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Metal Casting Technologies : March 2006
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: email@example.com ® TRUBOND is a registered trademark. All rights reserved. ©2003 Worldwide: www.metalcaster.com If plane strain is not achieved during the test the "fracture toughness" determined will be greater than KIc and the crack propagation resistance of the material will be overestimated. The variation of fracture toughness with section thickness is illustrated in Figure 5 (7, 8). The presence of shear lips on the fracture surface indicates that part of the failure occurred under plane stress and mixed mode conditions, not under plane strain, because the test specimen thickness was insufficient resulting in a misleading high fracture toughness value. For a thin specimen (plane stress condition) there is less plastic constraint at the crack tip than for much thicker specimens so the plane stress fracture toughness Kc is higher than the plane strain fracture toughness KIc. It is very important to note that the size of specimen needed to achieve plane strain conditions depends on the yield strength of the particular material being tested. Experience has shown that for plane strain conditions, i.e. for a valid KIc determination then where σys is the yield strength. If this condition is not met then the thickness of the test specimen must be increased in further testing. In the strictest sense the approach to fracture outlined above assumes that no yielding occurs i.e. it can only be applied to linear elastic materials. Hence it is termed Linear Elastic Fracture Mechanics (LEFM). However this LEFM approach can be used with confidence for metallic materials where the local plastic deformation (yielding) that occurs at the crack tip is limited. This is not the case for low yield strength tough materials such as wrought constructional steels where the LEFM approach would require the use of excessively large test specimens, and equally very high capacity test machines. For the interested reader Knott (4, 5) provides clear worked examples of this problem and explains the need for a general yielding fracture mechanics approach, e.g. measurement (9) of crack tip opening displacement (CTOD) and the energy based J Integral method. Figure 5. Variation in measured fracture toughness Kc with test specimen thickness (7, 8). In thin sheet the plane stress condition allows plastic deformation resulting in a high measured toughness: shear lips are seen on the fracture surfaces. As thickness is increased plane strain conditions at the central zone restrict plastic shear to the edges of the specimen giving mixed-mode fracture behaviour. When the test specimen is sufficiently thick to give plane strain throughout the section the measured fracture toughness is at a minimum i.e. at the plane strain fracture toughness value KIc.