Crack Propagation Simulation of Variable Amplitude Load Fatigue: Case Study 1

Materials :

               25CrMo4  axle steel             Yield= 550 mpa

               34CrNiMo6  high strength steel       = 950 mpa

Curves for da/dN baseline data were generated with tests using load ratios of
R=-1.0 and R=0.1 for both materials. R=-1.0 da/dN data from the reference
was fitted visually to the as shown if figure 1 and 2. (black points)

Fig. 1: Fitted da/dN points of 25CrMo4 Steel.

Fig. 2: Fitted da/dN points of 34CrNiMo6 Steel.

The original paper by Luke et al did not report any initiation tests.

Geometry

Load History:

The sub-blocks are divided into sub-block segments and the arrangement of
the segments is described in the text file

testLoads.txt

A program was used to convert the segment descriptors into a half cycle by
half cycle sequence. The program is available here:

makeHist.f.txt

Click to enlarge

A peak by peak history of membrane (Pm) and bending (Pb=0.0) are available
in the file

c100histPm.txt (1 Mb)

In the history the "100" value for the peak is proportional to the test reference stress
for sub-block 1 (the lowest stress sub-block). The arrangement of the sub-blocks and
their maxima stresses is a bit complicated and the reader is referred to the
paper by Luke et al for a full explanation.

Results:

34CrNiMo6 Steel:                                  Nf       af

             Delta S= 200 mpa  :  Fracture at    277000   7.7 mm
             Delta S= 200 mpa  :  Fracture at    298000   8.4 mm

             Delta S= 120 mpa  :  Fracture at   1380000   8.3 mm

25CrMo4 Steel:

              Delta S= 200 mpa :  Fracture at    156000   6.7 mm
              Delta S= 200 mpa :  Fracture at    159000   6.4 mm

              Delta S= 120 mpa :  Fracture at   1450000  10.0 mm


  (See Luke et al.  for experimental  crack length vs. cycles data plots )

Simulation/Predictions Available:

1. Original authors
   
   
2. Contributed Predictions (a)