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Method of calculation and experimental estimation of crack growth resistance under cyclic elastoplastic deformation is proposed. This method is based on measuring of local plastic strain near the crack tip and plotting the cyclic elasto-plastic fracture diagram for a specimen with a crack. Analysis of two types of the cyclic elasto-plastic fracture diagrams and their parameters is made. Ex-perimental D-diagrams of cyclic elasto-plastic fracture for the plastic carbon steel are given.

A significant influence on strength and life of the structures is conditioned by manufacturing defects and operational nature which often become the cause of fatigue cracks. Evaluation of the crack resistance of structures made from plastic steel based on the approaches of nonlinear fracture mechanics is problematic due to non- compliance of the conditions of plane strain. One solution to the problem is in the extrapolation of formulas of linear elastic fracture mechanics for stress intensity factor (SIF) on essentially nonlinear stage of deformation using functions of plasticity amendments. Analytical and experimental method for the estimation of crack growth resistance under cyclic elasto-plastic deformation [

Considering that plastic steel was subjected to test an estimation of applicability of basic formulas of linear elastic fracture mechanics was made. Observance of flat deformation conditions was checked by criteria [

where

It has appeared that conditions Equation (1) and Equation (2) are not satisfied for the investigated steel in the upper part of the fatigue crack growth diagram. Formulas of linear elastic fracture mechanics for the estimation of SIF value of the standard compact tension specimen [

where

is also correct for elastic deformation under preservation of flat deformation conditions. In order to apply them to elasto-plastic domain it is necessary to correct them for plasticity.

It can be realized by taking into account in Equation (4) the actual sizes of dangerous cross-section of the specimen, i.e. those sizes that take place under plastic deformation [

Let us multiply and divide the relation

mation of dangerous cross-section of the specimen in function Y it is necessary to accept the actual thickness

where

Thus Equation (4а) considers not only geometry of the specimen and its scheme of loading but also integrally the size of plastic strain in dangerous cross-section. And in Equation (3а) the local measure of damage of a specimen with a crack

It should also be stressed out that the measure

There are two types of CEPF-diagram [

The CEPF-diagram generally consists of two curves: a curve of cyclic elasto-plastic destruction (sections ОВС in D-diagram and _{1}) corresponds to the beginning of

cyclic rupture and is characterized by parameter

It is offered three expressions for the analytical description of ОВС curve at D-diagram [

where

The second dependence for the description of a curve of cyclic elasto-plastic destruction ОВС looks like:

where

be defined for them as such value _{w} is defined also as value corresponding to value

curve (see

found from the same graph as a tangent of an angle of an inclination of the received straight line to an axis of abscises.

For obtaining the third expression it is accepted that experimental points in an average part of ОВС part of the D-diagram are approximated by a straight line in co-ordinates

where