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Metal Casting Technologies : March 2005
reference in any failure investigation [5,11-12]. Figure 1 presents a simple example of the differences between ductile and brittle behaviour in a low C steel. The determination of valid Fracture Toughness data and its correct application has made an invaluable contribution in design, inspection, in-service monitoring and hence safe materials usage. Using such data it is possible to calculate whether or not a defect or a sub-critical crack (e.g. a fatigue crack) of a given size will propagate in a catastrophic (fast fracture) manner under service stresses . Hence, within limitations, if the size and shape of flaws are measured by non-destructive means the level of stress that will cause fracture can be found. Alternatively the maximum allowable flaw size can be calculated for a given design stress. The limitations are imposed by the difficulties of precise determination of applied stresses, e.g. at stress concentration points, and of the confident measurement of defect/s size and shape. Measurement problems include those presented by a cluster of defects and limitations in resolution of the available NDE techniques. In spite of such difficulties the quantitative design against fracture approach ensures safer operation of critical plant and equipment resulting in a reducing number of actual services failures. THE STEPS IN A FAILURE ANALYSIS INVESTIGATION The need for a disciplined, methodical approach to the analysis of a service failure cannot be overstated since it is easy to rush a specimen onto our latest "all singing-all dancing" instrument after paying little attention to the information that can be obtained by simple visual examination. Premature cleaning or sectioning may completely destroy important evidence. The disciplined approach also guards against management, public, or other pressures to "get on with it" in order to provide a quick answer to the cause of failure. The procedure and number of steps in a failure analysis will depend on the nature and the complexity of the failure. Some investigations require the efforts of a team of people such as materials experts, construction engineers, stress ana- lysts, chemists, etc. In some situations the investigation can be shortened by making use of an "expert systems" approach . Detailed practice is described together with techniques, precautions and practical examples from an wide range of applications in a number of publications [2-4,14-15]. The steps can be outlined as: 1. Obtain all the background information on the failure. Visit the failure site and get the failed items as soon as possible before any changes occur due to mishandling, repair, corrosion, removal of evidence ,etc. Dig out all possible information: time, conditions, witness observations, sequence of events, damage. Interview site Supercharge Your Green Sand System Green sand systems work harder and last longer with TRUBOND®. These sodium bentonite grades mull in quickly and undergo a controlled hydration to develop a stronger, more elastic adhesive. TRUBOND delivers excellent green compressive and hot strength properties and maintains its working bond in repetitive exposure to high heat loads and metal temperatures. SODIUM BENTONITE Unimin Australia Limited Tel.: +613 9586 5400 Fax: +613 9586 5411 E-mail: firstname.lastname@example.org ® TRUBOND is a registered trademark. All rights reserved. ©2003 Worldwide: www.metalcaster.com