Fatigue of Welds. Professor Darrell F. Socie Darrell Socie, All Rights Reserved

Transcription

7 Nominal Stress Nominal stress approaches are based on extensive tests of welded joints and connections. Weld joints are classified by type, loading and shape. For example, a transversely loaded butt weld. It is assumed and confirmed by experiments that welds of a similar shape have the same general fatigue behavior so that a single design SN curve can be employed for any weld class. The designer need only determine the nominal stress and select a weld class. There is no need to directly consider the stress concentration effects of the weld. Fatigue of Welds Darrell Socie, All Rights Reserved 6 of 65

8 Structural Stress Structural stress approaches are often referred to as "hot-spot methods". The structural stress includes the macroscopic stress concentrating effects of the weld detail but not the local peak stress caused by the notch at the weld toe. There are various methods used to determine the structural stress. They involve extrapolating the computed or measured stresses from two points near the weld to a structural stress at the weld toe. This method works in situations where there is no clear definition of the nominal stress. Fatigue of Welds Darrell Socie, All Rights Reserved 7 of 65

9 Local Stress Strain Local stress or strain approaches include both the macroscopic stress concentration due to the weld shape and the local stress concentration at the weld toe. To apply traditional methods of fatigue analysis to welds, an appropriate value of the stress concentration factor and residual stress must be selected. Although the smallest radius produces the largest stress concentration factor, its effect in fatigue is smaller because of the gradient effect. As a result there is a critical radius for fatigue that can be used to compute the fatigue notch factor. Fatigue of Welds Darrell Socie, All Rights Reserved 8 of 65

10 Crack Growth Many weld details have planar lack of fusion defects. This is particularly true of fillet welds. In this case fracture mechanics models for crack growth are the most appropriate fatigue technology. Fatigue of Welds Darrell Socie, All Rights Reserved 9 of 65

14 Strain-Life Fatigue Analysis Material Data Component Geometry Cyclic stress strain curve σ 0 σ ε 1 ' σ σ n = + ' E K ε log ( ε/2) 4 ε f σ 1,1' 5,5' 7,7' 3 8 2,2' 6 Strain-life curve ' ε σ f = + 2 E ( 2 b ' N ) ( 2 f ε f N f ) ε c Service Loading ε p 0 ε ε ε e = σ/ε σ σ f /E σ e / E 0 2N log (2N f ) 2N e Fatigue of Welds Darrell Socie, All Rights Reserved 13 of 65

18 Crack Growth Fatigue Analysis Material Data σ(x) Component Geometry a a Service Loading Stress distribution along crack path in an un-cracked body Fatigue of Welds Darrell Socie, All Rights Reserved 17 of 65

32 Fatigue Analysis of Welds Material Data Uncertain, but unimportant Component Geometry Uncertain, but very important Service Loading Uncertain, but important How do we deal with these uncertainties? Fatigue of Welds Darrell Socie, All Rights Reserved 31 of 65

38 Crack Growth Data Crack Growth Rate, m/cycle σ yield K, MPa m Ferritic-Pearlitic Steel: da dn da dn da dn = = = Martensitic Steel: ( KMPa m) 3. 0 ( KMPa m) Austenitic Stainless Steel: ( KMPa m) Barsom, Fatigue Crack Propagation in Steels of Various Yield Strengths Journal of Engineering for Industry, Trans. ASME, Series B, Vol. 93, No. 4, 1971, Fatigue of Welds Darrell Socie, All Rights Reserved 37 of 65

40 Crack Growth Data Crack Growth Rate m/cycle T3 m/cycle 3X 3 1 A533B m/cycle Cyclic Stress Intensity, MPa m Steel welds are 3 times stronger than aluminum Fatigue of Welds Darrell Socie, All Rights Reserved 39 of 65

44 Mean Stress Effects As welded structures usually have the maximum possible mean stress Stress relief, peening, etc. will have a substantial effect on the fatigue life Fatigue of Welds Darrell Socie, All Rights Reserved 43 of 65

45 Butt and Fillet Weld Test Data 1000 The good welds Stress Range, MPa % survival with 95% confidence Failures Run outs Fatigue Life, Cycles Fatigue of Welds Darrell Socie, All Rights Reserved 44 of 65

47 Sources of Inherent Scatter Weld quality Mean, fabrication and residual stresses Stress concentrations (geometry) Weldment size Material properties Opportunities for Improvement! Fatigue of Welds Darrell Socie, All Rights Reserved 46 of 65

48 The Good and Bad Good weld design Local stress concentration from weld toe Poor weld design Macroscopic stress concentration from a geometry change Fatigue of Welds Darrell Socie, All Rights Reserved 47 of 65

53 All Welds Contain Microcracks 100 K, MPa m 10 da dn = C K m ~ 3 m B C D m ~ 3 E 100 F F2 G W Fatigue Life, Cycles Crack Growth Rate, m/cycle Same slope means same mechanism, crack growth Fatigue of Welds Darrell Socie, All Rights Reserved 52 of 65

66 Things Worth Remembering Local weld toe stresses, geometry and flaws control the life of weldments There are many ways to improve the fatigue strength of welded structures. Fatigue of Welds Darrell Socie, All Rights Reserved 65 of 65

67 Fatigue of Welds