ECF15 FRACTURE TOUGHNESS OF STRUCTURAL COMPONENTS. INFLUENCE OF CONSTRAINTS Andrzej Neimitz, Jaroslaw Galkiewicz Kielce University of Technology Al.1000-lecia PP 7 25-314 Kielce, Poland [email protected] , [email protected] Abstract In the paper the influence of the in- and out-of-plane constraint on fracture toughness was analyzed theoretically and experimentally. It was assumed that the Q stress is a measure of in-plane constraint and Guo’s Tz function is a measure of out-of-plane constraint. The general formula was proposed to compute the fracture toughness when two or more fracture mechanisms are active in parallel and they are represented by their individual fracture toughness’. The simple, analytical formulas were derived to compute fracture toughness’ for different fracture mechanisms acting individually. They are functions of three parameters: J IC , Q, Tm. Experimental program was carried out to measure fracture toughness of the specimen, J c . 2D and 3D FE analysis was performed to compute all necessary quantities. Theoretical models proved to be correct when compared with the experimental results for tested materials. Introduction Due to the high geometrical constraint the fracture toughness measured in the laboratory, according to national or international standards assumes values among the lowest for variety of other geometrical configurations. Therefore, structural integrity assessment procedures using the K IC or J IC or similar toughness measures provide, by definition, conservative results. It is, in principle, correct approach leading to “save” decisions. However, safety although the most important requirement imposed on structural integrity assessment procedures, is not a unique one. The structure should be not only safe but cheap as well. It turns out, that for structures without high constraint in front of crack the fracture toughness can be several times higher than for specimens with high constraint. In practice, it is not possible to measure fracture toughness for all structures (different shapes and sizes) made of given material. However, if one could define some measures of constraint (in-plane and out-of plane) computable for different geometries, these measures could be used to propose the physically based formulas defining fracture toughness for arbitrary component. 1. In-plane and out-of-plane constraints measures 1.1. In plane constraint. For the Ramberg-Osgood (R-O) material the second term of asymptotic expansion of the stress field weakly depends on the distance from the crack tip. O’Dowd and Shih [1] proposed the simplified, two-terms formula for the stress field in front of the crack, in the form: ij ij n n o o o ij Q n r I J δ σ θ σ σ αε σ θ σ 0 1 1 ) , ( ~ ) 0 ( + ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = = + (2)
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