Advances in Thick Section Composite and Sandwich Structures

A review of some recent advancements in knowledge of composite shell and sandwich structures subjected to extreme loading conditions in complex air and underwater environments is presented. Studies include polyurea (PU) coatings for mitigation of the response of these structures. Composite structures subjected to in-air blast loading included E-Glass Vinyl-Ester (EVE) plates, EVE sandwich structures with single and graded Corecell™ foam cores, and EVE plates with PU coatings. An in-depth analysis of the fluid-structure interaction between the shockwave and the structure is presented and applied to predict reflected pressure profiles, with close correlation to experimental results. Advances in underwater implosion research of both thin shell and sandwich composite structures are also presented. The mechanics of the hydrostatic collapse, as well as the emitted pressure pulses released during implosion, are characterized for the two structures respectively in free-field. Mitigation strategies are explored to reduce the strength of the implosion pulse from the collapse of the shell composites. Studies which address implosions in composite materials initiated by shock or explosive loading are also presented.

The response of composite materials and structures while subjected to highly transient loading in the form of underwater explosions has been studied through advanced experimental methods with corresponding computational simulations. The work conducted over approximately the past decade represents a progression in terms of loading conditions, structural geometries, and the effects of material ageing. The influence of elastomeric coatings has also been examined. Overall the research program was initiated through the study of curved composite plates subjected to far field underwater explosion (UNDEX) loading, and was followed by an investigation of flat plates undergoing near field blast loading. These efforts were followed by a detailed study into the highly complex loadings of cylindrical bodies subjected to near field blast conditions. Most recently, the effects of material ageing due to long term seawater immersion on the shock response of composites was considered. In each individual study, detailed experiments were conducted which subject the composite materials to controlled loading while capturing the response in real time through the use of high speed photography and optical methods. Furthermore, each individual study contains the development of detailed computational models which are shown to capture the complex fluid structure interactions while also accurately simulating the material response and damage characteristics.

This chapter details the experiments performed and simulations developed regarding the blast loading of composite sandwich structures by Dr. Hari Arora, Dr. Mark Kelly and Dr. Emily Rolfe over the past 10 years, with support from George Irven, Rob Quinn, Dr. Haibao Liu and Dr. Paul Hooper. Material and geometrical parameters have been varied, including face-sheet material, polymer interlayers, core material, thickness and graded density cores. Furthermore, the effect of stand-off distance has been investigated during air and underwater blast along with repeated loading during air blast. High-speed photography was employed throughout the air blast experiments, in conjunction with Digital Image Correlation (DIC), to monitor the deformation of these structures. Damage has been revealed using DIC and confirmed in post-test inspection. Strain gauge data was used to monitor the response of panels subjected to underwater blast. The effect of the backing medium (air or water) of the panel has been identified during underwater blast loading. The accumulated results illustrate how blast resilience of composite sandwich panels can be improved. Numerical simulations have been developed to support experimental investigations. These simulations have been validated against experimental data and can be used during the design process, thereby reducing the number of large scale experiments required. Furthermore, the simulations highlight the importance of boundary conditions with regards to blast resistance design and show the importance of damage development. The inherent blast resilience of composite sandwich structures has been demonstrated by these blast investigations and their associated results. The increasing demand for composite sandwich structures in marine, aerospace and automotive applications will continue to drive ongoing improvements.

This chapter presents an experimental investigation on the explosive blast response of four types of sandwich composites commonly used in naval ships, including hull and topside structures. Air blast tests were performed using plastic explosive charges on square target sandwich plates made of carbon or glass fiber-polymer facesheets with either an end-grain balsa or closed-cell PVC foam core. The sandwich composites were dynamically loaded by air shock waves of increasing pressure and impulse generated by plastic explosive charges, and the deformation, damage and post-blast mechanical properties were determined. The amount of out-of-plane deformation and blast-induced damage for a given shock wave impulse depends on both the type of fiber reinforcement used in the facesheet laminates and the type of core material. For a given shock wave impulse, glass sandwich composites have higher resistance to deflection and blast-induced delamination cracking and tow rupture then carbon fiber sandwich composites. Also, the balsa core composites are more resistant to out-of-plane deformation then the PVC core composites at low blast impulses. Damage initiates in the balsa core composites as core cracking and splitting, and facesheet-core debonding, whereas damage in the PVC core composites initiated as front facesheet compressive failure. Blast-induced damage to the skins and core causes large reductions to the mechanical properties of the sandwich composites.

During the last decade, our research has focused on developing experimental techniques to better understand the dynamic response of Navy relevant polymers. Two types of dynamic loading conditions were considered in this research: underwater shock wave loading and direct impact. The underwater shock load experiments utilized convergent water-filled sections of varying thickness and materials in which the fluid structure coupling during and after the shock event could be studied in detail using simultaneous strain gages and high-speed imaging techniques. The impact experiments were performed using a projectile launched from a gas gun that impacted the edge of the specimen. Also, in this case, detailed analysis was performed of the dynamic response using strain gages and high-speed imaging techniques. In particular, this later type of experiments was used to probe the mode-I and mode-II dynamic fracture response. The samples were either dry or had been subjected to an aging process while submerged in water thus leading to a significant water uptake, in some cases resulting in more than 1 wt%.

Dynamic response and failure of composites of naval relevance is important to assess the safety of warships. While the damage and residual properties of composite materials under low velocity impact has been widely investigated, ballistic performance of composites of naval relevance has not yet been fully understood. This chapter shows recent developments of high-speed impact performance of composites, including both analytical and numerical models as well as experimental data of ballistic response of carbon, glass and hybrid carbon/glass fiber reinforced vinylester composites at different temperatures and environmental conditions. Results include residual velocity against impact velocity as well as ballistic limit for all materials investigated. A good agreement is observed between experimental and analytical results.

Fluid-Structure Interaction (FSI) plays an important role on dynamic responses and failures of polymer composite structures as they are subjected to dynamic loading while in contact with fluids, mainly water. This chapter presents various case studies associated with FSI of composite structures in order to understand and predict the composite structural behaviors under FSI. The study considered open structures in contact with water on only one or both sides, closed structures containing fluids, separated structures coupled by fluids, and structures travelling in fluids. Both experimental and numerical studies were conducted to complement each other. Because the effect of FSI on polymer composite structures is so significant, FSI can change the dynamic motions, critical failure loads, potential failure locations, etc.

This chapter seeks to provide an overview on the dynamic behavior of Navy-relevant composite materials to low velocity impact, at room and extreme temperatures. The study focuses on carbon fiber laminates toward establishing a compelling body of empirical results, which could support numerical and semi-empirical models for the prediction of dynamic behavior. Experiments were carried out using a modular falling weight tower with a thermal chamber, for different energy levels. Results were collated in terms of maximum load, penetration extent, absorbed energy, indentation, delamination, and residual strength. The main novelty of the research lies in the experimental scheme which was realized to dynamically load marine panels, across a range of experimental temperatures in the presence of the water simulating realistic operating conditions of marine vessels. The setup is based on a modified falling weight machine, in which an instrumented impactor falls on a clamped specimen, resting upon a water column or it is immersed in it, at a controlled temperature. The effectiveness of two non-destructive techniques (ultra sound and electronic speckle pattern interferometry) in detecting barely-visible and non-visible impact damage was investigated. In agreement with our intuition, the presence of the water was found to critically shape the dynamic loading experienced by the panel. Preliminary insight from a physically-based theoretical model was presented to shed light on the underlying fluid-structure interaction and parametrically investigate the role of geometric and physical parameters on the dynamic response of water-backed panels subjected to low velocity impact.

An improved understanding of the fluid-solid interaction associated with a solid body slamming on the water surface is central to the design of naval and aeronautical structures. Of critical importance is the quantification of the hydrodynamic loading experienced by a slamming hull, in relation with its geometric and physical properties along with the conditions of the impact. This book chapter summarizes research supported by the Office of Naval Research, Solid Mechanics Program, at New York University to establish a reliable experimental methodology for the spatially-distributed, temporally-resolved inference of the hydrodynamic loading experienced by a slamming structure. Through the use of particle image velocimetry (PIV), we demonstrate the possibility to infer the hydrodynamic loading on rigid and compliant hulls that enter and exit the water surface, with varying inclination angles and different geometries.

We first review works on characterizing loads produced by underwater and in-air explosions/blasts. We then summarize the work of Batra’s group on studying transient deformations of doubly-curved sandwich structures by using a third-order shear and normal deformable theory. For a given areal mass density we find structural designs that maximize the first failure load and then ascertain their ultimate failure loads by progressively degrading the material moduli. Subsequently, we briefly outline the work on fluid-structure interaction related to water slamming for deformable hulls and high-speed viscous flows interacting with rigid solids.

The Extended High Order Sandwich Panel Theory (EHSAPT) is a structural theory that includes the transverse shear and the in-plane rigidity of the core as well as the compressibility of the core in the transverse direction. It was introduced several years ago and was proven to be the most accurate sandwich structural theory by comparing its predictions to the ones from corresponding elasticity solutions. It is especially noteworthy that this theory is very accurate not only for the softer cores but also for the cases of stiffer cores, for which cases the other available sandwich structural theories cannot predict correctly the stress fields involved. Thus, this theory can be used with any combinations of core and face sheets and not only the very soft cores that the other theories demand. The theory is derived so that all core/face displacement continuity conditions are fulfilled. In this review article the basic premises of the theory are outlined in both its nonlinear static and dynamic versions for a one-dimensional (beam or wide plate) configuration and its accuracy is validated by considering the cases of static distributed loading, dynamic blast loading and wrinkling behavior. These results are compared with the corresponding ones from the elasticity solution. Furthermore, the results using the classical sandwich model without shear, the first order shear and the earlier High-Order sandwich panel theory (HSAPT) are also presented for completeness. Finally, it should be noted that the theory is formulated for sandwich panels with a generally asymmetric geometric layout.

The chapter summarizes selected work by the author and her collaborators on mechanics based modeling of composite and sandwich materials and structures for marine applications. Models and results are presented on various aspects of the mechanical response of these systems. Closed form solutions for elastic response and wave propagation and dispersion of layered plates subject to thermo-mechanical loadings, also in the presence of interfacial damage and imperfections, are derived using the theory of elasticity, matrix methods and a multiscale homogenization technique. Interface fracture mechanics solutions are derived, which are useful for the characterization of the fracture properties of sandwich composites, and account for the effects of shear on energy release rate and mode mixity. Evolution and interaction of damage mechanisms in sandwich beams with compressive yielding cores subject to time-dependent loading are investigated and energy barriers to the propagation of face-sheet delaminations identified. A multiscale modeling strategy for mode II dominant delamination fracture of laminated and sandwich beams is presented which does not require a through-thickness discretization of the problem and captures, using the same kinematic variables of a classical equivalent single-layer theory, local and global effects caused by the layered architecture and the interaction of the delaminations.

A pressure vessel experiment was developed to determine the triaxial, elastic-plastic and hysteresis behavior of Divinycell PVC H100 foam. Arcan butterfly and tensile dogbone specimens were encased in the air chamber of a cylindrical pressure vessel, which was designed to work within an MTS servohydraulic machine. Digital Image Correlation was used to measure strains in the specimen during the experiments. Material tests under cyclic loading of the foam under uniaxial compression and tension, shear, biaxial compression and shear, triaxial compression, triaxial compression-tension and triaxial compression and shear, were performed in both out-of-plane and in-plane directions. The foam, which was transversely isotropic, exhibited elastic-plastic response followed by damage and viscoelastic hysteresis. It was shown that only the Tsai-Wu quadratic failure criterion, with its 12 material constants, was able to predict the correct yield behavior under triaxial stress states. Tsai-Wu plasticity with anisotropic hardening was combined with a linear viscoelastic, damage mechanism to describe the elastic-plastic and hysteresis behavior of the foam. Good agreement was found between the proposed elastic-plastic-viscoelastic-damage constitutive model and experimental results.

Syntactic foams are hollow particle filled lightweight composite materials that are widely used in structural applications in underwater marine vessels. Additive manufacturing (AM), also called 3D printing, methods are now being developed for printing parts of syntactic foams. These methods provide advantage that the entire part can be printed without the requirement of machining or joining and eliminates stress concentration locations. The present work is focused on describing the method of creating a syntactic foam filament for fused filament fabrication type printers and then developing parameters for printing syntactic foams parts using commercial printers. High density polyethylene resin is used as the matrix material with fly ash cenospehres and hollow glass microballoons as the fillers for creating syntactic foams. One of the major challenges is to minimize the fracture of hollow particles during filament manufacturing and 3D printing, which is addressed by parameter optimization during processing. Results show that the syntactic foam specimens are successfully printed and their properties are comparable to the injection molded specimens of the same compositions.

The presence of face-core debonds can seriously reduce the load-carrying capacity and remaining lifetime of sandwich structures. During the past 30 years considerable progress has been made in the modelling and experimental testing of sandwich structures for marine applications, especially with regard to the effects of such debonds. Much of this research has been supported by the US Office of Naval Research. This chapter summarises these developments, with special emphasis on the progress made during the past 10–15 years concerning foam-cored sandwich for naval applications. Fracture mechanics based analysis methods, experimental techniques for characterising face-core interfaces under mode I, mode II and mode III deformations and their combinations, and analysis and testing of structural elements such as beams, columns and panels in the presence of debonds are described. The assessment of observed debond damage in naval sandwich structures is addressed, and the implications for ensuring acceptable damage tolerance are discussed.

Polymeric sandwich composites are appealing for lightweight structures that require high strength and stiffness such as parts of aircraft, marine vessels and wind turbine blades. During service, polymeric sandwich composites are subjected to static and cyclic mechanical loading, in addition to a constant exposure to hostile environments. In addition, one of the characteristics of polymers is their prominent viscoelastic response when subjected to mechanical loading and the viscoelastic response of polymers becomes more pronounced at elevated temperatures and high humidity. Coupled mechanical loading and hostile environments cause the constituents of the sandwich structures to experience different time-dependent behavior and degradation, leading to complex failure mechanisms in sandwich composites. The aim of this study is to be able to predict the performance of sandwich composites subjected to mechanical loading histories and various environmental conditions, by incorporating knowledge of the behavior of each constituent (skins and core). We present experiments and numerical analyses in order investigate the overall mechanical response of two sandwich composite systems and their constituents, immersed in fluid at 50 °C. It is seen that different failure behaviors are observed in the two sandwich composites, which are strongly influenced by the response of the constituents.

Polymer and composite materials are being considered for marine applications where long term durability is critical. Unfortunately, for most of these applications it is not possible to perform test programs for ten or more years in order to verify performance, so accelerated testing is widely used. This chapter describes the parameters that can be used to accelerate aging tests and discusses their limitations. Two examples are shown from campaigns lasting over 6 and 8 years, in order to provide data that reveal long term behavior.

The tensile strength along the longitudinal direction of unidirectional CFRP constitutes important data for the reliable design of CFRP structures. This study establishes a method of predicting statistical creep failure time under tension loading along the longitudinal direction of unidirectional CFRP based on the matrix resin viscoelasticity. First, the accelerated testing methodology (ATM) to predict the long-term creep failure time of CFRP laminates statistically from the statistical static strength of CFRP and the creep compliance of matrix resin measured at various temperatures was proposed based on Christensen’s theory for viscoelastic crack kinetics. A formulation for the prediction was established. Second, the statistical long-term creep failure times obtained under tension loading along the longitudinal direction of unidirectional CFRP were predicted based on our proposed methodology using our developed testing system for resin-impregnated CFRP strands as specimens. Third, results clarified that two fracture modes existing in CFRP strands under static and creep loading and viscoelastic parameter n_R showing sensitivity to the matrix resin viscoelasticity change drastically with the fracture mode.

This chapter provides a summary of author’s findings from the research sponsored by the United States navy in the past decade associated with the durability aspect of the sandwich structures with thin face sheets made of carbon fiber vinyl ester composite facings and a thick core section of closed cell polymeric foam core material. Exposure to sea environment and coupled effects with temperature change to ship structures over extended period of time often leads to degradation of its mechanical properties. In this research, vinyl ester based carbon fiber composite facings manufactured using VARTM approach are evaluated for the fiber and resin dominated properties and corresponding degradation in static, fatigue, and fracture behavior. The mechanical response of carbon fiber vinyl ester composites immersed in simulated sea water and coupled degradation effects from simultaneous exposure to low temperature are reported. H100 PVC foam core degradation was evaluated under tension and torsion tests. Interfacial delamination fracture response for the sandwich structures due to combined effects of seawater and low temperature effects are also investigated including the weight gain and associated expansional hygroscopic strains. The effect of confined and one sided sea water exposure on the cyclic fatigue behavior yielded failures under much lower number of cycles of loading when fatigued under immersed conditions surrounded by sea water than in air for tension-tension fatigue. Even with the condition of one sided sample face exposed to sea water, a considerable reduction in the fatigue life, corresponding to approximately 50% was observed. A detailed study on the compression response of these naval composite sandwich structures is ongoing and is also included here.

The rise in demand and interest in arctic exploration has brought new challenges with regard to the mechanical behavior of lightweight offshore structures with fiber reinforced composite materials. These materials experience drastic changes and degradation in their macro-and-microstructures when exposed to seawater and cold temperatures during service. Therefore, it is critical to have a detailed comprehension of the mechanical behavior and failure mechanisms of these materials in arctic conditions. Within the scope of the current study, low-velocity single and repeated impact behavior of carbon fiber/vinyl ester composites in arctic temperature (-50 °C) is investigated. Impact responses, such as the contact force, displacement and absorbed energy, at four impact energies of 20 J, 25 J, 30 J and 35 J under single impact loading and repeated impact loading until perforation are determined at −50 °C and compared against those at room temperature (25 °C). For the repeated impact cases, the number of impacts required for perforation, the rate of reduction in impact force, the degree of damage and the failure mechanisms change significantly with varying impact energies and in-situ ambient temperatures, and are elucidated in detail in this paper.

Advanced composite materials play vital structural roles in automotive, aerospace and marine industries. Drastic reduction in arctic sea ice region over the last three decades has opened new sailing routes which are more efficient and economical. This has resulted in the increased use of marine and naval vessels in extreme low temperature arctic conditions. The fundamental challenge of operating in such cold and harsh environment lies in the understanding of how materials and structures behave and perform in extreme low temperature. In this chapter, the behavior of composite materials and composite sandwich structures in low temperature arctic conditions is presented. Composite sandwich structure far exceeds classical composite laminates in terms of flexural capability and performance. In this work, we experimentally investigate the impact and post-impact compressive and flexural response of Divinycell H-100 foam core sandwich panel with woven carbon fiber reinforced polymer (CFRP) facesheets. Specimens were conditioned and impacted over a temperature range (from room temperature down to −70 °C). Results show that exposure to low temperature inevitably causes more severe damage in the specimens. Post-mortem inspection using x-ray micro-computed tomography revealed complex failure mechanisms in the composite facesheets (such as matrix crack, delamination and fiber breakage) and foam core (core crushing, core shearing and interfacial debonding).

Compression-after-impact (CAI) test results showed that low temperature environment severely influenced the residual strength of composite structures, leading to significant drop in CAI strength with decreasing test temperature. Post-impact three point bending test reveals residual flexural properties are more sensitive to the in-plane compressive property of the CFRP facesheet than the in-plane tensile property. Results also indicate that degradation of flexural rigidity of the sandwich composite panel strongly depends on existing damage state of prior impact test. Analogous to impact behaviour, specimens have much reduced flexural properties when exposed to extreme low temperature conditions.

Understanding material properties and structural performance would subsequently lead to new methods to enhance impact performance in arctic condition. The findings in this study would form fundamental understanding of composites behavior at extreme low temperature environment and the correlation of low temperature effects and dynamic damage behavior of composite sandwich structures.

Due to the heterogeneity at different length scales, composite materials have a complex mechanical behavior. While many studies have been devoted to understanding its failure mechanism, much remain unknown. One of the main reasons is the lack of an effective experimental tool to monitor, in situ, the internal stress/strain field as the deformation progresses leading to failure. By taking advantage of the volumetric imaging capability of an X-ray-CT (Computed Tomography), we have developed a new tool called DVSP (Digital Volumetric Speckle Photography) whereby we can map quantitatively the interior deformation of almost any opaque material. In this study we describe in detail the theory of this technique and its application to mapping the internal deformation fields of two different types of composites: fiberglass reinforced woven composite and sandwich composite made of a polymeric foam core and two fiberglass face sheets.