Papers by Keyword: Fractography

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Authors: Dominik Krewerth, Anja Weidner, Horst Biermann
Abstract: The present paper illustrates a comparison of infrared thermography during ultrasonic fatigue testing of cast steel 42CrMo4 and cast aluminium alloy AlSi7Mg. Against the background of different material properties (e.g. mechanical properties as well as thermal properties) the benefit of this non-destructive material testing method in terms of determining the crack initiation point and time during fatigue testing as well as crack propagation is evaluated and discussed. Moreover, correlations between fractography and infrared thermography are performed for both materials.
Authors: Fabio Sorbello, Peter E.J. Flewitt, A.G. Crocker, Gillian E. Smith
Abstract: It is well established that within the lower-shelf temperature range of Fe2-3Si polycrystalline steels, the brittle fracture occurs predominately by transgranular cleavage, unless subject to embrittling heat-treatments. The cleavage fracture develops on the well established {001} planes of the bcc structure. In this paper we revisit the growth, of these cleavage cracks by considering crack propagation in single crystals of Fe2Si steel. Three point bend specimens manufactured from oriented crystals have been tested by impact loading at a temperature of -196°C. High spatial resolution focused ion beam imaging combined with ion milling is used to examine in detail the crack propagation path and has provided a new insight into the mechanisms involved. In particular it has been established that within the process zone of the propagating cracks local strain is accommodated by the formation of {112} twins. The results are discussed with respect to the overall crack propagation mechanism.
Authors: Hynek Lauschmann, Ondřej Kovářík
Abstract: The reference texture is a subset of the image texture in SEM fractographs of fatigue fractures. It is common to all fractures caused by loadings in which significant events occur sufficiently regularly and frequently. The reference crack growth rate is unambiguously related to the reference texture. A particular loading is characterized by the ratio of the reference and conventional crack growth rates called reference factor. Its value may be related to the sequence of successive sizes of cyclic plastic zone, while the mechanism of the effect of overloads follows the models of Wheeler and Willenborg. Application to a set of three test specimens from stainless steel AISI 304L loaded by various loading regimes is shown.
Authors: Wilson Luiz Guesser, Luis Carlos Guedes, Ailton L. Müller, Vagner B. Demetrio, Alexsandro Rabelo
Abstract: Fatigue strength and fracture of high strength cast irons, gray iron grade 300 and CGI grade 450, used for producing lightweight cylinder blocks, were studied. The results show endurance ratios of 0.27-0.28 and 0.38 for gray irons and CGI, respectively. The fracture surfaces in cast irons in general show the predominance of graphite and graphite/matrix interface; however, in CGI there is a larger proportion of fractured pearlitic matrix than in gray iron. This fact, and the differences in the morphology of the graphite/matrix interface, flat in gray iron, rough in CGI, explain the higher results of fatigue strength in CGI compared to gray iron. The results of fatigue strength are compared with the literature and with previous works.
Authors: Len Davidson
Abstract: From 1998 until 2001 a series of engine failures occurred in a fleet of large marine diesel engines due to the failure of precombustion nozzles. The nozzles are screwed into the head of the engines and are held in place by a tightening torque and a high temperature ceramic adhesive. The preliminary findings of an engineering investigation after one such failure concluded that cracking occurred in the nozzle at the time of the installation, due to poor installation technique, and that this cracking resulted in the loss of tightening torque causing the nozzle to become loose. However, the investigation also recommended that fractography be undertaken to verify the cause of failure since this would influence the engineering solution. The current paper focuses on the fractography that was conducted on the failed nozzle to determine its mode of failure and provides a reminder of the importance of fractography in failure analysis.
Authors: George D. Quinn
Abstract: The evolution of the science of fractography of brittle materials initially was driven by failure analysis problems. Early analyses focused on general patterns of fracture and how they correlated to the loading conditions. Many early documents are simply descriptive, but the curiosity of some key scientists and engineers was aroused. Scientific or engineering explanations for the observed patterns gradually were developed. Advances in microscopy and flaw based theories of strength and fracture mechanics led to dramatic advances in the state of the art of fractographic analysis of brittle materials. Introduction: This author was drawn backwards in time as he researched the current state of the art of fractographic analysis of brittle materials for his fractography guide book.[ ] Others have written about how the fractographic analysis of metals evolved (e.g., [ , , , ]), but there is no analogue for ceramics and glasses. The key scientists, engineers, and analysts who contributed to our field are shown in Fig. 1. Other work done by industry workers who were unable or loathe to publish is now lost, inaccessible, forgotten, or even discarded. It is the goal of this paper to review the key publications and mark the noteworthy advances in the field. Some deem fractography as the study of fracture surfaces, but this author takes a broader view. Fractography is the means and methods for characterizing fractured specimens or components and, for example, a simple examination of the fragments and how they fit together to study the overall breakage pattern is a genuine fractographic analysis.
Authors: Xiu Hai Zhang, Can Wei Lai, Man Feng Xian, Guang Cai Su
Abstract: The thermal fatigue behaviors of traditional Al-Si-Mg casting alloy and optimized Al-Si-Mg casting alloys at different thermal fatigue temperatures were investigated. Fatigue cracking appeared on the surface of traditional Al-Si-Mg alloy after 450 thermal cycles at 300 °C thermal fatigue temperature. However, the fatigue cracking was not found on the surface of optimized Al-Si-Mg alloy at the limited thermal fatigue cycles (less than 4450 times). Moreover, the optimized Al-Si-Mg alloy only occurred to elastic deformation and could not emerge in irreversible deformation. The grain refinement in the optimized Al-Si-Mg alloy could make the thermal fatigue cracking appeared much later and propagated much slowly. These results showed that the thermal fatigue resistance of optimized Al-Si-Mg alloy was superior to that of traditional Al-Si-Mg alloy.
Authors: Tatsuo Sakai, Shoichi Kikuchi, Yuki Nakamura, Noriyuki Ninomiya
Abstract: In order to use a low flammability magnesium alloy as structural components, very high cycle fatigue properties of this alloy (AMCa602) were investigated. S-N properties obtained in both rotating bending and axial loading were compared with each other. It was found that S-N curve in the axial loading appeared a little lower than that in the rotating bending due to the differences of stress distributions and critical volumes for both loading types. Moreover, the statistical aspect on the fatigue property was analyzed as P-S-N characteristics in the rotating bending. After fatigue tests, fracture surfaces of failed specimens were observed by means of a scanning electron microscope (SEM) and the microstructures at the crack initiation site and the propagation path were also observed by combining FIB technique and EBSD analysis. Thus, it was found that some specimens failed from surface inclusions and their fatigue lives were lower in comparison to those of the specimens without surface inclusions. In addition, the fracture surfaces of this alloy revealed very rough in the usual life region, whereas a characteristic smooth area was observed on the fracture surfaces of specimens failed in the surface inclusion-initiated fracture and in very high cycle regime. A stress intensity factor range at the front of the smooth area (ΔKsmooth) tended to a definite value so that the fracture mechanism of this alloy was governed by a concept of ΔK.
Authors: José M. López-Cepero, Sheldon M. Wiederhorn, António Ramirez de Arellano-López, Julian Martínez-Fernández
Abstract: Rhombohedral r-plane fracture surfaces in sapphire are analyzed by optical microscopy and by atomic force microscopy. Features of special interest include steps, lines and angles on the surface that appear to have crystallographic origins. A classification and description of these features is given over a scale ranging from hundreds of micrometers to tens of nanometers. Preferential directions in the surface are identified and related to the crystalline orientation of the sample; an attempt is made to identify the underlying phenomenology behind the appearance of each kind of feature.
Authors: Olivier Berteaux, Roger Valle, Monique Raffestin, Marc Thomas, G. Henaff
Abstract: The fatigue behaviour of a powder-metallurgy (PM) + heat-treated Ti49Al47Cr2Nb2 alloy is investigated using detailed SEM characterization. Based on the results of comparative static and cyclic loadings at RT, the fracture origins for the different test specimens is investigated. Conventional fractographic analyses revealed that internal structural defects inherent to powder metallurgy such as compaction defects, porosities and non-metallic inclusions can promote crack initiation. However, the fracture surface of test specimens is markedly affected by the microstructure, which is indicative of the microstructure dependence on crack initiation and propagation. In sub-transus heat treatment conditions, the detrimental effect of structural defects is illustrated by clear crack initiation sites onto the fracture surfaces. In super-transus conditions, crack propagation from defects can be blunted due to crack deflection, branching through lamellar interfaces, thus leading to lower defect sensitivity. Correlation of the results of these microfractographic examinations with the stress-strain curves corresponding to the various specimens allows identifying the role of such structural defects on the static and cyclic deformation behaviours. Finally, implications of such dependencies will be assessed relative to the requirements for aerospace gas turbine applications.
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