F. Gutiérrez-Solana

Determination of the mechanical properties of normal and calcified human mitral chordae tendineae.

The aim of the present research is to determine the influence of the calcification of human mitral valves on the mechanical properties of their marginal chordae tendineae. The study was performed on marginal chords obtained from thirteen human mitral valves, explanted at surgery, including six non-calcified, four moderately calcified and three strongly calcified valves. The mechanical response of the chords from the non-calcified and moderately calcified valves was determined by means of quasi-static tensile tests (the poor condition of the strongly calcified valves prevented them from being mechanically characterised). The material parameters that were obtained and analysed (the Youngs modulus, the secant modulus, the proportional limit stress, the ultimate strength, the strain at fracture and the density of energy stored up to maximum load) revealed noticeable differences in mechanical behaviour between the two groups of mitral chordae tendineae. Large scatter was obtained in all cases, nevertheless, considering the mean values, it was observed that the normal chords are between three and seven times stiffer or more resistant than the moderately calcified ones. On the contrary, the results obtained for the strain at fracture showed a rather different picture as, in this case, no significant differences were observed between the two families of chords. A scanning electron microscopy study was conducted in order to find out the relevant features of the calcium deposits present in the calcified chordae tendineae. In addition, the general aspects appreciated in the stress vs. strain curves were correlated with the collagen morphological evidences determined microscopically. Finally, the calcium content present in the three groups of chords was quantitatively determined through atomic absorption spectroscopy; then, the relation between the mechanical properties of normal and moderately calcified chords as a function of its calcium content was obtained. This analysis confirmed the existence of a strong correlation between calcium content and stiffness or resistance whereas the influence on the ductility seems to be negligible.

Use of Small Punch Notched Specimens in the Determination of Fracture Toughness

In recent years an increasing interest has been shown in materials characterisation techniques based on the use of reduced dimension samples. One of these techniques, first developed in the early 80’s, is Small Punch testing, in which a punch acts over a plane small specimen deforming it until fracture. So far, this test has been used as a tool for the estimation of mechanical properties, for characterising ductilebrittle transition or for knowing the evolution of these properties with ageing phenomena as neutron irradiation. The Small Punch Test has even been used in the determination of fracture toughness by means of indirect correlations or complex methodologies based on damage models and Finite Element simulations. In this paper, a direct methodology for the determination of fracture toughness from Small Punch tests is proposed. The approach is based on the use of simply modified Small Punch specimens in which a lateral notch was machined. Attending to energy considerations, the J-Δa curve for a ship-building Grade A steel, as well as for a structural steel, E690, has been obtained using this type of samples and the results have been compared with the calculated results from conventional fracture tests.Copyright

The effects of microstructure, strength level, and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.

A strain-based fracture model for stress corrosion cracking of low-alloy steels

The stress corrosion cracking (SCC) behavior of 4135 steel under different heat treatments is analyzed in an attempt to relate microstructural characteristics with macroscopic measurements of SCC resis-tance, especially the very impressive improvements associated with changes from intergranular (IG) to transgranular (TG) fracture paths. Considering that local hydrogen embrittlement at the crack tip causes SCC processes, a local cracking criterion, based on a critical strain depending on hydrogen concentration, is assumed to control the process. Stress corrosion cracking is viewed as a discontin-uous series of unstable crack extensions through the locally embrittled regions. The model developed on this basis explains the macroscopic behavior observed at the threshold situation and partially at stage II propagation and clarifies the role of the metallurgical variables in each of the types of fracture detected.

Modelling the stress corrosion cracking of low alloy steels

Abstract Based on a precise characterization of the stress-corrosion cracking behaviour of 41xx steels after various heat treatments, a model of the micromechanisms, on a microstructural scale, present in the cracking processes during stress corrosion, has been defined. The model establishes that stress-corrosion cracking occurs by a series of isolated and unstable local fractures inside the plastic zone, controlled by local embrittlement due to absorbed hydrogen. These local fractures occur when a critical strain is reached at an appropriate microstructural feature, and are controlled by the strain field and by the hydrogen concentration and distribution in the plastic zone. The model explains the intergranular fracture behaviour of dislocated and lightly tempered martensites, associated with the plastic-zone fitting within the grain facets under critical conditions, also the intergranular-transgranular transition at a critical temperature of tempering and the transgranular fracture behaviour, either by microvoid coalescence for tempered martensites or by cleavage for bainites.

An elastic–plastic fracture mechanics based methodology to characterize cracking behavior and its application to environmental assisted processes

Cracking processes suffered by new structural and piping steels when used in petroleum or other energy installations have demonstrated the need for a cracking resistance characterization methodology. This methodology, valid for both elastic and elastoplastic regimes, should be able to define crack propagation kinetics as a function of their controlling local parameters. This work summarizes an experimental and analytical methodology that has been shown to be suitable for characterizing cracking processes using compact tensile specimens, especially subcritical environmentally assisted ones, such as those induced by hydrogen in microalloyed steels. The applied and validated methodology has been shown to offer quantitative results of cracking behavior and to correlate these with the existing fracture micromechanisms.

Predicting crack arrest behaviour of structural steels using small-scale material characterisation tests

Abstract The suitability of small-scale tests to characterise the crack arrest behaviour of structures is assessed in this work. Empirical correlations based on instrumented Charpy tests results have been used to obtain the main parameters that define the crack arrest properties, comparing the predicted values with those experimentally obtained. Furthermore, an energy approach has been applied to determine relationships, as a function of the test temperature, between the propagation energy and the crack arrest toughness. The experimental work has been performed on two different structural steels.

Correlation between impact resistance and fracture toughness in aged duplex stainless steels

Abstract As a part of a more extensive study [1] of aging embrittlement at low temperature (280–400°C) of cast duplex stainless steels, a close characterization of the evolution of toughness of three duplex steels with different ferrite content (12,18 and 22%) has been performed. Fracture toughness characterization was based on the determination of JR curves, in accordance with the European Recommendations ESIS P1-92 and following the unloading compliance single specimen method. Impact toughness was determined with an instrumented Charpy pendulum, which permits the load-deflection and energy deflection curves to be obtained. From the obtained results, a phenomenological model based on the presence of brittle ferrite in the fracture path has been developed. This model enables the impact toughness and the fracture toughness to be determined as a function of aging time and aging temperature for a duplex steel, and therefore to predict fracture toughness values from Charpy, results. Finally, a comparison has been made between energy predicted from impact and experimentally obtained fracture toughness for the three steels.

Analysing the Notch Effect Within the Ductile-to-Brittle Transition Zone of S275JR Steel

This paper analyses the notch effect in ferritic-pearlitic steel S275JR in a range of temperatures within the material Ductile-to-Brittle Transition Zone (DBTZ). The notch effect is evaluated in terms of load-bearing capacity, apparent fracture toughness (modeled here using the Theory of Critical Distances) and fracture micromechanisms. The concept of Master Curve in notched conditions is also presented.To this end, experimental results obtained in S275JR notched specimens are presented, together with Scanning Electron Microscopy (SEM) fractographies. The analysis is performed at −50 °C, −30 °C and −10 °C, the material Transition Temperature (T0) being −26.1 °C, with the notch radii ranging from 0 mm (crack-type defects) up to 2.0 mm.The results show how the lower the temperature the larger the notch effect, and also that the evolution of both the load bearing capacity and the apparent fracture toughness is directly related to the evolution of fracture micromechanisms. Moreover, the proposed Master Curve in notched conditions has provided good predictions of the experimental results.Copyright

An extension of the application of elastic-plastic assessment to cracked pipework systems

Abstract Some of the current methods for assessing cracked structures made of high toughness materials come under the scope of elastic-plastic fracture mechanics (EPFM). However, these methods only evaluate flawed sections and do not consider their inclusion into framed structures. Stress distributions in redundant structures vary with the stiffness of their elements. As the stiffness of cracked elements changes with crack length, the structures stress distribution is modified by crack extension. This paper presents a method for assessing high toughness redundant cracked piping systems taking into account the interaction between stress distribution and crack extension. The methodology is illustrated by solving some examples using the proposed method and comparing the results with those that would have been obtained using conventional practices.