Crack Tip Opening Displacement Values Research Articles

Fracture toughness and its development in high purity cast carbon and low alloy steels

Fracture studies conducted using linear elastic fracture mechanics, crack opening displacement, and J-integral methods have shown that high room temperature toughness can be achieved in plain and low alloyed medium carbon cast steels. The high quality, carefully controlled, and heat treated steels prepared often yielded Jlc values which exceeded 0.1 MPa-m, or Klc values exceeding 150 MPa-m 1/2 Correspondingly, crack tip opening displacement values were of the order of 0.24 to 0.26 mm. Of the several elements examined chromium is the most effective for developing optimum properties at medium carbon levels. However, in general, chemical composition, in terms of major alloying elements, was found to be only a minor variable in determining fracture toughness of the heat treated steel. Fractographic measurements revealed that a squared power relationship exists between the stretched zone width and relative toughness (Klc/Φys) at both low and high toughness levels.

A new approach to determination of CTOD and axis of rotation in small scale yielding situation

Abstract A theoretical model for predicting crack tip opening displacement (CTOD) in small scale yielding situation has been developed by combining the elastic solution of Muskhelishvili with Irwin's idea of notional crack. The model is used to calculate the CTOD from the displacement at the mouth of the crack, without the use of rotational factor, r . An attempt has also been made to relate the rotational factor, r with the yield strength of the material. The above model has been used to compute CTOD values in a low alloy steel with yield strength ranging from 400–2100 MPa and the results have been compared with Wells' as well as Xiao's CTOD values. The significance of the factor m ' in the K -CTOD relationship has been investigated in the light of the results observed in the present investigation.

On a relationship between stretched zone parameters and fracture toughness of ductile structural steels

Quantitative stereofractographic analysis of the stretched zone at the crack tip is performed on specimens of two ductile steels used for determining fracture toughness. The effect of temperature, loading rate, stress-state mode and specimens size on the stretched zone formation is studied. A correlation is obtained of the stretched-zone height and width with fracture toughness of materials and the crack tip opening displacement value which is determined by means of various mathematical models.

An experimental investigation of the effect of specimen type on the crack tip opening displacement and J-integral fracture criteria

The effects of specimen type (precracked tension or bend) on the relationship between the J-integral and the crack tip opening displacement (COD) and on crack initiation values of J and COD were investigated. The material used was a mild steel. COD was measured by infiltrating the crack with a catalytically hardening silicone rubber. The J-integral was measured both by the compliance technique and using analytical formulae. No significant effect of specimen type was noted. Measurements of J from the analytical formulae used agreed well with compliance measurements. Simultaneous J and COD values were also measured on a low alloy steel. The results indicate that the J/G/COD relationships are material dependent, possibly being controlled by the material's work-hardening rate.

The relationship between crack tip opening displacement, local strain and specimen geometry

A method is described whereby values of crack tip opening displacement (COD) can be measured at any section of a pre-cracked or slotted specimen. The relationship between COD and the degree of transverse strain at the notch root of a Charpy specimen has been investigated. A specimen will show some portion in plane strain up to a COD of about one twenty-fifth of the thickness. Transverse contraction results in smaller COD values, for a particular clip gauge displacement, at the outside than at the specimen midsection. An equation is presented relating COD to longitudinal notch root strain for Charpy specimens with root radius 0.002 in. COD and notch root strain are found to be not directly proportional. This finding is discussed in relation to the concept of a “miniature tensile specimen” at the notch root. The effect of specimen geometry on the value of COD at fracture initiation, (COD)c, has been investigated. Neither a change in specimen thickness nor a change from a fatigue pre-crack to a 0.002 in. root radius notch has been found to significantly affect (COD)c. The material used in this research was, for the most part, a low alloy pressure vessel steel, A533B.