Key Engineering Materials Vols. 462-463

Abstract: The paper proposes first to study the behavior of honeycomb alone under uniform cyclic compression-relaxation loading. It is found that the behavior is linear until it reaches the maximum force following by a sudden drop of force and at the end following by a constant force during the compression. This force-displacement behavior is observed to be similar for all material of honeycomb such as; Nomex (with different densities and cell dimensions), fiber-glass and aluminum. From experimental study, the behavior becomes significantly non-linear in the area of constant force (flat zone) especially the spring-back behavior. A mathematical model is proposed to simulate completely compression-spring-back behavior of honeycomb. This mathematical model describes the behavior of honeycomb in compression and spring-back loading only in function of the maximum depth due to impact. The proposed mathematical model is then integrated to FEA model to simulate the spring-back behavior of sandwich structure with metallic skins and honeycomb core. After integrating the influence of skin-honeycomb interaction, a complete mathematical model which includes non-linearity of compression-spring-back behavior and rotation due to skin-honeycomb interaction is proposed to simulate the behavior of sandwich structure subjected to low energy/ low velocity impact (indentation) loading using spherical impactor. Some simulations of indentation and relaxation on sandwich structure with nomex honeycomb using different diameters of spherical indenter and different thickness of metallic skin are obtained with a good comparison with experimental results.

Abstract: In this study, a theoretical analysis for predicting the mechanical properties of three dimensional lattice structures under compressive loading is proposed, and verified by comparing the analytical predictions with FEM results. This theory for estimating the initial stiffness E* is based on the classical beam theory, and the one for estimating the plastic collapse strength reflects the stress state for each lattice structure. In particular, effects of inner geometry (strand’s diameter-to-length ratio and micro-architecture) on the mechanical behaviour are discussed.

Abstract: This paper presents the study of prismatic columns of different cross sections subjected to low velocity impact, which are commonly used as energy absorber components in vehicles. The impacts of the columns were numerically analyzed using FEM. Four cross sections were considered, i.e. square, hexagonal, octagonal and circular. For each cross section, columns with several combinations of perimeters and thicknesses were analyzed. The results showed that, for columns with equal perimeter and thickness, those with circular cross sections have the highest mean crushing force and those with square cross sections have the lowest crushing forces. Furthermore, keeping all other parameters constant, columns with thicker wall have significantly higher crushing force while columns with longer perimeter have only slightly higher crushing force. This parametric information will be very useful for modern automotive industry in designing front longitudinal members within an acceptable safety level.

Abstract: In this work, a probabilistic fracture mechanics analysis of multiple cracks in a cylindrical pressure vessel was conducted. The analysis was performed to predict service life of a pressure vessel with a certain level of reliability if the vessel has a multiple internal surface cracks that interact each other. The stress intensity factor of multiple cracks configuration was determined from the stress intensity factor of a single surface crack in a plate subjected to uni-axial load and the interaction factor between the cracks. In this work, the Swift’s crack link-up criterion was employed. These parameters together with several other stochastic parameters, i.e. initial crack size, Paris’s crack propagation constants and fracture toughness, were then used to calculate the probability of failure with a certain level of reliability. The failure probability was simulated using guided direct simulation, for cycle-by-cycle crack propagation, to find the expected service life and the mode of failure (leak or break). A case study of a high-pressure vessel having different initial crack sizes have been simulated and the service life with 99,99% reliability were determined.

Abstract: Prediction of external stability for segmental retaining walls reinforced with geogrid and backfilled with residual soil was carried out using statistical methods and artificial neural networks (ANN). Prediction was based on data obtained from 234 segmental retaining wall designs using procedures developed by the National Concrete Masonry Association (NCMA). The study showed that prediction made using ANN was generally more accurate to the target compared with statistical methods using mathematical models of linear, pure quadratic, full quadratic and interactions.

Abstract: The stress intensity factor (SIF) under the combined bending and torsion loading were studied using a finite element (FE) analysis ANSYS. A 20-node iso-parametric element was used to model the crack tip and the square-root singularity of stress/strain was employed by shifting the mid-side node to the ¼ position to the crack tip. Different crack geometries and loading ratios were used and due to the non-symmetrical analysis involved, a full FE model was developed and analyzed. Remotely applied bending and torsion moment were subjected to the FE model and the SIF were then calculated along the crack front under such loadings. The SIF calculated using the finite element analysis (FEA) was compared with those results obtained using an effective combined SIF method. According to the comparisons, the discrepancies were dependent on the normalized coordinate, x/h, the relative crack depth, a/D, the crack aspect ratio, a/b and the loading ratio, .

Abstract: The aim of this study was to investigate the thermal performance, water absorption and dimension stability against water of the green insulation boards. The results show that the thermal conductivity decreased with increasing fibre contents and reached its minimum value (0.0535 w/mk) for the 60/40 kenaf / PU weight %. Contrarily, thermal resistance increased with increasing fibres contents, up to its maximum value (0.09 k.m2/w) for the 60/40 kenaf / PU weight %. The minimum water absorption percentage and thickness swelling were recorded at a weight of 50% kenaf fibres. The effects of the alkaline treatment were significant enough to increase the thermal conductivity.

Abstract: The life prediction information is useful for improving the component design methodology at the early developing stage. Many thick cylinders are subjected to complex cyclic loading spectrum ranging from small vibration to large load induction. This paper presents modeling of fatigue crack growth behavior in thick wall cylinder for outer surface cracked pipe subjected to internal pressure. There are many factors affecting fatigue crack growth such as; crack length, orientation of crack, thickens of the cylinder and the load ratio. Fatigue crack growth as consequence of service loads depends on many different contributing factors. With the help of a simulation fatigue crack growth in three-dimensional structures can either be predicted or explain for already existing failures. The simulation results showed that, more studies on the thick wall cylinder structure need to be performed in order to obtain more accurate fatigue life.

Abstract: The purpose of this study was to develop effective green insulation boards fabricated from polyurethane (PU) reinforced with Kenaf fibres. Biocomposites having three different weight contents (40/60, 50/50 and 60/40 Kenaf / PU weight %) were manufactured. A fourth type was made from 60/40 NaOH-treated Kenaf / PU weight %. The results show that the elastic properties increased with Kenaf fibre content. The optimal performance was observed at a weight of 50% Kenaf fibres. In addition, kenaf fibres treated with NaOH exhibited significantly improved mechanical properties.

Abstract: The initial compressive residual stresses induced or inherent in a component will not remain stable during the life of the component, it relax and redistributed. In design of the component, it is important to consider the relaxation of residual stress phenomenon. In this study, equations to predict residual stress relaxation of 2024 T351 aluminium alloy specimens were proposed. The equations developed from the experimental data of 2024 T351 aluminium alloy specimens that were shot peened under three different shot peening intensities and undergoing cyclic tests for two load magnitudes for 1, 2, 10, 1000 and 10000 cycles. The residual stress, cold work and microhardness results were recorded after each cyclic load as well as the initial state. The presented model incorporates parameters including the degree of cold work, initial induced residual stress and the number of applied loading cycles.