In-situ repair of composite sandwich structures using cyanoacrylates

A novel method for the in-situ repair of composite sandwich structures using microvascular networks and cyanoacrylate (CA) adhesive systems has been presented. Upon a damage event, the vascules become ruptured, providing a route for the introduction of adhesive directly into the damage site. The efficacy of the two repair agents was first assessed under static and fatigue conditions using a modified double cantilever beam (DCB) method. Once baseline fracture behaviour of the cyanoacrylates has been established, they were further assessed by injection into a series of pre-damaged T-joint specimens. The presence of the vasculature was shown to have no detrimental impact on mechanical performance, whilst both of the cyanoacrylates were shown to be highly effective in the recovery of stiffness and ultimate strength of the T-joint specimens. (C) 2016 Elsevier Ltd. All rights reserved.

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

Fracture and Fatigue of Adhesively-Bonded Fiber-Reinforced Polymer Structural Joints

Ye Zhang

Being good structural replacement for other conventional material, the pultruded glass fiber reinforced polymer (GFRP) profiles are being increasingly used in civil engineering structures. The connection between components is considered the most suspect area for failure initiation. The adhesive bonding is preferred for FRP composite structures, rather than the mechanical fastening, due to the brittle failure nature of composite materials. During past decades, many efforts have been made by researchers to better understand the mechanism of adhesive bonding, to analyze the stress distributions and to improve the strength of composite structural joints. However there is still no commonly accepted design code/standard existing for adhesively-bonded joints in civil engineering infrastructures since several important knowledge gaps are to be filled. Besides the joint strength at failure, the characterization and modeling of the progressive failure process, in particular involving the so-called crack initiation and propagation phases, is also an important concern. By employing the strain energy release rate (SERR) as the fracture parameter, the linear-elastic fracture mechanics (LEFM) approach is considered an efficient method to model the fracture behavior of structural joints. However, due to the uncontrollable crack initiation and the complex geometric configurations, the crack measurement techniques and the calculation method for the SERR are to be validated. In fracture mechanics, the fracture of a material or component can be described by a single mode or the combinations of the following three basic modes: opening mode (Mode I), shearing mode (Mode II), and tearing mode (Mode III). During the fracture of a structural joint, crack initiation and propagation are driven by combined through-thickness tensile (peeling), and shear stresses, thus resulting in a mixed mode fracture. In order to use the fracture results of structural joints to form the mixed fracture criterion for a specific composite material, a feasible analytical or numerical method are to be developed to determine the Mode I and II components of the SERR during fracture. Although many efforts have been made to better understand the short-term behavior of structural joint under quasi-static loading, the long-term performance under fatigue loading and different environmental conditions is a more demanding task when adhesively-bonded joints are applied in a real structure. Most of structural failures occur due to mechanisms that are driven by fatigue loading and for composite structures, the fatigue produced by the repeated application of live load is more critical due to its lighter self-weight, in other words the lower dead load. Besides the fatigue loading, a structure in practice may also experience the combined environmental effects of two basic factors: temperature and humidity. These environmental conditions may directly affect properties of structural joints, including the failure mechanism, the stiffness and strength, the crack initiation and propagation and etc.. Thus, the missing knowledge and confidences in the long-term behavior under cyclic loading and the durability under different environmental conditions are the main obstacles to the further development of FRP composites in civil engineering infrastructures. In this research, the mechanical and fracture behavior of adhesively-bonded double-lap and stepped-lap joints (DLJs and SLJs) composed of pultruded GFRP laminates and an epoxy adhesive were experimentally and numerically investigated under both quasi-static and fatigue loadings. The crack measurement techniques and the calculation methods for the SERR were validated for DLJs and SLJs. The LEFM approach was successfully applied to characterize and model the progressive failure process of structural joints. The Mode I and II components of the SERR of DLJs and SLJs were determined using the Virtual Crack Closure Technique in finite element analysis. Combining with the results of pure Mode I and II experiments, a mixed mode fracture criterion for pultruded GFRP composite was formed. Under fatigue loading, the fatigue behavior of structural joints was successfully modeled by using the stiffness-based and fracture mechanics approaches, besides the F-N curves. Based on the stiffness degradation, a linear and a sigmoid non-linear model were established and the fatigue live corresponding to the failure and allowable stiffness degradation can be predicted. Concerning fracture mechanics approach, the Fatigue Crack Growth (FCG) curves were formed for DLJs and SLJs and the corresponding fracture parameters were obtained. Similarly to stiffness-based approach, fatigue lives corresponding to the failure and allowable crack length can be predicted. The environmental effects on both short- and long-term performances of structural joints were experimentally evaluated and numerically modeled based on experimental results. The temperature-dependent joint stiffness can be predicted using the finite element analysis based on the thermomechanical properties of constituent materials. A relationship between the equivalent quasi-static joint strength under different environmental conditions and the cyclic stresses and the fatigue life was established.

EPFL2010

Monitoring strain gradients in adhesive composite joints by embedded fiber Bragg grating sensors

John Botsis, Luis Pablo Canal Casado, Hans Georg Limberger, Véronique Michaud, Roohollah Sarfaraz Khabbaz, Georgios Violakis

The adhesive bonding of composite materials can be applied to assemble light-weight structures for aeronautical applications. However, the application of adhesive bonds in primary load-bearing structures is limited by the lack of adequate techniques to monitor the quality of the joint during service. This work evaluates the feasibility of using Fiber Bragg Grating (FBG) arrays to monitor the distribution of strains developed along the bonded region in single lap shear specimens. Three different positions for the FBGs were both experimentally and numerically studied namely in the first ply, at the composite/adhesive interface, and in the adhesive. The mechanical performance of the joint was not modified due to the embedding of the optical fiber at the different positions. The simulations and the measurements showed good agreement for the three positions. The optical sensor placed at the composite/adhesive interface captured the strongest strain gradient. (C) 2014 Elsevier Ltd. All rights reserved.