Monotonic Tensile Properties Research Articles

Some useful approximations for wrought aluminum alloys based on monotonic tensile properties and hardness

AbstractThe objective of the present paper is to derive some useful approximations for estimating the strain‐controlled fatigue properties and cyclic deformation of wrought aluminum alloys from hardness and monotonic tensile properties. A variety of relationships and correlations among monotonic tensile properties, Brinell hardness, cyclic deformation and strain‐controlled fatigue properties are developed for wrought aluminum alloys. A simple method is proposed for prediction of the strain‐life curve requiring only ultimate tensile strength and modulus of elasticity. Prediction capability of the proposed method is evaluated for 25 kinds of wrought aluminum alloys with ultimate tensile strength between 120 MPa and 650 MPa. The proposed method provides good approximations of the strain‐life curve.

Effect of Sandblasting on Low and High-Cycle Fatigue Behaviour after Mechanical Cutting of a Twinning-Induced Plasticity Steel

In the last years, car bodies are increasingly made with new advanced high-strength steels, for both lightweighting and safety purposes. Among these new steels, high-manganese or TWIP steels exhibit a promising combination of strength and toughness, arising from the austenitic structure, strengthened by C, and from the twinning induced plasticity effect. Mechanical cutting such as punching or shearing is widely used for the manufacturing of car body components. This method is known to bring about a very clear plastic deformation and therefore causes a significant increase of mechanical stress and micro-hardness in the zone adjacent to the cut edge. To improve the cut edge quality, surface treatments, such as sandblasting, are often used. This surface treatment generates a compressive residual stress layer in the subsurface region. The monotonic tensile properties and deformation mechanisms of these steels have been extensively studied, as well as the effect of grain size and distribution and chemical composition on fatigue behaviour; however, there is not so much documentation about the fatigue performance of these steels cut using different strategies. Thus, the aim of this work is to analyse the fatigue behaviour of a TWIP steel after mechanical cutting with and without sandblasting in Low and High-Cycle Fatigue regimes. The fatigue behaviour has been determined at room temperature with tensile samples tested with a load ratio of 0.1 and load amplitude control to analyse High-Cycle Fatigue behaviour; and a load ratio of -1 and strain amplitude control to determine the Low-Cycle Fatigue behaviour. Samples were cut by shearing with a clearance value of 5%. Afterwards, a part of the cut specimens were manually blasted using glass microspheres of 40 to 95 microns of diameter as abrasive media. The results show a beneficial effect of the sandblasting process in fatigue behaviour in both regimes, load amplitude control (HCF) and strain amplitude control (LCF) tests, when these magnitudes are low, while no significant differences are observed with higher amplitudes.low-cycle fatigue, high-cycle fatigue, mechanical cutting, sandblasting, high manganese steel, TWIP steel

Model for predicting fatigue life and limit of steels based on micromechanics of small crack growth

A model for predicting the fatigue life and limit of steels with banded ferrite-pearlite microstructures is proposed based on the micromechanics of small crack behavior. The model was simplified to a two-step 2D problem: (i) determination of crack initiation site on the surface plane; (ii) fatigue life evaluation by simulating crack growth on the inside plane. A multiscale model synthesis approach was used to integrate three elemental models of (i) macroscopic finite element analysis, (ii) geometries and spatial distributions of ferrite grains and pearlite colonies, and (iii) crack initiation and growth. The required input data consisted of only (i) microstructural information, (ii) monotonic tensile properties, and (iii) test conditions. Model validation was performed by comparing the experimental results using steels with various volume fractions of pearlite phases and grain sizes. The predicted fatigue lives and limits were in good agreement with the experimental data for all the steels. In addition, the model could accurately simulate the crack growth behavior observed by optical microscopy. The framework of the proposed model may serve as a basis for predicting the fatigue lives of metallic structural materials.

Forecasting Low-Cycle Fatigue Performance of Twinning-Induced Plasticity Steels: Difficulty and Attempt

We find the existing empirical relations based on monotonic tensile properties and/or hardness cannot satisfactorily predict the low-cycle fatigue (LCF) performance of materials, especially for twinning-induced plasticity (TWIP) steels. Given this, we first identified the different deformation mechanisms under monotonic and cyclic deformation after a comprehensive study of stress–strain behaviors and microstructure evolutions for Fe-Mn-C alloys during tension and LCF, respectively. It is found that the good tensile properties of TWIP steel mainly originate from the large activation of multiple twinning systems, which may be attributed to the grain rotation during tensile deformation; while its LCF performance depends more on the dislocation slip mode, in addition to its strength and plasticity. Based on this, we further investigate the essential relations between microscopic damage mechanism (dislocation–dislocation interaction) and cyclic stress response, and propose a hysteresis loop model based on dislocation annihilation theory, trying to quickly assess the LCF resistance of Fe-Mn-C steels as well as other engineering materials. It is suggested that the hysteresis loop and its evolution can provide significant information on cyclic deformation behavior, e.g., (point) defect multiplication and vacancy aggregation, which may help estimate the LCF properties.

Beneficial effect of prestrain due to cold extrusion on the multiaxial fatigue strength of a 27MnCr5 steel

Cold extrusion is a process commonly used to manufacture drive train components in the automotive industry. Large plastic strains can be applied during this operation (up to 1.5) and greatly changes the mechanical properties of the resulting material. This study focuses on the impact of cold extrusion process parameters on the multiaxial fatigue behaviour of steel components. A specific set of forward rod extrusion tools was developed to get original fatigue specimen able to characterise the effect of the manufacturing process on the fatigue behaviour. The specimens were extruded from two different initial diameters, giving two different reductions in cross-section of 18% and 75% respectively.To understand the influence of cold extrusion, the following analyses have been undertaken for each condition and on the initial material: monotonic tensile properties, microstructure, EBSD, residual stresses and hardness. Simulation of the forming process and microstructural observations show that the plastic strain is homogeneous in the specimen section. For both reduction factors, the forming process has a positive effect on the components properties: induced residual stresses in compression and improved hardness and roughness (Ra decreasing). Tension, plane bending and torsion fatigue tests show that the fatigue strength is about 30% higher for the batch with 75% reduced cross-section. All investigations show that strain hardening is the principal material parameter responsible for the increase in fatigue strength. A multiaxial fatigue criterion taking into account the effects of the forward rod extrusion process was also developed.

Tensile and Compressive Properties of Woven Kenaf/Glass Sandwich Hybrid Composites

Monotonic (tensile and compression) properties of woven kenaf/glass reinforced unsaturated polyester sandwich hybrid composites have been experimentally investigated. Five types of composites laminates were fabricated using a combination of hand lay-up and cold press techniques, postcured for two hours at 80°C and left for 48 hours at room temperature. The hybrid composites contained fixed six layers of glass as a shell, three on each side, whereas the number of core kenaf layers was changed in three stages to get S1, S2, and S3 hybrid composites. Composites specimens with pure glass and kenaf were also fabricated for comparison. It was found that one kenaf layer replaced about 20% of total fiber weight fraction of the composite; this leads to reducing the density of final hybrid composite by 13%. Besides, in mechanical properties perspective, there are less than 1% reduction in compression strength and 40% in tensile strength when compared to pure glass composite. Generally, the results revealed that the best performance was observed in S1, which showed a good balance of all mechanical properties determined in this work.

An improved method for estimation of Ramberg–Osgood curves of steels from monotonic tensile properties

AbstractIn the present study, first a method for estimating cyclic yield strength is improved and compared using the experimental data of 121 steels from literature. Correlations between cyclic deformation properties (K′ and n′) and monotonic tensile data are then investigated, and a simple method requiring only the monotonic tensile properties is developed for estimation of the Ramberg–Osgood curve. Prediction capability of the proposed method is not only evaluated using the aforementioned 121 steels, but also compared with several commonly used methods that are available in the literature. The proposed correlations are shown to predict the cyclic deformation properties of most of considered steels reasonable well.

Effect of constituent-particles distribution on mechanical behavior of an AlMgSi alloy

In the present work, the effect of constituent particles distribution on mechanical behavior of a high strength AlMgSi alloy is investigated. The study consists of the tensile and fatigue testing of the material in longitudinal and transverse directions. Monotonic tensile properties, in both the directions showed virtually identical yield and tensile strengths. However, higher values of elongation and reduction in area were observed in the longitudinal direction. SN curves produced after fatigue testing displayed a shorter fatigue life in transverse direction as compared to the longitudinal direction. Microstructural analysis exposed that the material had clusters of constituent particles. Post fracture analysis of tensile and fatigue samples, in SEM, revealed that the topographical features change with the sample orientation. The present study revealed that the distribution of constituent particles in longitudinal and transverse directions has a pronounced effect on the tensile and fatigue behavior of the alloy and also induce anisotropy in the material.

Low-cycle fatigue behavior of a high manganese austenitic twin-induced plasticity steel

The monotonic tensile properties and deformation mechanisms of Fe–Mn–C twinning-induced plasticity (TWIP) steels have been extensively studied; however, the low-cycle fatigue (LCF) properties of this series of advanced steels have not been well understood. The present paper addresses the cyclic deformation behavior and the deformed microstructure of an as-annealed TWIP steel. Fully reversed push–pull LCF tests were performed at room temperature under total strain amplitude control with a strain rate of 0.006s−1 and strain amplitudes ranging from 0.002 to 0.01. The results show initial rapid cyclic hardening within the initial 10% of the fatigue life at all strain amplitudes, and demonstrate an obviously enhanced cyclic yield strength. Different types of cyclic stress responses were revealed, which are featured by initial cyclic hardening followed by cyclic saturation, or followed by cyclic softening and saturation, or followed by cyclic softening without saturation till the final fracture, depending on the strain amplitude applied. The microstructure prior to and after fatiguing were examined by means of optical and transmission electron microscopy. The typical optical microstructure of fatigued samples is characterized by increases in slip band density with increasing strain amplitude or number of cycles at a given strain amplitude applied. The substructures of the deformed samples are featured by the formation of stacking faults and vein/labyrinth dislocation structures, while fine twins and cell or wall dislocation structures, besides those generated at lower strain amplitudes, are formed at high strain amplitudes.

A method of predicting cyclic stress–strain curve from tensile properties for steels

Cyclic loading can significantly alter the monotonic deformation resistance of a material. Knowledge of the cyclic deformation behavior of a material is essential for reliable life calculations of cyclically loaded structures. As monotonic tensile properties are commonly available or easily obtainable from a simple tension test, the objective of this study was to derive a method for estimating the cyclic deformation properties of steels from common tensile properties and hardness. A variety of correlations and relationships among hardness, tensile properties, and cyclic deformation properties were developed for fifty-seven steels included in this study. Several correlations and relations from the literature among the aforementioned properties were also examined and compared to the correlations and relationships proposed in this study using a different set of data from sixty-six steels. Implications of the proposed cyclic deformation behavior estimation method on fatigue life predictions are also discussed.

A neural network approach for predicting steel properties characterizing cyclic Ramberg-Osgood equation

This paper attempts to demonstrate the applicability of artificial neural networks to the estimation of steel properties, cyclic strain-hardening exponent and cyclic strength coefficient, characterizing cyclic Ramberg-Osgood equation on the basis of monotonic tensile test properties. For this purpose, steel tensile data were extracted from the literature and two separate neural networks were constructed. One set of data was used for training the two networks and the remaining for testing purposes. Regression analysis and mean relative error calculation were used to check the accuracy of the system in the training and testing phases. Comparison of the results obtained from the neural networks and the values obtained from direct fitting of experimental data, indicated the reasonable prediction of cyclic strain-hardening exponent and cyclic strength coefficient, which are often used to characterize the cyclic deformation curve by a Ramberg-Osgood type equation.

Weighted error criterion to evaluate strain–fatigue life prediction methods

Experimental determination of strain–fatigue life curve is expensive considering the time and effort involved. Several empirical relations based on monotonic tensile properties and/or hardness is available for estimating strain–fatigue life properties. Existing criteria to evaluate the accuracy of these empirical relations are limited by the data points in the strain–fatigue life curve. A new criterion based on the prediction errors of fatigue constants is proposed to overcome the limitation. Around eight prediction methods are evaluated for two hundred twenty-six steel grades from literature using the existing and the proposed criteria. It is observed that the conclusions from the new criterion are more reliable than the existing criteria.

Effects of annealing treatment on the ratcheting behavior of extruded AZ31B magnesium alloy under asymmetrical uniaxial cyclic loading

The objective of this investigation is to study the effects of annealing treatment on the ratcheting behavior of extruded AZ31B magnesium alloy. First, the microstructures and monotonic tensile properties of the extruded and annealed alloys were assessed. The results showed that the grain size increased slightly with increasing annealing time until an annealing time of 6 h after which abnormal grain growth happened. Accordingly, the ultimate tensile strength of the Mg alloy decreased with increasing annealing time, while the tensile yield strength and elongation percentage of the Mg alloy increased with annealing time until the annealing time reached 2 h. The cyclic softening/hardening behavior of the annealed AZ31B Mg alloy was similar to that of the extruded alloy: first an apparent cyclic softening was observed, then a cyclic hardening occurred, and finally a stable state was reached. The annealing treatment delayed the occurrence of the cyclic hardening. It was also shown that the effects of the annealing time on the ratcheting strain of the Mg alloy depended of the loading path.

Influence of niobium alloying on the low cycle fatigue of cast TiAl alloys at room and high temperatures

The low-cycle fatigue properties of TiAl alloys with 2 at.% and 7 at.% of Nb with nearly lamellar microstructure were studied at room temperature and 750 °C. Cyclic hardening/softening curves, cyclic stress-strain curves (CSSC) and fatigue life curves were obtained at both temperatures as well as monotonic tensile properties. Both materials show stable cyclic response at 750 °C. At room temperature, significant cyclic hardening characteristic for 2 at.%Nb alloy is not observed for 7 at.% Nb alloy. This difference is attributed to higher Nb content which increases the stacking fault energy and thus prevents intensive twinning. Parameters of the CSSC, the Manson-Coffin law and the Basquin law were determined. The cyclic strength of the alloy with 7 at.% Nb is substantially higher in comparison with 2 at.% Nb, probably due to lower thickness of the lamellas.

A novel approach to the estimation of strain life fatigue parameters

Most of the existing methods for the estimation of strain life parameters were developed by correlating experimentally determined strain life parameters, directly and independently of each other, with one or more readily obtainable monotonic tensile material properties. In some cases, this resulted in relatively complicated expressions for the calculation of fatigue parameters, while in others, because of the poor correlation between certain fatigue and monotonic properties, constant values were assigned to some of these fatigue parameters. Since in the strain life approach the fatigue life of the material is defined by Δε/2−2Nf relationship, it is suggested that the estimation of the strain life fatigue parameters should be based on the analysis and connection of actual strain life data to an appropriate monotonic property. For the purpose of an initial verification of the proposed approach, estimation of fatigue parameters σf′, b, εf′ and c was performed using data gathered for normalized and quenched and tempered high-strength low-alloy steels AISI 4140 and AISI 4142. Brinell hardness HB was chosen as a required monotonic parameter. Numbers of load reversals 2Nf calculated using experimentally determined and estimated fatigue parameters were analyzed and compared. They were found to be in very good agreement, thus confirming good predictive capabilities as well as general validity of the proposed approach to the estimation of strain life fatigue parameters.

Tensile properties and fracture behaviour of carbon fibre filament materials

Monotonic tensile properties and fracture behaviour of carbon fibre filament materials, namely single/mono- and multi-filaments (two and four filaments) as well as virgin carbon tows have been evaluated and discussed. Micro composite or single fibre approach is used in this study, which facilitated the evaluation of tensile properties and nature of fracture of carbon filament materials in a relatively short time with a large number of inexpensive trials. Tensile tests have been conducted on these filament materials at ambient temperature and laboratory air atmosphere. Load–elongation and the corresponding stress–strain plots thus obtained have been analysed to understand the tensile behaviour. The peak tensile strength of single carbon filament is found to be 3.8 GPa, and the value of the resilience obtained is 19 MJ/m3. The peak tensile strength was found to increase moderately with further increase in number of filaments. However, the value of resilience was found to increase with increase in the number of fibres, which is attributed to the controlled failure of multi-filaments. On the other hand, the tensile strength of virgin carbon tow without matrix was found to be 1.13 GPa, and the value of the fracture energy was determined to be 9.9 MJ/m3, which is nearly one fourth or even less than the corresponding values of the mono- and multi-filaments. The data obtained in the case of the virgin carbon tows were further analysed to evaluate the Weibull statistical parameters.

Spatially Resolved Materials Property Data From a Uniaxial Cross-Weld Tensile Test

Application of electronic speckle pattern interferometry (ESPI) is described to measure the spatial variation in monotonic tensile stress-strain properties along “cross-weld” specimens machined from a stainless steel three-pass welded plate. The technique, which could also be done with digital image correlation, was applied to quantify how the material 0.2%, 1%, 2%, 5%, 10%, and 20% proof stress varied with distance from the center-line of the weldment for parent and weld material associated with the first and final passes. The stress-strain curves measured by the ESPI method correlated closely with stress-strain data measured using conventional test specimens. The measured results are consistent with the hypothesis that thermo-mechanical cycles associated with the welding process work harden previously deposited (single-pass) weld metal and the surrounding parent material. The stress-strain response of the heat affected zone adjacent to the first weld pass is consistent with an accumulated (equivalent monotonic) plastic strain of 6.5% and that of the first pass weld bead was consistent with an accumulated plastic strain of approximately 4% greater than the state of the final pass weld metal.

Effect of loading rate on the monotonic tensile behavior and tensile strength of an oxide–oxide ceramic composite at 1200 °C

The influence of loading rate on monotonic tensile behavior and tensile properties of an oxide–oxide ceramic composite was evaluated in laboratory air at 1200 °C. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tensile tests conducted at loading rates of 0.0025 and 25 MPa/s revealed a strong effect of rate on the stress–strain behavior as well as on the ultimate tensile strength (UTS), elastic modulus and failure strain. At 0.0025 MPa/s, increase in stress results in non-monotonic change in strain, with the rate of change of strain reversing its sign at stresses ∼25 MPa/s. Several samples were subjected to additional heat treatments prior to testing in order to determine whether this unusual stress–strain behavior was an artifact of incomplete processing of fibers in the as-received material. The unusual material response in the 0–30 MPa stress range was further investigated in creep tests conducted with the applied stresses ≤26 MPa. Negative creep (i.e. decrease in strain under constant stress) was observed. Porosity measurements indicate that a decrease in matrix porosity and matrix densification may be taking place in the N720/A composite exposed to 1200 °C at stresses <30 MPa for prolonged periods of time.

Low-cycle fatigue behavior of two magnesium alloys

Magnesium alloys possess many excellent performances, such as low density, high specific strength, excellent thermal conductivity, and machinability. Magnesium alloys, including Mg-Al alloy family, are increasingly considered for structural components, especially in automotive industry. AZ91 alloy with about 9 wt.% Al and 1 wt.% Zn is the most common magnesium die-casting alloy. It has excellent castability and good room-temperature mechanical properties. AE42 alloy with about 4 wt.% Al and 2.5 wt.% rare earth elements shows good creep resistance to 150 ◦C, and is often used for automotive engine components. For both AZ91 and AE42 alloys, few low-cycle fatigue data are available even though their monotonic tensile and creep properties have been well documented [1–6]. In this paper, the low-cycle fatigue behavior of two magnesium alloys including AZ91 and AE42 were investigated. The rectangular fatigue samples with dimensions 14 × 5 × 5 mm in gauge section were directly machined from die-cast plate of each alloy (designated F). Some fatigue samples were subjected to solution treatment in argon at 415 ◦C for 12 hr (designated T4). Fully-reversed pull-push fatigue tests were performed at room temperature in laboratory air under total strain amplitude control mode using Shimadzu servohydraulic testing machine. All tests were run to final separation of sample, and the corresponding cycling number was defined as low-cycle fatigue life. The fatigue life data (total strain amplitude, et/2, versus number of cycles to failure, Nf) of two alloys with F and T4 status are shown in Fig. 1a and b, respectively. It can be seen that at most of total strain amplitudes except for the lowest strain amplitude of 0.3%, the AZ91-T4 alloy gives greater resistance on strain cycling than the die-cast AZ91 alloy. On the other hand, the AE42 alloy subjected to solution treatment exhibits longer fatigue life than the die-cast AE42 alloy at all total strain amplitudes imposed in this investigation. The above-mentioned fact implies that solution treat-

Statistical evaluation of strain-life fatigue crack initiation predictions

Most of the existing methods for estimating εN parameters are based on a relatively limited amount of experimental data. In addition, sound statistical evaluation of the popular rules of thumb used in practice to estimate fatigue properties is scarce, if available. In this work, an extensive statistical evaluation of the existing Coffin–Manson parameter estimates is presented based on monotonic tensile and uniaxial fatigue properties of 845 different metals, including 724 steels, 81 aluminum alloys, and 15 titanium alloys. The studied Coffin–Manson estimates include the methods proposed by Muralidharan and Manson, Bäumel and Seeger, Roessle and Fatemi, Mitchell, Ong, Morrow, Raske, as well as Manson’s universal slope and four-point correlation methods. From the collected data, it is shown that all correlations between the fatigue ductility coefficient ε′ f and the monotonic tensile properties are very poor, and that it is statistically sounder to estimate ε′ f based on constant values for each alloy family. Based on this result, a new estimation method which uses the medians of the individual parameters of the 845 materials is proposed.