Imposed Strain Level Research Articles

Investigations of dislocation-type evolution and strain hardening during mechanical twinning in Fe-22Mn-0.6C twinning-induced plasticity steel

In this study, for the first time, the evolution of geometrically necessary dislocation (GND) and statistically stored dislocation (SSD) densities, as well as their roles in strain hardening during mechanical twinning, was experimentally investigated in a tensile-deformed Fe-22Mn-0.6C twinning-induced plasticity (TWIP) steel. GND and SSD densities were estimated via EBSD-acquired orientation data and a modified strain hardening model, respectively. The analysis demonstrates that the GND density increases non-linearly due to mechanical twinning. The SSD density increases much faster than the GND density, which shows that multiplication of the SSDs is heavily dependent on the imposed strain level. It is revealed that the GND density is higher at early strain stages (below 0.14 true strain), dominating dislocation hardening, but thereafter the SSD density contributes more. It is also found that the GND density is several times higher in this TWIP steel than in metals or alloys, which deform through dislocation slip only. We attribute this difference to the planar slip of dislocations and the occurrence of mechanical twinning, which leads to much more pile-ups of the GNDs at/near boundaries. Mechanical twinning directly contributes less than 100 MPa to flow stress increment in the studied true strain range of 0 to 0.34. Consequently, depending on dislocation types, dislocation multiplication governs strain hardening at all deformation ranges. The findings provide insight into the evolution behaviors of GNDs and SSDs in TWIP steels, which are particularly important for further understanding of the dynamic Hall-Petch effect and useful for TWIP alloy design efforts.

Effect of strain level on strength evaluation of date palm fiber-reinforced sand

Conventional researches on the behavior of fiber-reinforced and unreinforced soils often investigated the failure point. In this study, a concept is proposed in the comparison of the fiber-reinforced with unreinforced sand, by estimating the strength and strength ratio at different levels of strain. A comprehensive program of laboratory drained triaxial compression test was performed on compacted sand specimens, with and without date palm fiber. The fiber inclusion used in triaxial test specimens was form 0.25%-1.0% of the sand dry weight. The effect of the fiber inclusion and confining pressure at 0.5%, 1.0%, 1.5%, 3.0%, 6.0%, 9.0%, 12%, and 15% of the imposed strain levels on the specimen were considered and described. The results showed that, the trend and magnitude of the strength ratio is different for various strain levels. It also implies that, using failure strength from peak point or the strength corresponding to the axial strain of approximately 15% for evaluating the enhancement of strength or strength ratio, due to the reinforcement, may cause hazard and uncertainty in practical design. Therefore, it is necessary to consider the strength of fiber-reinforced specimen at the imposed strain level, compared to the unreinforced specimen.

Fatigue Analysis of A356-TiB2 (5wt%) in-situ Nano composites

Abstract In the present work, Low-cycle fatigue (LCF) tests were performed on A356 Aluminium alloy and A356-TiB2(5wt%) in-situ nano-composites to estimate their fatigue performance. The tests were performed by strain controlled with a strain ratio of Re = -1. The cyclic stress–strain curves were determined for each imposed strain levels. The LCF and their characterization in the cyclic plastic response of both the alloy and composites were analysed. It is interesting to note that the type of cyclic deformation behaviour in Al-Mg-Si alloys seems to be influenced by the dispersed phases and TiB2 particles. The LCF values for the A356-TiB2(5wt%) composites were found to be higher than those of the A356 alloy. The separation fatigue life’s of the A356-TiB2(5wt%) in-situ nano-composite and A356 alloy was found to be 15652 and 12350 cycles respectively. This is attributed due to the presence of minute TiB2 particles which obviously affected the microstructure, hardness, tensile strength and fatigue life of the composites compared to the base alloy. It is further noted that the LCF values of the present composites were found to be higher than those of the alloys reported in the literature.

An investigation of the strain dependence of dynamic precipitation in an Al-Zn-Mg-Cu alloy

The use of high strength Al-Zn-Mg-Cu alloys in automotive components is restricted by their low formability. Warm forming in the under-aged state improves formability, but induces a strain-dependent microstructure evolution due to dynamic precipitation. We present a new methodology to quantitatively evaluate the dynamic precipitation strain dependence which is applicable to strain rates relevant to forming. The plastic strain is varied using a tapered tensile sample and the precipitation state is measured in terms of size and volume fraction using spatially resolved small-angle X-ray scattering, showing its evolution during straining is dependent on the imposed strain level and rate.

Kinetics of the deformation induced memory effect in polyamide-6

Nascent polyamide-6 shows a peculiar and irreversible effect; the quiescent crystallization kinetics on cooling are accelerated upon deformation in the melt, even after full relaxation of the melt. This phenomenon, known as the orientation (or better, deformation) induced memory effect of polyamide materials, is explored in a step-by-step manner using in-situ wide-angle X-ray diffraction. For this purpose, unique polyamide-6 samples were used, which were created by compression moulding the virgin pellets just below the average melting temperature, defined by the peak position in a DSC-curve. The experimental data shows that only the imposed strain level controls the irreversible change in the quiescent crystallization kinetics. During cooling of the undeformed melt, an unusual crystallization behaviour is observed, i.e. the stable α-phase converges into the less stable β-mesophase. Possible causes for the memory-effect are discussed and related to hypotheses from the literature.

Influence of cryorolling on microstructure and mechanical properties of a cast hypoeutectic Al–Si alloy

Cast Al–Si alloys often show poor tensile strength, less ductility and poor fatigue performance due to cast microstructure and presence of porosity. An attempt has been made to improve mechanical properties of a cast hypoeutectic Al–Si alloy (A356) by cryorolling. As cast Al–Si alloy (A356) processed by cryorolling at different imposed strain levels was subjected to microstructural analysis and mechanical testing. Cryorolling resulted in break up and uniform distribution of coarse acicular eutectic particles in the aluminium matrix, elimination of casting porosity, microstructural refinement and significant accumulation of dislocation density. These microstructural changes resulted in significant improvement in the strength (248%) and ductility (37%). The influence of cryorolling on mechanism of microstructural modification, strengthening mechanism and failure mechanism are studied in detail.

Microstructural, compositional and mechanical investigation of Shielded Metal Arc Welding (SMAW) welded superaustenitic UNS N08028 (Alloy 28) stainless steel

Shielded Metal Arc Welding (SMAW) was performed on UNS N08028 (Alloy 28) superaustenitic stainless steel sheets. In the present work, the microstructure and the mechanical properties of base metal (BM), weld metal (WM), and welded joint (WJ) are investigated. Optical micrographs show that the base metal presents austenitic grains, and the weld metal exhibits a fully austenitic dendritic structure, confirming the Schaeffler diagram estimations. Microhardness measurements indicate that the hardness increases in the weld bead due to the rapid cooling and thermal cycle during welding procedure. The measured mechanical properties and the analysis of the fracture profiles show that the two materials are ductile but the ductility is less pronounced in the weld metal. Consistently the yield stress, the plastic strength and the impact toughness are lower than in the base metal. In addition, the BM presents a higher cyclic hardening and plastic strain compared to those of WM. Cyclic stress–strain hysteresis loops show that WM and WJ have almost the same cyclic behavior and especially at high imposed strain levels.

A feasibility study of strain hardening fiber reinforced fly ash-based geopolymer composites

Fly ash-based geopolymer has been intensively studied as a promising alternative to ordinary cement materials. While geopolymer concrete has good strength and excellent material greenness, applications have been limited to niche or small scale applications. In order to use geopolymer for large scale structural applications, the inherent brittleness should be addressed. In this study, strain-hardening ductile fiber reinforced geopolymer composites were developed by using randomly oriented short Poly-Vinyl Alcohol (PVA) fibers. Subsequently, their mechanical properties were investigated by cube compressive and dogbone tensile testing. Tensile strain hardening behavior with very high ductility of over 4% was experimentally demonstrated for the developed composites. These performances were found to be further improved by utilizing temperature curing methods. Crack width distributions were also investigated by using the Digital Image Correlation technique. The analysis indicated that the maximum and average crack widths are 117μm and 45μm, respectively, even at a high imposed strain level of 4.5%. Therefore, the feasibility of strain-hardening ductile geopolymer composites was established.

Loading frequency effect on stiffness, damping and cyclic strength of modeled rockfill materials

This paper studies the loading frequency and waveform effects on shear modulus ( G) and damping ratio ( D) of three high compacted modeled rockfill materials by conducting large-scale triaxial testing. The laboratory test results have shown that the shear modulus, and especially the damping ratio behavior, is influenced by the frequency of loading. The excess pore water pressures generated in cyclic loading are less dependent on loading frequency, and most dependent on imposed strain level. The G and D values for sinusoidal waveform are slightly higher than those for the triangle waveform. Moreover, the D value of rectangle waveform is considerably higher than sinusoidal and triangle waveforms. Results indicated that when induced double amplitude axial strain exceeds 0.1%, the effect of the number of cycles in degradation of cyclic strength is significant. A failure criterion for 100% pore pressure ratio for the dense rockfill materials (i.e. initial liquefaction) is developed based on 2% double amplitude axial strain in 15 cycles of loading.

Viscoelasticity of aspen wood strands during hot pressing: Experimentation and modeling

Abstract To improve the fundamental understanding of wood composite consolidation, the viscoelastic behavior of aspen wood strands is experimentally investigated for a range of temperatures (20–200°C) and moisture contents (0–15%). The results show that the strand stress relaxation modulus and time follows a linear relationship in a log-log plot. The strand stress relaxation rate is highly dependent on the imposed strain levels and the environmental conditions. A model for predicting the stress relaxation of wood strands is developed and compared with the experimental results. It will be useful to further predict the stress relaxation response of strand-based wood composite mats during hot pressing.

Analysis of low cycle fatigue in AlMgSi aluminium alloys

In this study low-cycle fatigue tests were performed in two AlMgSi aluminium alloys with different chemical composition, namely 6082-T6 and 6060-T6 alloys, using standard round specimens and tube specimens, respectively. The tests were undertaken in strain control with a strain ratio R ε =−1. The cyclic stress–strain curves were determined using one specimen for each imposed strain level. The low-cycle fatigue results are used for the characterisation of the cyclic plastic response and the fatigue live of the alloys. Moreover, the geometry of the hysteresis loops and the occurrence of Masing behaviour are also analysed. The observed behaviour is discussed in terms of the chemical composition of the alloys (Mg 2Si hardening particles and Mn dispersoid content) and fracture mechanisms. Alloy 6060-T6 exhibits nearly ideal Masing behaviour, while alloy 6082-T6 presents significant deviations from the Masing model. The type of cyclic deformation behaviour in AlMgSi alloys seems to be influenced by the dispersoid phase.

Influence of prestraining and ageing on the fatigue properties of a dual phase sheet steel with tensile strength of 410 MPa: Fredriksson, K., Melander, A. and Hedman, M. Scand. J. Metall. 1989 18, (4), 155–165

In this study, for the first time, the evolution of geometrically necessary dislocation (GND) and statistically stored dislocation (SSD) densities, as well as their roles in strain hardening during mechanical twinning, was experimentally investigated in a tensile-deformed Fe-22Mn-0.6C twinning-induced plasticity (TWIP) steel. GND and SSD densities were estimated via EBSD-acquired orientation data and a modified strain hardening model, respectively. The analysis demonstrates that the GND density increases non-linearly due to mechanical twinning. The SSD density increases much faster than the GND density, which shows that multiplication of the SSDs is heavily dependent on the imposed strain level. It is revealed that the GND density is higher at early strain stages (below 0.14 true strain), dominating dislocation hardening, but thereafter the SSD density contributes more. It is also found that the GND density is several times higher in this TWIP steel than in metals or alloys, which deform through dislocation slip only. We attribute this difference to the planar slip of dislocations and the occurrence of mechanical twinning, which leads to much more pile-ups of the GNDs at/near boundaries. Mechanical twinning directly contributes less than 100 MPa to flow stress increment in the studied true strain range of 0 to 0.34. Consequently, depending on dislocation types, dislocation multiplication governs strain hardening at all deformation ranges. The findings provide insight into the evolution behaviors of GNDs and SSDs in TWIP steels, which are particularly important for further understanding of the dynamic Hall-Petch effect and useful for TWIP alloy design efforts.

Stress Relaxation Characteristics of Saturated Clays

A series of undrained triaxial stress relaxation tests on four saturated remoulded clays consolidated under isotropic stress condition were performed to investigate the influence of imposed strain level and strain rate prior to the start of relaxation test on the principal stress difference and pore pressure versus elapsed time relationships. Experimental results show that the principal stress difference versus logarithm of time relationship is approximated by the two straight lines independent of strain level, strain rate and mode of stress application in the preceding undrained shear process, and that the stress state corresponding to the point of intersection of the two straight lines locates close to the effective stress path calculated by the Cam-clay model proposed by Roscoe et al. (1963). A sign that the principal stress difference finally approaches to a limiting equilibrium value is found from the test results. It is also shown that the stress points corresponding to the equilibrium state lie below the Cam-clay effective stress path and the curve linking these stress points makes a similar shape of the path. The slope of the initial straight line on the principal stress difference versus logarithm of time plot depends on the magnitude of strain imposed in the preceding undrained shear process. Plastic component of the change of pore water pressure, which was defined by Lo (1969), during stress relaxation test depends not only on the strain level but also on the strain rate prior to the test.

Effect of imposed strain and moisture content on some viscoelastic characteristics of cowpeas ( Vigna unguiculata)

Cowpeas ( Vigna unguiculata) were equilibrated at relative humidities of 33, 57, 64·5, 67, 75, 79 and 86% over saturated salt solutions at 25°C. Uniaxial compressive loading and relaxation tests were done on a Universal testing machine at these relative humidities. The modified Hertz equation was employed to calculate the modulus of elasticity and the viscoelastic parameters. The modulus of elasticity of cowpeas was found to be independent of the imposed strain levels up to 2%. It was also found that the viscoelastic behaviour of the cowpea samples tested can be represented by two Maxwell models in parallel.

Relationship of strain hardening and damage in ductile fracture

Abstract The majority of theories of ductile fracture consider the values of the imposed strains needed to initiate and grow voids. However, in describing the microstructural damage resulting from the production and growth of voids, it is important to consider the spatial distribution of the damage in relation to the microstructure. It is shown that the process of void initiation is dominated by the location and size of second-phase particles rather than the average imposed strain level. In the growth stage the use of recovery anneal cycles indicates that the accumulation of microstructural damage depends not only on the local level of the flow stress and the imposed strain rate, but on the work-hardening rate of the matrix. If the work-hardening capacity can be restored it is possible to coax the material to much larger strains prior to failure.

Crack Propagation in Double-Base Propellants

Crack propagation tests were conducted on a composite modified double-base (CMDB) propellant with the use of center-cracke d strip biaxial specimens. Constant strain rate tests were conducted at several temperatures (40-105°F) and crosshead rates (0.02-200 in./min) to define the crack initiation and propagation characteristics for monotonically increasing strain history. The tests were conducted at ambient, 250, and 500 psig pressure to evaluate the effect of pressure on initiation and crack velocity. A second series of tests was conducted to evaluate the effect of a prestrain damage history on crack propagation. In the second series, the samples (without precut cracks) were initially prestrained to 15-25% and held for a period of time to induce material damage. After load release and sufficient recovery time, cracks were inserted in the specimens and they were then pulled to failure at a constant strain rate. Similar tests were conducted on round, notched tensile samples to define the critical stress intensity factor (Klc) and to provide a comparison between uniaxial and biaxial fracture initiation. Schapery's viscoelastic fracture theory was used to evaluate the crack velocity data under constant strain rate conditions. One important result of the study was the finding that the crack velocity depended rather strongly on imposed strain level. I. Introduction C RACK initiation and propagation in polymeric materials has been given considerable attention during the last 15 years, stimulated primarily by their unique viscoelastic behavior and also by the use of polymerics as solid propellant rocket fuels. Filled polymers are used extensively as solidpropellant grains which must undergo a variety of environmental loading conditions, during motor storage and handling and during actual operational firing conditions, which impose pressure loads at high rates and for relatively long times. The consequence of a crack can be the failure of the rocket motor as a result of overpressuriz ation, case burnthrough, erratic pressure-time response, or a variety of events related to structural or ballistic performance failure. The primary work over the last decade has been aimed at defining the laws which govern crack initiation, propagation, and trajectory under motorlike operational conditions to arrive at an assessment of overall crack criticality for a given rocket motor system. A number of investigators have studied viscoelastic crack propagation in polymers and solid propellants. l~l°