Weld Fracture. How Residual Stresses Affect Prediction of Brittle Fracture. Outline. Residual Stress in Thick Welds

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1 How Residual Stresses ffect Prediction of Brittle Fracture Michael R. Hill University of California, Davis Tina L. Panontin NS-mes Research Center Weld Fracture Defects provide location for fracture initiation Residual stress impacts fracture Opening stress Driving Force Here we consider only the influence of residual stress (RS) on weld fracture Residual Stress in Thick Welds Outline Depth (mm) Gunnert (9) Two-sided butt-weld in plate Center of weld length and width Stress (kg/mm ) Long. Trans. Perp. Long. (z) Perp. (y) Trans. (x) Background on residual stresses and fracture approach Constraint effects in fracture Micromechanical fracture prediction Fracture simulation FEM with micromechanical failure theory Girth weld fracture specimen testing Comparison of predictive methods

2 Predicting RS Influence on Fracture Previous research focuses on driving force effects Suggested by codes ccounts for contribution of RS to J or K I J total = J el + J pl = (K appl + K RS ) /E + J pl J pl and K appl from reference solution K RS from weight function solution Fracture predicted when J total = J c Constraint effects ignored Effect of Constraint on Crack-tip Stress Under certain conditions, crack-tip stresses are not predicted by J SSY (Infinite body) Large structure Constraint altered by: Q Size Loading mode Crack geometry Define the parameter Q Finite Body Test specimen Increasing Load 3 r/(j/σ o ) Indication of constraint and magnitude of hydrostatic stress Q usually negative J-Q locus can be used in fracture prediction σ yy σ o Problem Definition Prediction of -weld Fracture Large diameter girth weld fracture Geometries Structural -, D o /t =, t =. inch Lab specimen - Tools FEM - Refined meshes to compute J and crack-tip fields Residual stresses introduced using eigenstrain Micromechanical failure theory Predict fracture from local crack-tip conditions Predict fracture using global parameters xial load, mild steel (5-7) Two-sided weld External girth flaw, a/t =.3 t =5mm Section - a a/t=.3 b=t-a Girth weld Toughness data from specimen,, yy,y P a/w =.3 S Z 58 mm B W W = B = 5mm S = W R 58 mm P

3 Residual Stresses in Girth Residual Stresses in Specimen Residual stress assumed independent of θ Stress computed by imposing eigenstrain.5.5 σ/σ o Weld Center xial Hoop Radial Distance from the inner surface (r/t).8. σ/σ o.. Z 58 mm xial Hoop Distance from the centerline (z/t) R Surface Stresses 58 mm Residual stress opening the crack largely unchanged Residual stress acting along the crack front is changed This leads to different constraint influence caused by residual stress Through Thickness, x/w Through Thickness, x/w Normalized Transverse Stress Out-of-plane Stress Fracture Simulation Refined finite element models Symmetric, blunt-notched mesh Crack-tip : xisymmetric, r o =.5, nodes : 3D, r o =.5, 9977 nodes y J z plasticity, finite strain formulation Three analysis steps: Residual stress introduction Crack extension Loading to failure x,,, yyy Symmetry planes, y,, yy,, yy Crack-tip stress-strain history computed, J-integral estimated (including residual stress) Micromechanical Fracture Prediction Continuum micromechanical damage model RKR model for cleavage σ f * and l* FEM can provide crack-tip stress 5-7 σ f * = 3.5σ y l* = 3 grain diameters Micromechanics defines fracture (local) σ yy /σ o Global parameters J c and Q at predicted fracture. σ f */σ o r = l* σ yy σ f * over r l* r/(j/σ o ) 8

4 Results - Global Results - Constraint RS alters J-integral RS causes drop in load at fracture, P c - 5% - 5% J RKR criteria satisfied J-integral Normalized by for which J c = 7. kn/m....8 Load / Limit Load J-Q analysis quantifies constraint change Residual stress increases constraint Constraint change is much larger for the structure J/(bσ o ) ( -3 ) at rσ o /J = Residual stress causes drop in J at fracture, J c - % - 3% Q = (σ yy - σ yy ssy )/σ o testing does not bound structural behavior Results - Results - Find J from FEM Failure load at J = J c J c defined from Non-conservative failure prediction relative to RKR by % by % Configuration RKR....8 Failure Load / Limit Load Prediction affected by geometric constraint Toughness Source Conservative Non-conservative.5.5 Failure Load For the ( / RKR) Geometrically corrected toughness grossly non-conservative Prediction using is fortuitous Constraint-loss + Constraint-addition

5 Conclusions Residual stress changes crack-driving force Residual stress changes constraint J-Q theory helpful is in quantifying the constraint effect of residual stress ignores constraint imposed by residual stresses Can cause large errors in fracture prediction Micromechanical approach is valuable Includes effect of residual stresses on J Includes effect of residual stresses on constraint Effects to be shown experimentally