Overall stress–strain relationship of cold rolled transformation induced plasticity multiphase steels

Abstract

As a result of transformation induced plasticity, TRIP assisted steels possess favourable mechanical properties such as high strength, ductility and toughness. In the present work, the flow stress of a cold rolled TRIP multiphase steel has been calculated from the stress of individual constituent phases on the basis of a continuum model whereby the internal stress produced by the inhomogeneous distribution of plastic strain is considered. For the first time, account is taken of how the volume fraction of constituent phases changes with strain. A comparison of stress–strain curves determined from the model and experimentally derived stress–strain curves for an Si–Mn type TRIP800 steel gives satisfactory agreement. Variation of the strain hardening exponent of the TRIP steel with strain is also discussed.     

Strength and stress–strain characteristics of traditional adobe block and masonry

Traditional unstabilized adobe low-rise buildings are common in many Chinese small towns and villages. This paper presents a study on the uniaxial compressive strength and stress–strain behavior of traditional unstabilized adobe blocks and masonry prisms with various compositions. The adobe blocks were manually produced by Chinese traditional technique in various proportions of natural soil and sand. The influence of various proportions on unconfined compressive strength, dry density and initial tangent modulus are discussed. Following this, soil mortars in three different proportions were used to construct adobe masonry prisms, with the purposes of understanding the influence of mortar strength to block strength ratio on compressive strength and stress–strain characteristics. The result shows that the compressive strength, initial tangent modulus and Poisson’s ratio of prism are influenced by the ratio of mortar strength to block strength. In addition, tangent modulus and Poisson’s ratio increase with the ratio of stress to peak strength. It was also found that although coefficients of variation of experimental results are reduced by load–unload cycles, peak strains are largely increased.     

Influence of shear bond strength on compressive strength and stress–strain characteristics of masonry

The paper is focused on shear bond strength–masonry compressive strength relationships and the influence of bond strength on stress–strain characteristics of masonry using soil–cement blocks and cement–lime mortar. Methods of enhancing shear bond strength of masonry couplets without altering the strength and modulus of masonry unit and the mortar are discussed in detail. Application of surface coatings and manipulation of surface texture of the masonry unit resulted in 3–4 times increase in shear bond strength. After adopting various bond enhancing techniques masonry prism strength and stress–strain relations were obtained for the three cases of masonry unit modulus to mortar modulus ratio of one, less than one and greater than one. Major conclusions of this extensive experimental study are: (1) when the masonry unit modulus is less than that of the mortar, masonry compressive strength increases as the bond strength increases and the relationship between masonry compressive strength and the bond strength is linear and (2) shear bond strength influences modulus of masonry depending upon relative stiffness of the masonry unit and mortar.     

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.     

Mechanical anisotropy and stress–strain rate characteristics in superplastic deformation of titanium alloys

Abstract

Uniaxial tension and compression tests have been carried out on two titanium based alloys, Ti–6Al–4V in the form of extruded tubes and forged plates and Ti–3Al–10V–2Fe sheets, to study anisotropic behaviour during superplastic deformation. The following were observed: (i) originally round cross-section became elliptical after deformation; (ii) the flow stresses and strain rates were dependent on the orientation of the specimens; and (iii) the strain anisotropy became less severe as the strain rate increased. These characteristics of anisotropy were related to the original microstructure (e.g. the mechanical fibring of the α grains) and the microstructural evolution during superplastic deformation. New constitutive equations for describing anisotropic superplastic deformation have been proposed to explain the effect of strain rate or stress on anisotropy.     

Influence of strain rate on recrystallisation– precipitation interaction in V,Nb, and V–Ti microalloyed steels

Abstract

A study has been made of the recrystallisation–precipitation interaction in three microalloyed steels, containing respectively V, Nb, and V–Ti, applying two different strain rates. Recrystallised fraction v.time curves were determined and used to draw recrystallisation–precipitation–time–temperature (RPTT) diagrams. The influence of strain rate has been shown to be similar in the three steels. On the basis of the results the value of }}SB0·19 has been found for the exponent of the strain rate, following Dutta and Sellars'model for the parameter t _0·05 , which differs from the value }}SB0·5 proposed by these authors. Simultaneously, the influence of strain rate on the static recrystallis ation critical temperature has been determined, it being observed that an increase in the former leads to a drop in the latter. Furthermore, strain rate is shown to have an influence on the recrystallisation–precipitation interaction, acting on those parameters that best contribute to defining RPTT diagrams In this sense, it was found that an increase in the strain rate led to a drop in the curve nose temperature T _N and a reduction in the time necessary for precipitation to finalise t _0·95 , as well as an increase in the recrystallisation rate.     

Structure–property relationships in intercritical heat affected zone of low-carbon microalloyed steels

Abstract

The conditions for martensite formation in the intercritical heat affected zone (HAZ) of two low-carbon microalloyed steels have been investigated using optical and transmission electron microscopy. Based on Charpy V-notch testing of a large number of thermally cycled specimens, it is concluded that embrittlement within the intercritical HAZ of such steels is closely related to the development of twinned martensite during the weld cooling cycle. The reduced HAZ toughness probably arises from the associated stress concentrations developed in the surrounding ferrite matrix, which give rise to the initiation of brittle fracture in the ferrite.

MST/634     

Determination of local stress–strain properties of resistance spot-welded joints of advanced high-strength steels using the instrumented indentation test

For spot-welded joints, the resistance to mechanical stress depends on the local strength properties and gradients in the weld area. The commonly used methods for investigating the stress–strain behaviour across the weld area are connected with a high level of sample preparation and with considerable limitations in local resolution. A promising method for determining locally resolved stress–strain curves is the instrumented indentation test in connection with the method of representative stress and strain (RS) and the method of artificial neural networks (NNs). The stress–strain properties of the weld nugget and the base metal determined by these two methods are compared and discussed, additionally in relation to the stress–strain curves obtained from the tensile test. The measured Vickers hardness across the weld area is compared with the evaluated local stress–strain properties. Three steels used in automobile manufacturing are investigated: mild steel DC04 and two advanced high-strength steels (TRIP steel HCT690T and martensitic steel HDT 1200M). The results of the two methods (RS and NN) show good correspondence for the base metal area but some significant differences for the weld nugget. Comparing the data across the weld area, no evidence of the presence of residual stress (which would influence the results) was found.     

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.     

Cyclic deformation and microstructural behaviour of reduced activation ferritic–martensitic steels

Abstract

The present paper presents results about cyclic behaviour and the evolution of the dislocation structure of reduced activation ferritic–martensitic steels and commercial martensitic steels AISI 410 and 420. The variation of the free dislocation density within subgrains and subgrain size was mainly analysed during the cyclic softening of EUROFER 97 steel. From the analysis of the flow stress components, the friction and back stresses, and the information of the evolution of the dislocation structure, it could be concluded that the softening of tempered martensitic steels at 20°C is produced by the contribution of the friction stress and aided later by the back stress.     

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.     

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.     

Effect of prestrain on aging behaviour of directly air cooled copper added titanium–boron microalloyed steels

Abstract

The present investigation concerns effect of prestrain on the precipitation of Cu during aging of directly air cooled Ti–B microalloyed steels. The differential scanning calorimetry studies allow to identify that precipitation of Cu is most prominent between 350 and 500°C. Prestraining of the directly air cooled samples has broadened the temperature range for Cu precipitation with perceptible overlapping with the peaks due to other low temperature reactions like recovery and/or tempering. Fifty per cent prestraining of 1·5 wt-%Cu added steel before aging triggered the precipitation reaction at a temperature ～320°C with pronounced decrease in the activation energy values from 172 to 116 kJ mol^?1. Microstructural changes due to prestraining are evident in the micrographs obtained from transmission electron microscope as deformed ferrite laths and formation of cell structures therein. While 15–50% prestraining has increased the strength without much deterioration in ductility; aging of the prestrained samples has improved the strength and ductility concomitantly.     

Evaluation of stress–strain curve estimates in dynamic experiments

Accurate measurements of the forces and velocities at the boundaries of a dynamically loaded specimen may be obtained using split Hopkinson pressure bars (SHPB) or other experimental devices. However, the determination of a representative stress–strain curve based on these measurements can be challenging. Due to transient effects, the stress and strain fields are not uniform within the specimen. Several formulas have been proposed in the past to estimate the stress–strain curve from dynamic experiments. Here, we make use of the theoretical solution for the waves in an elastic specimen to evaluate the accuracy of these estimates. It is found that it is important to avoid an artificial time shift in the processing of the experimental data. Moreover, it is concluded that the combination of the output force based stress estimate and the average strain provides the best of the commonly used stress–strain curve estimates in standard SHPB experiments.     

Cyclic stress–strain response and crystal plasticity finite element analysis of AISI 9310 steel in biaxial fatigue loading

There are still many gaps in the research on the multiaxial fatigue failure mechanism of the gear shaft. In this paper, cyclic stress–strain response and biaxial fatigue damage characteristics of gear steel AISI 9310 were investigated. The specimens showed obvious cyclic softening characteristics at all phase angles, and the softening rate was directly associated with the initiation and propagation of cracks. The fractographies at different phase angles revealed that the specimens under out-of-phase loading suffered fatigue failure caused by a single crack source on the surface, while the fatigue crack under in-phase loading was gathered together by the propagation of different crack sources. Finally, the established crystal plastic finite element model showed a good prediction of the plastic strain energy density at different phase angles, and the maximum error was 13.03%. Furthermore, a biaxial fatigue life prediction method was proposed, with a maximum error of 39.5%.     

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.     

Cement stabilised rammed earth. Part B: compressive strength and stress–strain characteristics

Strength and behaviour of cement stabilised rammed earth (CSRE) is a scantily explored area. The present study is focused on the strength and elastic properties of CSRE. Characteristics of CSRE are influenced by soil composition, density of rammed earth, cement and moisture content. The study is focused on examining (a) role of clay content of the soil on strength of CSRE and arriving at optimum clay fraction of the soil mix, (b) influence of moisture content, cement content and density on strength and (c) stress–strain relationships and elastic properties for CSRE. Major conclusions are (a) there is considerable difference between dry and wet compressive strength of CSRE and the wet to dry strength ratio depends upon the clay fraction of soil mix and cement content, (b) optimum clay fraction yielding maximum compressive strength for CSRE is about 16%, (c) strength of CSRE is highly sensitive to density and for a 20% increase in density the strength increases by 300–500% and (d) in dry state the ultimate strain at failure for CSRE is as high as 1.5%, which is unusual for brittle materials.     

A non-local fractional stress–strain gradient theory

International Journal of Mechanics and Materials in Design - A generalized non-local stress–strain gradient theory is presented using fractional calculus. The proposed theory includes as a...