engleski

Spreading of Plastic Zones by Heterogeneous Fracture Toughness Specimens Cracked in the HAZ

The heat affected zone (HAZ) is in many welded structures considered to be the preferred location for the initiation and propagation of cracks. In order to evaluate the fracture tuoghness and possible causes of fracture, the stress-strain field at cracks located at HAZ must be understood. In this investigation a high strength low allowed steel (HSLA) with 700 MPa strength class was used as a base material (BM). The root of X-welded joint was produced with two passes of overmatched metal with strength mismatch factor M=1, 13 and filler was produced with M=1, 22. Three-point bending test specimens with 36 mm of thickness were extracted from the welded joints. Surface crack depth to specimen width ratio (a/W) was 0, 3. Both, fracture tests and plane stress finite element modelling are performed. The HAZ thickness modelled is 1, 5 mm. Mechanical properties of HAZ determined from microhardness are higher with respect to the weld metal (WM) properties. The comparison between experimental and numerical values of CTOD (d5) displacements shown good agreement. Plastic zones around HAZ crack tip for three levels of loading are presented. The highest yielding region is observed in the soft BM and the smallest in the HAZ. Similar asymmetric behaviour around crack tip shows the distribution of 0, 2 % of equivalent plastic strains. The yielding is directed from the HAZ toward base material, but HAZ with its high strength restricts further plastic deformation and locally rises the stresses. The results for principal stresses distribution shown that highest maximum principal stress is reached just under the HAZ crack tip in direction of BM. Higher stresses are observed in the hard HAZ than in the softer WM. The stress will reach its peak value at a distance less than two times the local CTOD's. In the case of less brittle HAZ, an overmatched weld metal will protect the weldment and favour yielding in the base material.

weldments; HAZ; fracture toughness; strength mismatch; FEA; plastic zones

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