Presented by Robert Hurlston

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

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

3. 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

4. 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?

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

6. 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?

7. Work Undertaken

8. Punches x R I = Indentation W (= 50mm) y Notch a 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

9. 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

10. Finite Element Modelling (Residual Stresses Generated) a/W = 0.2 a/W = 0.4 • 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)

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

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

13. Experimental (Loading) • 3-point bend testing carried out at -140oC • Good agreement between experiment and Finite Element data

14. Results

15. 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

16. 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 J Constraint corrected J (Jc) J*c Q 0

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

18. 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

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

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

21. Summary

22. 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

23. Questions??? • References: • Hill M R and Panontin T L. Effect of residual stress on brittle fracture testing. Fatigue and Fracture Mechanics29, ASTM STP 1332. 1998 • Sumpter J. The effect of notch depth and orientation on the fracture toughness of multi-pass weldments. Int. J. Pres. Ves. and piping 10. 1982 • 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 75 1516–1534. June 2007