Crack Paths 2006

fy = 650 M P a

fy > 650 M P a

Figure 9. Samples of curvilinear and straight crack paths observed in small scale “L”

plate specimens.

N U M E R I CMAOLD E L L I N G

In order to better understand the initial residual stress state, the finite element method

was used to simulate the cold forming operation for a structural tube [7]. The computed

through-thickness distribution of tangential residual stresses is shown in Fig. 10.

Numerous simplifying assumptions were made in this simulation and, as a result, the

computed residual stresses were not in full agreement with the stresses measured by X

ray diffraction. The errors were mainly due to the overly simple material model

implemented and the use of a 2D simulation. The true cold forming process for a

C F R H Stube is 3Dand difficult to simplify.

+

–

Figure 10. Tangential residual stresses of C F R H Saccording to FE-simulation.

Residual stresses along two crack paths were assessed. Paths were chosen based on

the observed crack paths in the laboratory test specimens and the in-service failure. The

first path was “S” shaped (see Fig. 1) while the second was straight through the

thickness in the corner region (see Fig. 7). The stress intensity factors (SIF’s), KI and

KII, as a function of crack length due to the residual stress alone were calculated. These

are shown in Fig. 11. For calculations residual stress distribution with crack growth was

assumed, however, contact between the crack faces was not included so SIF values