Brittle fracture of high-strength steels at very low temperatures

High-strength steels are used to increase the load carrying capacity of components. However, to guarantee a safe design, it is also necessary to combine high strength with adequate fracture toughness. In this paper, fracture toughness of three high-strength steels with yield strengths ranging from 460 to 890 MPa has been studied at very low temperatures. Taking into account experimental evidence, a new mechanism of cleavage at very low temperatures is proposed. This mechanism considers the possibility of reaching the ideal strength (the stress at which the lattice of a single crystal losses its stability) in the immediate vicinity of the fatigue crack tip. Moreover, a computational model able to calculate the external load needed to produce a catastrophic failure of these steels has been developed.     

陶瓷的断裂韧性与缺口半径 Ⅰ.断裂韧性测试技术

本文用一种新的简单方法制备了不同半径的尖缺口,用单边切口梁试件测试了不同缺口半么氧化铝陶瓷的断裂韧性,地七咱不同测试方法测得的,同一种材料的断裂韧性进行了对比,讨论陶瓷材料断裂韧性的最佳测试方法和测试中对缺口半径的要求。     

陶瓷材料断裂韧性与缺口半径 Ⅱ 断裂韧性估算方法

在陶瓷材料裂纹尖端存在一个断裂过程区,当断裂过程区内平均应力达到断裂强度时,裂纹扩展。本文由理论推导结合实验数据,得到了新断裂过程区的大小是平均晶粒直径的四倍。并由平均应力断裂模型,给出了陶瓷材料断裂韧性和缺口半径及平均晶粒直径之间的关系式,由此关系式可以用宽缺口试件测出的断裂韧性去估算陶瓷材料的本质断裂韧性。     

陶瓷的断裂韧性与缺口半径 Ⅰ.断裂韧性测试技术

本文用一种新的简单方法制备了不同半径的尖缺口，用单边切口梁试件测试了不同缺口半径氧化铝陶瓷的断裂韧性.对七种不同测试方法测得的、同一种材料的断裂韧性进行了对比，讨论陶瓷材料断裂韧性的最佳测试方法和测试中对缺口半径的要求.     

A statistical model for cleavage fracture in notched specimens of C–Mn steel

A statistical model for cleavage fracture in notched specimens of C-Mn steel has been proposed. This model is based on a recently suggested physical model. This statistical model satisfactorily describes the distributions of the cumulative failure probability and failure probability density of 36 notched specimens fractured at various loads at test temperature of −196 °C. The minimum notch toughness has also been discussed.     

Steam oxidation of ferritic steels – laboratory test kinetic data

Abstract

The oxidation resistance of 9–12% chromium steels in steam-containing environments simulating the service conditions of steam power plant has been investigated for exposure times ranging from 1 h up to 10 000 h. In the long-term experiments the steels were evaluated by the mass changes determined in exposure tests, which were interrupted every 250 h, thus providing information concerning the scale growth and spalling characteristics. The morphologies of the oxide scales were examined using optical microscopy, scanning electron microscopy, secondary neutrals mass spectrometry and Raman spectroscopy. Different mechanisms of oxidation were observed for the various steels in different temperature regimes and exposed for different durations. For some steels, the steam oxidation resistance increased with increasing exposure temperature. The scale thickness at which spalling was observed varied according to the exposure temperature and the interval between thermal cycles, so that a critical scale thickness for spalling cannot be derived. The cracking and spallation of scales was correlated with the type, morphology and growth of pores and voids in the scale and could be influenced by the steel microstructure.     

Strength of a half space weakened by a plane circular crack

The methods of the mechanics of brittle fracture and the theory of generalized cleavage fracture are used for the approximate solution of the problem of strength of a half space weakened by a plane circular crack. A simplified computational scheme is proposed for the evaluation of the fracture load in the case of free (tensile) fracture. The results of theoretical investigations are experimentally confirmed by testing concrete specimens. __________ Translated from Problemy Prochnosti, No. 6, pp. 122–132, November–December, 2006.     

Effects of notch geometry on the local cleavage fracture stress σf

An elastic–plastic finite element method (FEM) is used to analyse the stress and strain distributions ahead of notches with various depths and flank angles in four-point bending (4PB) specimens of a C–Mn steel. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ _f is measured. By increasing the notch depth and notch flank angle from 2.25 to 8.25 mm and 10 to 90°, respectively, the distributions of high stress and strain at the moment of fracture show considerable variations. However, the value of σ _f stays relatively constant. The critical fracture event is thus shown to be identical, i.e. the propagation of a ferrite grain-sized crack into the neighbouring matrix. It is concluded that σ _f is mainly determined by the length of the critical microcrack, while the notch geometry and its associated stress volume have little effect on the value of σ _f . The cleavage site ahead of a notch is determined by the stress distributions and the positions of the weakest grains.     

Fracture toughness diagrams of a notched body

To describe fracture toughness diagrams of notched bodies, a model of the cohesion zone near the notch root and an averaging criterion of stresses in this zone were employed. The geometric stress concentration factor and biaxiality coefficient affect greatly the shape of fracture toughness diagram. The notch root critical stress intensity factor is a decreasing function of geometric stress concentration factor. __________ Translated from Problemy Prochnosti, No. 5, pp. 142–148, September–October, 2006. Report on International Conference “Dynamics, Strength, and Life of Machines and Structures” (1–4 November 2006, Kiev, Ukraine).     

Modelling ductile fracture behaviour from deformation parameters in HSLA steels

In this work, an attempt is made to model the ductile fracture behaviour of two Cu‐strengthened high strength low alloy (HSLA) steels through the understanding of their deformation behaviour. The variations in deformation behaviour are imparted by prior deformation of steels to various predetermined strains. The variations in parameters such as yield strength and true uniform elongation with prior deformation is studied and was found to be analogous to that of initiation fracture toughness determined by independent method. A unique method is used to measure the crack tip deformation characterized by stretch zone depth that also depicted a similar trend. Fracture toughness values derived from the stretch zone depth measurements were found to vary in the same fashion as the experimental values. A semiempirical relationship for obtaining ductile fracture toughness from basic deformation parameters is derived and model is demonstrated to estimate initiation ductile fracture toughness accurately.