PSI - Issue 14

Ilyin A.V. et al. / Procedia Structural Integrity 14 (2019) 964–977 Ilyin A.V., Filin V.Yu. / Structural Integrity Procedia 00 (2018) 000 – 000

967

4

r

factor of cyclic asymmetry

r i r c

actual local value of cyclic asymmetry in a concentrator, usually in WTZ

factor of cyclic asymmetry at Stage 2 fatigue

RWS residual welding stress R z

surface roughness, parameter of unevenness height full thickness of a structural (load transmitting) element cyclic yield stress for the least strength metal (HAZ or weld metal)

S

S c

SIF

stress intensity factor

T

thickness of an attached element shortening displacement after welding

U x 0

WTZ weld toe zone x

in-plane direction transverse to the weld axis coordinate in the direction normal to the surface function accounting for the shape of a flaw-containing weld

y

Y 1

2. How the stress-strain state in welded joints depends on structural and technological factors

The fatigue strength of a welded joint is determined by the following stress-strain state parameters:  Stress and strain concentration factors, K t and K  (different under elastic-plastic deformation) in WTZ that determine an amplitude of a local cycle of deformation at the initial stages of failure;  RWS distribution  res ( y ) determining the mean stress in a cycle as in WTZ as along the whole fatigue crack path. In turn, these parameters depend on the complex of design features of a welded joint and applied welding process:  K t depends on the type of joint, shape and thicknesses of welded elements, weld reinforcement dimensions and WTZ micro geometry stipulated by the welding procedure parameters. K t together with the ratio of the design stress range to yield stress of base or weld metal determine the amplitude of local plastic strain in WTZ.  RWS distribution depends on the welded joint type, beads allocation, a combination of thermal properties for base and weld metals, and the rigidity of welded elements. These dependences have been predominantly received from the finite element method (FEM) solutions of elastic, elastic-plastic and thermoplastic problems. Physical methods such as strain measurement with small strain gauges, X-ray, ultrasonic and neutron diffraction; shrinkage displacements measurement after welding, were applied to check the main FEM results of examinations with an account of the measurement base disposable for each method. (2) Such a representation at first sight coincides with the known standard Hot Spot Stress method (HSS), however essential refinements are suggested by Ilyin and Sadkin (2013). First it was obtained that K w should be found for the joint in question considering its cross-section containing the assessment point regardless the joint type and its simple two-dimensional or more complicated three-dimensional geometry, Figure 1. Second, K s should be considered as a parameter of stress non-uniformity along the weld in the assessment point neighborhood. It can be lesser than 1.0, for example, for a hole reinforcement joint. To get a K s estimate independent of FEM mesh size the formulae suggested by Ilyin and Sadkin (2013) should be used:       S tension y dy S K 0 s ( ) 1 , (3a)     S bend y S y dy K s ( ) 6 . (3b) 2.1 Stress and strain concentration in WTZ Generally K t can be defined as follows: s w t K K K   .

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2

S

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0