A B S T R A C T Experimental fatigue data for butt-welded joints in as-welded condition and under constant amplitude tensile loading were analysed using the effective notch stress system and a new master curve analysis that takes the local stress ratio, R local , into account. The local stresses needed for computation of R local are calculated with the notch strain approach in conjunction with the reference radius concept. The main focus was to predict with the derived master curve the fatigue strength of peened butt-welded joints. The lowest surface residual stresses after peening were first estimated based on reported measurements and an analytical lower bound result. The predictions showed quite similar strength dependences and FAT values as reported for high-frequency mechanical impact treated welds for applied stress ratio R = 0.1. The benefits of peening reduce faster for higher strength steels when R increases. When R = 0.5, the FATs are practically the same for all steel grades.Keywords butt joint; effective notch stress; fatigue; improvement; local stress ratio; residual stress. N O M E N C L A T U R EASW = as-welded condition ENS = effective notch stress approach FAT = IIW fatigue class, fatigue strength corresponding to two million cycles GMAW = gas metal arc welding HAZ = heat affected zone HFMI, HFP = high frequency mechanical impact treatment (generic term) HSS = high strength steel IIW = International Institute of Welding MSSPD = minimization of the sum of squared perpendicular distances from a line NS = nominal stress approach QC = quenched and cold-formable steel SINTAP = structural integrity assessment procedure SWT = Smith-Watson-Topper approach TIG = tungsten inert gas welding UP = ultrasonic peening (device name) C = fatigue capacity E = Young's modulus f u = ultimate strength, nominal f y = yield strength H = cyclic strain hardening coefficient K f = fatigue effective fatigue stress concentration factor between notch stress and nominal stress K HP = improvement factor K m = structural stress concentration factor Correspondence: T. Nykänen.
A B S T R A C T First, fatigue tests were performed on butt-welded joints made of novel direct quenched ultra high strength steel with high quality welds. Two different welding processes were used: MAG and Pulsed MAG. The weld profiles, misalignments and residual stresses were measured, and the material properties of the heat-affected zone were determined. Fatigue tests were carried out with constant amplitude tensile loading both for joints in as-welded condition and for joints after ultrasonic peening treatment. Finally, in fatigue strength predictions, the crack initiation phase was estimated using the procedures described by Lawrence et al. [Lawrence F V, Ho N J and Mazumdar P K (1981) Predicting the fatigue resistance of welds. Annu. Rev. Mater. Sci,11,. The propagation phase was simply estimated using S-N curves for normal quality butt welds, which may contain pre-existing cracks or crack-like defects eliminating the crack initiation stage.Keywords fatigue; local approach; ultra high strength steels; weld profile. N O M E N C L A T U R Ea * = material parameter ASW = as-welded condition A 5 = elongation, permanent extension of the gauge length after fracture, % b = fatigue strength exponent c = fatigue ductility exponent C = fatigue capacity CEV = carbon equivalent value e = axial misalignment E = Young's modulus FAT = fatigue class, fatigue strength corresponding to two million cycles GMAW = gas metal arc welding HAZ = heat-affected zone HB = Brinell hardness number [kg/mm 2 ] HV10 = Vickers hardness number obtained using a 10 kgf force [kg/mm 2 ] IIW = International Institute of Weldingnotch factor K m = stress concentration factor caused by misalignments K m,e = axial magnification factor K m,α = angular magnification factor K t = stress concentration factor Correspondence: T. Nykänen. Fatigue Fract Engng Mater Struct 36, 469-482 469 470 T. NYKÄNEN et al.K w = stress concentration factor of weld l = half distance between clamps LCF = low cycle fatigue LEFM = linear elastic fracture mechanics m = exponent of S-N curve n = static strain-hardening exponent n = cyclic strain hardening exponent NDT = non-destructive testing N f = total life N i = initiation life N p = propagation life QC = quenched and cold-formable steel R = stress ratio, the ratio of minimum to maximum applied stress in a cycle R p0.2 = 0.2% proof stress R m = ultimate tensile strength SLM = structured light method t = plate thickness t 8/5 = cooling time between 800 • C and 500 • C UP = ultrasonic peening treatment α = angular misalignment angle, survival probability β = two-sided confidence interval ε = local strain range ε e = elastic strain range ε p = plastic strain range K = stress intensity factor range S = nominal stress range σ = local stress range ε f = fatigue ductility coefficient θ = weld notch angle ρ = notch root radius ρ c = critical notch root radius σ f = fatigue strength coefficient σ m = mean stress, a combination of applied and residual stresses σ res = residual stress Sub indexes: geo = mean value of log-normal distrib...
This study considers the effect of bending loading and the symmetry of joints on the fatigue strength of transverse non‐load carrying attachments. Conventionally, the fatigue strength of a welded joint has been determined without taking these factors into account. Experimental and finite element analyses were carried out and both methods showed that both loading type and symmetry have an influence on the fatigue resistance of a welded joint. Under tensile loading, the fatigue strength of asymmetric T‐joints was higher than that of symmetric X‐joints. Respectively, the fatigue resistance of tested joints improved explicitly when the external loading was bending. The finite element analysis was in good agreement with the test results in the joints subjected to tension but gave very conservative results in the joints subjected to bending.
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