Influence of crack tip plasticity on fatigue behaviour of laser stake-welded T-joints made of thin plates

Abstract

The paper investigates the crack tip plasticity of laser stake-welded T-joints when the thickness of the face plate is varied. For thin joints, mixed-mode condition is dominant and the Minimum Plastic Zone Radius (MPZR) criterion is employed to estimate the crack initiation direction. Subsequently, the same first-order plastic zone size r y as defined by Irwin is kept equal for the selected load levels in the MPZR direction, and finite element (FE) analyses are carried out to investigate the stress gradient over r y comparing different thicknesses. The results allow us to explain why the fatigue slope m is different for thin and thick joints. Moreover, a new method is proposed that permits the number of cycles to failure of thin joints to be derived directly from the fatigue curve of thick joints, employing an effective J-integral. The results prove that the method is able to give an estimation of the number of cycles to failure for thin joints with acceptable error.

Mechanical and fracture properties of aluminium cylinders manufactured by orbital friction stir welding

“…Such behaviour is also obvious by referring to the obtained load–displacement curves (Figure 8). The existence of significant plastic deformation around the crack tip can affect noticeably the crack growth resistance and fatigue life of welded parts as illustrated in previous works 46,47 . Closer inspection of these curves shows a significant nonlinear region before the ultimate load, which is demonstrated in Figure 8, that reveals the ductile fracture of all tested materials (i.e., fabricated from BM and FSW joint section of aluminium cylinders) in both T‐ and L‐directions.…”

Generalized Formulation for Fatigue Assessment of Laser Stake-welded T-joints Varying Thicknesses and Loading Conditions

“…The force F 1 depicted in Fig. 2 is assumed as the crack driving force, and can be used to quantify the stress gradient variation between joints of different loading conditions or thicknesses [13,14]. The crack initiation angle θ 0 is evaluated by using the Minimum Plastic Zone Radius (MPZR) criterion and defined as the minimum distance to the crack tip, i.e., in the direction of a relative MPZR where the circumferential stress is tensile according to Refs.…”

“…The crack initiation angle θ 0 is evaluated by using the Minimum Plastic Zone Radius (MPZR) criterion and defined as the minimum distance to the crack tip, i.e., in the direction of a relative MPZR where the circumferential stress is tensile according to Refs. [14,18,19]. The elastic-plastic boundary, instead, is defined according to the von Mises yield criterion.…”

“…The stress field analysis ahead of the crack tip also proved that the mixed-mode ratio is strongly influenced by the face plate thickness. Recently, Gallo et al [13,14] showed that the difference in the fatigue slope of laser stake-welded Tjoints was explained by analysing the stress distribution and plastic zone ahead of the crack tip.…”

“…The MPZR criterion, instead, permits to evaluate the direction of the crack initiation rapidly and therefore to define the correct orientation under mixedmode (I+II) loading condition. The methods initially formulated by Gallo et al separately for thick joints (only for tension and bending loads) [13], and later for joints of different thicknesses [14], is reformulated here in a unified formulation that applies indistinctly to all configuration of loads and/or thicknesses. The method is then verified with experiments taken from the literature [8,9].…”