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structural fracture at the maximum loading from the beginning of service or after some

periods of service during which the crack can grow up to the critical size for nominal

durable loading of the structure.

This also illustrate non-adequacy of conventional design praxis for the realization of

fracture safe structures of their integrity guarantied in service, which neglects the

existence of cracks and the possibility of their growth to the critical size, and in this way

the possibility of the unexpected fracture in spite of applied recommended safety factors

against the yield strength or similar parameters of “ideal” structure material.

The aim of this paper is short but sufficiently detailed review of fracture mechanics

and their applications, exhibiting exceptional power and potential, but at the same time is

susceptible to the accepted restrictions, based on the simplifications allowed by

theoretical solutions used.

2. BASIC ELEMENTS OF FRACTURE MECHANICS

Although the calculations for the evaluation of basic fracture mechanics parameters

are complex, they nevertheless do not take into account all aspects of the problems. These

calculations are only 2-dimesional, do not take into account redistribution and load

relaxation around the crack tip due to the material plasticity and accept the assumption of

material homogeneity. In addition, they started from an inherent assumption that the

crack driving force can be quantitatively characterised by only one parameter (stress

intensity factor,

path independent

integral or crack opening displacement,

). All

these assumptions are only partially fulfilled and can lead, in the dependence of the actual

case, to the significant errors in application, if the corresponding limits were not

considered. However, the latest is not always so simple.

In spite of this, the fracture mechanics became important discipline in the praxis.

Although not prefect, it offers many answers to the engineering practical problems that

were until now not attainable. Awkwardness due to the possible errors (of course not

these concerning wrong data for calculation and pure calculation) should not influence

the fracture mechanics application. Is it in this respect more favourable to calculate beam

with elastic stresses and with the addition of the safety margin declare it as safe, as has

been done more or les successfully during last hundred years? Of course, this elastic

calculation does not give exact results concerning the actual beam stresses, but was

successful after long-standing application and corresponding experiences. However, for

current conditions of light structures, high strength materials and tightness requirements

concerning both economy and performances are no more satisfied.

In all that we are confronted with the notorious fact of the selection between accurate

solution far from the reality and approximate solution satisfying practical requirements.

The solution for Hutchinson-Rice-Rosengren (HRR) problem appeared to be impor-

tant, because it established

-integral as the most convenient fracture mechanics para-

meter, taking in consideration elastic-plastic material behaviour at crack tip, but, in addi-

tion by recognised difference between plane stress and plane strain conditions. HRR

calculations have found that the stresses in the plastic zone are much higher in plane

strain than in plane stress condition, what is the theoretical explanation for empirically

observed thickness effects to the 2D-parameters of fracture mechanics. The LEFM

solution based on

do not apply this difference – the stresses are the same for both cases.

However, the HRR-solution is not complete and it is subjected to the corresponding

limitations. Just near the crack tip, at the distance

→

0, the solution is invalid because it