Surface layer stiffness effects on fracture of polymer multilayers: a linear elastic model

A tough thermoplastic polymer may show a transition to brittle behaviour when a skin of different properties forms on, or is painted or bonded onto, its free surfaces. A small-scale yielding, linear elastic analysis of the core material, in combination with an axisymmetric plate analysis of the skin, is used here to explore the role in this phenomenon of skin-core modulus inequality. When applied to the homogeneous (equal modulus) case, this very simple constraint model appears to provide independent support for the ASTM thickness criterion for plane-strain LEFM test validity. When applied to previously published impact fracture data from inhomogeneous (polyethylene-polypropylene) sandwich plates, the model successfully explains the shift in brittle-tough transition temperature precipitated by bonding a polypropylene skin to a polyethylene core. The model offers specific predictions for the effect, on transition temperature shift, of variables such as skin thickness and core properties; these predictions remain to be verified.     

Effect of aggregate size on the fracture and mechanical properties of a simple concrete

The influence of the aggregate size on the fracture energy, tensile strength and elasticity modulus in different types of concrete are analyzed. For this purpose, nine simple cement-based composites have been designed, manufactured and tested, with one objective to provide experimental results that can be used as a benchmark for checking numerical models of concrete fracture, as this simple composite (a matrix, spherical aggregates of the same radius, and two types of matrix-aggregate interface) is amenable to modelling. All in all, 44 specimens were tested. From notched beam tests, values of the fracture energy and modulus of elasticity were obtained. The tensile stress was deduced from indirect standard tensile test. Data for bilinear softening functions extracted from the experimental measurements are also provided. Comparison with available experimental data is also included and discussed.     

Three-Dimensional Analyses by Finite Element Method of a Spur Gear: Effect of Cracks in the Teeth Foot on the Mesh Stiffness

In this paper, a finite element method with a three-dimensional survey is presented. The effect of crack dimension and the direction of crack propagation, in the teeth foot, on the mesh stiffness is studied. For spur gears, the mesh stiffness is affected in a meaningful manner by the presence of a foot crack of one or more teeth. This study is an attempt to estimate the effect of crack size, position, and direction on the spectrum of the gear mesh stiffness.     

Effect of non‐uniform reactor cooling on fracture and constraint of a reactor pressure vessel

In the lifetime prediction and extension of a nuclear power plant, a reactor pressure vessel (RPV) has to demonstrate the exclusion of brittle fracture. This paper aims to apply fracture mechanics to analyse the non‐uniform cooling effect in case of a loss‐of‐coolant accident on the RPV integrity. A comprehensive framework coupling reactor system, fluid dynamics, fracture mechanics, and probabilistic analyses for the RPVs integrity analysis is proposed. The safety margin of the allowed RT_NDT is increased by more than 16°C if a probabilistic method is applied. Considering the non‐uniform plume cooling effect increases K_I more than 30%, increases the failure frequency by more than 1 order of magnitude, and increases the crack tip constraint due to the resulting higher stress. Thus, in order to be more realistic and not to be nonconservative, 3D computational fluid dynamics may be required to provide input for the fracture mechanics analysis of the RPV.     

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications. Part II: Resistance to fatigue crack propagation and fracture

Investigations were continued on the dissimilar laser beam welds of AA6056 and Ti6Al4V, fabricated by inserting Ti‐sheet into the profiled Al‐sheet and melting AA6056 alone. By using microstructure, hardness and strength as the criteria, sites exhibiting non‐uniform microstructure and localized plastic deformation due to strength mismatch were investigated in two orientations: ? crack parallel to the weld and ? crack perpendicular to the weld for fatigue crack propagation and fracture toughness at room temperature. Effect of temper of AA6056 on these properties was studied for two conditions; welding in T4 followed by post weld heat treatment T6, and welding in T6 and naturally aged for a defined period. The orientation “crack parallel to the weld” was investigated in 3 locations on the side of AA6056: the interface and the two changeovers on the Al‐side. Firstly, between the fusion zone and the heat affected zone (3 mm from the interface) and secondly, between (primary) heat affected zone and towards the base material (7 mm from the interface). Although brittle intermetallic TiAl₃ had been formed at the interface, uncontrolled separation or debonding at the interface was not observed. Insofar the bond quality of the weld was good. However, the ranking of interface was the lowest since fatigue crack propagation was relatively faster than that in the fusion zone and heat affected zone, and fracture toughness was low. Therefore, unstable fatigue crack propagation is observed when the crack propagates perpendicular to the weld from AA6056 towards Ti6Al4V. The results have shown that the dissimilar joints exhibit improved performance when laser beam welded in the T6 condition.