Crack Paths 2006

that in the intermediate stress intensity, Kmax > 8 MPa(m)1/2, all the data fall on a

straight line with a narrow scatter band. This indicates that the growth rate in the

intermediate Kmax (or 'K) is independent of particles. However, in the near threshold

region, there are three distinct growth curves for each material. The threshold stress

intensity factor varies from 4.5 MPa(m)1/2 for the matrix to 6.5 MPa(m)1/2 for the 10%

volume fraction composite to 7.5 MPa(m)1/2 for that of 20%. Thus, the threshold stress

intensity increases by about 44% for the first 10% particulate reinforcement and by

about 22%for the next 10%reinforcement compared to the matrix threshold value.

Figure 7. Variation of the crack growth rate with the maximumstress intensity, Kmax, at

load ratio of R = 0.4, for the A12O3/6061-T6 Al composites and for the 6061-T6

aluminum alloy, from [11].

This is a fairly substantial increase in the threshold stress intensity which indicates an

increased resistance to crack growth at the low (near threshold) values due to the

presence of particles.