Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.     

Stress-relaxation in bending of Inconel 718, at 773 and 823 K

Stress-relaxation data in Inconel 718, both at 773 and 823 K and up to times of the order of 4000 h, in specimens with different thermal treatments, are presented. No substantial differences are encountered, between 773 and 823 K, in the stress-relaxation behaviour of the age-hardened material. In the as-received and the solution-annealed specimens, on the contrary, the stress-relaxation curves indicate that the structure does not remain stable during relaxation and the internal stress decreases continuously as the applied stress decreases. The structure is stable, for all the thermal treatments, during stress relaxation at 773 K. Finally, the data are interpreted in terms of an interaction between gliding dislocations and precipitates as indicated by the large activation volumes involved and the negligible influence of thermal activation.     

Some factors exerting an influence on the coaxing effect of austenitic stainless steels

The present paper describes some factors exerting an influence on the coaxing effect of austenitic stainless steels. Particularly, the influence of prestrain was investigated in detail. The materials used were austenitic stainless steels, type 304 and 316. Type 304N2 was also used to examine the properties of the stabilized austenitic phase in type 304. Two types of rotating bending fatigue tests, i.e. the conventional constant amplitude tests and stress‐incremental tests, were performed using the specimens subjected to the several tensile‐prestrain levels. Under the constant amplitude tests, the fatigue strengths of type 304 and 316 increased with increasing prestrain. Under the stress‐incremental tests, type 304 showed a remarkable coaxing effect, where the fatigue failure stress significantly increased regardless of the prestrain level. The coaxing effect in the unprestrained specimens was larger than those of the prestrained ones. Type 304N2 showed lower coaxing effect than type 304. In addition, the strain‐induced martensitic transformation did not occur because of the higher stability of austenitic phase in type 304N2. In type 316, the coaxing effect was dependent on the prestrain level, i.e. below 15% prestrain the coaxing effect became smaller with increasing prestrain, whereas above 25% prestrain the coaxing effect reappeared. Based on the tests results, it was considered that the coaxing effect in austenitic stainless steel was due to the mechanisms such as work hardening, strain ageing and strain‐induced martensitic transformation. The contribution of these mechanisms to the coaxing effect was different among type 304, 304N2 and 316.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 2—Microstructural study

Abstract

The morphologies and characteristics of microstructure, including dislocations, mechanical twins and α' martensite, in 304L stainless steel deformed under various strain, strain rate range from 10² to 5 × 10³ s^-1 for different prestrain levels at room temperature were examined by a split Hopkinson bar and TEM. The evolution of microstructure correlated with dynamic mechanical behaviour are presented and discussed in terms of prestrain and applied strain rate. The results show that characteristics of dislocations, mechanical twins and α' martensite varied with prestrains, strains and strain rates. They dominate the strengthening effects on the 304L stainless steel. Dislocation cell structures can be observed in all tested specimens. At larger prestrain under dynamic loading, the formation of elongated dislocation cells becomes evident. The presence of elongated dislocation cells leads to different work hardening behaviour. Twinning occurred at all testing conditions except for the 0·15 prestrain specimen deformed at 0·1 strain and 8 × 10² s^-1 strain rate. The formations of α' martensites were found to be confined to the microshear bands and were barriers of dislocation movement. As the heavy loading is imposed, irregular and blocky α' martensites were observed. Quantitative measurement revealed that dislocation and twin density, as well as the volume fraction of α' martensite increase with the prestrain, strain and applied strain rate, but a decay of twin density occurred as the prestrain of 0·5 is applied. These microstructrual changes can be related to the different work hardening stress (σσ_y and strengthening nature. The observed strengthening effect resulted from the dislocation multiplication, twin formation and α' martensite seems to reflect an enhancement of hardness. However, the increased hardness is less sensitive to the twin formation.     

Effects of morphology and strength of martensite on cyclic deformation behaviour in dual-phase steels

Abstract

Tension–compression cyclic deformation behaviour in dual-phase steels has been studied. Three different ferrite (α)–martensite (α′) microstructures, i.e. isolated α′-colonies dispersed in α-matrix (I), continuous α′ (C), and laminated α–α′ (L), were prepared by appropriate heat treatments, keeping the α′ volume fraction at ~0·3. The work hardening and the Bauschinger effect are found to be greater in microstructure C or L than in I when they are compared at an arbitrary forward (tension) prestrain level. An increase in the hardness of α′ enhances the Bauschinger effect and then narrows the stress–strain hysteresis loop. The stress evolved as a result of the Bauschinger stress (defined as the difference between forward prestress and backward (compression) 0·1% proof stress) is found to be almost independent of microstructure and hardness when it is compared at an arbitrarily fixed prestress level. The slip lines are very fine and relatively straight in microstructure C, but wavy in microstructure I. These findings are discussed from the standpoints of the accumulation of the average internal stress resulting from inhomogeneous plastic flow between two constituent phases and the plastic relaxation.

MST/382     

Microstructural evolution and threshold stress for superplastic flow in the Zn-Al eutectoid

The microstructural evolution and the stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy were investigated by prestraining specimens at two strain rates corresponding to Regions I and II. Even though the scale of microstructure was similar, the stress-strain rate curves of differently prestrained specimens were distinctly different in the lower strain-rate regime. While Region I of low rate sensitivity was more prominent when prestrained at a lower strain rate of Region I, it was less distinct because of prestrain in Region II. The threshold stress for superplastic flow, as assessed by an extrapolation procedure, varied with the nature of prestrain. The interphase boundaries were more rounded (higher mean curvature) on prestraining on Region II, compared to Region I. The correlation between the changes in the mean curvature of phase boundaries and the threshold stress arising from the nature of prestrain was consistent with the boundary-migration controlled sliding mechanism to interpret the threshold stress for superplastic flow.     

On specimen design for size effect evaluation in ultrasonic gigacycle fatigue testing

Literature datasets showed that gigacycle fatigue properties of materials may be affected by the specimen risk‐volume, i.e., the part of the specimen subjected to applied stress amplitudes above a prescribed percentage of the maximum applied stress amplitude. The paper proposes a Gaussian specimen shape able to attain large risk‐volumes for gigacycle fatigue tests, together with a general procedure for its design: wave propagation equations are analytically solved in order to obtain a specimen shape characterised by a uniform stress distribution on an extended length and, as a consequence, by a larger risk‐volume. The uniformity of the stress distribution in the Gaussian specimen is numerically verified through a finite element analysis and experimentally validated by means of strain gauge measurements.     

Numerical simulation of prestrain history effect on ductile crack growth in mismatched welded joints

Pipe reeling may lead to plastic pre‐deformation (prestrain) around existing cracks in components; therefore, investigating whether this process accelerates or counteracts ductile crack growth, especially for strength mismatched welded joints, is warranted. This study focused on the effect of prestrain history on ductile crack growth in mismatched welded joints. A single‐edge‐notched tension specimen was selected for numerical study, and the crack was assumed to have existed before a prestrain history was applied. Crack growth resistance curves for plane strain and mode I crack growth under large‐scale yielding conditions have been computed using the complete Gurson model. Meanwhile, symmetrical and non‐symmetrical prestrain cycle modes with different loading levels were applied to the overmatched specimens. The outcome demonstrated that the mismatch ratio (the ratio between the yield stress of the weld metal and base metal) showed a significant effect on fracture resistance regardless of the stage at which the prestrain cycle loading was located. By contrast, the processing of the crack growth was weakened by the increase of prestrain values, and the symmetrical prestrain cycle resulted in greater plastic damage than the non‐symmetrical prestrain cycle did. However, the initial crack length had a non‐significant effect on the ductile fracture considering the prestrain and mismatch effects.     

Investigation of Non‐Linear Springback for High Strength Steel Sheets by ESPI

Abstract: The springback of the sheet metals after large deformations during deep drawing is not a strongly linear process with a constant Young’s modulus, but the stress–strain behaviour during the unloading phases shows considerably non‐linear and inelastic effects. Unloading of two types of steel sheets for cold forming, a cold‐rolled high‐strength microalloyed steel and a low‐carbon steel sheet, has been analysed using the method of electronic speckle pattern interferometry (ESPI). The specimens were investigated by uniaxial tension tests, and the influences of different testing parameters upon springback were analysed. The experimental measurements showed that the stress–strain curve during unloading is non‐linear, the influence of the prestrain path upon unloading is minor, and the secant moduli of unloading curves decrease with increasing prestrain. When the prestrain value becomes high enough, a saturated value for the secant modulus is approached. An empirical relation was found to describe the changes in the unloading modulus in accordance with the prestrain value.     

Effect of mean stress on residual stress relaxation in steel specimens

Abstract

Residual stress relaxation was investigated by subjecting specimens with various mean stress levels to strain controlled cyclic loading. The material studied was mild steel in three different conditions. The mean stress levels ranged from 100 to 200 MPa, and two strain amplitudes, 0·05 and 0·06%, were studied in detail. The residual stresses in the specimens were measured before and after mean stress relaxation experiments. It was found that experimental factors such as temperature variations and crack growth have a significant influence on the results. Based on the experimental results, it is proposed that the mean stress relaxation exponent should be divided into two parts: mean stress dependent and mean stress independent. The first includes the contribution of quasi-static relaxation, i.e. mean stress dependent plastic deformation. The second part includes the contribution of cycle dependent mean stress relaxation, which does not depend on the mean stress.     

Plasticity of columnar-grained CoSi2 with the C1 structure

Columnar-grained CoSi₂ crystals with the C1 structure (CaF₂ type) have been deformed in compression in vacuum at high temperatures up to 1400 K. The yield stress increases steeply with decreasing temperature, and the fracture precedes yielding below 700 K. Above 900 K, specimens can be compressed to a strain above 80% without fracture. From the strain-rate sensitivity determined by the stress relaxation test and the temperature dependence of the yield stress, the activation volume and the activation enthalpy of plastic deformation have been analysed. The activation volumes at high stresses (^*>100MNm^–2) are less than 10b ³ (whereb is the magnitude of the Burgers vector), indicating that the deformation is controlled by the Peierls mechanism. The total activation enthalpy is about 3 eV. The possibility of the dissociation of 1/2110 dislocations is proposed.     

Effect of prestrain on tensile properties and ratcheting behaviour of Ti-stabilised interstitial free steel

Effect of prestrain ranging between 2.5 and 15 percent on tensile properties, and ratcheting behaviour of an interstitial free steel has been studied at two different stress combinations. It is found that while yield strength increases in two distinctly different stages, the increase of tensile strength follows perfect linear relationship with increase in the amount of prestrain. The ratcheting strain accumulation direction during initial stage of asymmetric cyclic loading at constant tensile mean stress depends upon imposed maximum stress and the amount of prestrain. Number of cycles for accumulation of 16.30 pct true ratcheting strain increases with the amount of prestrain following perfect exponential relationships for both the stress combinations; but it increases in a perfectly bilinear manner with tensile yield strength of prestrained specimens. With 16.30 pct accumulated ratcheting strain the amount of back stress is found as 110 MPa irrespective of the amount of prestrain. Marginal variation in post-ratcheting tensile properties as a function of tensile prestrain has been observed.     

Viscoelastic properties of poly(butylene terephthalate)/poly(ethylene naphthalate) blends

Melt blends of poly(butylene terephalate) (PBT) and poly(ethylene naphthalate) (PEN) with 30 and 60 wt% PEN were prepared using a single screw extruder and an injection moulding machine. Stress relaxation tests for the specimens of PBT/PEN blends and the homopolymers were carried out using an Instron testing machine in an Instron environmental chamber. The Taguchi method of experimental design analysed how different levels of temperature, PEN content and initial stress affected the relaxation behaviour of PBT/PEN blends and homopolymers. From the response tables and analyses of main and interaction effects, it was shown that the most significant factor was temperature, followed by PEN content and then the initial stress. Consequently, high temperature, low PEN content and high initial stress speeded up stress relaxation rate of specimens. Interaction effects between factors were insignificant. To fit the relaxation curves of the PBT/PEN blends and the homopolymers at different temperatures, PEN contents and the initial stresses, four different equations were attempted with Matlab™, which determined the coefficients of these functions using the experimental data of stress change with time. The simulated curves from the most suitable function among them were shown using the calculated coefficients to predict the relaxation behaviour of PBT/PEN blends (50% PEN) at temperatures of 30 and 60°C with an initial stress of 7 MPa.     

Influence of temperature and stress-relieving treatment of the stress relaxation in bending of Zircaloy-4 near 673 K

Stress relaxation data in bending and at 633 and 673 K, for stress-relieved, cold-worked and annealed Zircaloy-4, are reported. The data can be described by a creep model that involves jog-drag and cell-formation and the ratio of cell diameter to dislocation spacing, obtained from the stress relaxation curves, is shown to be dependent on the thermomechanical treatment given to the specimens, prior to the stress relaxation tests. Finally, the valuesH _v87 kJ mol^–1 andD ₀3.3×10^–15m²sec^–1, for the activation enthalpy and the pre-exponential factor for self-diffusion, respectively, were obtained from the stress relaxation curves measured at the two temperatures.     

Stress Relaxation of Nonlinear Thermoviscoelastic Materials Predicted from Known Creep

A method to predict the stress relaxation response of nonlinear thermoviscoelastic materials from known creep data is presented. For given nonlinear creep properties, and creep compliance represented by the Prony series, it is shown that the Schapery creep model can be transformed into a set of first order nonlinear differential equations. By solving these equations the nonlinear stress relaxation curves for different strain and temperature levels are established. The strain/temperature-dependent constitutive equation can then be constructed for any nonlinear thermoviscoelastic model, as needed for engineering applications. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of dynamic strain aging on resistance to strain reversal as assessed through the Bauschinger effect

The Bauschinger effect of three commercially produced medium carbon bar steels representing different microstructural classes with similar tensile strengths and substantially different yielding and work-hardening behaviors at low-strain was evaluated at room temperature and in situ at temperatures up to 361 °C. The influence of deformation at dynamic strain aging temperatures as a means to produce a more stable dislocation structure was evaluated by measuring the resistance to strain reversal during in situ Bauschinger effect tests. It was shown that the three medium carbon steels exhibited substantial increases in strength at dynamic strain aging temperatures with the peak in flow stress occurring at a test temperature of 260 °C for an engineering strain rate of 10⁻⁴ s⁻¹. Compressive flow stress data following tensile plastic prestrain levels of 0.01, 0.02 and 0.03 increased with an increase in temperature to a range between 260 °C and 309 °C, the temperature range where dynamic strain aging was shown to be most effective. The increased resistance to flow on strain reversal at elevated temperature was attributed to the generation of more stable dislocation structures during prestrain. It is suggested that Bauschinger effect measurements can be used to assess the potential performance of materials in fatigue loading conditions and to identify temperature ranges for processing in applications that utilize non-uniform plastic deformation (e.g. shot peening, deep rolling, etc.) to induce controlled residual stress fields stabilized by the processing at temperatures where dynamic strain aging is active.     

Characterization of thermally activated dislocation mechanisms using transient tests

Two methods of repeated transient tests, namely relaxation and creep, are shown to be ideal techniques for the characterization of plastic deformation processes. They yield similar information about a microscopic activation volume, which is the signature of the operating dislocation mobility mechanism. Microstructural parameters are also obtained, the values of which are different in creep and stress relaxation. They characterize work-hardening during the transient and dislocation exhaustion rates respectively. The equations describing the transients and the assumptions involved are presented. Experimental results on Ni₃Al polycrystals illustrate the possibilities of both tests and support the above assumptions. In particular, crystals which work-harden exhibit high dislocation exhaustion-rates, as shown by comparison of Ni₃Al with TiAl, Ge and Cu. The respective contributions to the strain-rate of the mobile dislocation densities and velocities can also be estimated.     

Effect of prestrain paths on mechanical behavior of dual phase sheet steel

The tensile and fatigue performance of dual phase (DP600) sheet steel was investigated with specimens, as-received, and with two different prestrain path conditions, uniaxial and plane strain. First, tensile tests of the as-received condition of DP600 were performed to obtain mechanical properties, specifically the uniform elongation, for determining the prestrain levels of the specimens. Then three prestrain levels from each strain path were applied onto the sheet steel. Tensile and fatigue specimens were prepared from the prestrained coupons. Mechanical properties were obtained from the uniaxial tests of the as-received and prestrained specimens for comparison. Fatigue testing was also conducted with strain controlled to acquire fatigue properties. The fatigue life curves were plotted as a function of strain range and Neuber factor. The uniaxially prestrained specimens exhibited higher fatigue strength than that of the as-received ones for the long life region, but the opposite effect was observed for the short life region of less than 10⁴ reversals.     

Post-forming monotonic and cyclic behavior in a HSLA steel sheet after large deformations by in-plane compression

The effects of large prestrains (18–40%), produced by in-plane compression, on the asymmetry and the anisotropy of the stress response and on the fatigue life are investigated under fully reversed axial strain for a 345 MPa yield strength V–N high strength low alloy steel sheet. After prestraining, the hysteresis loops are asymmetric and the stress response is anisotropic, i.e., the response differs in directions parallel and perpendicular to that of the compressive prestrain. To understand the cyclic flow stress asymmetry, monotonic tension and compression tests were conducted in these two directions after prestraining. It is shown that the loop asymmetry is related to the Bauschinger effect after prestraining. Two cyclic stress strain curves, one corresponding to the tension side of the hysteresis loops and the other to the compression side, are defined to accurately describe the post-prestraining behavior. The amount of strengthening gained by prestraining is partially retained after cycling. Prestraining increases the fatigue life at low strain amplitudes but decreases it at high strain amplitudes.     

Electrically powered repeatable air explosions using microtubular graphene assemblies

Controllable rapid expansion and activation of gases is important for a variety of applications, including combustion engines, thrusters, actuators, catalysis, and sensors. Typically, the activation of macroscopic gas volumes is based on ultra-fast chemical reactions, which require fuel and are irreversible. An “electrically powered explosion”, i.e., the rapid increase in temperature of a macroscopic relevant gas volume induced by an electrical power pulse, is a feasible repeatable and clean alternative, providing adaptable non-chemical power on demand. Till now, the fundamental problem was to find an efficient transducer material that converts electrical energy into an immediate temperature increase of a sufficient gas volume. To overcome these limitations, we developed electrically powered repeatable air explosions (EPRAE) based on free-standing graphene layers of nanoscale thickness in the form of microtubes that are interconnected to a macroscopic framework. These low-density and highly permeable graphene foams are characterized by heat capacities comparable to air. The EPRAE process facilitates cyclic heating of cm³-sized air volumes to several 100 °C for more than 100,000 cycles, heating rates beyond 300,000 K s⁻¹ and repetition rates of several Hz. It enables pneumatic actuators with the highest observed output power densities (>40 kW kg⁻¹) and strains ∼100%, as well as tunable microfluidic pumps, gas flowmeters, thermophones, and micro-thrusters.