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A yield/failure criterion is coordinated with a separate fracture criterion to provide a comprehensive three dimensional description of failure for isotropic materials. Two properties provide the entire calibration for the two criteria.

Purpose of this study was to understand the effect of core material thickness () on the core deformation constraint and the associated mode I fracture toughness in Double Cantilever Sandwich Beam (DCSB) specimens. Specimens were made from woven roving glass fiber/vinyl ester composite face sheet with PVC core, whose thickness ranged from 3.18 mm to 40.6 mm. The specimens were tested in mode I loading and measured fracture initiation () and resistance () toughnesses. The was found to be practically same for core thicknesses from 3.18 to 40.6 mm. The was found to be 1.02, 0.88 and 0.91 kJ/m for ’s 3.18, 6.25, and 12.7 mm. For ≥ 25.4 mm, the crack grew by only few mm’s before it deflected to face sheet. Larger for = 3.18 mm is probably due to resin densification of foam cells in the co-cure processing of panels. Three dimensional, material nonlinear finite element (FE) analysis very well simulated the test data. The integrals from FE analysis agreed well with from the test. The analysis revealed that the deformation constraint was nearly the same for all core thicknesses considered and thus resulting in nearly identical fracture toughness.

Combining NDE techniques can prove to be beneficial in assessing the physical state of health of composite sandwich structures. Several NDE techniques were examined from which vibration and thermal responses were selected. A Neural Network (NN) was chosen as a means to interpret and classify the information such that the type of damage, severity and location could be identified. Numerical simulations were used to train the NN and experimental measurements were used to test and validate the approach.

Results of sandwich composite static and fatigue loading are presented. To discern cracking in various constituents of the sandwich composite, AE technique is used. Core damage has been found to be the predominant failure activity. Fiber rupture triggered the onset of catastrophic failure. Mode I cracking was observed in the core while fiber rupture took place in mode I.

Local effects occurring near junctions between different cores in sandwich structures are considered. Two groups of sandwich beams with conventional butt and reinforced butt core junctions, respectively, were examined experimentally in three-point bending under static and fatigue loadings. A rigorous statistical treatment of the obtained data has shown unambiguously that sandwich beams with modified, that is reinforced, butt junctions display superior structural performance compared with sandwich beams with the traditional core junctions.

This paper deals with fatigue of closed cell foams. The main idea is to use a few simple tests to predict the tension-tension fatigue properties of foams. The required testing consists of crack propagation rate measurements and one tension-tension fatigue test performed at yield stress for the foam. This data can then be combined to construct a synthetic S-N curve for the foam. Tests on three densities of Divinycell H-grade foam are performed and the results support this approach. Some preliminary results from two densities of Rohacell WF-grade are given as well. Static properties of foams scale with relative density and once this scaling can be obtained through various static tests and the same scaling appears to be valid for both crack propagation rates and fatigue properties of foams. The implication of this is that once the fatigue behaviour of one relative density foam is established, one can predict the fatigue behaviour of all other relative density foams within the same class of materials.

Sandwich structures are widely used in marine, automotive, and aerospace structures because of their high stiffness and strength to weight ratio. In all of these applications, core plays an important role in controlling the extent of damage in sandwich structures especially when subjected to repetitive dynamic loading. When a sandwich structure is subjected to transverse loads, the face sheets carry bending moments as tensile and compressive stresses and the core carries transverse forces as shear stresses. The core is typically the weakest part of the structure and is first to fail in shear. Hence strengthening of core materials will essentially enhance the overall performance of sandwich structures. In this study foam core materials have been strengthened with the infusion of acicular nanoparticles such as carbon nanotubes and carbon nanofibers in the polymer precursor. This infusion has been carried through a sonic cavitation process. Once the core was modified, sandwich composites were fabricated through a traditional resin transfer molding (RTM) process. Shear fatigue behavior of sandwich composites having both pure and nanophased polyurethane foams as core materials have been investigated. The density of the core materials was identical in both cases. Static shear tests reveal that nanophased foams are more ductile, have higher strength and stiffness, and better crack propagation resistance when compared to pure foams. Shear fatigue tests were conducted at room temperature, at a frequency of 3 Hz and at a stress ratio, = 0.1. S-N curves were generated and shear fatigue characteristics were determined. The number of cycles to failure for the nanophased sandwich was substantially higher than that of the neat ones. SEM micrographs show that the cell structures of nanophased polyurethane foams are stronger and larger in size with thicker walls and edges. These stronger cell structures subsequently strengthen the sub interfaces when the sandwich composite is fabricated. The high intrinsic toughness of the sub interface delays the initiation of fatigue cracks and thereby increases the fatigue life of the nanophased sandwich composites. There was no volume change for either the neat or the nanophased foam during shear deformation, and the material failed by shearing in the vicinity of the centerline of the specimen along the longitudinal axis. In both cases numerous 45° shear cracks formed across the width and the cracks traversed through the entire thickness of the specimen signaling the final failure event during fatigue.

The ingress of sea water and its damaging effects on polymeric foams was investigated experimentally and explained by a mechanics model. Similarly, the reduction in delamination toughness at the core/facing interface was recorded and explained by means of fracture mechanics.

This paper deals with the linear static and buckling analysis of an asymmetric and orthotropic stiffened 3-layered square sandwich plate. Three different analytical formulas (Model 1, 2 and 3) have been taken from Ref. [1], slightly modified to account for unequal faces and different Poisson’s ratios and applied to a square sandwich plate of fixed dimensions but different face layer thickness. The obtained deflections and buckling loads are compared with the results of a 3D analysis [2] and of Finite Element calculations. Objectives are to evaluate the limits of validity of the formulas in view of thick face layers and orthotropic properties. The properties of the two considered specimen are listed in Tables 1 and 2. The obtained results might serve as benchmark for further studies.

The aim of this paper is to evaluate classical models for sandwich beams. These models are compared in the static and the dynamic fields. In all cases the Finite-Element-based solution is considered as reference.