Fatigue Crack Paths 2003

D A M A GA EC C U M U L A TUINODNENRO NS I M I L ALR O A D I NMGO D E S

To apply the critical plane damage model to sequence containing loading modes of

different types, let us use the fatigue data from tests on two mild steels. For the first one,

the C36, observations of the crack path at the free surface will be carried out while for

the second one, the C18 tested by Froustey and Lasserre [9], blocks of combined

bending and torsion are introduced in a sequence containing also blocks of bending and

blocks of torsion.

Materials and Test Procedures

Tension, torsion blocks applied to C36

The variable amplitude fatigue tests conducted on the mild steel C 36 are carried out by

subjecting the specimen to a block loading, each block consisting of a number of push

pull cycles and torsional cycles. The sequence is built so that within each block the

fatigue damage produced by each mode if applied separately is approximately the same

(Τa = 175 M P ain torsion and Σa=250 M P ain tension lead to a mean fatigue life of 8105

cycles). Moreover, two different lengths of each individual load condition (torsion or

tension) are used : 105 cycles and 5.104. For all the tests the first applied loading is

tension.

The fatigue properties of this mild steel under constant amplitude loading have been

estimated from tests carried out on a vibrophore test machine under two loading

conditions : purely reversed tension and purely reversed torsion. The two fatigue limits

estimated from a stair case method on at least 15 specimens are : tension s-1 = 240 M P a

and torsion t-1 = 169 MPa.

Tension and torsion crack growth on C36

Whenapplied successively tension and torsion, one can expect to face the same kind of

crack initiation and growth mechanisms than in constant amplitude [8]. Six specimens

were then subjected to tension and torsion block sequences. For three of them the length

of each block was 105 cycles whereas the remaining three others were submitted to 5

104 cycles blocks. The failure of the specimens was defined as when the surface cracks

were approximately 2 m mlong. Figure 2 shows the crack on the specimen surface after

8 105 cycles of a sequence composed by 105 cycles blocks. The numbers given above

the crack correspond to the block number of the applied sequence. The last block n° 9

composed of tension is readily recognized since for that crack length, stage II (mode I)

is expected and hence the crack growth plane is perpendicular to the specimen axis. In

the previous block torsion is applied. Crack growth in stage II (mode I) occurs then on a

plane oriented 45° to the axis. Tension occurs again in block 7 and the corresponding

crack is transversal ( stage II, mode I). Finally, the first six blocks are required to create

a 200 μ mcrack. During these first 6 105 cycles, it seems that after nucleation of the first

crack a tension stage I crack interacts with torsion stage I followed by torsion stage II

cracks. The same kind of damage accumulation and crack orientation in each block is