v predicted weldment fatigue behavior

AM 11/03 1 V Predicted Weldment Fatigue Behavior AM 11/03 2 Outline n Heavy and Light Industry weldments n The IP model n Some predictions of the IP model AM 11/03 3 n Heavy industry AM 11/03 4 n Heavy industry AM 11/03 5 Side shell Torsion from web frame In-plane bending and shear fromlateral load (axial load in deck & bottom) Longitudinal Bending Web shear Panel stiffener Axial load (bending at bulkhead detail) Web frame n Complex weldment AM 11/03 6 Light Industry There are about 4000 to 6000 spot welds in the average automobile. For the most part these are executed by welding robots. AM 11/03 7 Weld quality The civil engineering view: Ever- present small flaws virtually eliminate fatigue crack initiation life N I of weldments. AM 11/03 8 Welding procedure A Welding procedure B Weld quality and welding procedure Cold laps virtually eliminate the fatigue crack initiation life (N I ) Such weldments may have an appreciable fatigue crack initiation life (N I ) “Nominal” “Ideal” AM 11/03 9 Two rather different paradigms Symbol Low (simple)High (redundant) Weldment Complexity Small < 1/2-inBig > 1-inWeldment size High: Flaws > 0.01-inLow: Flaws < 0.1-inWeld Quality Mechanical Eng. “Light Industry” “Ideal” Mass production Moderate reliability Civil Eng. “Heavy Industry” “Nominal” One-of-a-kind Very high reliability Application AM 11/03 10 Outline n Heavy and Light Industry weldments n The IP model n Some predictions of the IP model [...].. .Fatigue of a component The fatigue life of an engineering component consists of two main life periods: Initiation or nucleation of a fatigue crack (NI) And Its growth to failure (NP) A smooth specimen AM 11/03 11 Initiation-Propagation (IP) Model N T = N N + N P1 + N P2 Total Fatigue Life ai = ? NN N P1 N P2 Actual a i - smooth specimen ? N P2 NI Nucleation and "early" growth Invisible Crack... component notch root plasticity log fatigue life, N (cycles) The severity of a notch in fatigue, that is the magnitude of Kf can be measured… The effect of notch-size can be estimated using Peterson’s equation Kf = 1 + AM 11/03 Kt − 1 a 1+ r Alternatively, for generally elastic body, the local stress can be estimated analytically using Neuber’s Rule (Kt S)2 σε = E 17 Perfect weldments: concept of the worst-case... remote stress range (? S, ksi) Fatigue crack propagation coefficient (C, ksiv in) Fatigue crack propagation exponent (n) Notch depth (D, in) Geometry correction factor for an edge crack (Y): 10 10 1x10-9 3 0.1 1.12 Fatigue crack propagation life: m  1− m 1−  2 2   af − ao       NP = AM 11/03 1 C (1 − m / 2) (Y∆S √ π) m 26 IP Model example: NI = ? ?S Kt r D The fatigue crack initiation life... (KIC, ksiv in ): Fracture toughness @ 0°F (KIC, ksiv in ): Geometry correction factor for an edge crack (Y): Linear-elastic fracture mechanics (LEFM): AM 11/03 KIC 10 10 70 45 1.12 = YSf (πaf)1/2 25 IP Model example: NP = ? ?S af D x Calculate the number of cycles to propagate the fatigue crack from an initial length (xi) of 0.01 in to the length at which it fractures (af), that is, estimate the fatigue. .. Nucleation The relative importance of fatigue crack initiation and propagation (SAE 1045 steel after Socie) 20% 10% 0% 1E+02 1E+03 1E+04 1E+05 1E+06 Total Life, Nt (cycles) AM 11/03 13 IP Model details Estimation of NI Laboratory fatigue tests on smooth specimens ε’f σ’f b c σm time K K’ n n’ Set-up cycle σ σm Mechanics FEA analysis ε σm Kfmax Kt NT = NI + NP Kfmax Mk Estimation of NT Fatigue notch size... considering the elliptical notch to have a fatigue notch factor (Kf) which can be estimated using Peterson's equation which estimates size effect of the notch in fatigue from the elastic stress concentration factor (Kt) −1 / b 1  σ'f − σ m  NI =   K ∆S / 2   2 f  Notch depth (D, in.): Notch root radius (r, in.): Elastic stress concentration factor of an ellipse (Kt ): Fatigue notch factor (Kf ) - Peterson's... root mean stress (σm, ksi): Fatigue strength coefficient (σ’f, ksi): Fatigue strength exponent (b): AM 11/03 0.1 0.01 0.01 10 5 Sm Kf 110 -0.1 1+ 2 1+ D r Kt − 1 a 1+ r 27 IP Model example: NT = ? af (in.) = 5.14 Kt = 7.32 Kf = 4.16 NI (cycles) NP (cycles) 38% = 1,051,000 62% = 650,000 NT = NI + NP (cycles) = 1,701,000 AM 11/03 100% 28 Outline n Heavy and Light Industry weldments The IP model n Some... cracks Visible Long Crack Growth NI + N P1 ) + N P2 = N I + N P 2 IP Model The total fatigue life of a component may be considered to be the sum of three periods: crack nucleation (NN), short crack growth (NP1) in which the crack closure phenomenon is a function of crack length, and long crack propagation (NP2) in which the crack closure phenomenon is more or less independent of crack length 12 Relative... 20° away from the point of tangency (Can also get stresses along crack path for NP calculation from this analysis.) AM 11/03 20 Events during the first load Notch-root residuals after set-up cycle application The difference between the monotonic (tensile)and cyclic σ−ε behavior causes the notch root mean stresses to be altered during the first load application!!!! Set-up cycle analysis AM 11/03 S t... Failures Runouts Propagation Life (ai = 0.01 in.) Total Life 1 10 4 10 5 10 6 10 7 8 10 Fatigue Life, N (cycles) AM 11/03 30 IP Model predictions and experimental results ∆ S (ksi.) 100 NP 10 NT Failures R = 0.1 Runouts 1 10 4 10 5 10 6 10 7 10 8 Fatigue Life, N (cycles) AM 11/03 31 IP model predictions for a cruciform weldment IP model predicts notch-root (local) stresses and strains Restraint ²Sb Sr . 1 V Predicted Weldment Fatigue Behavior AM 11/03 2 Outline n Heavy and Light Industry weldments n The IP model n Some predictions of the IP model AM 11/03 3 n Heavy industry AM 11/03 4 n Heavy. robots. AM 11/03 7 Weld quality The civil engineering view: Ever- present small flaws virtually eliminate fatigue crack initiation life N I of weldments. AM 11/03 8 Welding procedure. B Weld quality and welding procedure Cold laps virtually eliminate the fatigue crack initiation life (N I ) Such weldments may have an appreciable fatigue crack initiation life (N I ) “Nominal” “Ideal” AM