The influence of stress-controlled tensile fatigue loading on the stress–strain characteristics of AISI 1045 steel

This study analyses the influence of fatigue loading on the residual tensile properties of AISI 1045 steel. The fatigue tests were carried out under stress-controlled tensile loadings at a stress ratio equal to 0. The maximum applied stresses were within the range from 550 MPa to 790 MPa. An analysis of ratcheting strain and plastic strain amplitude evolution due to fatigue loading was performed on the experimental data. In the next stage of this study, the initial fatigue loadings were introduced. Two maximum stresses, 550 MPa and 750 MPa, and three cycle lengths, 25%, 50% and 75% of the total number of cycles required to fracture the material at a given stress, were used. The pre-fatigued specimens were subjected to tensile testing at strain rates from 10⁻⁴ to 10⁰ s⁻¹. A large number of fatigue cycles, equal to 75% of the fatigue life, induces material softening as well as a drop in elongation and a reduction of area. Pre-fatigue at maximum stress equal to 550 MPa results in the increase of the elastic limit and offset yield point as well. Both parameters reach almost constant value after number of cycles equal to 25 % of the fatigue life. The further increase in the number of cycles does not affect elastic limit and offset yield point in a clearly visible way. The increase of maximum stress of the initial fatigue loadings up to 750 MPa induces similar but stronger effect i.e. increase and stabilization of elastic limit and offset yield point values, however decrease of both parameters value is observed at large number of pre-fatigue cycles corresponding to 75% of the fatigue life.     

The effect of low cycle fatigue,ratcheting and mean stress relaxation on stress–strain response and microstructural development in a dual phase steel

This study examines the cyclic plastic deformation behavior and microstructural development of a dual phase steel in both symmetric and asymmetric cycling in strain and stress control modes. The low-cycle fatigue (LCF) and mean stress relaxation (MSR) tests show very similar fatigue lifetimes. However, fatigue lifetimes reduce and prominent accumulation of directional strain was observed in ratcheting. A microstructural analysis has revealed that the type of cyclic test carried out has a noticeable impact on the substructural development, and this has been correlated with differences in accumulated tensile strain. Electron backscatter diffraction investigation has shown larger in-grain misorientation for ratcheting specimen in comparison with LCF and MSR specimens. The orientation of ferrite grains was found to have very little effect on their substructural development, and strain localization commonly occurred in the ferrite at the ferrite/martensite interface.     

Constitutive modeling of cyclic plasticity deformation and low–high-cycle fatigue of stainless steel 304 in uniaxial stress state

For constructing a theory that adequately describes the effects of cycling loading, we initially analyze an experimental plastic hysteresis loop of the stainless steel SS304 and allocate on it three backstress types responsible for yield surface center displacement. Evolutionary equations per each backstresses type are formulated based on the equation of plasticity flow theory at combined (isotropic-kinematic) hardening. Material functions (parameters) closing the theory are defined, and basic experiment and identification methods of material function are formulated. Comparison of design results and experiments testifies their reliable compatibility.     

Low-velocity impact response of fibre–metal laminates – Experimental and finite element analysis

In this contribution, the impact dynamic response and failure modes of fibre–metal laminated panels subjected to low velocity impact were investigated and presented. The fibre–metal laminate in this paper comprised of a layer of glass fibre-reinforced plastics sandwiched between two layers of aluminium alloy. Two different types of glass fibre-reinforced plastics were used for the fabrication: unidirectional and woven. A fairly extensive experimental investigation was conducted in conjunction with a detailed finite element analysis. The experiments were conducted using a standard drop-weight test machine and the finite element analysis was carried out using a commercially available finite element software. The results of maximum contact force, contact duration and corresponding failure modes are presented, compared and discussed in this technical paper.     

Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.     

Effects of strain rate and temperature on the stress–strain response of solder alloys

To ensure reliable design of soldered interconnections as electronic devices become smaller, requires greater knowledge and understanding of the relevant mechanical behavior of solder alloys than are presently available. The present paper reports the findings of an investigation into the monotonic tensile properties of bulk samples of three solder alloys; a lead–tin eutectic and two lead-free solders (tin–3.5 copper and a tin–3.5 silver alloy). Temperatures between–10 and 75°C and strain rates between 10^–1 and 10^–3 s^–1 have been studied. Both temperature and strain rate may have a substantial effect on strength, producing changes well in excess of 100%. Strength is reduced by lowering strain rate and increasing temperature, and Sn–37 Pb is usually most sensitive to the latter. Expressions for strain and strain rate hardening have been developed. The Sn–0.5 Cu alloy is usually the weakest and most ductile. Sn–37 Pb is strongest at room temperature but with increasing temperature and lower strain rates it becomes inferior to Sn–3.5 Ag. Ductility changes with temperature and strain rate for all three alloys are generally small with inconsistent trends. The role of such data in stress analysis and modeling is considered and the paramount importance of employing data for conditions appropriate to service, is emphasized.     

Inverse analysis of flow behaviour of AISI 304 stainless steel casing and Nd–Fe–B magnet at different strain rates

Abstract

Nanocrystalline Nd–Fe–B magnets are the most promising permanent magnets for high value applications. Strain rate and temperature have been established as the important parameters under dynamic processing for the development of texture and optimisation of magnetic properties. Experiments have been carried out at the constant true strain rates of 0·01, 0·5 and 5·0 s^–1 at 800°C. Flow stress generated from the experiments represents the combined effect of casing and magnet. To isolate the effect of stainless steel casing, composite deformation has been studied using finite element method. In this way, flow behaviour of steel and Nd–Fe–B magnet has been evaluated.     

Description of stress–strain curves for stainless steel alloys

There is a wide variety of stainless steel alloys, but all are characterized by a rounded stress–strain response with no sharply defined yield point. This behaviour can be represented analytically by different material models, the most popular of which are based on the Ramberg–Osgood formulations or extensions thereof. The degree of roundedness, the level of strain hardening, the strain at ultimate stress and the ductility at fracture of the material all vary between grades, and need to be suitably captured for an accurate representation of the material to be achieved. The aim of the present study is to provide values and predictive expressions for the key parameters in existing stainless steel material models based on the analysis of a comprehensive experimental database. The database comprises experimental stress–strain curves collected from the literature, supplemented by some tensile tests on austenitic, ferritic and duplex stainless steel coupons conducted herein. It covers a range of stainless steel alloys, annealed and cold-worked material, and data from the rolling and transverse directions. In total, more than 600 measured stress–strain curves have been collected from 15 international research groups. Each curve from the database has been analysed in order to obtain the key material parameters through a curve fitting process based on least squares adjustment techniques. These parameter values have been compared to those calculated from existing predictive models, the accuracy of which could therefore be evaluated. Revised expressions providing more accurate parameter predictions have been proposed where necessary. Finally, a second set of results, containing material parameters reported directly by others, with information of more than 400 specimens, has also been collected from the literature. Although these experimental results were not accessible as measured raw data, they enabled further confirmation of the suitability of the proposed equations.     

Determination of biaxial stress–strain diagram for gas pressure superplastic forming

Abstract

The success of a gas pressure superplastic forming operation depends on accurate formulation of a pressure–time diagram which in turn needs an accurate stress–strain relationship evaluated preferably under multiaxial or biaxial conditions. The present analysis describes a technique of generating such curves from gas pressure cone forming tests and subsequent manipulation of the data. The method also includes an innovative technique of online monitoring of strain during the forming process by measuring the volume of displaced air from the die during progress of forming.     

Fatigue behavior of ferritic–pearlitic–bainitic steel in loading with positive mean stress

The effect of positive mean stress on the fatigue behavior of ferritic–pearlitic–bainitic steel has been studied. Specimens, produced from a massive forging, were cycled with two constant stress amplitudes and various positive mean stresses. Plastic strain amplitude and cyclic creep rate were measured during cyclic loading and the effect of the mean stress on saturated plastic strain amplitude and mean strain at half-life was established. Plastic strain amplitude is weakly dependent but creep strain increases with the mean stress exponentially. Fatigue life decreases with the mean stress for both stress amplitudes. The contributions of cyclic plastic strain and cyclic creep to the fatigue damage were evaluated and discussed in relation with the Manson-Coffin curve.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

Strain rate and orientation-dependent strain hardening of Mg–9Li–Al using crystal plasticity

A crystal plasticity finite element model was proposed considering slip and twinning interactions. The grain morphology and crystallographic orientations were introduced into the model to describe the microstructure of duplex polycrystalline Mg–9Li–Al. The activation of the slip systems and the strain localisation with respect to initial grain orientations were investigated. In addition, the effects of phase distributions and volume fractions on the macroscopic responses and on strain hardening rates were analysed. The results show that the strain hardening is rate-dependent but the texture is less sensitive to strain rate. The distribution of a phase and its volume fraction play primary roles in governing the mechanical response.     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

Tensile stress–strain and work hardening behaviour of 316LN austenitic stainless steel

Abstract

The true stress (σ)–true plastic strain (?) data of a type 316LN austenitic stainless steel tested at nominal strain rates in the range 3×10^-5–3×10^-3 s^-1 and temperatures of 300–1123 K were analysed in terms of flow relationships proposed by Hollomon, Ludwik, Swift, Voce, and Ludwigson. The applicability of the particular flow relationship is discussed in terms of ‘complete’ and ‘applicable’ range fits of the experimental σ–? data. At all strain rates, in the case of the complete range fit, the Ludwigson equation followed the stress–strain data most closely at 300 K, while in the temperature range 523–773 K, the flow behaviour was described equally well by both the Ludwigson and Voce equations. In the temperature range 823–1023 K, the Voce equation described the flow behaviour most accurately in the case of the complete range fit of σ–? data at all strain rates. The analysis of σ–? data employing the Ludwigson equation in the applicable range fit covering low and intermediate strains, and the Hollomon equation at high strains provided close simulation of the observed flow behaviour in the temperature range 823–1023 K. At high temperatures of 1073 and 1123 K, the Ludwigson equation reduces to the Hollomon equation. The variations in different flow parameters of the Ludwigson and Voce equations with temperature and strain rate exhibited anomalous behaviour at intermediate temperatures because of dynamic strain aging.     

Constitutive analysis of the hot deformation behavior of Fe–23Mn–2Al–0.2C twinning induced plasticity steel in consideration of strain

In this study, constitutive analysis has been carried out on Fe–23Mn–2Al–0.2C twinning induced plasticity (TWIP) steel. For this purpose, hot compression tests were conducted on a Gleeble-3500 thermo-mechanical simulator in the temperature range of 900–1150 °C and the strain rate range of 0.001–20 s⁻¹. The effects of deformation heating and friction on flow stress were analyzed and corrected. On the basis of Sellars–Tegart–Garofalo equation, the strain-dependent constitutive equations of the steel were derived. The results show that deformation heating has a significant influence on the flow stress at lower temperatures and higher strain rates, while the frictional effect is slight even at the highest strain level investigated. Comparison of the calculated flow stress with the experimental data suggests that the developed constitutive equations can adequately describe the relationships between the flow stress, strain rate, temperature and strain of the steel during hot deformation. This is supported by a high correlation coefficient (R = 0.996) and a low average absolute relative error (AARE = 3.31%) for the entire deformation condition range investigated.     

Uniaxial stress–relaxation and stress–strain responses of human amnion

The mechanical behavior of human amnion is examined under uniaxial tensile loading conditions. Monotonic strain-to-failure and stress-relaxation tests are described for membrane strip samples of amnion obtained by removing the chorion cell layer from specimens of whole chorioamnion. The monotonic behavior of the amnion is characterized by a large stress-free strain (approximately 10%) prior to a quadratic load-displacement response. Substantial stress relaxation behavior (ranging from 20-80%) is observed, described by a two time-constant exponential decay. The effects of the application of a topical antiseptic and of prior straining and relaxation on subsequent monotonic failure properties are examined. The results suggest that while amnion is a remarkably resilient tissue material, its mechanical behavior is typical of nonlinear viscoelastic materials, and depends strongly on its history.     

Grain-scale deformation in a Mg−0.8 wt% Y alloy using crystal plasticity finite element method

Magnesium (Mg) alloys with hexagonal close-packed (HCP) structure usually have a poor ductility at room temperature. The addition of yttrium (Y) can improve the ductility of Mg alloys. To understand the underlying mechanism, crystal plasticity finite element method (CPFEM) was employed to simulate the tensile deformation of a Mg−0.8 wt% Y alloy. The simulated stress‒strain curve and the grain-scale slip activities were compared with an in-situ tensile test conducted in a scanning electron microscope. According to the CPFEM result, basal slip is the dominant deformation mode in the plastic deformation stage, accounting for about 50% of total strain. Prismatic slip and pyramidal 〈a〉 slip are responsible for about 25% and 20% of the total strain, respectively. Pyramidal ⟨c + a⟩ slip and twinning, on the other hand, accommodate much less strain.