Damage and Crack Propagation at a Microstructural Scale
- PDF / 2,886,441 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 114 Downloads / 226 Views
315
Mat. Res. Soc. Symp. Proc. Vol. 578 © 2000 Materials Research Society
1. STRUCTURE OF DAMAGE CAVITIES Damage is known to have a central role in ductile fracture : after nucleation due to cleavage of brittle second phase precipitates or to decohesion between matrix and such precipitates for example, cavities grow under the influence of the triaxiality of the local stress [ 17]. The main crack, which progresses through successive deflections at small length scales propagates through junctions with the cavities at intermediate length scales. It has been shown in Molecular Dynamics simulations that this fracture mode is also the one observed in the case of Si 3N4 and of amorphous SiO 2. In both cases, the low density regions act as nucleation sites, which concentrate the stresses, grow and coalesce with the main crack. In the case of Si 3N 4, it has been shown that the dynamic fracture surfaces exhibit, at the nanometer scale, the two self-affine regimes with exponents 0.5 and 0.8 [18,19] that other materials, fractured in a quasi-static way, exhibit at significantly larger scales [20]. All the cavities (not linked to the main crack) were found to be self-affine with an exponent 0.5 [18]. This suggested that the non trivial 0.78 could characterize the structure of the ensemble of opened cavities lying ahead of the main crack, while 0.5 characterized the growth of a single cavity. This is the idea we have tested experimentally. In this section, we show the results of our observations of damage cavities in two different aluminum alloys broken either in monotonous tension or in fatigue. Chevron notched bar specimens of a rapidly quenched Al-Cr-Zr alloy were submitted to tension mode I loading, while fatigue cracks (R=0. 1, f=-I OHz) were grown in Compact Tension samples of the 7010 aluminum alloy. In both cases, crack propagation was stopped before complete failure. The samples were cut and polished within a plane containing the crack (Fig.l a). Cavities of two types were examined in both cases, with an Atomic Force Microscope
0.
examined area
S0.1
MM
3411 nm
S0 0. nm
Figure 1. a) is a sketch of the region of observation valid for both cases. b) is an AFM image of the AICrZr sample. The main crack observable on the upper part of the image is linked to a big cavity (lower part of the image). Circles indicate the presence of "secondary" cavities, of significantly smaller sizes, which are very likely independent from the main crack. (AFM). In the case of the AlCrZr alloy, the various cavities which were explored are indicated in Fig. lb. "Large" anisotropic cavities (5 to 15 gim wide, 30-100 gtm long) visibly related to the main crack were analyzed, as well as significantly smaller cavities (3-5 gm) which appear to
316
be more isotropic and independent of the main crack. In the large cavity shown in Fig.Ib, four regions of size IOjim xlrpm were explored, aligned two by two in the two directions (parallel and perpendicular to the direction of crack propagation). Profiles of 1000 points each were registered in
Data Loading...
