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1 CONSTRAINT CORRECTED FRACTURE MECHANICS IN STRUCTURAL INTEGRITY ASSESSMENT Application to a failure of a steel bridge Anssi Laukkanen, Kim Wallin Safir.

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Presentation on theme: "1 CONSTRAINT CORRECTED FRACTURE MECHANICS IN STRUCTURAL INTEGRITY ASSESSMENT Application to a failure of a steel bridge Anssi Laukkanen, Kim Wallin Safir."— Presentation transcript:

1 1 CONSTRAINT CORRECTED FRACTURE MECHANICS IN STRUCTURAL INTEGRITY ASSESSMENT Application to a failure of a steel bridge Anssi Laukkanen, Kim Wallin Safir Interim Seminar, January 2005

2 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 2 Temperature K JC Baseline toughness Q effect Geometry related constraint T stress  Yielding related constraint Q  T stress effect Beyond basics “R&D” department

3 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 3 T-Stress Effect on the Master Curve

4 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 4 Uniform temperature Varying temperature Treatment of Surface Cracks in the MC Method Or divide crack in constant temperature sections.

5 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 5 Master Curve T 0, nearly linearly dependent on T-stress

6 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 6 Master Curve expression verified by analytical expressions Direct stress comparison Local approach prediction

7 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 7 Comparison of Local Approach and Constraint Corrected Master Curve

8 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 8 Master Curve analysis of the Point Pleasant bridge failure 213 m 116 m Originally built in 1928, bridge floor renovated in 1941.

9 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 9 Bridge failed December 15, 1967 at 5:10 PM, 46 lives were lost. T = -1°C ”Cracking” started 30 min before collapse. Cause of failure was identified to be brittle fracture of eye-bar 330 in joint C13N.

10 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 10 Failure iniatiated from semielliptic stress corrosion crack in eyebar Actual stress at the edge of the hole was estimated to be 585 MPa 3.2 mm 1.6 mm ≈ 10 mm

11 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 11 Heat-treated rolled carbon steel with forged heads,  Y = 520 MPa The material has low upper-shelf energy and high transition temperature. Resembles, in properties, a highly embrittled pressure vessel steel.

12 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 12 Master Curve analysis of fracture toughness results SEN(T) specimens have T-stress ≈ -130 MPa.

13 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 13 Application of Master Curve and constraint correction for real cracks Normally, the Master Curve parameters are determined using test specimens with "straight" crack fronts and comparatively uniform stress state along the crack front. This enables the use of a single KI value and single constraint value to describe the whole specimen. For a real crack in a structure, this is usually not the case. Normally, both KI and constraint varies along the crack front and in the case of a thermal shock, even the temperature will vary along the crack front. Standard MC

14 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 14 Application of Master Curve and constraint correction for real cracks A visualisation, that is in line with ASME practice, can be achieved by defining an effective stress intensity factor K Ieff corresponding to a specific reference temperature. The reference temperature can e.g. be chosen as the minimum temperature along the crack front. The procedure is to determine an effective driving force, which would give the same failure probability as a standard Master Curve presentation. K I   is obtained from the stress analysis as a function of location (  ). K 0Tref is the standard, high constraint, Master Curve K 0, corresponding to a reference temperature along the crack front.

15 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 15 Constraint correction The expression is directly applicable with the ASME Code Case N-629 fracture toughness reference curve, since it is written in terms of the standard deep specimen T 0 (RT To ). Constraint corrected

16 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 16 Battelle full scale eye-bar test a ≈ 9 mm, 2c ≈ 20 mm (s ≈ 30 mm) T = 0°C  f =393 MPa “K IC ” = 51 MPa  m (original) T stress ≈ -200 MPa

17 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 17 Battelle full scale eye-bar test Constraint corrected Full scale test decribed well with MC.

18 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 18 ASSESSMENT OF THE FAILURE

19 VTT TECHNICAL RESEARCH CENTRE OF FINLAND mm ≈ 10 mm Engineering assessment T 0 = +74°C  = 585 MPa T = -1°C  Y =540 MPa Flaw re- characterisation: SINTAP plasticity correction

20 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 20 SINTAP level 1 plasticity corrections: Conservative Non-conservative Master Curve analysis T-stress shallow flaw correction (T/  ≈-1) Realistic but safe prediction of eye- bar failure. ENGINEERING ASSESSMENT SAFE AND RELIABLE

21 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 21 FEM analysis of eye-bar 330  -local = 585 MPa  -global = 210 MPa

22 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 22 Without plasticity correction 3.2 mm 6.5 mm K Ieff25mm = 54 MPa  m s = 10 mm Best estimate of eye-bar 330 Best estimate prediction close to 50 % failure probability based on full MC analysis.

23 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 23 CONCLUSIONS Master Curve based methods for pressure vessel integrity assessment are applicable also for other structures. Method validated for catastrophic failure of the Point Pleasant bridge, containing: real shallow surface crack (constraint effects, variable K I ) brittle steel (resemble embrittled PV steel) high primary stresses (nozzle corner etc.)

24 VTT TECHNICAL RESEARCH CENTRE OF FINLAND 24


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