J. Toribio

A FRACTURE CRITERION FOR HIGH-STRENGTH STEEL NOTCHED BARS

Abstract In this paper a fracture criterion is proposed for high-strength steel subjected to multiaxial stress states produced by notches of very different geometries. The research includes a fractographic study of the microscopic modes of fracture by scanning electron microscopy, and a numerical analysis by the finite element method to compute the distribution of continnuum mechanics variables in the samples at the fracture instant. The results demonstrate that fracture takes place when the distortional part of the strain energy density reaches a critical value over a critical distance characteristic of the microstructure of the material.

Hydrogen-assisted micro-damage evolution in pearlitic steel

Fractographic analysis plays an important role in hydrogen induced fracture to clarify the physical micromechanisms governing the damage=failure process towards the critical situation and to determine the characteristics and extent of the region affected by hydrogen-assisted damage at the microscopical level [1, 2]. Previous research on hydrogen-induced fracture of pre-cracked and notched samples of high-strength pearlitic steel revealed the existence of a specific microscopic fracture mode: tearing topography surface (TTS), which can be associated with the hydrogen embrittlement process and thus can be considered as the microscopically-affected region [3]. This paper is focused on the microscopic appearance and progression of the TTS region from the initiation (sub-critical) to the fracture (critical) point, to elucidate on some specific features of the evolution of this hydrogen-affected region throughout fracture tests under a hydrogen environment in a triaxial stress state produced in the vicinity of a notch. The material used in this investigation was a hotrolled pearlitic steel supplied in bar form of 12 mm diameter. Chemical composition and mechanical properties appear in Tables I and II, respectively. The specimens were round notched samples of the four geometries sketched in Fig. 1. The dimensions of the samples were:

The reliability of the fracture mechanics approach to environmentally assisted cracking: 1. Uniqueness of the v(K)-curve

Abstract This paper analyzes the reliability of the fracture mechanics approach to environmentally assisted cracking in engineering design. A wide collection of experimental evidences of uncertainty in the fracture mechanics characteristics of the phenomenon—the crack growth kinetics v(K) -curve and the threshold stress intensity factor K th — is presented. Although these basic fracture mechanics items are supposed to depend solely on the material and the environment, they are notably sensitive to the influence of a wide family of test/service variables, producing loss of confidence in materials evaluation and structural integrity assessment.

Hydrogen embrittlement of prestressing steels: the concept of effective stress in design

Abstract The concept of effective stress is introduced for engineering design against hydrogen embrittlement of prestressing steels. This concept takes into account the important role of surface residual stresses in hydrogen assisted cracking. The analysis is performed by using a diffusion-based computer model which includes both the hydrogen transport equations and the fracture criterion to predict the life of prestressing steel wires under hydrogen embrittlement environmental conditions. The effective stress concept allows an analysis of the influence of stress level and test temperature on the time to failure of the steel wires in laboratory and field situations.

The reliability of the fracture mechanics approach to environmentally assisted cracking: 2. Engineering safe design

Abstract In Part 1 of this work, ample experimental evidence was presented of uncertainty in the fracture mechanics characteristics of environmentally assisted cracking. This paper addresses the soundness of the fracture mechanics approach to the phenomenon — the K -dominance over all the process constituents — and reveals the intrinsic variability of the crack growth kinetics curve v = v ( K ) and the threshold stress intensity factor K th . Suggestions to consolidate the approach by a strictly local treatment are outlined and a procedure is proposed for engineering safe design against environmentally assisted cracking based on the idea of the worst crack tip situation as the intrinsic one for the material-environment system.

Microstructure-based modelling of fracture in progressively drawn pearlitic steels

Abstract The fracture process of progressively drawn steels is studied. Results show that drawing affects the phenomenon, so that heavily drawn steels exhibit anisotropic fracture behaviour. A micromechanical model of fracture is proposed to rationalize the results on the basis of the microstructure of the steels after drawing. In slightly drawn steels, the Miller–Smith model of shear cracking in pearlite may be adequate to describe the fracture process. In heavily drawn steels, a fracture propagation step appears, and it may be caused by extremely slender pearlitic pseudocolonies aligned in the drawing direction, with anomalous (very high) local interlamellar spacing which makes them preferential fracture paths with minimum local resistance.

Two-dimensional numerical modelling of hydrogen diffusion assisted by stress and strain

This work is based on previous research on the one-dimensional (1D) analysis of the hydrogen diffusion process, and proposes a numerical approach to simulate the phenomenon in two-dimensional (2D) situations, e.g., near notches. The weighted residual method was used to solve numerically the differential equations set out when the geometry was discretized through the application of the finite element technique. This developed procedure can be a suitable practical tool to analyze hydrogen embrittlement phenomena in structural materials.

Stress corrosion behaviour of high-strength steel: design on the basis of the crack growth kinetics curve

Abstract This paper deals with stress corrosion behaviour of high-strenghth steel in aqueous solution. Work is focused on the analysis of the crack growth kinetics curve as a fundamental tool for design against stress corrosion cracking. In the case of hydrogen assisted cracking, a typical stage II (plateau) appears in the curve at the lowest values of stress intensity factor K , corresponding to pure slow crack growth and a microscopical appearance of TTS (tearing topography surface) and a second pseudo-plateau associated with cleavage-like propagation (clearly faster than TTS) up to the final fracture. SEM analysis of the cleavage crack path demonstrated that the crack grows breaking the lamellae of pearlite, which is consistent with the Miller-Smith mechanism. Under conditions of localized anodic dissolution there is no pure sub-critical plateau, and crack growth at the lowest levels of K was produced by chemical attack. A transition microscopical topography—consisting of cleavage facets with some evidence of chemical attack—was found at slightly higher levels of K . For values of K near the fracture toughness, critical crack growth by cleavage appeared, with no evidence of a dissolution process, probably because there was not enough time for it to develop.

Anisotropic Fracture Behaviour of Progressively Drawn Pearlitic Steel

This paper analyses the differences in fracture behaviour of two cold drawn pearlitic steels with different degree of strain hardening: a slightly drawn bar and a heavily drawn wire. The load-displacement curve F-u was analysed in the two cases, with special emphasis in the characteristic points of the plot: the load defining the end of linear behaviour (Fe), the final fracture load (Fmax) and, in the heavily drawn steel, the pop-in load (FY). Results demonstrate that slightly drawn steels exhibit isotropic fracture behaviour with crack propagation along its own plane (mode I propagation). On the other hand, heavily drawn steels exhibit a markedly anisotropic fracture behaviour with crack deflection (mixed mode propagation), and vertical fracture embryos suddenly appear associated with the pop-in instant. This special fracture mode is a consequence of the markedly oriented microstructure of the heavily drawn steel.

Strength Anisotropy in Prestressing Steel Wires

Cold-drawn prestressing steel wires exhibit strength anisotropy in the form of fracture path deflection towards a direction approaching the wire axis, or cold drawing line, as a consequence of the pearlitic microstructure orientation induced by the manufacturing procedure. Such a crack path deflection is initiated at certain nuclei (fracture origins) at which axial cracking appears in the cold drawing direction (or wire axis) in the form of micro-cleavage units producing a macroscopic phenomenon of pop-in in the load–displacement curve. This chapter shows that such fracture initiators appear at a certain distance from the fatigue pre-crack tip at which a local maximum of the cleavage stress is located.