Fatigue behaviour of a box‐welded joint under biaxial cyclic loads

Abstract

Multiaxial fatigue behaviour of box‐welded (wrap‐around) joints in a JIS SM400B steel (12‐mm‐thick plate) was examined using a biaxial fatigue test facility. For the specimen, two stiffeners were attached to a main plate by a CO2 semi‐automatic welding procedure. Residual stress measurements and finite element (FE) analyses were also performed. Fatigue tests were performed under both uniaxial and biaxial (mainly out‐of‐phase) cyclic loads, and both results were compared and examined. It was found that fatigue cracks in the biaxial fatigue test specimens were initiated at the boxing‐weld toes and propagated almost in the direction of the lateral loads. This is considered to be due to the dominant direction of tensile residual stresses from welding and the stress concentration in the vicinity of the boxing‐weld toe. From the relation between the strain range near a weld toe, Δε5 , and the fatigue lives, it was found that crack initiation life, Nc , was almost equivalent in the biaxial and uniaxial fatigue tests, while the failure life, Nf , was slightly longer in the biaxial tests. However, when the fatigue lives are put in order using the stress range near a weld toe, Δσ5 , the crack initiation life, Nc , in the out‐of‐phase biaxial tests (phase difference of π) is ~30% lower than in the in‐phase biaxial and uniaxial tests, while the failure life, Nf , was almost equivalent in the biaxial and uniaxial tests. From these results, it is concluded that an increase in Δσ5 (lowering of the minimum value of σ5 ), induced by the out‐of‐phase lateral loads, leads to an increase in fatigue damage where the high tensile welding residual stresses exist in the vicinity of the boxing‐weld toe. Finally, a simple life estimation for the biaxial fatigue tests was made using FE analyses and the results of the uniaxial fatigue tests, proving that the effects of the lateral loads should be taken into consideration.

Fatigue behaviour of a box‐welded joint under biaxial cyclic loading: effects of biaxial load range ratio and cyclic compressive loads in the lateral direction

“…In Ref. [9], the reference stress range Δ σ 5 in the x ‐direction 5 mm away from the box‐weld toe was mainly used for correlating the uniaxial and biaxial fatigue data. This parameter has been shown to be an effective parameter when considering the quantitative effects of loads in the y ‐direction and for estimating the biaxial fatigue lives from uniaxial test data.…”

“…However, only a few reports have been issued in which the fatigue of welded joints under orthogonal biaxial pulsating tensile loads was experimentally examined 8 . The present authors previously performed biaxial fatigue tests under pulsating tensile loads with a phase difference of π ; these being on box‐welded (wrap‐around) cruciform joint specimens which showed that biaxial fatigue lives were well correlated using a reference stress range, Δ σ 5 9 …”

“…An 8‐node isoparametric hexahedral element was chosen and a 1/8 solid model was used considering the symmetry of the model analyzed (Graphical presentation of the model was given in Ref. [9]). In the elastic analyses, Young's modulus E = 2.0594 × 10 5 N mm −2 and Poisson's ratio ν = 0.3 were used.…”

“…An arrangement of four servo‐hydraulic actuators (dynamic load capacity = 1000 kN each) symmetrically in a cruciform shape on a reaction floor made of pre‐stressed concrete created an experimental environment in which orthogonal biaxial load‐controlled fatigue tests were performed with the centre point of the specimen kept in a centrally located position (see Ref. [9] that provides details of the control system). In order to realize cyclic compressive loading in the y ‐direction, anti‐buckling jigs were specially designed and manufactured (Fig.…”

“…The load in the x ‐direction ( P x ) and the load in the y ‐direction ( P y ) were controlled as two sinusoidal waves with the same frequency and almost the same amplitude but with a phase difference of π radians 9 . When an elastic material is stretched and the length increases, the width of the material decreases according to the Poisson's ratio.…”