Crack Paths 2012

General microstructural crack growth

For an appropriate choice of a fracture mechanics approach to describe the crack growth

behaviour the following three main influences which have to be taken into account:

• Kind of material,

• Crack length and the

• Local and global loading situation.

Especially at the crack front, the forming and size of the plasticity zone plays an

important role in analysing short crack behaviour. Only whenthe plasticity zone is

small enough in relation to the microstructural short cracks the application of the linear

elastic fracture mechanic (LEFM)is justified. Eq. 1gives an approximation of the

dimension of the plasticity zone at the crack tip [5].

K

( ) 2 2 1 υ −

r

=

(1)

pl

2

2 πσ

y

σy the yield stress and K the stress intensity factor. The

where ν is the poisson's ratio,

yield strength for ADI1000 is about 700 MPa.

The L E F Mconnects the crack propagation to the loading situation with the stress

intensity factor range K, defined by Eq. 2.

a Y K π σ Δ = Δ

(2)

Δσthe outer load range and a the crack length.

where Y is the geometry factor,

In the case of the presented austempered ductile iron the L E F M(linear elastic fracture

mechanics) is applicable only whenmode1 cracks appears and the plasticity zone

doesn't exceed half of the grain size.

Surface preparation method of the fatigue specimens

To indentify small cracks under the microscope clearly, the specimen's surface has to be

prepared with special methods. The geometry for the short crack growth experiments

was a 140 x 12 m mround specimen with a test diameter of 4.5 m m(Fig. 6) and was

taken out of a heat treated Y-block according to DIN-EN-1564:1997.

Fig. 6: C N Cdrafting of a 4,5 m mMini-UBspecimen

The surface finishing comprehends several steps of grinding and polishing. After the

last polishing step the average surface roughness (RZ) measured and was about 11 μm.

Anadditional etching technique to uncover the microstructure was not applied on the

787