PSI - Issue 4

J. Maierhofer et al. / Procedia Structural Integrity 4 (2017) 19–26 J. Maierhofer/ Structural Integrity Procedia 00 (2017) 000–000

22 4

3. Experimental results and modelling

3.1. Residual stresses

Residual stresses are frequently belabored to explain discrepancies between crack growth predictions and experimental results. And indeed, residual stresses can influence the lifetime of components significantly. In general, differences in residual stresses are mainly due to manufacturing. In full-scale components residual stresses can occur due to forging, heat treatment, machining and other manufacturing processes. Laboratory specimens, on the other hand, are frequently free from residual stresses due to stress relaxation if they are machined from semi finished material or components with a significantly larger cross-section. Such differences between the residual stress states in laboratory specimens and quenched and tempered bar stock of 180 mm diameter used as pre material, both measured by means of the cut-compliance method, are shown in Fig. 2. While there are negligibly small residual stresses in the laboratory specimen (Fig. 2a), there are significantly higher residual stresses in the bar stock with around 50 MPa residual compressive stresses at the surface sinking to approximately 20 MPa in 10 mm depth (Fig. 2b).

Fig. 2. Residual stress states after manufacturing (a) laboratory SE(B) specimen; (b) quenched and tempered bar stock (180 mm diameter) pre material.

The influence of compressive residual stresses on the crack propagation rate can be enormous, as shown by small-scale experiments in Maierhofer et al. (2014b). Here the applied load leading to finite life of a specimen with residual compressive stresses (as shown in Fig. 3a) is approximately 4.5 times higher than in a specimen free from residual stresses (see Fig. 3). To consider crack retardation due to residual compressive stresses in computational models, we note that the local maximum and minimum stresses during one load cycle are composed by load stresses and residual stresses:

σ σ

=

σ

+

σ

max

max,load

res

(1)

=

σ

+

σ

min

min,load

res

This means that the local stress ratio is influenced by the local residual stress:

σ σ

+ +

σ σ

σ

max,load max,load

res res

max min

= R

=

(2)

σ

As the crack propagation threshold and rate depend on the stress ratio, Eq. (2) gives a natural explanation for the