has been studied. The microstructure evolution during the FSP has been characterized using electron back-scatter diffraction (EBSD) technique and scanning and transmission electron microscopes. Standard tension and hardness tests were used to characterize the mechanical properties. The results show that the FSP produced a refined microstructure composed of ferrite, bainite, martensite, and tempered martensite which in turn increased the hardness and strength magnitudes by a factor of 1.5. The initially 2.83 μm average grain size of ferrite has decreased to 0.79 μm in the pin effected zone of (PE-SZ-I) of the processed region. Both EBSD and TEM observations showed regions with high dislocation density and sub-structures region in the processed zone. The grain size became coarser, the density of both dislocations and low-angle grain boundaries decrease, away from the processed zone. Moreover, phase fractions and hardness values were predicted using CALPHAD thermodynamic based software based on commercial material properties. Although the prediction does not take into consideration the influence of severe plastic deformation, the results were within 10% uncertainties of the experimental findings. The present study demonstrates that an ultra-fine grained structure can be obtained through the thickness of a 1.5 mm thick D P600 steel sheet via FSP. FSP can produce a range of different hardness and strength values; which can also be predicted successfully by inputting the composition and local temperatures reached during the FSP.

Probing Local Mechanical Properties in Polymer-Ceramic Hybrid Acetabular Sockets Using Spherical Indentation Stress-Strain Protocols

Mechanical properties exhibited by the materials used in biomedical device components for articulating joints play an important role in determining the implant performance. In the fabrication of complex-shaped parts, the thermomechanical history experienced in different locations of the final part can be substantially dissimilar, which may lead to large differences in the local microstructures and properties. In many instances, it is not feasible to evaluate experimentally the local mechanical properties in the as-manufactured bioimplant prototypes using standardized tests, and use this information in refining the manufacturing cycle to develop implants with improved performance. In order to bridge this critical gap between materials development and manufacturing, we explore here the use of recently developed spherical indentation stress-strain analysis protocols for the mechanical characterization of local properties in the as-manufactured biomedical device prototype. More specifically, this paper presents two main advances: (i) extension of spherical indentation stress-strain analysis protocols needed to extract reliable estimates of elastic modulus and indentation yield strength from polymer matrix composite (PMC) samples and (ii) demonstration of the differences in the properties between samples produced specifically for the standard tension tests and the as-fabricated PMC acetabular socket prototype intended for total hip joint replacement applications. The results of the present study revealed large differences in the mean and variance of the measured moduli and indentation yield strengths in the acetabular socket and the tensile specimen. Based on the extensive micro-computed tomography (micro-CT) analysis, an attempt has been made to rationalize the local property differences on the basis of microstructural attributes.

CONSTITUTIVE MODEL FOR TIMBER FRACTURE USED FOR FE SIMULATION OF LVL ARCH

Non-linear finite element simulation of four-point bending test of a small-size arch is described in this paper. The arch of 780mm span is made of Yellow Poplar (<em>Liriodendron tulipifera</em>) laminated veneer lumber (LVL) and it is manufactured with a through crack parallel to fibers in the middle of its crown. 2D homogeneous orthotropic constitutive model of tensile and shear fracture in timber that has been recently developed and implemented into ATENA<sup>®</sup> element software by the authors is used for the numerical calculation. Both standard and compact tension (CT) test results are used for the material model calibration. Results show that the model successfully reproduces the increasing part of the load-displacement response. Furthermore, the model can capture the most distinctive features of the crack pattern.

A simplified stress-based forming limit criterion for advanced high strength steel (AHSS)

A methodology for stress-based forming limit analysis has been developed for advanced high strength steel (AHSS). It was proposed that localized necking occurs when a critical normal stress condition is met. Using a basic, isotropic material model (von Mises, power law hardening), the criterion was applied to various 980 Class AHSS. In most cases the simplified criterion adequately described the experimental strain-based forming limit curve (FLC). For AHSS with substantial volume fractions of metastable austenite, a more sophisticated material model and/or an adapted failure criterion will be required. A strong linear relationship was found between the critical normal stress and the measured true stress at maximum load in tension. This empirical functionality applies over a large range of strength levels and may form the basis for a methodology by which FLCs may be estimated from standard tension tests. Finally, in the context of the proposed failure criterion, the effects of work hardening behavior on the “shape” of the strain-based FLC are explored.

Dynamic behaviour of aluminium alloy plates with surface cracks subjected to repeated impacts

ABSTRACTThis paper investigates the behaviour of aluminium alloy plates with and without initial cracks under repeated impacts. Three series of impact tests were conducted to study the behaviour of circular plate that do not have any cracks (series B), have surface cracks with varying length (series L), and have cracks varying depths (series D). A hammer was dropped from the same height with a constant initial striking energy 60J for all tests. It was observed that plates with larger cracks carried smaller impact forces and assumed larger deformations. With the increase in impact number, the effects of crack lengths and depths on the dynamic behaviour became much more significant. Predictions using a rigid-plastic theoretical model were compared with these test results. With the stresses determined based on the true strain-stress curve obtained by standard tension test, the refined analytical formula provides better predictions that agree well with the lab tests.

Evaluation of tension-compression asymmetry of a low-carbon steel sheet using a modified classical compression test method

A modified classical compression test method was used to examine in-plane tension-compression asymmetry in a low carbon steel sheet. In this compression test method, interfacial friction between the compression platens and specimen surfaces was significantly reduced by use of polycrystalline diamond plates installed on the compression dies. Furthermore, crosshead displacement of the universal testing machine, which inherently includes deflection of the machine itself, was corrected to the net deformation of the specimen based on a series spring model. Consequently, precision of the compressive strains obtained with the present test method is equivalent to that attained in the standard tension tests using wire strain gauges. For tension and compression tests performed at in-plane directions of 0°, 45° and 90° relative to the rolling direction (RD), significant tension-compression asymmetry (i.e. strength differential effect (SDE)) was observed. In all the mechanical tests in the three testing directions, flow stresses in tension tests were smaller than those in compression tests.

Investigation on particle strengthening effect in in-situ TiB2/2024 composite by nanoindentation test

Mechanical properties of in-situ 5.0 vol% TiB2 particle reinforced 2024-T4 alloy matrix composite (in-situ TiB2/2024-T4 composite) synthesized through exothermic reaction process are experimentally investigated using different approaches. Standard uniaxial tension tests are conducted, and SEM fractography is made to characterize the fracture mechanisms. The nanoindentation experiment and Oliver-Pharr method are employed to obtain the micro mechanical properties (indentation modulus and hardness). To carry out comparative study, all kinds of experiments have also been done for 2024-T4 alloy. The indentation results demonstrate that compared to 2024-T4 alloy, the dispersive in-situ TiB2 nano particles do not evidently alter the Young's modulus of Al matrix of the composite, but elevate the hardness of it by 33.7%, which further means that the increase of Young's modulus of this composite over the 2024-T4 alloy is mainly due to the high stiffness of TiB2 particle, and the increase of yield strength and ultimate strength of the composite can be attributed to microstructure strengthening to its Al matrix.

Mechanical Properties of AISI 1045 Steel Subjected to Combined Loads of Tension and Torsion

The quasi-static standard tensile, torsional, and combined tension and torsion tests were performed at room temperature to investigate the mechanical properties of normalized AISI 1045 steel specimens. The performance of yielding, Young’s modulus, and modulus of elasticity in shear were analyzed via two kinds of experiments with sequence-given loading paths, such as tension-torsion (torsional response after tension) and torsion-tension (tensile response after torsion) tests, under various preloads. Additionally, time-variant coupled effects between the shear stress and normal stress responded similarly in tension-torsion and torsion-tension experiments. Results demonstrate that ultimate strengths of torsion and tension obtained by combined tension and torsion tests were consistent with those strengths achieved by standard uniaxial tests. Yield strengths derived by the Von Mises criterion and combined tension and torsion test were compared, and results showed maximum deviations of 23.01% and 43.28% in shear and normal stress, respectively. Results indicated that the material exhibited quite different mechanical properties under combined loads of tension and torsion from those under uniaxial loads.

Comparison of fracture toughness values of normal and high strength concrete determined by three point bend and modified disk-shaped compact tension specimens

The modified disk shaped compact tension test is a configuration derived from standard compact tension test that is used for measuring fracture mechanical properties of primarily metallic materials. The compact tension configuration is commonly used for measurement fracture mechanical properties as e.g. fracture toughness, Young’s modulus, work of fracture etc. The modified compact tension tests imply significant modifications of the specimen morphology in order to avoid premature failure. The modified compact tension test is not proper for quasi-brittle materials due to its complicated shape (steel-concrete interface), but it is easily extracted from drill core and we do not need large amount of material for obtaining fracture properties as we need for e.g. three- or four- point bend test. Since it is a new test method, a wide range of tests is needed to be done before it can be applied. In the paper the selected outputs of the experiments performed on normal and high strength concrete will be processed and the values of fracture mechanical parameters will be discussed.

Comparison of fracture toughness values of normal and high strength concrete determined by three point bend and modified disk-shaped compact tension specimens

The modified disk shaped compact tension test is a configuration derived from standard compact tension test that is used for measuring fracture mechanical properties of primarily metallic materials. The compact tension configuration is commonly used for measurement fracture mechanical properties as e.g. fracture toughness, Young’s modulus, work of fracture etc. The modified compact tension tests imply significant modifications of the specimen morphology in order to avoid premature failure. The modified compact tension test is not proper for quasi-brittle materials due to its complicated shape (steel-concrete interface), but it is easily extracted from drill core and we do not need large amount of material for obtaining fracture properties as we need for e.g. three- or four- point bend test. Since it is a new test method, a wide range of tests is needed to be done before it can be applied. In the paper the selected outputs of the experiments performed on normal and high strength concrete will be processed and the values of fracture mechanical parameters will be discussed.

Material Modeling of Concrete for the Numerical Simulation of Steel Plate Reinforced Concrete Panels Subjected to Impacting Loading

The steel plate reinforced concrete (SC) walls and roofs are effective protective structures in nuclear power plants against aircraft attacks. The mechanical behavior of the concrete in SC panels is very complicated when SC panels are under the action of impacting loading. This paper presents a dynamic material model for concrete subjected to high-velocity impact, in which pressure hardening, strain rate effect, plastic damage, and tensile failure are taken into account. The loading surface of the concrete undergoing plastic deformation is defined based on the extended Drucker–Prager strength criterion and the Johnson–Cook material model. The associated plastic flow rule is utilized to evaluate plastic strains. Two damage parameters are introduced to characterize, respectively, the plastic damage and tensile failure of concrete. The proposed concrete model is implemented into the transient nonlinear dynamic analysis code ls-dyna. The reliability and accuracy of the present concrete material model are verified by the numerical simulations of standard compression and tension tests with different confining pressures and strain rates. The numerical simulation of the impact test of a 1/7.5-scale model of an aircraft penetrating into a half steel plate reinforced concrete (HSC) panel is carried out by using ls-dyna with the present concrete model. The resulting damage pattern of concrete slab and the predicted deformation of steel plate in the HSC panel are in good agreement with the experimental results. The numerical results illustrate that the proposed concrete model is capable of properly charactering the tensile damage and failure of concrete.

Characterization of fracture in medium Mn steel

In the present study, the damage mechanisms operating during the tensile deformation of intercritically annealed Fe-0.3%C-6.0%Mn-3%Al-1.5%Si medium Mn steel were investigated. The steel was annealed at different temperatures to obtain a range of strain hardening properties in uniaxial tension by activating the twinning-induced plasticity and transformation-induced plasticity effects. The initial microstructure consisted of coarse δ-ferrite grains and an ultra-fine grained (UFG) constituent containing ferrite (α) and austenite (γ). However, the volume fraction of martensite (α′) increased significantly by phase transformation from austenite as the material deformed plastically. The internal damage and the fracture appearance after monotonic standard uniaxial tension tests and in-situ interrupted tensile experiments were characterized at macro- and micro-scale. The fracture features were analyzed as a function of the intercritical annealing temperature, which is the most important processing parameter for medium Mn steel.Three mechanisms contributed to the damage that developed in these materials. First, nucleation and growth of voids occurred at non-metallic inclusions. Second, debonding of the α-α′ and α′-α′ interfaces due to a local loss of interfacial strength was observed in the UFG constituent. While the voids initiated at non-metallic inclusions were in the order of several microns, the size of those initiated at the α-α′ and α′-α′ interfaces in the UFG constituent was limited by the initial grain size, with little or no growth. Finally, in addition to void damage, longitudinal cleavage-like cracks formed along the δ-ferrite layers, and parallel to the sheet plane, were observed in the fractured specimens. These longitudinal cleavage-like cracks were the consequence, but not the cause, of a fracture process triggered by plastic flow localization during uniaxial tension testing.

Simple and effective failure analysis of dissimilar resistance spot welded advanced high strength steel sheets

The failure of welded structures was analyzed considering dissimilar combinations of advanced high strength steels (AHSS) and a conventional mild steel. A method to characterize the mechanical properties of hardening behavior along with the deterioration associated with micro-void development and fracture criterion based on a modified damage model was applied by employing simple but effective experimental and numerical inverse procedure. The proposed procedure was solely based on standard and miniature simple tension tests for base sheets and weld nuggets, respectively. The identified plastic and failure properties were applied to the analysis of the failure mode and strength in the two well-accepted coupon tests of lap-shear and U-shape tension tests for DP980-TRIP980 and GMW2-TRIP980 dissimilar welded sheets. The analysis confirmed that the distinct failure behavior in the coupon tests for the two dissimilar weld cases was mainly due to the competition between the element with high strength/low ductility and the element with low strength/high ductility.

Tensile Fracture Behavior of Progressively-Drawn Pearlitic Steels

In this paper a study is presented of the tensile fracture behavior of progressively-drawn pearlitic steels obtained from five different cold-drawing chains, including each drawing step from the initial hot-rolled bar (not cold-drawn at all) to the final commercial product (pre-stressing steel wire). To this end, samples of the different wires were tested up to fracture by means of standard tension tests, and later, all of the fracture surfaces were analyzed by scanning electron microscopy (SEM). Micro-fracture maps (MFMs) were assembled to characterize the different fractographic modes and to study their evolution with the level of cumulative plastic strain during cold drawing.

Pressurized hole as a Stress Reducing feature

In many structures, cracks may appear during manufacturing process or in service. Stress intensity factor is used in fracture mechanics to more accurately predict the stress state (stress intensity) near the tip of the crack caused by remote load or residual stresses. In this work a rectangular plate with central hole & two cracks emanating from his hole is analyzed for remote tensile loading. Stress Intensity Factor is determined for this configuration using finite element based software ANSYS. This value is compared with the one obtained by analytical method. Then hole as a stress reducing feature is studied. The effect of hole location, hole size on the stress intensity factor is studied. Feasibility of pressurized hole as a stress reducing feature is studied for the best hole location & size obtained earlier. Further variation in the pressure was done to study effect of pressurized hole. Fracture Mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material resistance to fracture. Stress intensity factor, K, is used in fracture mechanics to more accurately predict the stress state (stress intensity) near the tip of the crack caused by remote load or residual stresses. It is theoretical construct applicable to a homogeneous elastic material and is useful for providing failure criteria for brittle materials. The stress intensity factor remains finite and provides a basis for determining the critical load. If the value of SIF is less than the toughness of the material, the structure is safe. If SIF value is more than the toughness of material, the structure fails. Typical Stress Intensity Factor Reduction Techniques used by earlier researchers are: Strip or patch repair method, Artificial crack closure by crack filling, Welding repair, Hole drilling. In this work, finite element analysis of a cracked plate is done. Initially a hole is used as a stress reducing feature. Then the effect of pressure applied in this hole is studied. II. LITERATURE REVIEW Miloud Souiyah, Abdulnaser Alshoaibi. et al (1) considered a rectangular plate with crack starting from a circular hole and double edge notched plate. Both geometries were in tensile loading and under mode-I conditions. In this paper a displacement extrapolation technique was employed particularly to predict the crack propagations direction and to calculate SIFs and validated with the relevant numerical and analytical results obtained by other researcher. Xiangqiao Yan (2) studied the stress intensity factors (SIFs) of cracks emanating from a rhombus hole in a rectangular plate subjected to internal pressure by means of the displacement discontinuity method with crack-tip elements (a boundary element method) proposed by the author. Moreover, an empirical formula of the SIFs of the crack problem was presented and examined. It was found that the empirical formula is very accurate for evaluating the SIFs of the crack problem. Junping Pu (3) studied a Boundary value problem of a plate with crack and defect such as the circular and/or elliptical holes. This kind of problem was suitable for solving by boundary element method with its higher precision. The sub-region method was used in this work. A center cracked plate subjected to remote tensile and shear loading was studied numerically. Levend Parnas and Omer G. Bilir (4) calculated Stress Intensity Factor by experimental procedures using strain gages. In this method, cracks were opened at the tip of crack starter slot on the standard compact tension test specimens by using Electrical Discharge Machining (EDM) and Wiring Discharge Machining (WDM). Strain gage data from the crack tip region were used to calculate stress intensity factors. But the values obtained were on lower side. This was partly due to unavoidable deformations in the adhesives used to fasten the strain gages to the specimen. This might be also due to the use of the relatively long strain gages, local yielding and three-dimensional effects and limited regions for strain gauge.

Biomechanical properties of a structurally optimized carbon-fibre/epoxy intramedullary nail for femoral shaft fracture fixation

Intramedullary nails are the golden treatment option for diaphyseal fractures. However, their high stiffness can shield the surrounding bone from the natural physiologic load resulting in subsequent bone loss. Their stiff structure can also delay union by reducing compressive loads at the fracture site, thereby inhibiting secondary bone healing. Composite intramedullary nails have recently been introduced to address these drawbacks. The purpose of this study is to evaluate the mechanical properties of a previously developed composite IM nail made of carbon-fibre/epoxy whose structure was optimized based on fracture healing requirements using the selective stress shielding approach. Following manufacturing, the cross-section of the composite nail was examined under an optical microscope to find the porosity of the structure. Mechanical properties of the proposed composite intramedullary nail were determined using standard tension, compression, bending, and torsion tests. The failed specimens were then examined to obtain the modes of failure. The material showed high strength in tension (403.9±7.8MPa), compression (316.9±10.9MPa), bending (405.3±8.1MPa), and torsion (328.5±7.3MPa). Comparing the flexural modulus (41.1±0.9GPa) with the compressive modulus (10.0±0.2GPa) yielded that the material was significantly more flexible in compression than in bending. This customized flexibility along with the high torsional stiffness of the nail (70.7±2.0Nm2) has made it ideal as a fracture fixation device since this unique structure can stabilize the fracture while allowing for compression of fracture ends. Negligible moisture absorption (~0.5%) and low porosity of the laminate structure (< 3%) are other advantages of the proposed structure. The findings suggested that the carbon-fibre/epoxy intramedullary nail is flexible axially while being relatively rigid in bending and torsion and is strong enough in all types of physiologic loading, making it a potential candidate for use as an alternative to the conventional titanium-alloy intramedullary nails.

Failure Performance Analysis of Resistance Spot Welded Advanced High Strength Steel Sheets

A numerical method to predict failure performance was developed for resistance spot welded AHSS (advanced high strength steel) sheets in this work and applied for a TRIP (transformation induced steel sheets) 980 sheet. For the numerical analysis, utilizing the numerical inverse calibration method, hardening data and fracture criteria were characterized for the base and the weld nugget based on the standard simple tension test and a newly designed miniature tension test, respectively. The characterized properties of the base and the weld nugget were then applied to analyze failure performance in the coupon tests of the lap-shear and U-shape tension tests carried out for welded sheets. The analysis was performed under the quasi-static condition in this early effort and showed reasonably good agreement with experiments both in failure modes and strength.