by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : March 2006
20 www.metals.rala.com.au TECHNICAL FEATURE FRACTURE TOUGHNESS INFORMATION ON CAST METALS For casting alloys fracture toughness information is normally given as KIc values. As shown schematically (10) in Figure 6, graphs can be plotted to show the maximum allowable stress for given crack lengths at different KIc levels. The higher the fracture toughness the higher is the allowable service stress for a given defect size, a. Fracture toughness data for various cast alloys have been conveniently summarized, as seen in Figure 7, in a discussion by Campbell (11). As for wrought metals, as the yield strength of an alloy is increased its fracture toughness is reduced, with low alloy steels giving the best combination of high strength and toughness. Increasing the temperature normally increases the fracture toughness of BCC and HCP metals but has little effect on FCC metals. Any increase in the size or proportion of brittle phase, such as carbides or other intermetallics, will reduce toughness. Clean melting and liquid metal handling and filling to minimize inclusions and oxide films will clearly improve toughness in the same way that fatigue life is improved. For materials which have relatively low toughness such as the Alloy White Irons EDM notches can be sharp enough to produce valid fracture toughness results and fatigue pre-cracking may not be necessary. In these cases fracture toughness can be measured using instrumented Charpy testing (dynamic fracture toughness, KId) or short rod tests (KIcSR). In summary, fracture mechanics is used to determine if a defect of a known size and shape in a component will grow in an unstable manner resulting in sudden fracture at the stress levels to be met in service. This approach enables quality assurance procedures to be set in place so that any defect of critical size, in relation to the intended service stresses, would always be detected before the part entered service. The intention in writing this short article was to give those foundry engineers, who may not be familiar with fracture toughness, a feel for the subject, hopefully without causing confusion. For a better understanding the interested reader should consult specialized texts on the subject and the ASTM or other standards on toughness testing. ● Figure 6. Schematic relationship between defect (crack) size and allowable stress level for materials with high and low fracture toughness (10). Figure 7. Relationship maps between fracture toughness and yield strength for selected cast alloys (11). REFERENCES: 1. E23-05 Standard Test Methods for Notched Bar Impact Testing of Metallic Materials, ASTM International. 2. A327M-91(2006) Standard Test Methods for Impact Testing of Cast Irons (Metric), ASTM International. 3. D.R. Askeland, "The Science & Engineering of Materials", 1989, Van Nostrand Reinhold International. 4. J.F. Knott, "The Fundamentals of Fracture Mechanics", Butterworths (London) 1973 5. J.F. Knott and P. Withey, "Fracture Mechanics Worked Examples (2nd Edition), 1993, Institute of Materials, UK. 6. E399-05 Standard Test Methods for Linear-Elastic Plane-Strain Fracture Toug Welding Handbook, Volume 1, 7th Edition, 1976, American Welding Society. 8. J.W. Jones, R.A. Flinn & P.K. Taylor, "Analysis & Prevention of Failure", Ch.19 in "Engineering Materials & Their Applications", 4th Edition, 1990, Houghton Mifflin, US. 9. E1290-02e1 Standard Test Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement. 10. G.E. Dieter, "Engineering Design", 2nd Edition, McGraw Hill International, US. 11. J. Campbell, "Castings", 1991, Butterworth Heineman, UK.