FRACTURE TOUGHNESS TESTING OF DUCTILE WELD METAL BY USING NORMALIZATION METHOD

CHAPETTI, MIRCO DANIEL; CARR, GUSTAVO EDUARDO; GUBELJAK, NENAD; LANDES, JOHN; PREDAN, JOZEF

Rosario

Congreso; 11º Congreso Binacional de Metalurgia y Materiales; 2011

Sociedad Argentina de Materiales / Sociedad Chilena de Materiales

<!-- @page { margin: 2cm } P { margin-bottom: 0.21cm } --> Relevant fracture toughness 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 and the weld material. Fracture toughness of welded joints is tested according to existing standards (ASTM E-1820-01, BS 7448: Part 4, etc.) 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. In this way, some 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. Besides, 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 normalization method seems a good alternative measurement and analysis technique, which indirectly correlates to surface displacement measurements, especially in case of miniature mechanical testing. The analysis was carried out using as base material SA516 Gr. 70 with 26 mm thickness and as weld material Elgacore DWA 55LSR, rutile filled wire of 1.2 mm diameter. The welding was carried out with a preheating temperature of 90 ± 10° C and a maximum interpass-temperature of 200° C. The welding procedure, according to DIN EN 24063 is 136 weldcar with average heat input of 0.75 kJ/mm, was used. The average tensile mechanical properties are σy (yield stress) = 528 MPa, Rm (UTS) = 568 MPa, σm (failure stress) = 648 MPa and n (hardening exponent) = 0.061 for the base material, and σy = 586 MPa, Rm = 632 MPa, σm = 715 MPa and n = 0.063 for the weld metal. SENB specimens with welded joint were prepared. These specimens were subjected to fatigue loading in order to obtain a fatigue pre-crack according to Standard ASTM E-1820. The total crack length (machined and fatigue crack) was between 0.45 and 0.70 W (W is the specimen height) for J integral and CTOD determination. With the use of the values of crack extension acquired using the normalization method, and of the equations for CTOD calculation, the curve can be derived. With the critical fracture toughness values at initiation points defined, the J-R resistance curve can be also determined [5]. The results clearly show that the standards for testing the fracture toughness of high toughness weld metal should be revised. The application of the normalization method can be an appropriate alternative method to determine the J-R curve of the material from the basic load-displacement record of the test. The fracture behavior can be described with the combination of the normalization method in the test standard and measurements of the stretched zone within the plastic region at the crack tip (size and shape). It is postulated here that a reasonable fracture toughness value resistance of the material can be described by using the standard (ASTM E-1820-01) normalization method even though the values of fracture resistance are greater than those allowed by the standard and hence out of the valid range. The value J = 575 MPa is calculated from plastic SZW measurement, having Ji a higher because it is given by the elastic and plastic energy stored till the crack initiation event. However, the measurement of SZW confirms that toughness of tested material in blunting stage is higher than the limit value Jmax.