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Metal Casting Technologies : March 2006
16 www.metals.rala.com.au Optimize Core Performance TECHNICAL FEATURE INTRODUCTION t meetings in Thailand and some neighboring countries, whenever there are presentations covering the mechanical properties of cast metals, my colleagues and I are quite often asked by delegates from industry to explain "what does fracture toughness mean and how do you measure it?" The basic answer given is that measurement of fracture toughness provides a quantitative description of the resistance of a material to the rapid propagation of a pre-existing defect or crack. Fracture toughness is sensitive to variations in microstructure and is of value in determining the seriousness of defects such as shrinkage voids, inclusions, hot tears, welding cracks, etc., and also, of fatigue or other cracks that may develop during service. It is measured using standardized test procedures under the most severe conditions for the material being tested: that is, the sharpest possible crack in a specimen whose size and geometry are sufficient to give the maximum possible constraint to plastic deformation in the crack tip regions. This article is intended to provide cast metals engineers with a little more information on fracture toughness and to give a short introduction to the subject of fracture mechanics. TOUGHNESS DETERMINED BY IMPACT TESTS Most foundries are familiar with routine tensile, hardness and impact testing as part of the quality assurance procedures needed to show that their castings meet the mechanical properties requirements of customers. Conventional impact tests such as Charpy and Izod are used to indicate the impact toughness of a material, i.e. its ability to plastically deform without fracture under impact loading. Impact toughness is measured in terms of the amount of energy that is needed to fracture a standard specimen. The impact loading energy is provided by a calibrated pendulum using a standard test procedure (1, 2). A tough ductile metal will absorb much more energy in fracturing than a brittle material when it is struck by the pendulum, so toughness can be quantified as the amount of energy, absorbed from the pendulum, which is required to cause fracture. Impact tests are easy to perform, even at sub-zero temperatures, since the specimens are relatively small, only 10mm square cross section. Most alloys are tested using specimens notched with simple machined V, U, or key-hole notches depending on test requirements but alloys with limited toughness may be assessed using un-notched test pieces. In the tougher materials the presence of a notch is used to initiate fracture due to the stress concentration at the notch. Impact tests are useful in ferrous foundries to determine the ductile to brittle fracture transition temperature range in Carbon & Low Alloy Steels and Ductile Irons, and to monitor the effectiveness of heat treatments applied to Low Alloy and Stainless Steels. Differences between the impact energy values for un- notched and notched specimens give an indication of the notch sensitivity of a material. Figure 1 illustrates the effect of lower temperatures, and the effect of the presence of a notch, on the impact toughness of a ferritic ductile iron and a flake graphite iron (3). Impact toughness data are normally expressed in Joules, J (or Nm), in place of the previous ft-lb unit, where 1J = 0.7376 ft-lb. Although impact toughness tests are relatively straightforward to perform, the information that they provide is somewhat limited in that the impact toughness values cannot be used in a quantitative manner in engineering design to predict service performance. For all alloys, test data from V-notched bars can be subject to scatter due to variations (from machining) in the sharpness of the notch, and hence stress concentration, at the notch root. In alloys such as High Strength Steels, Alloy White Irons, and some Non--Ferrous materials, impact tests cannot discriminate between toughness levels for different microstructural conditions in an alloy, or between alloys of similar type. FRACTURE BEHAVIOUR The first analytical approach to study the fracture behaviour of materials was the classical work by A.A. Griffith some 80 years ago. He showed that the stress required to cause fracture of glass depended on the size of the pre- existing flaws that were present in the glass, and hence he developed the well known Griffith equation for an embedded flaw: In which σ f = the fracture stress in MPa α = one half of the length of the flaw (or crack) in m E = Modulus of Elasticity of the glass in MPa γs = the surface energy needed to extend the crack by unit area in J/m2 The surface energy to create the new area of fracture face is provided by the elastic strain energy from the tensile loading. In a brittle material like glass there is no plastic deformation leading up to fracture so the only energy input required to produce fracture is that needed to provide the surface energy (γs) of the newly created fracture faces. This comes from conversion of the elastic strain energy during the tensile loading. However in a ductile material such as a metal some plastic deformation will always occur in the zone around the crack tip before failure takes place. Additional energy input, called the plastic work γp, is therefore required to fuel this plastic deformation. A Figure 1. The results of impact tests illustrate the effects of internal and external notches on impact toughness (3). Flake graphite cast irons contain sharp flakes of graphite that act as notches resulting in low impact toughness. Ductile irons, containing spheroidal not flake graphite, do not have such sharp internal notches and have greater toughness, however they are sensitive to the presence of external notches. By Dr. John Pearce An Introduction to Fracture Toughness