PSI - Issue 28

Wei Song et al. / Procedia Structural Integrity 28 (2020) 200–207 Author name / Structural Integrity Procedia 00 (2020) 000 – 000

204

5

In order to overcome some shortcomings of some criteria, a new damage parameter on the bases of the SWT criterion by the critical plane approach is proposed, which incorporates the stress concentration factor and loading non-proportional effect. The base fatigue material properties about base metal and undermatched welds obtained from previous investigations were used as initial input conditions. The multiaxial fatigue analysis for notch specimens were conducted considering the geometric variations in the ABAQUS and MATMAB software. The final output results of fatigue damage and fatigue lives of test specimens can be obtained from MATLAB. The related multiaxial fatigue analysis procedure has been given in Fig. 4. According to the notch gradient assessment procedure of strain energy density, it is important to calculate the effective critical distance of local positions. The fatigue zone contains the crack initiation caused by local high stresses. The effective critical distance of damage zone can be determined by the finite element simulations considering the variations of material properties and loading conditions. Fig. 5 shows the relationship of the effective critical distance along with nominal stress and the strain energy density under pure tension and torsion loadings, respectively. The fatigue damage parameter for notch specimens also can be evaluated based on the effective critical distance curves.

OUTPUT

ANALYSIS PROCEDURE

INPUT

Material properties

Basic properties: E, G, n e ,  y , K', n ' Fatigue properties: e f '( g f '),  f '( t f '), b(b 0 ),c(c 0 )

Critical plane methods Notch greident

3D Elastic-plasticity FE analysis (Notch specimens)

Fatigue damage

Notch specimens

Notch types Notch geometry Notch angles Notch radius

Fatigue life prediction

Loading condition Tension Torsion Uniaxial Multiaxial

Stress, strain, energy parameters

Fig. 4 Procedure of multiaxial fatigue damage parameters.

10 (b)

10 (a)

Exp.

Analy.

Exp.

Analy.

Tension Torsion

Tension Torsion

1

1

0.1 L eff (mm)

0.1 L eff (mm)

0.01

0.01

0.1

1

10

10

100

1000

Norminal stress

SED  W eff (mj/mm 3 )

(a) Gradient under tension

(b) Gradient under torsion

Fig. 5 Notch effective distance determination.

According to previous references, the damage parameters of multiaxial fatigue for notch specimens have been employed to assess the fatigue lives of notch components under service stress states. On the one hand, the fatigue damage models based on the critical plane concept are often put forward to calculate the multiaxial damage parameters considering the actual fatigue crack initiation on a specific plane, such as Fatimi-Soice model, Fatimi-Soice modified model, or related extension models. On the other hand, it is more suitable for assessing multiaxial low cycle fatigue by the concept of strain energy density than by the stress or strain components. Although the strain-based or energy based criterion of these models offers satisfactory prediction of fatigue life, it lacks the ability to describe the effect of