Influence of loading conditions during tensile testing on acoustic emission

The Acoustic Emission (AE) monitoring technique is widely used in mechanical and material research for detection of plastic deformation, fracture initiation and crack growth. However, the influence of AE features (such as signal amplitude, frequency, rise time and duration) on the fracture parameters (such as brittle or ductile mode of propagation and fracture propagation speed) is not completely understood. In this paper, the effect of loading conditions on fracture behavior was studied using AE monitoring during tensile testing of an aluminum alloy specimen. The fracture development was observed using a high speed video camera and was analyzed using the finite element method. The hardware and software produced by Physical Acoustics Corporation (USA) was used. Variations in AE parameters were analyzed and correlated to the stress-Strain curves obtained during testing. It is shown that the strain rate and the presence of a crack (modeled by a notch on the sample), affect the fracture mode (brittle or ductile) and a relative amount of the mode dependent AE signatures

Effect of solidification rate on microstructure evolution in dual phase microalloyed steel

In steels the dependence of ambient temperature microstructure and mechanical properties on solidification rate is not well reported. In this work we investigate the microstructure and hardness evolution for a low C low Mn NbTi-microalloyed steel solidified in the cooling rate range of 1–50 Cs−1. The maximum strength was obtained at the intermediate solidification rate of 30 Cs−1. This result has been correlated to the microstructure variation with solidification rate

Strengthening mechanisms in thermomechanically processed NbTi-microalloyed steel

The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly

Investigation of X70 line pipe steel fracture during single edge-notched tensile testing using acoustic emission monitoring

This paper presents an investigation of the fracture behavior of X70 pipeline steel using the acoustic emission (AE) monitoring technique. The AE monitoring technique is widely used in mechanical and materials research for detection of plastic deformation, fracture initiation and crack growth. However, quantitative dependences of the AE parameters (such as signal amplitude and frequency) on the fracture parameters of X70 pipeline steel (such as ductile and brittle fracture initiation and propagation) have not been studied in depth and are therefore not completely understood. In this paper, we report an investigation of the effect of loading conditions on the fracture behavior of X70 pipeline steel using AE monitoring and single-edge notch tensile (SENT) testing. The fracture development was observed using a high speed video camera. The AE was recorded and analyzed using the hardware and software produced by Physical Acoustics Corporation (USA). The variations in AE parameters with loading conditions were analyzed using the \u27Average Hit\u27 features and Wave Form and Power Spectrum methodologies and correlated with the load-displacement/load-time curves obtained during SENT testing. The strain rate, temperature, and features of the sample notch, that was used to simulate a crack, were shown to affect the fracture mode and the relative magnitude of mode-dependent AE signatures

Strain-induced ferrite formation and its effect on mechanical properties of a dual phase steel produced using laboratory simulated strip casting

Thermo-mechanical processing of a strip cast dual phase (DP) steel was carried out using a Gleeble thermo-mechanical simulator. The effect of deformation temperatures in the range from 1050 to 700 °C on the microstructure evolution was investigated using optical, scanning and transmission electron microscopy along with electron backscattering diffraction (EBSD). Strain-induced ferrite (SIF) formation was observed following austenite deformation (∼0.41 reduction) in the 800–700 °C temperature range, leading to a ferrite grain refinement down to 3.1 ± 2.3 μm. A novel segmentation procedure was applied to separate selected EBSD maps into polygonal ferrite, SIF and second phase regions (bainite/martensite). Following this, the microtexture, misorientation angle distribution and the deviation from the Kurdjumov–Sachs and Nishiyama–Wasserman orientation relationships of each microstructure constituent were analysed. Based on iso-work modelling analysis of tensile stress-strain curves, the SIF was found to enhance strength with a slight decrease in ductility compared to polygonal ferrite. The tensile mechanical properties after deformation at 750 °C reached the level of DP 600 produced in industry, highlighting the potential to manufacture DP steels via the strip casting technique

Microstructures and mechanical properties of dual phase steel produced by laboratory simulated strip casting

Conventional dual phase (DP) steel (0.08C-0.81Si-1.47Mn-0.03Al wt.%) was manufactured using simulated strip casting schedule in laboratory. The average grain size of prior austenite was 117. ±. 44. μm. The continuous cooling transformation diagram was obtained. The microstructures having polygonal ferrite in the range of 40-90%, martensite with small amount of bainite and Widmanstätten ferrite were observed, leading to an ultimate tensile strength in the range of 461-623. MPa and a corresponding total elongation in the range of 0.31-0.10. All samples exhibited three strain hardening stages. The predominant fracture mode of the studied steel was ductile, with the presence of some isolated cleavage facets, the number of which increased with an increase in martensite fraction. Compared to those of hot rolled DP steels, yield strength and ultimate tensile strength are lower due to large ferrite grain size, coarse martensite area and Widmanstätten ferrite

The effect of processing parameters on the microstructure and mechanical properties of low-Si transformation-induced plasticity steels

A base low Si, high-A1 transformation-induced plasticity (TRIP) steel and one with 0.03Nb and 0.002Ti (wt percent) additions were subjected to thermo-mechanical processing (TMP) and galvanising simulations.

Microstructures and mechanical properties of TRIP steel produced by strip casting simulated in the laboratory

Conventional transformation induced plasticity (TRIP) steel (0.17C-1.52Si-1.61Mn-0.03Al, wt%) was produced via strip casting technology simulated in the laboratory. Effects of holding temperature, holding time and cooling rate on ferrite formation were studied via analysis of the continuous cooling transformation diagram obtained here. A typical microstructure for conventional TRIP steels consisting of ~ 0.55 fraction of polygonal ferrite with bainite, retained austenite and martensite was obtained. However, coarse prior austenite grain size of ~80 μm led to large polygonal ferrite grain size of ~17 μm, coarse second phase regions of ~21 μm size, small amount of retained austenite (0.02-0.045) and the presence of Widmanstätten ferrite. Optimisation of the microstructure-property relationship was reached via a variation in the isothermal bainite transformation temperature. The highest retained austenite fraction of 0.045±0.003 with medium carbon content of 1.23±0.01 wt% was obtained after holding at 400 °C, resulting in the highest ultimate tensile strength of 590±35 MPa and largest total elongation of 0.27±0.05. The presence of TRIP effect in the studied steel was revealed through the analysis of strain hardening exponent and modified Crussard-Jaoul model. Effect of processing parameters on retained austenite retention and stress-strain behaviour was discussed

Bauschinger effect in microalloyed steels: part I. Dependence on dislocation-particle interaction

The Bauschinger effect (yield stress decreasing at the start of reverse deformation after forward prestrain) is an important factor in strength development for cold metal forming technology. In steels, the magnitude of the Bauschinger effect depends on composition, through the presence of microalloy precipitates, and prior processing, through the size and distribution of microalloy precipitates and presence of retained work hardening. In this article, the microstructures of two (Nb- and Nb-V-microalloyed) steel plates, in terms of (Ti,Nb,V,Cu)-rich particle distributions and dislocation densities, have been quantitatively related to the Bauschinger parameters for the same processing conditions. For the 12- to 50-nm effective particle size range, the Bauschinger stress parameter increases with the particle number density and dislocation density increase. The relative influence of these two microstructure parameters is discussed