Presented by Robert Hurlston UNTF Conference 2011 Characterisation of the Effect of Residual Stress on Brittle Fracture in Pressure Vessel Steel

Content Introduction –Residual Stress –Constraint Work Undertaken –Finite Element Modelling –Experimental Results –Finite Element Modelling –Experimental –Two-Parameter Analysis (J-Q) Summary

Introduction It is extremely important that the integrity of nuclear plant can be ensured Failure assessment –Fracture toughness of materials within the structure are commonly used in failure assessments –This can be difficult to evaluate where weld residual stresses are present Therefore, –We need to understand the effects of residual stress on fracture toughness

Residual Stress … is defined as: –stress existing in a material when it is under no primary load This can contribute to crack driving force How else does it affect crack-tip conditions?

Constraint It is well known that geometry and loading affect crack-tip constraint

Effect of Residual Stress on Constraint? Can residual stress affect constraint of crack-tip material? Yes! –It has been demonstrated by many authors However, these effects are not well understood –Problematic associated plastic strains Can we characterise these effects?

Work Undertaken

Out-of-Plane Compression Based on work by Mahmoudi et al. Double punch pair situated ahead of crack Developed to generate residual stresses with no associated plastic strain Rx y I = Indentation W (= 50mm) a Punches Notch

Finite Element Modelling (Models) Single edge notched bend specimens modelled with cracks of a/W = 0.2 and a/W = 0.4 (where W = 50mm) Circular features simulated punch contact with surface

Finite Element Modelling (Residual Stresses Generated) Out-of-plane compression used double, 5mm radius ‘punches’ Stress was generated ahead of crack-like notch before crack was grown to final length (5mm growth) a/W = 0.2 a/W = 0.4

Finite Element Modelling (Loading and J-Integral) Loading was simulated in 3-point bending (span = 200mm) –-140 o C to ensure cleavage fracture conditions A boundary layer model was also loaded in tension to simulate small- scale yielding conditions (for calculation of Q)

Experimental (Out-of-Plane Compression) Carried out to validate the Finite Element findings Out of Plane Compression

Experimental (Loading) 3-point bend testing carried out at -140 o C Good agreement between experiment and Finite Element data

Results

Constraint Based Fracture Mechanics Elastic-plastic crack-tip fields can be characterised via a two parameter approach where: –J describes the crack-tip driving force and –Q describes crack-tip constraint condition The approach allows ‘apparent’ fracture toughness to be determined

J-Q Space J-Q space Loading line (evolution of constraint with increasing J) Failure Line (J for failure increases as constraint is lost) Failure deemed to occur where lines intersect Constraint corrected J (J c ) 0 J Q J* c

No Residual Stress Crack-tip stress fields, generated during loading of the boundary layer model, are plotted at increasing J-integrals Finite Element Results a/W = 0.2 a/W = 0.4 Residual Stress

J-Q Analysis Using constraint based fracture mechanics: –Loading lines can be plotted –Their associated fracture toughness curves can be plotted using RKR Closed form equation is in good agreement

Experimental Results Specimens with residual stress fail at lower loads Large degree of scatter –A533B laminate microstructure

Experimental Validation Mean experimental results validate the use of unique toughness curve –All within 7% of the closed form failure curve

Summary

It is known that residual stresses can affect crack-tip constraint –How it does was not well understood This work has validated the use of a unique failure curve in J-Q space when residual stresses affect crack-tip conditions –Where no associated plastic strain is present –Using novel adaptation of out-of-plane compression Future work might consider the effect of plastic strain on constraint and the use of unique a material toughness curve –Allowing inclusion into failure assessment guidance

Questions??? References: Hill M R and Panontin T L. Effect of residual stress on brittle fracture testing. Fatigue and Fracture Mechanics29, ASTM STP Sumpter J. The effect of notch depth and orientation on the fracture toughness of multi-pass weldments. Int. J. Pres. Ves. and piping Mahmoudi A H, Truman C E and Smith D J. Using local out-of-plane compression (LOPC) to study the effects of residual stress on apparent fracture toughness. Engineering Fracture Mechanics –1534. June 2007