Fracture Behavior Of High Strength Steel Research Articles

Improved true stress-strain model of high-strength steels for fracture simulation in fire

Fracture prediction of high-strength steel at elevated temperatures is particularly essential for the performance-based design of high-strength steel structures in fire. Accurate prediction of fracture behavior of high-strength steels in fire requires a precise true stress-strain model up to a large strain level (>1.0 mm/mm). This research investigates improved true stress-strain models of Q550, Q690, Q890, Q960, S690, and S960 high-strength steels at elevated temperatures up to 800 °C. The true stress-strain models are calibrated from test data and finite element analysis results. The calibrated true stress-strain model is improved in the following two aspects. First, to ensure the rationality of the true stress-strain models, they were examined by Considère’ Construction together with the physical law of ductile metals. Furthermore, by modifying the weight function, the true stress-strain models can accurately simulate the post-necking behavior of high-strength steels at high temperatures. Based on the research results, the true stress-strain behavior of three regions, including the elastic region, strain-hardening region, and post-necking region, need different forms of functions to express. They were linear function, combined exponential and linear functions, and weighted upper-bound and lower-bound functions, respectively. It was observed that the variation of the weighting factor in the weighting function may be related to the post-necking softening behavior of high-strength steels. For the investigated high-strength steels, at lower temperatures (< 550 °C), the weighting factor decreased with the increase of true strain, resulting in a significant softening of high-strength steels after the onset of necking.

Ductile Fracture Investigation of High-Strength Steel SM570 under Low Stress Triaxiality

A comprehensive understanding of the fracture behavior of high-strength steel is of great significance for its structural application. In this study, experiments were conducted to investigate the ductile fracture mechanism of high-strength steel SM570, one type of conventional structural steel. Two types of shear specimens, one with symmetrical notches and the other with asymmetrical notches, were designed, and by changing the notch angles, a wide range of low-stress triaxiality could be obtained. Based on the discussion of the experimental results, crack initiation, and its propagation up to fracture failure were clarified. Compared with the fracture behavior of SM490 (one type of conventional normal-strength structural steel), the SM570 with higher yield stress has relatively severe stress concentration, the crack initiation appears earlier, and the brittle fracture is more likely to occur. Numerical simulations based on the finite element method (FEM) were performed with ABAQUS to obtain the stress triaxialities and equivalent plastic strain of the symmetrical and asymmetrical specimens. A modified N-VG model with a fracture criterion at a negative and low-stress triaxiality range from −0.6 to 1/3 was proposed for evaluating the fracture behavior of steel SM570.

Fatigue fracture behavior of high-strength low-alloy steel for flexible marine riser in the high cycle fatigue regime

The relationship between microstructure, stress ratio, and fatigue fracture behavior of high-strength steel for flexible marine risers was investigated by microstructure characterization and fatigue test. The microstructure characteristic, S-N curve, fatigue fracture morphology, and fatigue crack morphology after quenching and tempering treatment and cyclic heat treatment, respectively, were evaluated. The results revealed that after cyclic heat treatment, the microstructure was refined considerably, the grain boundary density and the content of high-angle grain boundary increased, which inhibited the fatigue crack propagation and improved the fatigue strength. The average stress increased by increasing the stress ratio, which promoted the crack initiation and propagation; consequently, the fatigue strength and fatigue life of the tested steel decreased.

Effect of Heat Treatment on Microstructure and Mechanical Properties of High-Strength Steel for in Hot Forging Products

High-strength steel is widely used in hot forging products for application to the oil and gas industry because it has good mechanical properties under severe environment. In order to apply to the extreme environment industry requiring high temperature and high pressure, heat treatments such as austenitizing, quenching and tempering are required. The microstructure of high-strength steel after heat treatment has various microstructures such as Granular Bainite (GB), Acicular Ferrite (AF), Bainitic Ferrite (BF), and Martensite (M) depending on the heat treatment conditions and cooling rate. Especially in large forged products, the difference in microstructure occurs due to the difference in the forging ratio depending on the location and the temperature gradient according to the thickness during post-heat treatment. Therefore, this study attempted to quantitatively analyze various phases of F70 high-strength steel according to the austenitizing temperature and hot forging ratio using the existing EBSD analysis method. In addition, the correlation between microstructure and mechanical properties was investigated through various phase analysis and fracture behavior of high-strength steel. We found that various microstructures of strength steel depend on the austenitizing temperature and hot forging ratio, and influence the mechanical properties and fracture behavior.

Analysis of fracture behavior of high-strength steels in tension after fire exposure

High-strength steels have been widely applied in high-rise and long-span structures. Exposure to high temperatures due to fire may lead to reduction in material properties and fracture behavior of high-strength steels, greatly affecting the post-fire behavior of high-strength steel structures. This paper investigates the post-fire fracture behavior of high-strength steels in tension to pave the way for safety assessment of high-strength steel structures after fire. The fracture models of Chinese high-strength steels (Q460, Q550, Q690, Q890) and European high-strength steels (S460, S690 and S960) are calibrated and validated against limited experimental results of tensile coupon tests in an exposure temperature range of 200 °C to 1000 °C. The effect of air-cooling and water-cooling methods on the post-fire fracture behavior of high-strength steels is investigated. The fracture behavior is represented by the post-necking true stress-strain curve, toughness index (resistance to fracture initiation) in the stress modified critical strain model (SMCS) and damage evolution law. It is found that different types of high-strength steels with similar yield strength may exhibit quite different fracture behavior after exposure to high temperatures. The maximum exposure temperature and cooling method are two critical factors influencing the post-fire fracture behavior of high-strength steels. The true stress-strain curves after onset of necking can be simply expressed by a straight line with a slope, which becomes temperature dependent when the exposed temperature exceeding a critical level (e.g. 600 °C). The toughness index in SMCS model is subjected to great reduction in a temperature range of 800 °C to 900 °C for Q460-Q890 steels and 750 °C to 900 °C for S460-S960 steels. The most rapid evolution of damage occurs after being exposed to temperatures in a range of 650 °C to 900 °C. Compared to air-cooling method, the water-cooling method may lead to great reduction in the slope of post-necking true stress-strain curves (by 50%), significant reduction in toughness index (by 80%), and even change of fracture mode from ductile fracture to brittle fracture. It is recommended to choose reasonable and appropriate post-fire material properties for fracture prediction of different high-strength steels.

Fracture behavior of high-strength steels at elevated temperatures

High-strength steels have been increasingly applied in different categories of structures due to their advantages of high strength and light weight. Fracture in high-strength steel members becomes more essential when exposed to fire, which governs the overall strength and deformation capacity of steel structures. This paper presents the studies on the elevated-temperature fracture behavior of high-strength steels with the yield strengths ranging from 460 MPa to 960 MPa. A fracture prediction approach is first proposed for Chinese Q690 steels. Using experimental data together with finite element analysis, true stress-strain curves considering post-necking behavior, stress modified critical strain model (SMCS) and damage evolution law are calibrated and validated for Q690 steels at elevated temperatures. The proposed fracture prediction approach is then applied to other high-strength steels (Q460, Q550, Q890, S460, S690 and S960). The results show that different high-strength steels with similar strength grade may exhibit quite different fracture behaviors at given temperature. However, their variation trends of the post-necking material properties including slope of post-necking true stress-strain curves and fracture toughness index with temperature were relatively consistent. These high-strength steels may exhibit different variations of damage evolution law with temperatures. The proposed post-necking material properties can reasonably predict the fracture behavior of these high-strength steels at elevated temperatures.

A method to investigate strain rate effects on necking and fracture behaviors of advanced high-strength steels using digital imaging strain analysis

Quasi-static and dynamic tensile tests are performed to investigate the effects of strain rate on the necking and fracture behavior of high-strength steels for automotive structures. A newly developed strain analysis system is applied to quasi-static and dynamic tensile tests to measure the strain propagation with a digital camera or a high-speed video camera. For the dynamic tensile test, a Hopkinson bar method utilizing a high-speed video camera is developed for the visualization of the strain propagation under a high strain rate of 102 s−1. The effect of strain rate on the stress–strain relations and strain localization behavior is investigated for steel sheets with tensile strengths ranging from 270 MPa to 1470 MPa, including advanced high-strength steels. To improve the strain measurement technique of the Hopkinson bar method, the digital imaging strain analysis method is applied to determine the dynamic stress–strain relations using the high-speed digital images. Regarding the strain-rate dependence of fracture behavior, the limit strains corresponding to the fractures are evaluated by measuring the fractured shapes. The local true stress–strain relation and the strain rate history in a necking region are investigated to estimate the material properties at large strains and high strain rates. Finite element analyses simulating the dynamic strain localization is performed for a mutual assessment of the experimental result and the numerical prediction.

Mechanical properties and fracture behavior of high-strength steels

Typical thicknesses of high-strength steels (HSS) sheets used in the car industry are inapplicable for standardized testing procedures. The aim of this study is to propose an appropriate methodology for testing and comparing of thin HSS sheets. Microstructures were observed by means of light and scanning electron microscopy. The modified Charpy impact tests and fracture toughness tests were used in order to compare the fracture properties of three different HSS sheets (Docol 1200 M, Multiphase 1200 and BTR 165). Ductile-to-brittle transition curves and tearing resistance (J − Δa) curves were measured. From the fracture toughness linked to the specimen thicknesses the value of fracture toughness KIc was estimated. Fractographic analysis of broken specimens has revealed that due to the fine microstructure of mixed ferrite-martensite fracture mechanism remains ductile even at low temperatures (down to −100°C).

Fatigue Fracture Behavior of High-Strength Steel in Super Long Life Range.

Long term cantilever-type rotational bending fatigue tests of up to 109 cycles were carried out on high carbon chromium bearing steel, SUJ2. The fatigue fracture behavior of SUJ2 in the super long life range was discussed based on scanning electron microscope observations and fracture mechanics. Fatigue failure occurred when the number of cycles exceeded 107. In the super long life range, the fish-eye-type fracture and the subsurface-type fracture were observed. In the fish-eye-type fracture, the stress intensity factor calculated from the area of the facet region was independent of the number of cycles to failure and was almost constant at 5.4MPa• m1/2. In the subsurface-type fracture, high carbon segregation was observed at the crack initiation area. The stress intensity factor for the carbon segregation area was close to 5.0MPam1/2. Pure fatigue crack was initiated from the area outside the facet region or the high carbon segregation area.