F.W. Brust

A combined numerical/experimental study of ductile crack growth after a large unloading, using T∗, J and CTOA criteria

Abstract The J-integral is commonly used to determine the resistance of plastically deformed structures to continued crack growth. This approach is only valid for monotonic loading and small crack growth; yet it is believed conservative for all general loadings. Another parameter, T ∗ , has been formulated, which is theoretically valid under a wide range of conditions. These two plastic fracture criteria, along with CTOA (crack tip opening angle), were examined after a cycle of loading/unloading to zero load followed by reloading. The results of the numerical/ experimental study showed that the T ∗ parameter accurately predicted the behavior (crack growth after 50% reloading), while the other parameters (J and CTOA) were seriously anticonservative.

Mixed-mode stress intensity factors for interacting semi-elliptical surface cracks in a plate

Abstract Mixed-mode stress intensity factor solutions are presented for two parallel, but not necessarily coplanar, interacting surface cracks in a plate. The plate is subjected to a remote uniaxial stress acting normal to the crack planes. The crack separation distances, measured normal and parallel to the crack planes, are varied to provide 24 crack geometries. The smallest separation distance is equal to a quarter of the plate thickness. The equal sized interacting cracks have fixed depth-to-thickness ratios of 0.5 and fixed half-width-to-depth ratios of 3. The finite element alternating method is used to develop the solutions. The tendency for the cracks to grow apart or to coalesce is discussed.

An estimation method for evaluating energy release rates of circumferential through-wall cracked pipe welds

Abstract A new methodology is proposed to estimate energy release rates of through-wall cracked (TWC) ductile pipe weldments subjected to pure bending loads. It is based on the deformation theory of plasticity, the constitutive law characterized by the Ramberg-Osgood model, and an equivalence criterion incorporating the reduced thickness analogy for simulating system compliance due to the presence of a crack in weld metal. A closed form solution is obtained in terms of elementary functions for approximate evaluation of the J- integral . The method utilizes the material properties of both base and weld metals, which are not considered in the current estimation methods. It is very general and can be applied in the complete range between elastic and fully plastic conditions. Several numerical examples are presented to illustrate the proposed technique. Comparisons of results with reference solutions from the finite element method indicate satisfactory prediction of energy release rates.

Load history effects on creep crack growth

Abstract In this paper the effects of load history on the high-temperature creep crack growth process are studied through a combined experimental and computational approach. The general features of constitutive response during cyclic creep are reviewed. Next, fracture parameters for creep crack growth are reviewed, with special emphasis on integral parameters. Finally, examples comparing computational predictions of experiments which experience history dependent load histories are presented. This includes displacement time comparisons and fracture parameter comparisons.

A model for predicting primary creep damage in axial cracked cylinders—I. Theory

Abstract Primary creep damage may occur at a crack tip in steel at room temperature and below. The effect of this time dependent damage is generally neglected. Recently developed experimental data clearly show that, for some materials, neglecting time dependent deformation and damage may be quite non-conservative and dangerous in certain practical applications. This paper presents a new primary creep crack growth damage model. The theoretical developments of the model were presented elsewhere [F. W. Brust and B.N. Leis, Int. J. Fracture54, 45–63 (1992)]. The model was verified by comparison of finite element solutions and primary creep crack growth tests on single edge notch and center crack panel geometries. Here the model is extended to be applicable to axial through-wall cracked cylinders. Comparisons of model predictions to experimental data are presented in the companion Part II paper [F. W. Brust and B.N. Leis, Engng Fracture Mech.43, 629–639 (1992)]. The new model can be easily placed in a probabilistic framework due to its accuracy and simplicity.

A model for predicting primary creep damage in axial cracked cylinders. II: Applications

Abstract Time dependent effects, which are normally neglected in steel structures which operate at or below room temperature, need to be considered under certain conditions if the body contains a crack. Part I of this effort [F. W. Brust and B.N. Leis, Engng Fracture Mech. 43 , 615–627 (1992)] provided the theory for a model which is used here, in Part II, to predict the effect of time dependent damage on the crack growth and failure process. Time dependent crack growth and failure for a number of different length axial flaws in two different size pipes is predicted using this model. These model predictions are then compared to experimental data. The analysis procedure and results presented here may be useful in developing optimum proof test strategies.

Creep crack growth under history-dependent loading

We discuss the influence of loading history on creep crack growth. Our attention is mainly focused on the following three aspects of this problem: (i) principal laws of history-dependent creep strain of materials; (ii) creep behavior of cracks, including the choice of suitable fracture parameters that may help to predict cracking; (iii) the importance of taking the history-dependent response of the material into account. We performed numerical calculations based on the use of an appropriate constitutive model and fracture theory for (1) and (2), respectively, to analyze results of tests for (3).