Fracture and Crack Propagation in Weldments. A Fracture Mechanics Perspective. Uwe Zerbst, BAM Berlin

Transcription

1 Fracture and Crack Propagation in Weldments. A Fracture Mechanics Perspective Uwe Zerbst, BAM Berlin

2 Outline Specific aspects of weldments Determination of fracture toughness Determination of the crack driving force Shallow crack propagation and fatigue strength

3 Outline Specific aspects of weldments Determination of fracture toughness Determination of the crack driving force Shallow crack propagation and fatigue strength

4 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch Residual stresses Misalignment

5 Fracture mechanics of weldments: Specific aspects Susceptibility to cracking

6 Weld imperfections Figure according to Gagg, 2005

7 ISO 5817: Arc welded joints in steel - Guidance on quality levels for imperfections 26 different types of weld imperfections Can be assigned to distinct groups from the perspective of mechanical integrity (a) Cracks and crack-like imperfections have to be avoided or if they occur are immediately subject to fracture mechanics analysis (b) Material imperfections which act as crack initiation sites of paramount importance for fatigue strength and fatigue life analyses (c) Geometric discontinuities increase the local stresses, affect crack initiation, propagation and final failure (d) Imperfections which probably are of no effect on fracture or fatigue life

8 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to Cracking

9 Material inhomogeneity Reason: Inhomogeneous cooling & TTT behaviour HAZ regions Figure according to Toyoda, 1998

10 Consequence Toughness scatter Specific requirements on toughness testing identification of specific microstructure number of test specimens Figure according to Toyoda, 1998

11 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch

12 Strength mismatch Unintended and intended mismatch Usually in steel: moderate overmatching Cases of undermatching: aluminium, high strength steels Pronounced mismatching: laser & electron beam welding M = σ YW σ YB W = Weld metal B = Base plate

13 Strength mismatch Effect on crack driving force Effect on crack path deviation UM OM Figures: Dos Santos et al., Koçak Factors affecting the mismatch effect Crack location (weld metal, fusion line etc.) Mismatch ratio (σyw /σyb) Global constraint (W-a)/H Residual stresses interdependency

14 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch Residual stresses

15 Welding residual stresses Reason: inhomogeneous cooling constrained shrinking solid state phase transformations External restraint macro-residual stresses (residual stresses of the first kind); vary within the cross section over a distance much larger than grain size Internal forces and moments are in equilibrium with respect to any cross section and axis respectively Figure according to Leggatt, 2008

16 Welding residual stresses Scatter and uncertainty in simulation and measurement Figures according to Bouchard, 2008

17 Welding residual stresses Dependency on location along the weld Further effect: Stop-start features Figures according to Hosseinzadeh and Bouchard, 2011; (b) Bouchard, 2008

18 Welding residual stresses Residual stress profiles Individual determination Compendia (upper bound curves to literature data) Membrane stress (as-welded: max. value: yield strength) Post weld treatment: p r σ + σ σ Y Membrane stress (yield strength at annealing temperature + correction for ratio of E modules at room & annealing temperatures Mechanical post weld treatment

19 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch Residual stresses Misalignment

20 Welding residual stresses Types of misalignment: (a) Axial misalignment between flat plates (b) Angular misalignment between flat plates (c) Angular misalignment in a fillet welded joint Consequence: Notch effect/local bending stress Strong effect of fatigue life and shallow crack propagation Effect on long crack fatigue propagation and (sometimes) on failure load

21 Outline Specific aspects of weldments Determination of fracture toughness Determination of the crack driving force Shallow crack propagation and fatigue strength

22 Fracture toughness determination Modifications compared to testing of non-welded material Specimen geometries most appropriate for weldments, e.g., shallow cracked bend specimens Weldment specific aspects of specimen preparation such as the introduction of the notch, minimisation of residual stresses and misalignment Generation of a straight crack front Validity criteria ISO Required number of test specimens Strength mismatch effects for testing in the net section yielding range

23 Fracture toughness determination: Scheme According to ISO 15653

24 Fracture toughness determination Adapted testing Perform test as much as possible representative with respect to the component in service. Relevant factors and parameters are: Welding process including filler material Base plate composition Joint thickness Preheat and interpass temperatures Heat input Detailed welding procedure Joint configuration Restraint Postweld treatment Time between welding and testing Environment Test temperature Hydrogen release heat treatment prior to testing can be necessary when the time between welding and the beginning of service is much longer than those between welding and testing.

25 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking

26 Fracture toughness determination Specific features because of inhomogeneous microstructure, metallography HAZ testing: Pre and post test metallographic examination In steel: crack tip no more distant than 0.5 mm from target microstructure Crack front should sample either 15% or at least 7 mm of the HAZ microstructure Both within the central 75% of the specimen thickness ISO 15653

27 Fracture toughness determination Specific features due to inhomogeneous microstructure: Weakest link approach (1) Randomly distributed small regions of low toughness ( weak links ) across the ligament; in weldments: HAZ brittle zones During load increase, when stress peak is shifted into the ligament to the location of the nearest weak link the whole specimen (or component) fails Due to the random distribution of the weak links in the ligament area the distance of the first one from the crack tip varies from specimen to specimen and so does the work necessary to shift the stress peak to the right position fracture toughness scatter

28 Fracture toughness determination Specific features due to inhomogeneous microstructure: Weakest link approach (2) The longer the crack front the higher the probability of a weak link next to it Toughness scatter becomes smaller for longer crack fronts but lower bound remains constant Same lower bound toughness can be determined by using few specimens with large crack fronts or by using many specimens with short crack fronts Usually: 3-Parameter Weibull distribution; e.g., Stage 2 and 3 Options of SINTAP Master Curve approach

29 Fracture toughness determination Specific features due to inhomogeneous microstructure: Weakest link approach (3) BS 7910: Minimum of 12 valid HAZ tests for ductile-to-brittle transition Figures according to Toyoda, 1998

30 Fracture toughness determination Pop-in behaviour Pop-in: Discontinuity in the load versus displacement curve in the fracture mechanics test displacement suddenly increases and load decreases Different reasons: Limited cleavage fracture propagation + arrest Out-of-plane slits Other reasons Fig.: Dos Santos et al., 2001 Criteria: > 4 (2) % of (W-a) crack propagartion Load drop more than x % Increase in compliance Problem: When is a pop-in event component relevant?

31 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch

32 Fracture toughness determination Specific features because of strength mismatch ISO 15653: Error in J integral or CTOD (standard equations) due to mismatch less than 10% as long as Weld metal testing: CTOD tests: J integral tests: 0.5 < M < < M < 1.25 M > 1.5 or 1.25: overestimation of J or CTOD M < 0.5 underestimation HAZ testing: Error ± 5% for J and -20% to +10% for CTOD as long as 0.7 < M < 2.5 Else mismatch specific η pl function in J 2 K U = + ηpl E B W a ( )

33 Fracture toughness determination η pl function for strength mismatch (EFAM, Schwalbe et al.) Some additional solutions in the literature

34 Fracture toughness determination Definition of weld width H for other than prismatic welds Proposals: (a) H = average of 2H 1 and 2H 2 (b) equivalent H, H eq, on the basis of the shortest distance between the crack tip and the fusion line along the slip lines emanating from the crack tip However: Systematic investigation still missing.

35 Fracture toughness determination Effect of strength mismatch on constraint and toughness According to Toyoda, 2002 Complex issue: Various constraint parameters According to Kim (Schwalbe et al., 1996) Damage mechanics simulation (e.g. GTN)

36 Fracture toughness determination Effect of strength mismatch on toughness and crack path deviation Electron beam weld, steel Kocak et al., 1999 Probability of crack path deviation decreases with longer crack front Laser beam weld, steel Heerens & Hellmann, 2003

37 Stress-strain curves Micro tensile tests e.g., Kocak et al., 1998 BS 7448: Estimation from hardness Base plate : R = 3.28 HV 221 for 160 < HV < 495 p0.2b Weld metal : R = 3.15 HV 168 for 150 < HV < 300 p0.2w

38 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch Residual stresses

39 Fracture toughness determination Specific features because of residual stresses Considered at applied side (crack driving force in component) Specimen if possible residual stress free (but not realistic) Specimen preparation in order to generate straight crack front From left to right: - Local compression - (Reverse bending) - High R ratio test

40 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch Residual stresses Misalignment

41 Fracture toughness determination Specific features because of misalignment Deformation of specimen wings in order to avoid bending However, no plastic deformation within a distance B from weld

42 Outline Specific aspects of weldments Determination of fracture toughness Determination of the crack driving force Shallow crack propagation and fatigue strength

43 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch

44 Crack driving force and fracture assessment Crack path simulation by damage mechanics methods, e.g., GTN model Local parameters for at least base plate, weld metal and HAZ Conventional fracture mechanics (finite element based and analytical) Negre et al., 2004 Lower bound toughness or R curve or probabilistic analysis Effect of mismatch and residual stresses on R curve or toughness scatter! (crack path deviation) }Mismatch corrected limit load Again: When are pop-in events component relevant?

45 Crack driving force: R6 type assessment FAD approach CDF approach K r K r = ( ) f L r = K K mat J J f L ( ) -2 = e r 2 Je = K E Example. Option 1B analysis (no Lüders plateau) ( ) ( 6 r = + r + µ r ) f L L exp L 0 L r 1 ( ) ( ) r r r ( ) f L = f L = 1 L N 1 2N ( ) L = 0.5 R + R R max r p0.2 m el µ = min 0.6 ( p0.2 m ) N = R R 0.001( E R p0.2 ) L 1 L L = F F = σ σ r Y ref Y Replace F Y by F YM r max r

46 Mismatch corrected limit load F YM Example Conservative option: F YM determined as F Y based on the lower yield strength of base plate and weld metal Individual determination F YM solutions as functions of global geometry, mismatch ratio M and (W-a)/H Limit states: long crack a and/or wide weld (large H) short crack and/or narrow weld (small H) plastic zone mainly in weld metal plastic zone mainly in base plate F Y based on σ YW gives good estimate F Y based on σ YB gives good estimate (e.g. laser or electron beam weld)

47 Mismatch corrected limit load F YM Examples UM OM

48 Fracture analyses including mismatch: Examples F c = 569 kn M = 1.5 F c = 589 kn F c (homogenous) = 550 kn

49 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch Residual stresses

50 Primary and secondary stresses Primary stresses σ p : Arise from the applied mechanical load, including dead weight or inertia effects contribute to plastic collapse Secondary stresses σ s : Result from suppressed local distortions, e.g., during the welding process, or are due to thermal gradients Self-equilibrating across the structure, i.e., net force and bending moment are zero do not contribute to plastic collapse K factor determination is based on both primary and secondary stresses but only the primary stresses are taken into account for the limit load F Y, However: Secondary stresses can act like primary stresses in the crack carrying section Treatment as primary conservativ

51 Crack driving force due to primary and secondary stresses Primary stresses only K = π a σ f n ( x) n a n n } T n x n n T σ = σ Primary + secondary stresses

52 Interaction factor V Small scale yielding: K = K p + K s However: because of rather high s σ s in as-welded structures K > K p + K s L r < 1 and because of stress relief K < K p + K s L r > 1 Although secondary stresses don t contribute to plastic collapse they contribute to ligament yielding p s K = K + V K = + FAD approach: CDF approach: K r = K p I + V K K mat s I p s 1 KI + V K I J = E f ( Lr ) 2

53 Determination of V Plasticity corrected K factor for secondary stresses s Kp V = ξ K s K factor for secondary stresses Fit function to finite element results Different options for determining s p K p ( r ) s p K K L e.g., plastic zone corrected K: s p s ( ) ( ) K = a a K a eff ( ) 2 s K a 3 plane strain 1 aeff = a + β = 2βπ σy 1 plane stress L r

54 Fracture analyses including residual stresses Example: Residual stress profile Transverse residual stresses (compendium) 2 3 T * z z z σr σ Y ( z t) = t t t z z z t t t

55 Fracture analyses including residual stresses Example: Critical load for stable crack initiation Reduction in critical load: ca. 25%

56 Fracture analyses including residual stresses Example: Fatigue crack propagation and residual lifetime No effect on K But on R = K min /K max Effect on crack closure behaviour Reduction in residual lifetime: ca. 25% Simplified assumption: R > 0.5 (BS 7910)

57 Fracture analyses including residual stresses Ongoing discussion on less conservative determination of V factor This workshop Including solutions Without elastic follow-up Large elastic follow-up for application to short crack propagation problems

58 Fracture mechanics of weldments: Specific aspects Inhomogeneous microstructure Susceptibility to cracking Strength mismatch Residual stresses Misalignment

59 Fracture analyses including residual stresses Misalignment Example: Angular distorsion Butt weld clamped ( β 2) α l tanh( β 2) σ tanh s 3y 3 = = σm t β 2 2 t β 2 2 l 3σ t E m β = 1 2 (rad!) Solution for bending stress σ s refered to membrane stress σ m Alternativ: Finite element stress distribution

60 Outline Specific aspects of weldments Determination of fracture toughness Determination of the crack driving force Shallow crack propagation and fatigue strength

61 Initial defects in engineering alloys Frequently: Inclusions at or close to surface are crack initiaton sites Further crack initiation sites: Primary phases Pores/cavities Crack initiation at inclusions in steel (42CrMoS4) (Figs. Pyttel) Corrosion pits Surface roughness (scratches) Welding defects

62 Weld discontinuities and defects Distinguish between geometrical discontinuities (considered at applied side) and material defects Applied side Material - Misalignment - Slag lines - weldment geometry - Pores - Undercuts - Lack of fusion - Overlap - Cracks Initial crack size and geometry (multiple cracks) Usually excluded Specified by weldment quality system Steel 350WT Crack initiation in WAZ 0.3 mm deep surfacerdefect (Josi, 2010)

63 Example: Weldment quality grades: VOLVO Group Weld Quality Standard , 2008 Discontinuity VD (normal quality) VC (high quality) VB (post weld treated) Overlap < 0,5 mm < 0,1 mm not permissable Lack of fusion not permissable not permissable not permissable Transition > 0,25 mm > 1 mm > 4 mm radius Undecut < 0,05 t (max 1 mm) < 0,025 t (max 0,5 mm) not permissable inadequate < - 0,2a (max 2 mm) weld thickness smaller not permissable smaller not permissable Misalignment < 0,1 t (max 2 mm) not permissable not permissable Single Pore 0,4 t (max 4) 0,3 t (max 4) 0,2 t (max 2) 0,3 t (max 3) 0,2 t (max 2) 0,1 t (max 1) Pores cluster 6% / 3% 4% / 2% 2% / 1%

64 Contributions to fatigue life Contribution to overall lifetime Nt: - Crack initiation N i - short crack growth N s - long crack growth N l Polak (CSI, 2003): Crack initiation stage N i Nt = Ni + Ns + Nl at smooth, nominally defect-free surfaces: - less than 5-20% of overall lifetime N t - even less for existing initial defects Allows to treat defects as initial cracks in a fracture mechanics model

65 Specifica of mechanically short cracks Long crack growth (a > 0,5 mm, 2c > 1 mm) Short crack growth K concept not applicable Alternatives: plasticity corrected K (e.g., plastic zone size corrected) J-Integral CTOD Gradual built-up of plasticity-induced crack closure effect:

66 Fracture and Crack Propagation in Weldments. A Fracture Mechanics Perspective Specific aspects of weldments Determination of fracture toughness Determination of the crack driving force Shallow crack propagation and fatigue strength Uwe.zerbst@bam.de