Fracture Toughness Testing of Ductile Weld Metal by Using Normalization Method

N. GUBELJAK; J. LANDES; J. DONOSO; M.D. CHAPETTI; J. PREDAN

Lima

Conferencia; nternational Conference on Welding and Joining of Materials (ICWJM) 2010N; 2010

Pontificia Universidad Católica del Perú

Relevant fracture mechanics values are essential for the structural integrity assessment of a structural component containing a crack-like defect Modern welding technology of the new high quality steels can assure high impact energy and high fracture toughness for both the base material and the weld material. Most often these materials exhibit Charpy impact energy of more than 350 J at room temperature. Fracture toughness of welded joints is tested according to existing standards (ASTM E-1820-01 or BS 7448: Part 4) using fracture mechanics parameters like J-integral or critical crack tip opening displacement (CTOD). To ensure the conditions under which the fracture toughness is considered to be a material property, independent of geometry, (shape of specimen, length of uncracked ligament) limits are imposed on the stable crack extension and maximum value of fracture toughness. These limits place the points between the blunting line and maximum stable crack growth. The above mentioned construction lines are limiting theoretical valid area for the so called resistance curve of the material. In the case of modern materials the resistance curve can be outside the prescribed area. This article studies the fracture behavior of a welded joint used in pressure vessels. The analysis has shown that current standards cannot be used for fracture toughness determination of highly ductile materials as a function of one fracture mechanics parameter. The fracture behavior can be described with the combination of standard normalization method and observation of plastic region at the crack tip (size and shape) during fracture test. In this paper it is postulated that the fracture toughness resistance of the material can be described by using standard (ASTM E-1820-01) normalization method in spite of the fact that values of fracture resistance are out of standard range. Due to this, fracture toughness tests of weld metal using existing procedures according to standards may give invalid results for the critical crack initiation toughness or fracture resistance curve in terms of the J-integral or the critical crack-tip opening displacement, CTOD. In such cases, the measured high toughness is the consequence of the gross yielding of a significant volume of un-cracked ligament, and can be measured only by using surface displacement measurement techniques. The results show that standards based on recording load vs. few measured clip gauge points and displacement are insufficient. The normalization method seems a good alternative measurement and analysis technique which indirectly correlates to surface displacement measurements, especially in case of miniature mechanical testing