Jozef Predan

Crack driving force in twisted plywood structures

Twisted plywood architectures can be observed in many biological materials with high fracture toughness, such as in arthropod cuticles or in lamellar bone. Main purpose of this paper is to analyze the influence of the progressive rotation of the fiber direction on the spatial variation of the crack driving force and, thus, on the fracture toughness of plywood-like structures. The theory of fiber composites is used to describe the stiffness matrix of a twisted plywood structure in a specimen-fixed coordinate system. The driving force acting on a crack propagating orthogonally to the fiber-rotation plane is studied by methods of computational mechanics, coupled with the concept of configurational forces. The analysis unfolds a spatial variation of the crack driving force with minima that are beneficial for the fracture toughness of the material. It is shown that the estimation of the crack driving force can be simplified by replacing the complicated anisotropic twisted plywood structure by an isotropic material with appropriate periodic variations of Youngs modulus, which can be constructed based either on the local stiffness or local strain energy density variations. As practical example, the concepts are discussed for a specimen with a stiffness anisotropy similar to lamellar bone. STATEMENT OF SIGNIFICANCE Twisted plywood-like structures exist in many natural fiber composites, such as bone or insect carapaces, and are known to be very fracture resistant. The crack driving force in such materials is analyzed quantitatively for the first time, using the concept of configurational forces. This tool, well established in the mechanics of materials, is introduced to the modeling of biological material systems with inhomogeneous and anisotropic material behavior. Based on this analysis, it is shown that the system can be approximated by an appropriately chosen inhomogeneous but isotropic material for the calculation of the crack driving force. The spatial variation of the crack driving force and, especially, its local minima are essential to describe the fracture properties of twisted plywood structures.

Application of Structural INTegrity Assessment Procedure to Nuclear Power Plant Component

Quite few analytical flow assessments methods as specific standard and guidelines there have been developed in recent years. Today, as one of a most comprehensive assessment procedure is SINTAP- Structural INTegrity Assessment Procedure. The SINTAP introduced the basic principles of R6 (rev. 3) and ETM. SINTAP procedure is possible to performed assessment for inhomogeneous configurations such as strength mis-matched weldments and an effect of residual stresses. Nevertheless, the SINTAP procedure take into account temperature transition region from ductile-to-brittle behavior of material. In the contribution the SINTAP procedure was applied to the failure analysis of cracked component. Structure integrity assessment is issued to failure prediction of cracked structural component regarding to increasing applied load or crack size. In order to ensure safe use of cracked structural component the many standards and procedures are available (e.g. SINTAP). The results obtained by using of these procedures consist always certain portion of conservatism, what leads to underestimation of the real carrying capacity of engineering component. In this paper a T-joint of 8 in. RHR nozzle on 29 in. RC hot leg as the part of a high pressured pipeline of the nuclear power plant has been considered as engineering component subjected to thermal shock.

Local Variation of Crack Driving Force in a Mismatched Weld

The paper deals with the assessment of the fracture resistance of an inhomogeneous welded joint. The material inhomogeneities create a difference between the near-tip crack driving force, J tip, and the nominally applied far-field crack driving force, J far. This difference is quantified by the socalled material inhomogeneity term, Cinh, which can be evaluated by a post-processing procedure to a conventional finite element stress analysis. Figure 1 shows a welded joint, where the weld metal is in half overmatched (OM) and half undermatched (UM) configuration. Such welds are commonly used for repair welding or for welded joints where the possibility of hydrogen assisted cracking exists. The crack is perpendicular to the mismatch interface. The material properties have a jump at the mismatch interface, but are assumed constant in the regions above and below. In a preliminary study it has been shown that the slant interfaces to the HAZ have no noticeable effect. For this reason, the geometry is simplified to that of a CT specimen with a biomaterial interface between the OM and UM weld metal. The material inhomogeneity term is given by

Crack Tip Shielding or Anti-shielding due to Smooth and Discontinuous Material Inhomogeneities

C_{inh} = - e \cdot \int_\Sigma {\left( {\left[\kern-0.15em\left[ \phi \right]\kern-0.15em\right]I - \left\langle \sigma \right\rangle \cdot \left[\kern-0.15em\left[ {grad{\text{ }}u} \right]\kern-0.15em\right]} \right) \cdot n{\text{ }}ds}

Improvement of fatigue life by compliant and soft interlayers

(1) where σ is the Cauchy stress, u the displacement, I the identity matrix, e the unit vector in the direction of crack growth, and n the unit normal to the mismatch interface Σ.