Core Laminates Research Articles

Low‐velocity impact characterization of polyurethane rubber/nano‐clay enriched sustainable sandwich composites: Synergy of experimentation and simulations

AbstractThis research proposes a novel sustainable composite using basalt (B), hemp (H) and polyurethane rubber (Pu) reinforced in nano clay functionalised epoxy matrix for sacrificial structural applications prone to low‐velocity impact (LVI). To this end, symmetric and asymmetric laminates such as HHHH, BBBB, BHHB, BHPuHB, BBPuHH and BPuBPuHPuH are fabricated using compression molding techniques and subjected to LVI at three different impact energies of 19.66, 39.39, and 59.05 J. The hemp fibers were treated with NaOH solution before fabrication. Material characterization such as X‐ray diffraction, Raman spectroscopy and morphological studies has been carried out. The impact and post‐impact properties of the proposed composites are experimentally evaluated and validated with the finite element (FE) results. The effect on the residual tensile strength degradation of laminates at different interlayers and energy levels is also investigated using the Caprino analytical model. The barely visible impact damages (BVID) are investigated through non‐destructive dye‐penetration tests, which facilitate easy identification of the prominent LVI damages like “Plateau” and “Cliff‐drop” impressions. Based on the impact energy absorption and residual tensile strength, proposed laminates followed BHPuHB > BBBB > BHHB > HHHH. The experimentation suggests that Polyurethane core laminates support maximum impact energy absorption by favoring a structural change in interlayers. Also, the residual tensile strength decreases as impact velocity increases.Highlights Low‐velocity impact behavior of sustainable composites is experimented. Basalt, hemp and polyurethane rubber are reinforced in nano clay epoxy matrix. A FE framework to validate the experimentation is proposed. Dye penetrant NDT is adopted to investigate the damages. Morphological studies are conducted to understand LVI responses.

Investigation of quasi-static indentation on sandwich structure with GFRP face sheets and PLA bio-inspired core: Numerical and experimental study

Composite materials incorporated in naval, aerospace, and automobile structures are exposed to unforeseen damages, which seriously damage their mechanical performance. In particular, the existence of hardly visual impairment on the in-service composite structures is a critical issue that highlights the severity of structural safety in an aircraft. The ability of these materials to sustain multiple impacts requires an understanding of the impact energy absorption to improve the material’s damage tolerance. The present study investigates the energy absorption, load response, failure modes, and damage mechanics under the quasi-static indentation of a sandwich structure. The structure comprises bio-inspired Poly-lactic Acid (PLA) core and Glass Fibre Reinforced Polymer (GFRP) laminates, used in real naval and aeronautical structures, under repeated low-velocity impact with reduced energies and quasi-static indentation. In addition, a lightweight sandwich structure’s damage performance is greatly influenced by its core geometry. Therefore, choosing an appropriate core structure is critical. A Numerical model is developed for the composite structure under quasi-static indentation loads, considering suitable material properties and damage models for the core and face sheets. These results are then compared with the experimental investigation. The load–displacement response and the energy absorption characteristics of the sandwich structure showed good agreement for the experimental and numerical analysis results.

On the impact damage resistance and tolerance improvement of hybrid CFRP/Kevlar sandwich composites

This work presents an experimental demonstration of the significant improvement in the damage resistance of carbon fiber reinforced polymer (CFRP) Kevlar sandwich composite laminates under impact and compression after impact loads. A core of Kevlar plies was sandwiched between two face sheets of CFRP, representing two-third the global laminate thickness. Hand lay-up and vacuum bagging were used to manufacture the sandwich composite structure. Tests of low velocity impact and compression after impact (CAI) were performed to characterize the damage resistance. The different damage modes and their sequence during low velocity impact were visualized using microcomputed tomography. The results showed different damage levels in CFRP/Kevlar core laminates at low and high impact energies. At low impact energy, 10 J, few matrix cracks and delaminations were the main damage modes inside CFRP/Kevlar core laminate, however, extensive delamination, matrix cracks and fiber cut at the lower part of the laminate were appeared inside the fully CFRP laminate. At high impact energy, 70 J, fiber cut was observed inside the whole CFPR laminate, however, for CFRP/Kevlar core laminate, the lower CFRP face sheets did not suffer from any kind of damage. This allowed CFRP/Kevlar core laminate to sustain large load after the load drop occurred when the maximum load capacity of the laminate is reached. Moreover, it allowed CFRP/Kevlar core laminate to sustain larger compressive load during CAI tests resulting in improved residual CAI strength. Owing to the low density of Kevlar fibers and lower damage level during impact (better damage resistance), CFRP/Kevlar core laminate showed up to 51% larger specific CAI strength than CFRP laminate (better damage tolerance).

Impact and post-impact response of lightweight CFRP/wood sandwich composites

Lightweight CFRP/wood sandwich composites receive considerable attention because they can be tailored to satisfy many specifications. Therefore, the damage modes during low velocity impact and compression after impact (CAI) of CFRP/wood sandwich laminate with different wood core types are investigated in this article. The effect of wood fiber orientation on the specific energy dissipation and CAI strength are studied. Results demonstrated that the laminate with balsa core wood, where the wood fibers are perpendicular to the CFRP face plies, shows higher impact load and dissipated energy due to the ability of wood cells to deform during impact and hence dissipate more energy. However, the laminate with birch core, where the wood fibers are parallel to the CFRP surface plies, reveals slightly larger CAI strength due to the role of wood fibers in sharing the load during compression. The damage initiated in the balsa core sandwich laminates at the impacted face and propagate to the unimpacted CFRP surface, while the opposite damage behavior occurs for the birch core sandwich laminate. The balsa core laminate showed larger specific impact load (load/density), dissipated energy (dissipated energy/density), and CAI strength (strength/density), reaching 2.21-, 2.24-, and 1.41-fold times compared to birch core laminates.

Effect of Silane Coupling Treatment on the Adhesion between Polyamide and Epoxy Based Composites Reinforced with Carbon Fibers

The increasing efforts aimed to design structures with reduced weight and better mechanical performances has led in recent years to a growing use of fiber reinforced polymer materials in several fields such as marine. However, these materials can be composed of chemically very different elements and, hence, may be difficult to joint. This research aims to improve the adhesion between a thermoplastic matrix of polyamide reinforced with short carbon fibers (PA12-CR) and a carbon fiber reinforced epoxy matrix (CFRP). Two different silane coupling agents, (3-Aminopropyl)trimethoxysilane (AM) and (3-Glycidyloxypropyl)trimethoxysilane (EP), were applied, through the spray deposition method, on the PA12-CR substrate to create a reactive layer between the adherents. Different deposition methods and coupling agents curing conditions were also investigated. The wettability of the PA12-CR surface as well as the chemical modifications induced by silane treatments were investigated through contact angle and Fourier Transform Infrared spectroscopy (FTIR) analyses. Furthermore, the interfacial adhesion between PA12-CR and CFRP substrates was evaluated through Mode I delamination tests (DCB). The effectiveness of the most promising treatment was finally verified on sandwich structures, having PA12-CR printed as internal core and CFRP laminates as external skins, through quasi-static three-point bending mechanical tests. Overall, the epoxy-based silane (EP) allowed significantly better resistance to the delamination up until the tensile failure of the CFRP substrate.

Post-impact flexural behavior of carbon-aramid/epoxy hybrid composites

Hybrid polymeric composites are currently used in aerospace structures due to their specific strength and stiffness as well as larger design space. This paper presents an experimental study on residual flexural strength of impacted Carbon-aramid/Epoxy hybrid composite laminates. Specimens are designed in a sandwich form in which plies of aramid/epoxy represent the core and carbon/epoxy plies play the role of face sheets. This design is expected to take advantage of the high energy absorption capabilities of aramid/epoxy composites. We pre-damage such composites by performing low-velocity impact at different energy levels. Three-point bending tests then are used to measure the residual flexural strength for the impacted specimens. The damage sequence during three-point bending is monitored using a camera and, later on, with computed tomography. The results show that, unlike the all-carbon/epoxy laminates, the carbon fiber plies in the lower part of the laminate (non-impacted face) are not fractured after either the impact test or the three-point bending test. The damage is locally concentrated at the impacted face and the upper part of the aramid plies core. As a result, the strength losses are smaller, as compared to available results in the literature for carbon/epoxy composites, glass/epoxy and carbon fibers with aluminium core laminates.

Fully water-blown polyisocyanurate-polyurethane foams with improved mechanical properties prepared from aqueous solution of gelling/ blowing and trimerization catalysts

AbstractFully water-blown polyisocyanurate-polyurethane (PIR-PUR) foams with improved mechanical properties have been prepared using aqueous solutions of metal-ammonia complex, Cu(Am) or Zn(Am), as gelling/blowing catalysts and potassium octoate (KOct) solution in diethylene glycol as a trimerization catalyst. Two catalyst mixtures, Cu(Am)+KOct and Zn(Am)+KOct, were obtained as homogeneous aqueous solutions. In comparison to commercial catalyst system, DMCHA+KOct (DMCHA = N,N-dimethylcyclohexylamine), Cu(Am) and Zn(Am) could be miscible with KOct solution and water easier than DMCHA. This miscibility improvement led Cu(Am)+KOct and Zn(Am)+KOct to show faster catalytic reactivity in PIR-PUR foam reactions than DMCHA+KOct. All obtained PIR-PUR foams showed self-extinguishing properties and achieved HF1 materials. However, PIR-PUR foams prepared from Cu(Am)+KOct and Zn(Am)+KOct at NCO:OH ratio of 2:1 had suitable density for industrial applications and showed higher compressive strength than that prepared from DMCHA+KOct. These foams have high potential to apply as insulations for constructions, core laminates in wall panel or storage tanks.

Evaluation of aging behavior under salt-fog spray conditions of green sandwich structures

ABSTRACTIn this paper, the aging resistance of sandwich structures made with cork agglomerate as core and flax-reinforced polymeric laminates as face sheets was investigated for the first time. In order to evaluate their durability in a hostile environment, a sandwich plate was manufactured by vacuum bagging technique and aged under salt-fog spray conditions, according to ASTM B 117 standard. The evolution of the sandwich mechanical properties during the aging exposition was analyzed by carrying out quasi-static flexural tests and Charpy impact tests both on unaged specimens and on specimens aged for 10, 25, and 60 days, respectively.The quasi-static experimental results showed that despite the salt-fog exposition leads to clear decrements in both maximum strength and modulus, the deformation capability of the sandwich structure evidences a noticeable improvement. According to this last result, it was found that the impact strength of the sandwich specimens was clearly increased after the salt-fog exposition evidencing that the studied green sandwich maintains good toughness properties in a hostile environment such as marine.

Free vibration analysis of multilayered composite and soft core sandwich plates resting on Winkler–Pasternak foundations

Free vibration of laminated composite and soft core sandwich plates resting on Winkler–Pasternak foundations using four-variable refined plate theory are presented. The theory accounts for the hyperbolic distribution of the transverse shear strains through the plate thickness, and satisfies the zero traction boundary conditions on the surfaces of the plate without using shear correction factors. Equations of motion are derived from the dynamic version of the principle of virtual work. Navier technique is employed to obtain the closed-form solutions of antisymmetric cross-ply, angle-ply, and soft core laminates or soft core sandwich plates resting on elastic foundations. Numerical results obtained using present theory are compared with three-dimensional elasticity solutions and those computed using the first-order and the other higher-order theories. It can be concluded that the proposed theory is not only accurate, but also efficient in predicting the natural frequencies of laminated composite and soft core sandwich plates resting on Winkler–Pasternak foundations.

Stiffness and strength degradation of damaged truss core composites

Truss core laminates display stiffness and strength/density ratios superior to those seen in foam cored laminates. However, this superiority is held only for ideal shaped struts. If the truss core is damaged, its performance rapidly decreases towards that of a foam. The present study investigates the stiffness and strength degradation with imposed core deformation/damage. This is done for a pyramidal core structure made by electro-discharge machining from AA5083 alloy. The experiments are compared with finite element predictions. The effect of the strain rate sensitivity is studied by performing the tests at different temperatures and by FE simulations with different material data sets. The results show reasonable agreement between experiments and modeling. The stiffness of a damaged truss core rapidly degrades and reaches the performance levels seen in foams after ≈8% of deformation. The results show that a high strain rate sensitivity significantly influences post-buckling core behavior and is able to decrease the stiffness and strength degradation rate.

Flexural behavior of stiffened syntactic foam core sandwich composites

Sandwich composite with phenolic syntactic foam as a core and glass polymeric laminates as face skins are widely used in ship hull structures because of its high specific properties and buoyant nature. In the present work, syntactic foam is prepared by uniform mixing of cenosphere and phenolic resin in equal proportions. The effect of stiffening the syntactic foam core by resin-impregnated paper honeycomb structure on flexural behavior of sandwich composites is experimentally investigated under flatwise and edgewise loading configuration. Microscopic features of fractured coupons are also investigated to predict the failure mode. The results reveal that use of resin-impregnated paper honeycomb structure in syntactic foam to form the core leads to considerable improvement in the flexural properties of sandwich composites in both flatwise and edgewise loading configurations. The span of sandwich composite has significant role in altering the magnitudes of core shear strength, skin bending strength and mode of failure. Coupons under edgewise configuration failed in progressive failure mode; however, in flatwise configuration coupons failed in sudden brittle mode.

Materials Selection and Processing Techniques for Small Spacecraft Solar Cell Arrays

Body mounted germanium substrate solar cell arrays form the faces of many small satellite designs to provide the primary power source on orbit. High efficiency solar cells are made affordable for University scale satellite programs as triangular devices trimmed from wafer scale solar cells. The smaller cells allow the array designs to pack tightly around antenna mounts and payload instruments, giving the board design more flexibility. We are investigating the reliability of solar cells attached to FR-4 and carbon core laminate printed circuit boards. FR-4 circuit boards have significantly higher thermal expansion coefficients and lower thermal conductivities than germanium. This thermal expansion coefficient mismatch between the FR-4 board and the components used cause major concern for the power system when considering a failure of the solar cells, such as a series of cracked cells or faulty solder joints. These failures are most likely to happen with a longer orbital lifetime and longer exposure to the harsh environment the satellite will experience while in orbit. Carbon core laminates provide an advanced alternative because the core thickness can be selected to more closely match the device substrate, or at least provide a wider thermal expansion coefficient range to match the components on the board. We are also comparing various methods of attaching the solar cells to the printed circuit boards, using solder paste alone and in parallel with a silicone adhesive, considering the application of these adhesives by comparing the solder joints under x-ray when applied by screen printing versus stencil printing, and looking closely at the cleaning processes for array assembly. Storage, vacuum exposure, thermal cycling, functional and vibration testing will be used to compare the survivability and performance of the solar arrays.

Experiences with peel strength

PurposePeel strength numbers are part of a laminate's specifications and should characterize the specific bond performance (copper adhesion) under test conditions. Unfortunately, from both a theoretical and a practical point of view they are not able to do that. This study seeks to address this issue.Design/methodology/approachThis paper has been written to show the main impacts on the measured peel strength numbers in the IPC‐TM‐650 peeling test. From an extensive database regarding peel numbers for diverse foil types, foil thicknesses and treatment roughnesses it is possible to show the influence of prepreg type, foil thickness and roughness on the measure peel strength.FindingsCopper adhesion to laminating resin is insufficiently described by peel strength data because of the impacts of foil thickness, stiffness on bending (physical bending work, stress distribution underneath the peeling line) and the treatment roughness. The latter works reinforcing regarding the (low) resin strength and this influence is measured on resin strength instead of real bond. Fracture due to peeling is cohesive, mostly with a totally intact copper‐resin interface. This is especially true in high performance laminates that show low peel strengths not because of bad copper bonding but because of brittle resins (filled and unfilled).Originality/valueUsers have to understand the limited benefit of the IPC peel test in characterizing copper‐resin bonds. Peel increase on (low bond) high performance resins by increased foil roughness is not a practical way in the field because of no bond improvement (interface) and heavy disadvantages in dielectric thickness (HiPot tests at thin core laminates), respectively.

The nearly isotropic behaviour of high-rank nonlinear sequentially laminated composites

The effective transport behaviour, in the context of electrical conductivity, of sequentially laminated composites with nonlinear behaviour of the constituents is considered. An expression for the effective electric potential of two-dimensional composites is developed from the classical variational principle. This provides the means for determining the effective potential of a rank– N sequentially laminated composite with arbitrary volume fractions and lamination directions of the core laminates, in terms of an N –dimensional optimization problem. The effective potentials of sequentially laminated composites with power–law behaviour of the constituent phases are determined. It is demonstrated that the behaviour of sufficiently high–rank sequentially laminated composites tends to be isotropic. Two families of composites with both resistive and conductive inclusions are considered. The overall behaviour of these composites is compared with corresponding available bounds and estimates. It is found that the behaviours of composites belonging to these two families are, respectively, more conductive and more resistive than the corresponding second–order estimates of Ponte Castan∼eda and Kailasam.

Design of Two-Dimensionally Curved Panels for Sandwich Cores

Alternatives to honeycomb and corrugated core sandwich designs are explored to provide inexpensive and comparable performing sub-laminates that can be easily manufactured into more complex shapes. The core designs utilize two-dimensionally curved1 panels that can be manufactured using common manufacturing methods such as rolling, thermoforming, and stamping. The panels are designed using mathematical formulas and algorithms to provide quasi-isotropic behavior in bending. Experimental results indicate that these designs can provide comparable performance to honeycomb core laminates.

High-rank nonlinear sequentially laminated composites and their possible tendency towards isotropic behavior

This work is concerned with the determination of the effective behavior of sequentially laminated composites with nonlinear behavior of the constituting phases. An exact expression for the effective stress energy potential of two-dimensional and incompressible composites is introduced. This allows to determine the stress energy potential of a rank- N sequentially laminated composite with arbitrary volume fractions and lamination directions of the core laminates in terms of an N-dimensional optimization problem. Stress energy potentials for sequentially laminated composites with pure power-law behavior of the phases are determined. It is demonstrated that as the rank of the lamination becomes large the behaviors of certain families of sequentially laminated composite tend to be isotropic. Particulate composites with both, stiffer and softer inclusions are considered. The behaviors of these almost isotropic composites are, respectively, softer and stiffer than the corresponding second-order estimates recently introduced by Ponte Castañeda (1996).