Abstract
Brittle fracture in carbon steel has a serious impact on the safety of steel structures. Thus, technology that arrests crack propagation is the final line of protection for such structures. It is such an important issue that conditions that can reliably stop crack propagation should be thoroughly clarified. Due to the social importance of the problem, many experimental and theoretical studies have been conducted from both the mechanical and microstructural viewpoints. Though it has been reported that the upper limit of the speed of brittle crack propagation is theoretically the Rayleigh wave speed, which is approximately 2,900 m/s in steels, the actual speed of brittle crack propagation in steels is approximately 1,000 m/s and lower. The reason for this difference is due to braking effects during crack propagation, for example, unevenness in the faceting, tear ridges, microcracking, twin deformation and side ligaments, which are the elements that dominate the arresting toughness. To evaluate the most fundamental element of the arresting toughness, the authors have studied the crack propagation resistance inside a single crystal and across a grain boundary by using a 3% silicon steel with a microstructure of single phase ferrite and a very large grain size of 4-5 mm. The crack propagation rate inside a single crystal is relatively large, but only half of the Rayleigh wave speed even under the highest stress intensity factor conditions. In this study, the change in the crack propagation rate was measured using small sized multiple-strain gauges that were pasted inside a single crystal along the crack line. From these measurements, crack propagation resistance and the role of grain boundaries are quantitatively discussed in this article.
Highlights
S topping brittle crack propagation in steel is an important issue in many industries; it is a consideration incorporated into the design of ships [1], low temperature storage tanks [2], hydroelectric power plants [3], and nuclear power plants [4]
Experimental results using PMMA (Dally et al [28]) showed that there is a limit on the speed that is considerably lower than the Rayleigh wave velocity and that as K is made extremely large, crack propagation velocity gradually approaches this speed limit
Conclusions were drawn as follows: 1) In the three-point bending test, the brittle fracture originated from a trigger point located slightly away from the tip of the notch like an ordinary polycrystalline steel material
Summary
S topping brittle crack propagation in steel is an important issue in many industries; it is a consideration incorporated into the design of ships [1], low temperature storage tanks [2], hydroelectric power plants [3], and nuclear power plants [4]. I n this study, a three-point bending test was conducted to measure the brittle crack propagation rate in coarse grained 3%Si - 2%Al steel. Investigation of the brittle crack propagation rate in a single crystal A strain gauge including multiple-strain gauge lines was attached to the surface of the grain that was approximately in the centre of the specimen (Fig. 4) to evaluate the crack speed history inside a single crystal. When the crack propagation speeds from this experiment are plotted versus Kstatic, as shown, they are located in the polycrystalline region, the measurements were from a single crystal This is because the targeted grain did not penetrate the through thickness, and the grains close to the strain gauge likely propagated before the grains on the opposite side, according to detailed observation of the opposite side fracture surface. If dynamic K is calculated by theoretical dynamic fracture mechanics, we think the change of K value is limited
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