Abstract
The applied model consists of a three step FE simulation procedure including elasticplasticstress analysis, diffusion analysis and finally elastic-plastic stress analysiswith hydrogen influenced cohesive elements implemented in the crack path. The applied FE code is ABAQUS Standard 6.5Constant load SENT testing at applied stresses of 450-550 MPa of not precharged25%Cr duplex stainless steel in 3.5% NaCl at 4°C and a corrosion protectionpotential of -1050 mVSCE revealed hydrogen induced stress cracking prior to finalfracture. Main findings:The stable crack growth rates ranged from 2.2 ·10-10 m/s to 1.5·10-8 m/s which isconsistent with the hydrogen diffusion rate.For prediction of crack initiation a finite element simulation procedure withhydrogen dependant cohesive elements in the crack path was carried out. Best fit toexperimental results was obtained for an initial critical cohesive stress of 2200 MPa(σc=3.7·σy) and a separation (δC) of 0.005 mm. The simulations indicate a lowerbound applied net section stress of 480 MPa giving a threshold stress intensityfactor (KHISC) of 20 MPa√m.In the simulations both lattice and trapped hydrogen contributes to a lowering of thestress necessary for the onset of fracture. A direct relation between plastic strainand trapped hydrogen concentration is implemented. The relation implies thatcompared to stress driven lattice diffusion plastic strain is the main contributor tothe simulated hydrogen concentration and crack initiation in the fracture processzone.
