IFMASS10

4. SIZE AND THICKNESS INFLUENCE

It is very important for successful calculation and results evaluation to be guaranteed

that the material resistance curve relative to the crack growth is independent on geometry.

For that it is necessary to know the limits for the parameters and used method, accurately

define its application boundaries and, if necessary, to correct the method for parameters

estimation in order to extend application limits and assure the use of existing methods.

Application of J-integral has been significantly extended in last years. However, some

uncertainties still exist that must be eliminated for further extension of application. Most

important is to consider the constraint effects when evaluate the parameters of the crack

growth for material. This is the preposition for successful application of described

methods also in case of 3-dimesional cracks and structures of different thickness.

When the structure or specimen containing a crack is exposed to increasing load, they

pass through different regimes and different methods could be adequately applied. The

conditions of small scale yielding (SSY) are established for the crack tip plastic zone

infinitely small compared to other dimensions and LEFM is applicable since the crack is

embedded in an elastic volume. However, by the further load level increase the plastic

deformations cause the reduction of constraint at the crack tip. In the moment when the

global and local deformation interact, the crack tip stresses and strains no more increase

in proportion to one another and in dependence on only single parameter. At these large

deformations, equivalence of single parameter characterisation of fracture driving force

(i.e.

K, J

and

) does not ensure identical stresses and strains distribution at crack tip for

different cracked geometries. The general term “size effect“ includes complex geometry

and loading effects on the stress state at crack tip and material fracture toughness, when

fracture can appear at different load level. In essence this effect is based on size influence

to the relationships between macroscopic fracture parameters and the crack driving force

at micro scale. Early fracture mechanics research addressed size effect to establish size

and deformation limits below which the geometry independence of fracture toughness is

assured. If the corresponding requirements are fulfilled, a single parameter is sufficient

for unique description of the stresses and strains state near crack tip. In this range the

LEFM application with the simple plasticity correction of crack size is appropriate. in

general this analysis produces over conservative results. However, increase in fracture

resistance in dependence on geometry could not be taken into account in this way and

other methods based on application of two parameters become necessary.

The breakthrough in this respect has come through the new methods and increased

computing power, which has enabled crack tip stress distributions to be examined in

detail and without simplifications concerning material non-linear behaviour. The series of

different procedures hade been developed, directed to the consideration of parameter

variations in dependence on constraint and difference in geometry. Decisions in this

respect are also supported with experimental results that showed large differences in

curves in dependence on geometry and loading conditions. Idea that the ductile tearing

resistance could be described by only one representative curve was given up.

The need for a quantification of the constraint and local stress states effect on ductile

fracture become evident, making unavoidable an additional parameter for fracture

resistance. The term “constraint effect” in fracture mechanics commonly designates the

influence on the crack-tip stress field imposed by the loading configuration and geometry.

The main cause of constraint is

the degree

of crack tip stress triaxiality, that is the size