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Constraint capacity corresponds to enforcing plane strain conditions, i.e. nil-levels of

lateral contraction across the crack front. This definition is best understood in terms of

the plastic zone developed at the crack front of a

specimen. If

can be measured,

then the plastic zone is very small because of plane strain conditions. When constraint

relaxation evolves, the plastic zone size increases and eventually becomes large related

to the section size, Figs. 10 to 12. The complexity of the problem is indicated not only by

different view of fractographs in Fig. 12, presenting final stretch zones for the weld and

HAZ of the same steel, but also by the their values obtained at 20

C and at -60

C (Table

1). It is clear that micro structure in the region of crack tip is heterogeneous, and the

response of material to the load is locally different. Crack tip is located in different micro

structures and after blunting crack grows in different conditions. It this case structural

integrity can be assessed by global consideration, i.e. energy dissipation.

2.3. Extension of application of fracture mechanics parameters

Once establish, and supported by mathematical background, basic parameters of

fracture mechanics, stress intensity factor, path independent contour integral (

integral)

and crack opening displacement, have found the application also in the other fields. In

addition to brittle behaviour, steel and metallic materials frequently exhibited significant

plastic deformation, and the application to crack growth beyond yielding, (elastic-plastic

fracture mechanics - EPFM) is helpful in this situation.

Well known Paris law based on stress intensity factor range, although empirical in its

nature, is a powerful tool to solving the problems in fatigue. Looking for solutions of

important problems with the crack in creep condition,

and

integrals are derived

base on

integral. The crack problem in stress corrosion environment is also considered

applying critical stress intensity factor,

Iscc

It is to emphasis that performed extensions were possible only after introduction of

series of assumptions, simplifications and approximations, necessary to cover uncer-

tainties and shortage in knowledge.

Following the experience and using it in a similar way, fracture mechanics and its

parameters are involved also for nano materials and nano structures /1/, but again with

series of assumptions, simplifications and approximations, and applying broadly methods

of simulations and modelling.

2.4. Structural integrity assessment by fracture mechanics

The achievement in fracture mechanics and numerical modelling of structures

enabled the development of methods for reliability evaluation and structural integrity

assessment. Applying these methods service safety of structures is increased and their

life extended, allowing in the same time significant economical saving. Numerical

modelling is today an inevitable tool in different structures design, manufacturing and

use, like steel structures, power and petrochemical plants, aircrafts, machinery, vehicles,

but also in the development of nano materials and structures. Welded structures have

very important position in many of these sectors, requiring special attention. The reason

is the possibility of crack occurrence in welded joints, which can endanger structural

integrity and service safety /15/.

Fracture mechanics approach was applied and has been formally accepted in the case

of Trans Alaska Crude Oil Pipeline /16/ for reliability assessment in the form of "fitness-

for-purpose". The investigation defined theoretically and verified experimentally maxi-