Origami-inspired carbon fiber-reinforced composite sandwich materials - Fabrication and mechanical behavior

A folding fabrication method inspired by origami for carbon fiber-reinforced composites to fabricate three-dimensional structures is presented. PMI foams serve as substrates, making it possible to manufacture this kind of origami-inspired composite sandwich materials in a single-step process. Analytical models are proposed to provide upper and lower bounds of the out-of-plane compressive strength of the origami-inspired composite sandwich materials. Finite element analysis (FEA) and experiments are conducted to characterize the compressive behaviors including the deformation history, failure mode, strength, etc., and verify the validity of the analytical models. By incorporating PMI foams, the out-of-plane compressive failure mode of the composite sandwich material at low relative density changes from buckling to wrinkling or crushing, thus greatly increasing the strength. Additionally, the energy absorption capacity is significantly enhanced due to the PMI foams and their interaction with the composite origami core. This work creates a new method to fold composites into lightweight sandwich materials with high strength and high energy absorption, paving the way for their applications in transport, energy and communication.

Ultra-light photovoltaic composite sandwich structures

Julien Rion

The ultra-light photovoltaic sandwich structure is a new multifunctional structure concept enabling weight and thus energy to be saved in high-tech solutions such as solar cars, solar planes or satellites. The novelty of this approach is to use solar cells as a load carrying element in the structure. The aim of this work was to investigate the failure mechanisms of such ultra-light sandwich structure and their correlation with microstructure, processing pressure, and strength in order to obtain optimal design and processing. To this end, composite sandwich structures were extensively studied with weights in the range of 650 – 850 g/m2, and comprising one 140 µm thick skin made of 0/90° carbon fiber-reinforced plastic (CFRP), one skin made of 130 µm thick mono-crystalline silicon solar cells, and a 29 kg/m3 honeycomb core. As a first step, core-to-skin bonding in a symmetric (CFRP / core / CFRP) sandwich, for which a design criterion was lacking, was especially studied. An adhesive deposition technique was developed enabling the adhesive weight used for core-to-skin bonding to be tailored. Based on adhesive contact angles, the formation of the adhesive fillets between honeycomb cell walls and skin was modeled. Core / skin debonding energy was measured and compared to core tearing energy measured with a new video-based method, and the failure mechanisms during skin peeling were investigated. It was thus ascertained that, to provide the highest debonding energy-to-weight ratio, the optimal adhesive weight was 35-40 g/m2. Furthermore, in contrast with classic sandwich structures with thicker skins, it was observed that the bending strength of the ultra-light sandwich panels increased with adhesive weight. This was due to the formation of adhesive fillets, which significantly increased the bending stiffness of the thin CFRP skin, and thus increased the compressive load causing local instability of the skin. Models taking into account the increased skin stiffness showed that the best adhesive quantity required to increase the strength-to-weight ratio was ∼40 g/m2. In a second step, the influence of processing pressure on the morphology and strength of symmetric (CFRP / core / CFRP) ultralight sandwich structures was investigated by using one-shot vacuum bag processing. This showed that higher processing pressures caused the formation of larger adhesive fillets and an increased waviness of the CFRP skin on vacuum bag side. These two effects had conflicting impacts on the strength of the structure. Waviness of the skin favored local instabilities, whereas adhesive menisci stabilized the skin. Modeling of the local instability of the skin by taking into account the waviness of the skin and the size of the menisci as a function of processing pressure enabled an optimal processing pressure of 0.7 bar to be identified, giving the highest strength-to-weight ratio. The third step of the study was devoted to the mechanical analysis of the mono-crystalline silicon solar cells. The brittle behavior of the cells was confirmed, and the Weibull failure probability curve was established with the mean tensile strength at 221 MPa. It was demonstrated by experimental testing and finite element modeling (FEM) that the low strength of the cells compared to the intrinsic strength of silicon (∼6.9 GPa) was not due to surface texturation of the cells used for increased efficiency, but to more severe surface or edge defects. FEM also showed that no significant reinforcing effect of the cells could be obtained with polymer encapsulation. In addition, thermo-mechanical stresses due to a mismatch of the coefficient of thermal expansion (CTE) between the Si cells and the polymer encapsulation were found to be negligible. In order to protect the cells against the environment, encapsulation of the cells was successfully carried out, using highly transparent fluoropolymer films treated with SiO2 plasma sputtering for improved adhesion, together with silicone adhesive. Finally, the integration of solar cells as a photovoltaic skin of an ultra-light sandwich structure was achieved using thin stress transfer ribbons to ensure load transfer between adjacent cells. It was observed that the cells were not damaged by sandwich panel processing, even in curved panels, thus showing that the processing windows of the different constituents were compatible. The asymmetric (Si / core / CFRP) photovoltaic sandwich structure with a weight equal to 800 g/m2 and a specific power density equal to ∼250 W/kg (i.e. 20 times more than standard commercial photovoltaic panels) demonstrated an equilibrated mechanical behavior, i.e. the CFRP skin, reinforcing ribbons, and solar cells had similar failure loads.

EPFL2008

Mixed-Mode Static and Fatigue Failure Criteria for Adhesively-Bonded FRP Joints

Moslem Shahverdi

Fiber-reinforced polymer (FRP) composites are increasingly used in many load-bearing structures. Joints are the most critical structural elements as they often become the weak links in engineering structures. Bonding and bolting are two of the main joining techniques used for composite elements but, although bolted and bonded joints have their advantages and disadvantages, adhesively-bonded joints are often preferred. This is thanks to their better performance in terms of cost-effective structures with uniformly distributed stresses without the need for cutting fastener holes that result in high stress concentrations in the FRP adherends at the edges of the holes. In addition, due to the absence of stress concentrations, adhesively-bonded joints exhibit better fatigue behavior. However, the efficient and reliable use of adhesively-bonded joints requires a thorough understanding of their failure mechanisms under mixed-mode quasi-static and fatigue loading conditions. In real structural applications, failure of adhesively-bonded joints occurs due to crack initiation and propagation under varying mixed-mode ratios. Since it is difficult to describe this progressive fracture in a structural joint, fracture joints with constant mode-mixity ratios are used to determine the strain energy release rates for crack initiation and propagation. Subsequently, mixed-mode failure criteria based on the determined strain energy release rates can be established and used to model/predict the fatigue and fracture behaviors of structural joints. The main objectives of this thesis were to understand the fracture and fatigue behaviors of adhesively-bonded FRP fracture joints under Mode I, Mode II, and mixed-Mode I/II loading conditions and, consequently, develop mixed-mode static and fatigue failure criteria applicable to structural joints. To fulfill these objectives a combined experimental-analytical-numerical study was undertaken. Double cantilever beam (DCB-Mode I), end-load split (ELS-Mode II), and mixed-mode bending (MMB-Mode I/II) specimens were examined under quasi-static and constant amplitude fatigue loading. Analytical approaches were used to determine the strain energy release rates under quasi-static and fatigue loading, whereas phenomenological models were developed to characterize the fatigue and fracture behaviors of the joints. A new approach designated the “extended global method” was established and applied for the analysis of the quasi-static and fatigue experimental data and the fracture mode partitioning in the mixed-mode experiments. In parallel, non-linear finite element models were developed to simulate the quasi-static fracture behavior and investigate the effects of both the existing asymmetry and the observed fiber bridging on the fracture behavior of the examined joints. The bridging zone was modeled by zero-thickness cohesive elements. An exponential traction-separation description of the cohesive zone model was shown to appropriately model the fiber bridging and calculate its contribution to the fracture energy under quasi-static loading. The FE models were successfully employed for the separation of the fracture parameters, i.e. strain energy released at the crack tip (Gtip) and the amount of energy released along the crack-bridging zone (Gbr), in the quasi-static mixed-mode failure criterion for crack propagation. In addition to the studies on the quasi-static fracture behavior of the joints, a new phenomenological fatigue crack growth formulation for the modeling and prediction of Rratio effects on the Mode I fatigue behavior of adhesively-bonded joints was also derived. The model was used for prediction of the fatigue behavior of the examined joints under different R-ratios. The results proved that the model could accurately simulate the fatigue behavior of the examined joints and was also capable of predicting behavior exhibited under unseen loading conditions, i.e. the R-ratios used for estimating the model parameters were different from those used for validating its prediction capability. The experimental results and numerical analyses combined with the phenomenological models were used to establish mixed-mode static and fatigue failure criteria for crack initiation and crack propagation for the adhesively-bonded joints. The derived mixed-mode failure criteria can be used for simulating/predicting the crack initiation and progressive crack propagation in structural joints comprising the same adhesive and adherends.

EPFL2013

Environmental assessment of radical innovation in concrete structures

Emmanuel Denarié, Amir Hajiesmaeili

In the building sector, the contribution of concrete structure to the overall emissions of greenhouse gases is significant. Switzerland is engaged in a 2050 energy strategy where the reduction of the embodied energy of buildings is a key aspect. In this study, we assess the environmental impact of different low energy concrete solutions. The study focuses on technologies that use cement with very high substitution rate (up to 65%) and other tensile resistant materials than steel in order to keep high durability targets. Hybrid wood-concrete structure, low carbon high performance concrete prestressed with carbon fiber reinforced polymer, and ultra-high performance fiber reinforced concrete with synthetic fiber reinforcement are among the studied options. The environmental assessment is done through life cycle analysis using Ecoinvent database for Switzerland and SimaPro software. Results of initial environmental assessment of production of the new technologies present huge energy and emission savings potential for the energy turnaround.