On the slope of the fatigue resistance curve for laser stake‐welded T‐joints

Abstract

A B S T R A C T The paper investigates the influence of highly localised stress distribution around the notch tips of the laser stake-welded T-joints to the slope of the fatigue resistance curve. The study considers experimental data of eight series involving joints under tension or bending loads. Various boundary conditions and plate thicknesses are considered. The stress distribution in the singularity-dominated zone ahead of the notch tips is investigated by means of the finite element analysis. The aim is to relatively distinguish the stress distribution from one case to another. The growth rate of the elastic singular stress with respect to the distance from the tip is described by the dimensionless gradient. This gradient is equal to the slope of the linear stress-distance function when presented in double-logarithmic scale. The slope of the fatigue resistance curve varies approximately from 4 to 8. It is observed that the change of the slope can be closely associated with the gradient of the maximum principal stress evaluated in the plane that is orthogonal to the crack path. The orthogonal plane corresponds to the maximum principal stress direction. In contrast, there is a large scatter in the relation between the slope and the gradient evaluated in the commonly assumed crack plane. The study shows that the dimensionless gradient exhibits sensitivity towards plate thicknesses, local weld geometry and the loading condition.Keywords laser stake-welds; fatigue testing; J-integral; stress gradient; stress state. N O M E N C L A T U R Ea 1 , a 2 = notch depths [mm] C = material constant [À] e weld = offset of weld [mm] E = Young's modulus [MPa] h g = distance between joint plates [mm] J =J-integral [MPa mm] k ϕ = rotational stiffness [kN] l p = length of panel [mm] m = slope of the fatigue resistance curve [À] n = exponent of a stress function [À] N f = number of cycles to failure [À] s = spacing of web plates [mm] t f = face plate thickness [mm] t w = web plate thickness [mm] t weld = thickness of weld [mm] u i = node translation [mm]; i = x, y, z ΔF = force range [kN] ΔJ =J-integral range [MPa mm] ΔK I = range of stress intensity factor for fracture mode I [MPa mm 0.5 ] ΔK II = range of stress intensity factor for fracture mode II [MPa mm 0.5 ] θ MAX = direction of the maximum principal stress [°] θ MTS = initial crack propagation angle [°]

Fatigue strength assessment of laser stake-welded T-joints using local approaches

“…This paper considers eight tested series from [6,[11][12][13] that are divided into four groups, shown in Fig. 3.…”

“…Regardless of the loading condition, one of the notch tips is more stressed and therefore critical from the fatigue point of view because this notch determines the fatigue strength of the joint [6]. The utilisation of local approaches requires accurate stress and strain fields around the critical notch tip and thus the response of the joint needs to be obtained by accounting precise notch depths [6,[11][12][13].…”

“…A systematic comparison of the above-mentioned stress and energy-based concepts showed that they can be used for the fatigue strength assessment of thick T-joints (t > 5 mm) [11]. Later, the J-integral was used for thin joints [6] and also for panels under different loading conditions [12,13]. However, the applicability of other approaches in studies [6,12,13] was not investigated further.…”

“…Later, the J-integral was used for thin joints [6] and also for panels under different loading conditions [12,13]. However, the applicability of other approaches in studies [6,12,13] was not investigated further. It was observed that the J-integral-based fatigue resistance curves can have slope values between 4 and 8, depending on the loading condition [12].…”

“…It was observed that the J-integral-based fatigue resistance curves can have slope values between 4 and 8, depending on the loading condition [12]. Later, it was demonstrated that different loading conditions cause different concentrations of the maximum principal stress at the critical notch tip and that the slope value seems to depend on the concentration level [13].…”