Key Engineering Materials Vols. 488-489

Abstract: Hypervelocity impacts on spacecraft in low earth orbit by meteoroids and space debris poses a threat to space missions, and the use of a shield can significantly decrease the probability of a catastrophic failure. Tests have identified that metallic foams have a good shielding performance against hypervelocity impact by micro-meteoroids and orbital debris. A metal-foam stuffed Whipple shield was presented under the concept of light-weight shield structure. A meso-structure model of geometry for metallic foams was set up simulating their manufacturing process and validated by comparison with experimental results using own SPH code. Three base materials of foam, including Al 7075-T651, Ti and Al ZL102, were researched for their performances as stuff of shield by means of numerical simulation. The results indicated that different base materials show the best shield performance at different impact speeds with other conditions the same. The foam of Al ZL102 stuffed can cause the strongest radial dispersion of the secondary debris cloud and is more likely to provide the best shield performance, which is proved at the higher part of the speed range investigated.

Abstract: CANMET-MTL has developed a low-constraint test designed to reduce unnecessary conservatism in the measurement of toughness for use in the assessment of flaws in pipeline girth welds. The design is based on tension loading using fixed (clamped) grips of a single-edge-notched BxB SE(T) specimen, side-grooved to promote plane-strain conditions. Equations have been developed to derive J-integral, CTOD and crack growth from measurement of load and crack-mouth opening displacement. Loading conditions (essentially distance between the grips) have been chosen to reproduce the crack-tip constraint of a circumferential surface flaw in a pipe in service under tensile or bending loads. In this paper, the development of the test and the principal findings from its use will be described.

Abstract: Ceramic laminates designed with strong interfaces have shown crack growth resistance (R-curve) behaviour through microstructural design (e.g. grain size, layer composition) and/or due to the presence of compressive residual stresses, acting as a barrier to crack propagation. The goal of the contribution is to model the mechanism of crack bifurcation in laminar ceramics with large compressive stress which still have not been satisfactory explained. Experimental observations of the crack path in the multilayered ceramics tested under several kinds of loading showed crack penetration (i.e. crack propagating normal to the layers followed by crack bifurcation when the crack propagated from the tensile to the compressive layer. Numerical results [1] show that the initiation of crack bifurcation can be explained by the near-tip J-integral, provided that micro-cracks exist near the crack tip. We revisit the problem using the concept of Finite fracture mechanics and the matched asymptotic expansion method in order to evaluate the energy release rate criteria describing the competition of the crack bifurcation and straight crack propagation near behind the bimaterial interface.

Abstract: In this paper, considering the material properties of the composite flywheel and the characteristics of the pre-stressed structure, stresses and strains induced by rotor rotation and interference fit were calculated by finite element (FE) method based on the plane stress hypothesis, in the commercial software ANSYS. Based on the given material properties and the main dimension with a certain speed of rotation, three 2D FE-models of hybrid composite flywheel rotors with two-layer rotor structure were built with the unit property of plane stress, axisymmetric and plane strain respectively. Followed, the radial stress, circumferential stress and radial displacement of the rotor were obtained. The three simulation results are almost accordant with the present theoretical results. It shows that the numerical analyses are reliable. It can be shown that is advisable to design and optimize the flywheel rotor.

Abstract: Cracks present a serious threat to the performance of beam-like structures. In this paper, the flexural vibration of a cantilever beam having a slant crack is considered. The beam natural frequencies are obtained for various crack locations, depths and angles, using the finite element method. These natural frequencies and crack specifications are then used to train a neural network. The input of the neural network is the crack specifications and the output is five natural frequencies of the beam. With the trained neural network, genetic algorithm is then used to determine the beam crack specifications by minimizing the differences from the measured frequencies. Simulations are performed to evaluate performance of the neural network. Results show that the proposed scheme can detect slant cracks in cantilever beams with good accuracy.

Abstract: Seismic isolation is a strategy to reduce damage of structures exposed to devastating earthquake excitations. Isolation systems, applied at the base of buildings, lower the fundamental frequency of the structure below the range of dominant frequencies of the ground motion as well as allow to dissipate more energy during structural vibrations. The effectiveness of the base-isolated buildings in damage reduction has been confirmed numerically for the models of structures with fixed supports. The aim of the present paper is to show the results of the non-linear analysis of the response of a base-isolated building supported on soft soil incorporating soil-structure interaction. The detailed study has been conducted for the building equipped with high damping rubber bearings used as isolation devices. The results of numerical simulations demonstrate that soil flexibility has a significant influence on the behaviour of isolated base of the structure. Considering the flexibility of soil significantly affects the rigid superstructure response lowering its potential to reduce structural damage.

Abstract: This paper provides an original experimental characterization of the shakedown state of a shape memory alloy structure under cyclic pseudo-elastic loading. This analysis is performed through the observation of the dissipated energy at a macroscopic scale as well as the temperature on the surface of the sample through infrared thermography measurement. Morevover, a deeper study is led thanks to acoustic emission to quantify the shifts between microscopic evolutions at a lower scale. The main conclusion is that these 3 quantities are correlated and enable us to identify different stages the structure crosses until the shakedown state.

Abstract: In the present paper 3D rate sensitive constitutive model for modeling of laminate composites is presented. The model is formulated within the framework of continuum mechanics based on the principles of irreversible thermodynamics. The matrix (polymer) is modeled using 3D rate sensitive microplane model. For modeling of fibers (glass) a uni-axial constitutive law is employed. The fibers are assumed to be uniformly smeared-out over the matrix. The formulation is based on the assumption of strain compatibility between matrix and fibers. To account for the de-lamination of fibers, the matrix is represented by the periodically distributed bands with non-uniform strength properties over the band width. The input parameters of the model are defined by the mechanical properties of matrix and fibers (elastic properties, strength and fracture energy), the volume content of fibers and by their orientation in 3D space. The model is implemented into a 3D finite element code. To assure mesh objective results, the localization limiter is based on the assumption of constant energy dissipation within each finite element, i.e. the crack band method is used. The performance of the model is shown on one numerical example for specimens loaded in uni-axial tension. It is demonstrated that the proposed model is able to realistically predict the resistance and failure mode of complex fiber-reinforced composite for different orientation of fibers.

Abstract: Experimentally, the evolution of several mechanic properties (hardness, density, Young’s modulus, fracture toughness) is observed in nuclear glasses under irradiation. In this work, classical molecular dynamics calculations are performed to better understand fracture mechanisms in simplified nuclear glasses at atomistic scale and to explain the radiation effects. Fractures are simulated in more disordered glasses, representative of irradiated samples, to reveal radiation effects. We observe a lower elastic limit and a greater plasticity in the irradiated glass that can explain its larger fracture toughness.

Abstract: Secure and cost effective joining methods are key points for practical applications of plastic pipelines. The morphology and material structure of welded joints are complicated in comparison with the base pipe material. The formation of the weld is highly dependent on both thermal history and stress state. Consequently, the material parameters characterizing the weld joint and corresponding heated zone influence the reliability and safety of the welded pipe system as a whole. In the contribution a welded polymer-polymer butt joint is considered and its possible damage caused by slow crack growing in the weld zone is numerically analyzed. The numerical model takes into account the geometry of the bead and changes in material properties inside the weld zone. The results obtained from welded specimens are compared with those for a smooth specimen from the base material. The conclusions described in the paper can be used for a better transfer of fracture mechanics characteristics between laboratory specimens and real pipes.