Thermo-mechanical characterisation of AA 6056-T4 and estimation of its material properties using Genetic Algorithm

This paper presents an experimental investigation of thermo-mechanical material properties of AA 6056-T4, which is used extensively in aeronautic applications. Monotonic tensile tests have been carried out on the dog-bone type specimens at temperatures ranging from room temperature (16 °C) to high temperature (450 °C) with two different strain rates; viz. high strain rate (∼0.002 s⁻¹) and low strain rate (∼0.0002 s⁻¹). Specimens were heated with the help of Joule heating system using Gleeble® 3500 machine at a temperature rate of 25 °C/s. Material properties which were investigated include the Young’s modulus, yield strength at 0.1% plastic strain and hardening modulus.     

Effect of temperature and hold time on internal hardening behavior of a near α titanium alloy under cyclic deformation

In the present work, the study of dynamic strain aging (DSA) in near α titanium alloy Timetal 834 is reported in terms of internal hardening variables (kinematic and isotropic hardening variable). Total strain controlled low cycle fatigue tests have been conducted in air at 300 °C and from 400 °C to 500 °C at a temperature interval of 25 °C at nominal strain rates of 6.67 × 10⁻³ s⁻¹. The alloy exhibits gradual cyclic softening till failure at 300 °C, whereas, it exhibits initial cyclic softening followed by marked cyclic hardening from 400 °C to 500 °C. The cyclic hardening is attributed to DSA phenomena, resulting due to increase in isotropic stress component. The observed maximum peak stress ratio, lower fatigue life and minimum half-life plastic strain range at 450 °C indicates the maximum effect of DSA at that temperature. The fatigue life of tensile and compressive hold at 450 °C was observed to be inferior as compared to pure fatigue tests.     

The static and high strain rate behaviour of a commingled E-glass/polypropylene woven fabric composite

In this paper the effect of strain rate on the tensile, shear and compression behaviour of a commingled E-glass/polypropylene woven fabric composite over a strain rate range of 10⁻³–10² s⁻¹ is reported. The quasi-static tests were conducted on an electro-mechanical universal test machine and a modified instrumented falling weight drop tower was used for high strain rate characterisation. The tensile and compression modulus and strength increased with increasing strain rate. However, the shear modulus and strength were seen to decrease with increasing strain rate. Strain rate constants for use in finite element analyses are derived from the data. The observed failure mechanisms deduced from a microscopic study of the fractured specimens are presented.     

Forming behavior and workability of 6061/B4CP composite during hot deformation

Compression tests of 6061/B₄C_P composite have been performed in the compression temperature range from 300 °C to 500 °C and the strain rate range from 0.001 s⁻¹ to 1 s⁻¹. The flow behavior and processing map have been investigated using the corrected data to elimination of effect of friction. The processing maps exhibited two deterministic domains, one was situated at the temperature between 300 °C and 400 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹ and the other was situated at the temperature between 425 °C and 500 °C with strain rate between 0.003 s⁻¹ and 0.18 s⁻¹.The estimated apparent activation energies of these two domains, were 129 kJ/mol and 149 kJ/mol, which suggested that the deformation mechanisms were controlled by cross-slip and lattice self-diffusion respectively. The optimum parameters of hot working for the experimental composite were 350 °C - 0.01 s⁻¹ and 500 °C - 0.01 s⁻¹. In order to exactly predict dangerous damaging mechanism under different deformation conditions exactly, Gegel’s criterion was applied to obtain processing map in the paper. The result showed that the processing map used Gegel’s criterion can be effectively to predict the material behavior of the experimental composite.     

Tensile deformation behaviour of forged disc of IN 718 superalloy at 650 °C

The tensile properties of forged disc of IN 718 superalloy were evaluated in the strain rate regime between 10⁻⁴ and 10⁻² s⁻¹ at 650 °C. Flow oscillations were observed in stress–strain curves in the strain rate regime investigated. These flow oscillations were identified as strain increments attributed to twining mechanism at all the strain rates. However, presence of well defined serrations, temperature insensitivity of yield strength and increase in strain hardening exponent confirmed the occurrence of dynamic strain aging at 10⁻³–10⁻² s⁻¹ strain rates. Deformation behaviour was observed to be planer in nature. Fracture features remained same (transgranular) in the strain rate regime studied.     

Hot compressive deformation behavior of a new hot isostatically pressed Ni–Cr–Co based powder metallurgy superalloy

The hot compressive deformation behavior of a new hot isostatically pressed Ni–Cr–Co based powder metallurgy (P/M) superalloy was studied in the temperature range of 950–1150 °C and strain rate range of 0.0003–1 s⁻¹ using Gleeble-1500 thermal simulator. The dynamic recrystallization-time–temperature (RTT) curve was developed and the constitutive equation of flow stress during hot deformation was established. The results show that the flow stress decreases with increasing deformation temperature and decreasing strain rate. The flow stress represents as the characteristic of dynamic crystallization with the increasing of strain at the deformation temperatures lower than 1100 °C and strain rates higher than 0.0003 s⁻¹. The beginning time of dynamic crystallization has no linear relationship with deformation temperature in the condition of strain rate lower than 0.01 s⁻¹. Besides, the experiments verify that the hyperbolic sine model including the variable of strain reflects the changing law of flow stress during the hot deformation process.     

Superplastic behavior of a TiAl alloy in its as-cast state

Superplastic behavior of a TiAl alloy was investigated in a temperature range between 800 and 1000°C and at a strain rate of 1×10⁻⁴ s⁻¹. The results show that the present alloy possesses very good superplasticity even in its as-cast state. A tensile elongation of 628% was obtained at 850°C. The observed superplastic behavior can be explained by the formation of a fine microstructure containing a metastable β-phase during solidification.     

Determination of strain rate dependent through-thickness tensile properties of textile reinforced thermoplastic composites using L-shaped beam specimens

The presented work focuses on a methodology to characterise strain rate dependent strength and elastic properties of textile reinforced composites in laminate through-thickness direction. Here, for the characterisation L-shaped beam specimens are used. The investigated composite is a fabric reinforced thermoplast made of hybrid E-glass/polypropylene yarns. The analytical solution for the determination of the through-thickness tensile strength as proposed by Lekhnitskii and Shivakumar is verified by means of an optical deformation analysis and is extended for thew determination of the through-thickness elastic modulus. Finally, the possibility of the strain rate dependent characterisation is investigated and a Johnson-Cook based modelling approach is used to represent the apparent strain rate dependency of the through-thickness failure onset. The methodology is successfully used to capture the material strain rate effects with the according strength values and model parameters over a strain rate range of 10 ⁻⁴ s⁻¹ to 10 s⁻¹ as well as the elastic modulus.     

Effect of Ni content on the tensile properties and strain-induced martensite transformation for 304 stainless steel

The effect of Ni content (8.3-12 wt.%) on the tensile properties and strain hardening behavior was studied on type 304 stainless steels (STS) used for the membrane of LNG storage tanks. The tensile test temperature was varied from 25 °C to −196 °C. At room temperature, the hardening and ductility indices (tensile strength, strain hardening exponent and elongation) increased with decreasing Ni content. For the 8.3-9.0 wt.% Ni STS, a lower yield point was observed at temperatures below −60 °C. It was due to the dynamic strain softening and/or transformation-induced plasticity (TRIP) that accompanied the rapid increase in the amount of strain-induced martensite (α′) at low strains. Neither dynamic strain softening nor TRIP was observed for the 12 wt.% Ni STS because only the ?-martensite transformation was produced at the low strains.     

Effect of temperature on the mode I and mixed mode I/III fracture toughness of SA333 steel

The effect of temperature on tensile properties, mode I and mixed mode I/III fracture toughness of SA333 Grade 6 steel was investigated. The variation of ultimate tensile strength and strain hardening exponent with temperature as well as the appearance of serrations in the stress-strain plots indicated that dynamic strain aging regime in this steel is in the temperature range 175-300 °C at a nominal strain rate of 3 × 10⁻³ s⁻¹. Both mode I and mixed mode I/III fracture toughness values were found to exhibit a significant reduction in the DSA regime. The mixed mode I/III fracture toughness was found to be significantly lower than the mode I fracture toughness at all temperatures. However, the difference between the two toughness values was much higher prior to the onset of DSA. The results are explained on the basis of the nature of deformation fields under mode I and mixed mode I/III loading as well as the fracture mechanism prevalent in these steels at different temperatures.     

Tensile and impact-toughness behaviour of cryorolled Al 7075 alloy

The effects of cryorolling and optimum heat treatment (short annealing + ageing) on tensile and impact-toughness behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its mechanical properties were studied by using tensile testing, hardness, and Charpy impact testing. The microstructural characterization of the alloy was carried out by using field emission scanning electron microscopy (FE-SEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain structure as observed from its FE-SEM micrographs. It is observed that the yield strength and impact toughness of the cryorolled material up to 70% thickness reduction have increased by 108% and 60% respectively compared to the starting material. The improved tensile strength and impact toughness of the cryorolled Al alloy is due to grain refinement, grain fragments with high angle boundaries, and ultrafine grain formation by multiple cryorolling passes. Scanning electron microscopy (SEM) analysis of the fracture surfaces of impact testing carried out on the samples in the temperature range of −200 to 100 °C exhibits ductile to brittle transition. cryorolled samples were subjected to short annealing for 5 min at, 170 °C, and 150 °C followed by ageing at 140 °C and 120 °C for both 40% and 70% reduced samples. The combined effect of short annealing and ageing, improved the strength and ductility of cryorolled samples, which is due to precipitation hardening and subgrain coarsening mechanism respectively. On the otherhand, impact strength of the cryorolled Al alloy has decreased due to high strain rate involved during impact loading.     

Effect of cooling rate on the high strain rate properties of boron steel

In this work, the effect of cooling rate on the high strain rate behavior of hardened boron steel was investigated. A furnace was used to austenize boron sheet metal blanks which were then quenched in various media. The four measured cooling rates during the solid state transformation were: 25 (compressed air quench), 45 (compressed air quench), 250 (oil quench) and 2200 °C/s (water quench). Micro-hardness measurements and optical microscopy verified the expected as-quenched microstructure for the various cooling rates. Miniature dog-bone specimens were machined from the quenched blanks and tested in tension at a quasi-static rate, 0.003 s⁻¹ (Instron) and a high rate, 960 s⁻¹ (split Hopkinson tensile bar). The resulting stress vs. strain curves showed that the UTS increased from 1270 MPa to 1430 MPa as strain rate increased for the specimens cooled at 25 °C/s, while the UTS increased from 1615 MPa to 1635 MPa for the specimens cooled at 2200 °C/s. The high rate tests showed increased ductility for the 25, 45 and 250 °C/s specimens, while the specimens cooled at 2200 °C/s showed a slight decrease. The Hollomon hardening curve was fit to the true stress vs. true strain curves and showed that the mechanical response of the high rate tests exhibited a greater rate of hardening prior to fracture than the quasi-static tests. The hardening rate also increased for the specimens quenched at higher cooling rates. Optical micrographs of the fractured specimens showed that the failure mechanism transformed from a ductile-shear mode at the lower cooling rates to a shear mode at the high cooling rates.     

Dynamic properties of pultruded natural fibre reinforced composites using Split Hopkinson Pressure Bar technique

The paper presents results on dynamic mechanical properties of jute, and kenaf fibre reinforced composites at various strain rates using compression Split Hopkinson Pressure Bar technique. The stress–strain curves for both pultruded natural fibre reinforced composites at strain rates of nearly 1400 s⁻¹ are illustrated and then compared with statically determines stress–strain curve (1.0 × 10⁻³ s⁻¹). Results show that the strain rate does affect the value of dynamic compressive properties of both pultruded natural fibre composites. Higher dynamic compression modulus and 2.5% flow stress were recorded for higher strain rates as compared to lower strain rate over the range of strain rates investigated. Under dynamic loading, jute fibre reinforced composites recorded the highest value of dynamic response in terms of compression modulus, 2.5% flow stress and compressive strength than that of kenaf fibre reinforced composites. In addition, kenaf fibre reinforced composites is more severely damaged as compared to jute fibre reinforced composites for all tested strain rate.     

Effect of temperature on deformation behavior and microstructures of TC11 titanium alloy

The isothermal compression deformation behavior of TC11 titanium alloy with beta microstructure was studied between 750 °C and 1100 °C under the strain rate ranging from 0.001 s⁻¹ to 10 s⁻¹ by THERMECMASTOR-Z simulator. In addition, the effect of temperature on microstructure was observed using optical microscope. The results showed that the temperature greatly affected the flow stress and microstructure of TC11 titanium alloy cooled from beta phase region in air. During hot deformation of TC11 titanium alloy, the steady state flow characteristic was observed at higher temperature or lower strain rate. In the α + β phase region, spheroidization fraction of α lamellar decreased with increasing temperature, while in near-β and β phase regions, dynamic recrystallization fraction increased with increasing temperature in all strain rates except at the strain rate of 0.001 s⁻¹.     

Notch and strain rate sensitivity of non-crimp fabric composites

The notch and strain rate sensitivity of non-crimp glass fibre/vinyl-ester laminates subjected to uniaxial tensile loads has been investigated experimentally. Two sets of notch configurations were tested; one where circular holes were drilled and another where fragment simulating projectiles were fired through the plate creating a notch. Experiments were conducted for strain rates ranging from 10⁻⁴ s⁻¹ to 10² s⁻¹ using servo hydraulic machines. A significant increase in strength with increasing strain rate was observed for both notched and un-notched specimens. High speed photography revealed changes in failure mode, for certain laminate configurations, as the strain rate increased. The tested laminate configurations showed fairly small notch sensitivity for the whole range of strain rates.     

Post-welding formability of AZ31 magnesium alloy

The plastic flow behaviour and formability of friction stir welded AZ31 magnesium alloys were widely investigated. Flow curves were obtained in extended ranges of temperature (250–350 °C) and strain rate (0.001–0.1 s⁻¹) by means of uniaxial tensile tests; furthermore, forming limit curves were determined using the hemispherical punch method in the same range of temperature but with a constant crosshead speed of 0.1 mm/s. The results were compared with those obtained, under the same experimental conditions, on the base material. The flow stress levels of joint and base material are very similar up the peak of the flow curve although the equivalent strains at the peak and to failure are usually lower than those of base material. However, at the highest temperature and lowest strain rate investigated (350 °C and 0.001 s⁻¹), the flow behaviour of the welded joint tends to be similar to the one of the base alloy. Finally, formability of the friction stir welded material, evaluated in terms of forming limit curves, is usually lower than the one of the base material.     

Polypropylene foam behaviour under dynamic loadings: Strain rate,density and microstructure effects

Expanded polypropylene foams (EPP) can be used to absorb shock energy. The performance of these foams has to be studied as a function of several parameters such as density, microstructure and also the strain rate imposed during dynamic loading. The compressive stress–strain behaviour of these foams has been investigated over a wide range of engineering strain rates from 0.01 to 1500 s⁻¹ in order to demonstrate the effects of foam density and strain rate on the initial collapse stress and the hardening modulus in the post-yield plateau region. A flywheel apparatus has been used for intermediate strain rates of about 200 s⁻¹ and higher strain rate compression tests were performed using a viscoelastic Split Hopkinson Pressure Bar apparatus (SHPB), with nylon bars, at strain rates around 1500 s⁻¹ EPP foams of various densities from 34 to 150 kg m⁻³ were considered and microstructural aspects were examined using two particular foams. Finally, in order to assess the contribution of the gas trapped in the closed cells of the foams, compression tests in a fluid chamber at quasi-static and dynamic loading velocities were performed.     

Materials modeling and finite element simulation of isothermal forging of electrolytic copper

Isothermal forging of electrolytic copper is modeled using finite element simulation and materials models involving kinetic analysis and processing maps with a view to validate their predictions. Forging experiments were conducted on a rib–web (cup) shape in the temperature range of 300–800 °C and at speeds of 0.01–10 mm s⁻¹. The processing map for hot working of electrolytic copper revealed two domains in the temperature and strain ranges of (1) 400–600 °C and 0.001–0.01 s⁻¹, (2) 650–950 °C and 0.3–30 s⁻¹, where dislocation core diffusion and lattice self-diffusion are the rate-controlling mechanisms, respectively. Finite element simulation using the relevant experimental constitutive equations, predicted load–stroke curves that correlated well with the experimental data. The simulation has shown that there is a strain variation from about 0.4 to 4 in the web and rib regions of the forged component, although the dynamically recrystallized grain structure is fairly uniform, suggesting that dynamic recrystallization (DRX) is not sensitive to strain once the steady state flow is reached. The DRX grain size in the component is linearly dependent on Z and is similar to that predicted by the materials model after discounting for the longer time taken for the component removal.     

Hot deformation behavior and microstructural evolution of Ag-containing 2519 aluminum alloy

The hot deformation behaviors of Ag-containing 2519 aluminum alloy were studied by isothermal compression at 300–500 °C with strain rates from 0.01 s⁻¹ to 10 s⁻¹. The microstructural evolution of the alloy was investigated using Polyvar-MET optical microscope and Tecnai G² 20 transmission electron microscope (TEM). It has been shown that the flow stress of the alloy increases with increasing the strain rate and decreasing the deformation temperature. When the strain rate is lower than 10 s⁻¹, the flow stress increases with increasing strain until the stress reached the peak value, after which the flow stress remains almost constant. This result indicates that dynamic recovery happens during deformation. When the strain rate is 10 s⁻¹ and the temperature is higher than 300 °C, serrated flow behavior is generally observed with the stress decreasing with increasing strain, a typical phenomenon of dynamic recrystallization.     

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.