Failure analysis and engineering mechanics modeling methods of a stitched ceramic sandwich structure

Designing highly stressed offshore renewable energy composite structures (e.g. wind and tidal turbine blades) necessitates characterisation of woven fabric composite under off axial loading. In this work a combined method of finite element analysis, digital image correlation and microscopy is used to assess the effect of ply orientation on the tensile/shear properties and failure modes of woven glass fibre reinforced polymer composites. Full field strain maps obtained by the digital image correlation method were used to evaluate the damage development and the inhomogeneity of strain localisation. The development of finite element models of mechanical test specimens is based on the analysis of micro-mechanical models of representative volume elements using a homogenisation technique in order to calculate the effective orthotropic properties. The agreement between numerically and experimentally calculated strains obtained in the elastic regimes indicates that stress analysis conducted by numerical methods is useful when characterising the effect of ply orientation on mechanical behaviour. Strain measurement conducted by the digital image correlation method indicated that there is a strong relationship between the strain distribution and the microstructure/ply orientation. In addition, it was found that the levels of localised tensile strain are higher than the global strain indicating the structural heterogeneity of the composite material. Finally, microstructural analysis of tension and shear test specimens showed that the main failure modes are de-bonded fibres, fibre pull out, in-plane/inter-laminar shear cracks and delamination.

Machine parts and structures of a change in the displacement and strain can be evaluated safety is one of the important factors. Typically the strain gauge has been employed to measure the displacement and strain. However, this contact-type measurement method has disadvantages that are not measured under condition of specific object shape, surface roughness and temperature. In particularly, 3 point bending and 4 point bending test not use strain gauge. So its test used cross head displacement and deflect meter. Digital Image Correlation measurement methods have many advantages. It is non contact-type measurement method to measure the object displacements and strain. In addition, it is possible to measure the Map of full field displacements and strain. In this paper, measured the 3 point bending deflection using the Digital Image Correlation methods. In order to secure the reliability, Digital Image Correlation method and universal test machine were compared.

Effect of damage on the interlaminar shear properties of hybrid composite laminates at cryogenic temperatures

Application of the DIC (Digital Image Correlation) method for the analysis of crack initiation and propagation in RC beams reinforced by BFRP bars (Basalt Fibre Reinforced Polymer) is presented in the paper. Two dimensional, single camera DIC system was used to observe strain and crack distribution in such RC beams loaded using four point bending setup. BFRP bars have arose as an alternative to steel bars for RC (Reinforcement Concrete) members in aggressive environments. The low modulus of elasticity of BFRP bars lead on to large crack widths and, as a result, the design of BFRP Reinforced Concrete beams is often governed by SLS (Serviceability Limit State) criterion. Due to this it is necessary to explore specific crack propagation in concrete beams. Process of such beams degradation can be identified by comparison of strains and displacements obtained by DIC method, with the database of experiment results.

Dynamic response of hemispherical-shell sandwich structures subjected to underwater impulsive loading

The purpose of this study was to design a light-weight sandwich panel for trailers. Strength calculations and selection of different materials were carried out in order to find a new solution for this specific application. The sandwich materials were fabricated using vacuum infusion technology. The different types of sandwich composite panels were tested in 4-point bending conditions according to ASTM C393/C393M. Virtual testing was performed by use of ANSYS software to simplify the core material selection process and to design the layers. 2D Finite element analysis (FEA) of 4-point bending was made with ANSYS APDL (Classic) software. Data for the FEA was obtained from the tensile tests of glass fiber plastic (GFRP) laminates. Virtual 2D results were compared with real 4-point bending tests. 3D FEA was applied to virtually test the selected sandwich structure in real working conditions. Based on FEA results the Pareto optimality concept has been applied and optimal solutions determined.

The application of metallic foam core sandwich structures in engineering components has been of particular interest in recent years because of their unique mechanical and thermal properties. Thermal spraying of the skin on the foam structure has recently been employed as a novel cost-efficient method for fabrication of these structures from refractory materials with complex shapes that could not otherwise be easily fabricated. The mechanical behavior of these structures under flexural loading is important in most applications. Previous studies have suggested that heat treatment of the thermally sprayed sandwich structures could improve the ductility of the skins and so affect the failure mode. In the present study the mechanical behavior of sandwich beams prepared from arc sprayed alloy 625 skin on 40 ppi nickel foam was characterized under four point bending. The ductility of the arc sprayed alloy 625 coatings was improved after heat treatment at 1100°C and 900°C while the yield point was reduced. Heat treatment of the sandwich beams reduced the danger of catastrophic failure.

Sandwich structures can be classed as composite materials in that they consist of two or more individual components of differing properties which when combined result in a high‐performance material. In contrast to monolithic composites—which consist of an intimate mixture of fibers (glass, Kevlar, carbon etc.) supported within a continuous matrix (e.g., thermoplastic or thermoset resin)—sandwich structures show a discrete structure in which a core material is bonded to, and faced with, a skin material. The skin material usually shows a high stiffness, whereas the core typically has high compressive and shear strength. The primary advantage of a sandwich composite is very high stiffness‐to‐weight and high bending strength‐to‐weight ratio. The sandwich enhances the flexural rigidity of the structure without adding substantial weight. First of all, the core should possess low density in order to add as little as possible to the total weight of the sandwich construction. In this paper, three different core configurations are been studied: honeycomb, inversed honeycomb and Kagome bonded to aluminum skins. The cores are made from PLA using the FDM (fusion deposition method) procedure, on a 3D printer.

Herein, a single-camera stereo digital image correlation (DIC) method is developed for dynamic deformation measurement, and a dual-biprism high-speed stereo camera system is proposed to capture surface images. Then, full-field displacement and strain distributions are obtained after correlation and differential calculation using the stereo-DIC algorithm. After a typical experiment is performed to verify the accuracy of the proposed system, full-field displacement and strain distributions of sandwich panels subjected to low-velocity impacts are determined. These results show that the proposed system is an efficient and convenient tool for dynamic deformation measurement.

Effective stiffness concept in bending modeling of laminates with damage in surface 90-layers

The effect of interlaminar deformation due to interlaminar shear stresses and delamination on the in-plane behavior of FRP laminates was investigated. The interlaminar shear stresses considered were induced by in-plane external load and stiffness mismatch. In order to develop a simple lamination theory which could take account of the effect of interlaminar shear deformation, a simplified laminate model and linear shear slip theory were employed. The lamination theory was applied to a series of two-layer angleply laminate tubes and the results show that interlaminar shear deformation affects significantly the mechanical behavior of the angle-ply laminate tubes with imperfect interfacial bonding.

In the present paper, criterion of interlaminar strength for unidirectional CFRP laminates was discussed. Generally, interlaminar shear strength (ILSS) of the CFRP laminates is evaluated by 3-point bending test, but the criterion of the fracture depends on not only interlaminar shear stress but also interlaminar normal stress in the specimens. In order to obtain the limit curve which is given by the relation between normal stress and shear stress, two types of ILSS test have been employed in the present study. The one is a 'compression type ILSS test' by a simple 3-point bending. The another one is a 'tension type ILSS test' applying the shear tensile loading using a shear pin. Employing 2-dimensional finite element analysis the criterion of the CFRP laminates and CFRP toughened by carbon nanofiber were investigated in detail using a modified 'Tsai-Wu' criterion.

The aim of this study is to numerically investigate the low‐velocity impact behavior of carbon fiber reinforced orthogonal woven fabric composite sandwich structures with five different geometric configurations and four different curve angles. Low velocity impact simulations were performed in LS DYNA finite element program to investigate the effects of core configuration and curve angle on peak contact force, energy absorption efficiency and damage mode. A progressive damage analysis based on the combination of the Hashin damage criterion and the Cohesive Zone Model (CZM) with bilinear traction‐separation law was performed using the MAT‐54 (ENHANCED_COMPOSITE_DAMAGE) material model. Among the five different cores, Trapezoidal core has the highest peak force average of 2.9 kN while Triangular core sandwich structure has the lowest with 1.23 kN. Absorbed energy efficiency average was highest for rectangular core with 0.95 and lowest for sinusoidal core with 0.69. The specific absorbed energy value was highest for Triangular core with 0.312 J/g and lowest for Sinusoidal core with 0.178 J/g. The damage area on the structure increased with increasing curve angle. It was determined that the core structure and curve angle is an effective parameter on peak force and energy absorption efficiency.Highlights Impact behavior of sandwich composite structures was examined. The effects of core type and curve angles on peak force, energy absorption efficiency and specific absorption energy were investigated. Specific absorption energy rates were compared with studies in the literature.

<div class="htmlview paragraph">This paper presents the design and manufacture a sandwich structure bumper beam that could withstand at least the same load required to have plastic deformation in a 2002 Jeep Wrangler bumper beam at a lower weight. The dimensions from a bumper beam were scaled down in order to match the limiting length of the sandwich structure specimens. Theoretical optimization calculations were conducted in order to find the optimal dimensions and face thicknesses for the hybrid structures. Sandwich panels were based on Glass Fiber Reinforced Polypropylene (Twintex) and an Aluminum foam core (Alporas). Three point bending tests were performed on the sandwich structures. The resulting failure modes were revealed and found to be in agreement with those offered by the analytical predictions.</div>

Past studies of dynamic failure in sandwich structures have mostly been concerned with post-mortem investigations of failure patterns resulting from the dynamic loading of such structures through projectile impact. The final fracture patterns have typically been very complex and their time sequence, interdependence, and detailed nature (opening versus shear) were unclear. In the present work we concentrated on the real-time investigation of the generation and the subsequent evolution of dynamic failure that occurred within microseconds after impact. Model sandwich structures involving a combination of transparent polymers and metals (typically tri-layers bonded together by weak or strong adhesives) were designed and subjected to impact loadings to study the failure evolution mechanism. High-speed photography and dynamic photoelasticity were utilized to study the nature and the sequence of failure modes. A series of complex failure modes was observed. In most cases, interfacial cracks (dynamic delamination) appeared first. These cracks were shear dominated and were often intersonic. Thus, initial delamination was found to be shear driven and happen at extremely short time scales even at moderate impact speeds ranging between 20 to 50 m/s. The transition behavior between interlayer crack growth and intralayer crack formation was also observed. Opening (subsonic) intralayer cracks kinking from the dynamic shear delamination propagated into the core layer of the model sandwich structure and eventually branched as they reached a high enough growth speed.