Key Engineering Materials Vols. 348-349

Abstract: Using damage mechanics, cyclic damage evolution has been described and evaluated in a non-crimped glass epoxy fabric composite. A fundamental fatigue study has been carried out by progressively monitoring the fatigue damage modulus and crack density throughout the life of an [0,+45,90,-45]2 (antisymmetric) laminate cycled at a stress ratio R (minimum stress/maximum stress) of 0.1. Development of damage can be separated into two main stages. Initially, damage increases very quickly during the first 10% of life (Stage I). Afterwhich, it increases more slowly at a relatively constant rate to failure (Stage II). The changes in the fatigue modulus and crack density both show the same behaviour. A large amount of damage in the form of transverse matrix cracks develops during the first cycle. These then remain constant throughout life. By contrast, the number of shear matrix cracks increase continually. The crack density is cycle, not stress dependent. This behaviour is reflected by changes in the fatigue modulus. Using damage mechanics, a representative equation has been applied to express the progressive evolution of damage. The significance of which is that the amount of fatigue damage at the end of Stage I for any stress level can be used to predict fatigue life and the stress-life diagram for the laminate.

Abstract: Adhesive in joints will have complex stress state rather than bulk adhesives. This will lead to the assumption behind bulk adhesive that triaxiality function (Rv) is equal to one (uni-axial stress state) is not valid anymore. In this paper, new procedure to find damage parameters α and β for single-lap joints has been developed based on global damage of adhesive joints. With this procedure, damage parameters α and β have been found. Validating the procedure by calculating the number of cycles to failure (Nf) has been performed successfully. The accuracy of the damage evolution equation is less than 2 %.

Abstract: In polycrystalline materials grain boundaries provide an important contribution to the resistance to the propagation of both brittle and ductile cracks. In this paper we describe experimental measurements of brittle cracks developed within both small punch and matchstick test specimens of polycrystalline hcp zinc. These specimens were tested over the temperature range 77 to 423K. Fractography undertaken using focussed ion beam imaging provides detail of the propagation from grain to grain and across {10-12} twins of (0001) basal and {10-10} prismatic cleavage cracks. The results are discussed by comparison with the predictions from previously described 3-D geometric modelling applied to this hcp polycrystalline material.

Abstract: In recent years, increased loading and low weight requirements have led to the need for automatic crack tracing software. At MTU a purely hexahedral code has been developed in the nineties for Mode-I applications. It has been used extensively for all kinds of components and has proven to be very flexible and reliable. Nevertheless, in transition regions between complex components curved cracks have been observed, necessitating the development of mixed-mode software. Due to the curvature of the crack faces, purely hexahedral meshes are not feasible, and therefore a mixture of hexahedral elements at the crack tip, combined with tetrahedral in the remaining structure has been selected. The intention of the present paper is to compare both methods and to point out the strength and weaknesses of each regarding accuracy, complexity, flexibility and computing time. Furthermore, difficulties arising from the out-of-plane growth of the crack such as the determination of the crack propagation direction are discussed.

Abstract: In the present paper, the repair of a cracked structure under dynamic load using the electromechanical admittance (EMA) approach is investigated. Conceptually, appropriate electrical field are applied on the outer surfaces of piezoelectric (PZT) patches to effect closure of the crack. This has the effect of altering the electromechnaical (E/M) admittance signature, extracted at the electrical terminals of a specific PZT patch, considered as an admittance calculating sensor (ACS) patch, towards that of the healthy structure, which is the criterion concept used for the repair in this paper. To demonstrate the present repair methodology, a cantilever 3D beam numerical example is considered in combination with a FEM-based minimization of the difference between the healthy and cracked structure’s (E/M) admittance signature, for specific frequency ranges.

Abstract: Nickel base superalloys are the primary class of materials used in the manufacture of high temperature components for gas turbine aeroengines, including combustion casings and liners, guide vane and turbine blades and discs, etc. These components are subjected to complex cyclic loading induced by the combination of mechanical loading, changing temperatures and thermal gradients, inducing plastic deformation and creep, that ultimately may lead to crack initiation and propagation. The purpose of the present paper is to provide a necessarily brief overview of recent modeling activities in this field, including polycrystalline crystal plasticity modeling for the study of crack initiation, coupled non-local damage-plasticity modeling for crack initiation and propagation studies, and the incorporation of time and environment dependent processes (creep and oxidation) in the predictive modeling of fatigue crack growth rates in nickel base superalloys.

Abstract: We conducted work to investigate fatigue cracking and repair in bone, in which we discovered the mechanism by which bone is able to detect the presence of microscopic cracks and thus initiate repair processes. This investigation has made use of theoretical and applied fracture mechanics, in combination with cell biology. It is the first example of a completely-understood mechanism showing how living cells can respond to mechanical stimuli.

Abstract: In this paper, the basic damage mechanisms and the primary reasons for the property fluctuation of recycled aggregate concrete are investigated experimentally. By a comprehensive literature study and systematic laboratory tests, the interactions between the old and the new interfaces in recycled aggregate concrete are analyzed. In particular, the damage initiation and evolution mechanisms on the old and the new interfaces are studied in details. The essential factors affecting the fluctuation and its extent are investigated from the point of view of material sciences. The present results imply that the qualitative and quantitative changes of the old and the new interfaces during the loading process induce a notable fluctuation of the mechanical and the physical properties of recycled aggregate concrete. To reduce the fluctuation and improve the mechanical properties of the recycled aggregate concrete, effective controlling and processing measures are suggested and discussed.

Abstract: The paper contains results of an experimental programme aimed at an evaluation of fatigue crack growth rate and threshold conditions in a reactor pressure vessel steel. Though the main target of the work was to gain a data basis for possible future needs of defect and risk assessment, an emphasis was put on an evaluation of crack growth mechanisms, too. It was shown that despite some recent works infirming crack closure phenomenon itself or methods of its evaluation, crack closure explained near-threshold fatigue crack behaviour in the specific case of the reactor steel in air conditions and was in a direct consistency with results of fractographical analyses. A fairly recent model of partial crack closure was very suitable for an explanation of an unexpected fatigue crack growth behaviour in water environment, when fatigue crack growth rates were rather irregular and significantly lower that in air.

Abstract: This paper presents a crack analysis of linear magnetoelectroelastic materials subjected to static loading conditions. To this end, an efficient boundary element method (BEM) is developed. Unlike many previous investigations published in literature, two-dimensional (2-D) linear magnetoelectroelastic materials possessing fully coupled piezoelectric, piezomagnetic and magnetoelectric effects are considered in this paper. A combination of the displacement BEM and the traction BEM is used in the present formulation. The displacement BEM is applied for the external boundary of the cracked solid, while the traction BEM is used for the crack-faces. A regularization technique is implemented to compute the strongly singular and hypersingular boundary integrals in the BEM. The electric displacement intensity factor (EDIF), the magnetic induction intensity factor (MIIF), the stress intensity factors (SIF), the mechanical strain energy release rate (MSERR) and the total energy release rate (TERR) are evaluated directly from the computed nodal values at discontinuous quarter point elements placed next to the crack tip. The accuracy of the BEM is verified by analytical solutions known in literature. Results are presented for a branched crack in a bending specimen subjected to combined magnetic-electric-mechanical loading conditions.