Monotonic Uniaxial Loading Research Articles

The study of the fatigue crack propagation in mixed mode crack growth

This work is devoted to analysis of thermodynamics properties of the fatigue cracks propagation in metals. A theoretical description of the elastoplastic condition at the fatigue crack tip is proposed on the base of an elastic solution and a secant elastic modulus. An experimental confirmation of the theoretical approach to the heat flux calculation at the fatigue crack tip is carried out. The character of heat dissipation at different stages of crack propagation is studied. The investigation of the fatigue crack propagation was carried out on flat samples with stress concentrator made from stainless steel AISE 304. The stress concentrator was the side notch. Infrared thermography method and the contact heat flux sensor based on the Seebeck effect are used to monitor the dissipated thermal energy. The stress intensity factor was constant during the loading. The plastic zone shape under monotonic uniaxial loading was calculated theoretically.

Thermomechanical response of a TWIP steel during monotonic and non-monotonic uniaxial loading

The tensile properties of a Fe-18%Mn-0.6%C-1.5%Al Twinning-Induced Plasticity (TWIP) steel were investigated at different strain rates in three loading modes, i.e. uniaxial monotonic loading, stress relaxation and loading-unloading-reloading. Infrared thermography was used to investigate the effect of the dynamic strain aging, the strain rate and the temperature on the flow stress. In addition to the standard, i.e., non-isothermal tensile tests, isothermal uniaxial tensile tests were performed at 25°C, 45°C and 65°C. While the non-monotonic loading modes resulted in an increase of the total elongation at a low strain rate of 10−3s−1, no increase was observed for strain rates higher than 6×10−3s−1. The temperature gradients observed during non-isothermal tests were reduced when non-monotonic loading conditions were used. Temperature changes were found to influence the hardening behavior, and consequently the ductility, of the TWIP steel. Deformation twinning also had a significant influence on the results as its kinetics in TWIP steel are determined by the temperature dependence of the stacking fault energy.

Experimental Study on Direct Tensile Behavior of Concrete under Various Loading Regimes

An experimental study of the stress-strain behavior of concrete under monotonic and cyclic uniaxial tensile loading is carried out. Complete monotonic and cyclic stress-strain curves are obtained by six various loading regimes. The uniqueness of envelope curve is validated for the concrete subjected to monotonic and cyclic tensile loading. Residual strain accumulation and the change of initial elastic modulus were used to evaluate damage accumulation and stiffness change with the increasing of cyclic loads. The stress drop versus number of cycles to specified maximum strain is investigated and the result shows that the rate of stress drop decreases as the cycling continues and finally tends to stable. The proposed semi-empirical mathematical expression model to simulate the stress-strain curve for unloading and reloading has been developed by fitting the experimental stress-strain curves. The analytical results show a good agreement with the experimental results for all loading regimes.

Experimental study of damage evolution in cuboid stirrup-confined concrete

This paper presents an experimental study on cuboid stirrup-confined concrete specimens under uniaxial monotonic loading and cyclic loading. The effects of stirrup volume ratio, stirrup yield strength, and concrete strength on damage evolution of the stirrup-confined concrete were investigated. The experimental results showed that the strength and ductility of concrete are improved by appropriate arrangement of the stirrup confinement. Firstly, with the increase of the stirrup volume ratio, the damage evolution of concrete can be relatively restrained. Secondly, higher stirrup yield strength usually leads to larger confining pressures and slower damage evolution. In contrast, higher concrete strength leads to higher brittleness, which accelerates damage evolution. Based on the experimental data, a plastic strain expression is obtained through curve fitting, and a damage evolution equation for stirrup-confined concrete is proposed by introducing a confinement factor (C). The comparisons results demonstrated that the proposed damage evolution curve can accurately describe the experimental results.

Strain intermittency in shape-memory alloys

We study experimentally the intermittent progress of the mechanically induced martensitic transformation in a Cu-Al-Be single crystal through a full-field measurement technique: the grid method. We utilize an in- house, specially designed gravity-based device, wherein a system controlled by water pumps applies a perfectly monotonic uniaxial load through very small force increments. The sample exhibits hysteretic superelastic behavior during the forward and reverse cubic-monoclinic transformation, produced by the evolution of the strain field of the phase microstructures. The in-plane linear strain components are measured on the sample surface during the loading cycle, and we characterize the strain intermittency in a number of ways, showing the emergence of power-law behavior for the strain avalanching over almost six decades of magnitude. We also describe the nonstationarity and the asymmetry observed in the forward versus reverse transformation. The present experimental approach, which allows for the monitoring of the reversible martensitic transformation both locally and globally in the crystal, proves useful and enhances our capabilities in the analysis and possible control of transition-related phenomena in shape-memory alloys.

Investigation on partition of plastic work converted to heat during plastic deformation for reactor steels based on inverse experimental-computational method

Abstract This paper aims at studying the ratio of plastic work converted to heat, β (Taylor and Quinney, 1934) in pressure vessel steels used in VVER-440 energetic reactors by an inverse-experimental and computational analysis. The experiments were performed with different strain rates on a thermal-mechanical testing machine from DSI (model GLEEBLE-3800). Because of the magnitude of strain rate during the experiments it cannot be modelled as an adiabatic process, also the heat transfer during the tests has to be taken into account. Analytical model was carried out to account for the conduction, convection and radiation heat losses that occur during the tests. A thermodynamically based thermo-elasto-plastic model is used to describe both the stress–strain behavior and temperature evolution in these steels under monotonic uniaxial loading. The governing equation system of thermo-elasto-plasticity was implemented in MATLAB software. It is found that β is always smaller for compression than that of tension. The results for 15Ch2MFA (bainitic structure with fine grains) show a rather strong dependence on strain rate, whereas 08Ch18N10T (austenitic structure with coarse grains) is practically independent. The results obtained by the investigation correspond to the literature.

Effect of frequency of free level fluctuations and hold time on the thermal ratcheting behavior

Investigation of cyclic strain accumulation behavior of a thin cylindrical shell (SS 316L) due to thermal ratcheting, in the framework of time independent (Model-1) and dependent formulations (Model-2) is carried out. The effect of frequency of free level fluctuations by varying cycle time (CT) is compared for Model-1 and Model-2. Contribution of strain due to high frequency and low frequency level fluctuations is quantified. Further, the contribution of ratcheting strain with hold time is evaluated to highlight the effect of free level hold on radial deformation of the cylinder. Improvement in predicting ratcheting strain is observed using semi-implicit plasticity integration method. Implicit plastic increment formulation is derived using Newton's method. Validation of code for Model-1 is done by comparing the results with the existing experimental results. Strain controlled cyclic characteristics and uniaxial monotonic loading at different strain rate is analyzed to validate the code for Model-2.

CFRP-confined concrete columns under different environmental conditions

The behaviour of carbon fibre reinforced polymer-confined cylindrical and prismatic concrete specimens subjected to several different environmental conditions was investigated in four groups. The first group of specimens was used as a reference. In the second group, specimens were exposed to dry heating–cooling (H–C) cycles in the environmental test chamber (ETC), whereas specimens in the third group were kept in water while simultaneously being subjected to H–C cycles in the ETC. Finally, the last group of specimens was kept outside of the laboratory to determine the outdoor environmental effect. After the ageing phase, specimens were tested under monotonic uniaxial loading to determine the change in their mechanical properties. Cylindrical specimens were observed not to be significantly affected by H–C cycles with the exception of their ultimate strain, which decreased under outdoor conditions. As for the prismatic specimens, dry H–C cycles were found to reduce strength by up to 24% and strain by up to 33%. Wet H–C cycles and outdoor exposure were seen to decrease further the strength and ultimate strain of prismatic specimens.

Microstructure, mechanical behaviour, damage mechanisms of polypropylene/short hemp fibre composites: Experimental investigations

This article is devoted to the study of mechanical behaviour of a polypropylene reinforced with short hemp fibres available industrially. The main objective of this study is to understand the major physical phenomena to be taken into account to propose, in an upcoming work, a relevant modelling of these composites. Important experimental investigations were performed and revealed quasi-isotropic microstructure at fibres scale in terms of spatial distribution and orientation using X-ray tomography. Influence of hemp fibres on behaviour of polypropylene/hemp composites was examined at controlled temperature upon monotonic and complex path uniaxial tensile loading at different strain rates (10−4, 10−3 and 10−2 s−1) and fibres weight fraction (0, 5, 10 and 20%w/w). Resulting stress–strain curves pointed out a reinforcing effect of hemp fibres at initial stage. Furthermore, the noted high strain rate sensitivity is accounted for quasi-brittle failure. Use of infrared thermography technique highlighted the presence of ‘two breaking modes’ depending on strain rate, i.e. at low strain rate one single damaged zone appeared, on the contrary at higher strain rate many damaged areas occurred in all calibrated zone. In accordance of all conducted experiments, non-linear behaviour of polypropylene/hemp composites is mainly attributed to matrix behaviour.

Compressive behavior of reinforced concrete columns with recycled aggregate under uniaxial loading

The mechanical properties of recycled aggregate concrete (RAC) may be inferior to those of conventional concrete that contain natural aggregate, but they are sufficient nonetheless for some practical applications in terms of mix design, design specifications, etc., based on results found in the literature. The use of recycled aggregate serves to promote the recycling of concrete waste in the construction industry as well as to preserve natural resources and the environment. This study investigates the compressive behavior of reinforced concrete columns that are made from recycled aggregate and are subjected to monotonic uniaxial loading. Seventeen columns of various types, quality and quantity of recycled coarse and fine aggregate are tested. The failure mode, compressive strength, and ductility of the columns are examined. In particular, the maximum compressive strength of the columns is compared to existing equations found in American Concrete Institute (ACI) 318-08 [1]. A comparison of the test results and the calculated strengths for the axial load capacity of the RAC columns using the above approach indicates that RAC columns fulfill the ACI design strength criteria.

3D atomistic simulation of fatigue behavior of a ductile crack in bcc iron loaded in mode II

We present new results of 3D molecular dynamic (MD) simulations of a mechanical response of an edge crack (1¯10) [110] (crack plane/crack front) in a single crystal of bcc iron cyclically loaded in mode II at room temperature and 0K under different boundary conditions. Our previous 3D simulations showed this crack emitted blunting dislocations in the inclined 〈11¯1〉{1¯12} slip systems under monotonic uniaxial tension loading in mode I and was stable up to very high loads. These findings were in qualitative agreement with experiments and continuum predictions. Our current study questions how the cyclic loading in mode II changes crack behavior and its stability. The paper also presents information on comparisons with fatigue behavior of the crack loaded in mode I and mode III.

Effect of Hydrogen on Mechanical Properties of Aluminum AA 6061 Alloy Hardened by Precipitation

ABSTRACTThe present work reports on the effect of hydrogen on mechanical properties of aluminum AA 6061 hardened by precipitation. Test specimens are solution treated in an electric induction furnace at 803 K during 1 hour and then precipitation hardened during 4 hours at 423, 448 and 473 K. The heat treated specimens are subjected to a hydrogen cathodic charging process during 0, 24 and 96 hours and then tested under monotonic uniaxial tensile loading according to ASTM E8-03. Scanning electron microscopy characterization of tensile fracture surfaces reveals that hydrogen charging of precipitation hardened specimens enhances intergranular cracking by microvoid coalescence and intragranular cleavage fracture. The results show that, although hydrogen charging causes embrittlement of precipitation hardened aluminum AA 6061, the effect is a function of time; short time cathodic charging increases the ductility of the alloy.

High-Strain-Induced Deformation Mechanisms in Block–Graft and Multigraft Copolymers

The molecular orientation behavior and structural changes of morphology at high strains for multigraft and block–graft copolymers based on polystyrene (PS) and polyisoprene (PI) were investigated during uniaxial monotonic loading via FT-IR and synchrotron SAXS. Results from FT-IR revealed specific orientations of PS and PI segments depending on molecular architecture and on the morphology, while structural investigations revealed a typical decrease in long-range order with increasing strain. This decrease was interpreted as strain-induced dissolution of the glassy blocks in the soft matrix, which is assumed to affect an additional enthalpic contribution (strain-induced mixing of polymer chains) and stronger retracting forces of the network chains during elongation. Our interpretation is supported by FT-IR measurements showing similar orientation of rubbery and glassy segments up to high strains. It also points to highly deformable PS domains. By synchrotron SAXS, we observed in the neo-Hookean region an approach of glassy domains, while at higher elongations the intensity of the primary reflection peak was significantly decreasing. The latter clearly verifies the assumption that the glassy chains are pulled out from the domains and are partly mixed in the PI matrix. Results obtained by applying models of rubber elasticity to stress–strain and hysteresis data revealed similar correlations between the softening behavior and molecular and morphological parameters. Further, an influence of the network modality was observed (random grafted branches). For sphere forming multigraft copolymers the domain functionality was found to be less important to achieve improved mechanical properties but rather size and distribution of the domains.

Uniaxial nonlinear viscoelastic viscoplastic modeling of polypropylene

This paper presents the application of the Schapery viscoelastic and the Perzyna viscoplastic models to strain recovery data of polypropylene. In a previous study, the recovery of strain after monotonic uniaxial tensile loading was measured to gather information on the viscoelasticity and viscoplasticity. The viscoplastic strains from several load histories were determined and are used to calibrate the viscoplastic model. The parameters of the one-dimensional Schapery model are then found by nonlinear optimization using the strain recovery history. The prediction of stress relaxation and creep behavior is investigated.

3D atomistic simulation of fatigue behaviour of cracked single crystal of bcc iron loaded in mode III

We present new results of 3D molecular dynamic (MD) simulations of a mechanical behaviour of an edge crack ( 1 ¯ 1 0 ) [1 1 0] (crack plane/crack front) in a single crystal of bcc iron cyclically loaded in mode III at room temperature. Our previous 3D molecular dynamic simulations showed that this crack under monotonic uniaxial tension loading in mode I emitted blunting dislocations in the inclined 〈 1 1 ¯ 1 〉 { 1 ¯ 1 2 } slip systems and it was stable up to very high loads, in qualitative agreement with experiments and continuum predictions. Our question is how the cyclic loading in mode III changes the crack behaviour and its stability. The paper also brings information on how the distribution of external forces in mode III influences the fatigue crack behaviour and how the defects generated by the crack can produce experimentally observed advances of the crack front in local modes I, II, I + II and II + III.

Experimental Research on Relationship of Load and Displacement of Universal Testing Machine

The related components of material testing machine (e.g., pressure plate, pressure head, compression bar, etc.) produce distortion in the experiment. It is obviously irrational to use the stroke to replace the specimen displacement, which can’t be measured directly by the extensometer in the destructive test. Therefore, the evolution of specimen displacement, machine displacement and stroke of 45 steel, duralumin alloy and sandstone was analyzed under uniaxial monotonic loading and uniaxial cyclic loading. The results show that the deformation of the related components of universal testing machine, which increases/decreases along with the increase/ decrease of load, can’t be ignored. And there is a good one-to-one correspondence between the machine displacement and the load. It also has nothing to do with the types of specimen materials and the experiments. The relationship of machine displacement and the load can be used to determine the specimen displacement indirectly.

3D atomistic simulation of fatigue behavior of a ductile crack in bcc iron

We present results of 3D molecular dynamic (MD) simulations of the ductile–brittle behavior at an edge ( 1 ¯ 1 0 ) [1 1 0] crack in bcc iron loaded cyclically in mode I at temperature of 300 K. This crack in 3D bcc iron crystal under monotonic uniaxial tension loading in mode I emits dislocations in the inclined 〈1 1 1〉{1 1 2} slip systems and it is stable up to very high loads, in qualitative agreement with experiments and continuum predictions. Our question is how the cyclic loading changes the crack behavior and its stability.

Damage Evolution in Structural Steel at Different Loading Conditions

This paper presents an experimental and numerical characterization of ductile damage evolution in steel subjected to large plastic deformations. The main objective of this research is to better understand damage initiation and evolution in structural steel throughout the deformation process at different strain rates. The proposed study relies on a continuum damage mechanics approach that involves characteristic parameters to describe the accumulation of plastic strain, the damage variable, and the strain rates. The work was divided into experimental, and simulation phases. The experimental phase involved testing under monotonic uniaxial tensile loading under varying strain rates. The obtained material parameters are then used as the basic data in the simulations that are performed afterwards. Finally, this model was implemented as a new user defined material in the finite element analysis software ABAQUS where damage was quantified. Initial results of this research showed that a simple model with substantial cost and time saving can be developed for damage assessment in steel. The rate of loading is a main sensitive parameter that affects both damage initiation and propagation, as they increased significantly with increasing loading rate. Beyond the ultimate load, the strain energy was sufficient to cause the damage to increase without any further applied load.

Internal variable constitutive relations and their application to description of hardening in single crystals

The paper briefly considers the structure of internal variable constitutive relations. The mesoscale model required for determination of macroscale internal variables is taken to be one of the crystal plasticity (Lin's model), in which critical shear stress along slip systems assumes great importance. In this work, evolution equations for critical shear stress that take into account dislocation annihilation and reactions with the formation of Lomer-Cottrell barriers are proposed thus making possible description of the Bauschinger effect and additional hardening under complex loading. The potentialities of the model are demonstrated by numerical simulation of monotonic and cyclic uniaxial loading of polycrystals.

Simulation of perforation and penetration in metal matrix composite materials using coupled viscoplastic damage model

In the first part of the two companion papers, theoretical formulation of the multiscale micromechanical constitutive model that couples the anisotropic damage mechanism with the viscoplastic deformation is presented. In the second part of these companion papers the numerical simulation of the computational aspects of the theory are elaborated. The perforation and penetration problem of metal matrix composites (MMCs) due to high impact loading is simulated. In this sense, the computational aspects of the developed theory are elaborated here. First, the verification of the developed model is performed through its numerical implementation in order to test the model predictions of the material characteristic tests. This encompasses uniaxial monotonic loading and unloading under different strain rates, uniaxial cyclic loading, and uniaxial loading and relaxation. The verified material routine of the developed model is then implemented in the explicit finite element code ABAQUS via the user defined subroutine VUMAT at each integration point in order to analyze the projectile impact and penetration into laminated composite plates.