An iterative analytical/experimental study of bridging in delamination of the double cantilever beam specimen

Large scale bridging in mode-I delamination of layered composites is an important toughening mechanism. While its extent depends on constituent materials, it is also influenced by specimen geometry. Here, an analytical approach is adopted to express the through the thickness longitudinal strains in a double cantilever beam (DCB) specimen with the applied load and bridging tractions. The expression for the strains is confronted with strain data from embedded Bragg grating sensors to obtain bridging tractions in specimens with different thicknesses. The results show that the maximum stress of traction-separation in the bridging zone and maximum crack opening displacement at the end of the bridging zone are independent of thickness. However, the form of the bridging traction depends on thickness and is not a material property. The identified bridging relations are appended in a cohesive zone model to simulate delamination. The proposed approach predicts fracture of DCB specimens with different thicknesses. (C) 2014 Elsevier Ltd. All rights reserved.

Active sensing and repair composites

Eva Kirkby

This thesis describes the development of a smart carbon fibre polymer composite that is capable of sensing its own damage and self-healing. Structural composites are used in the aerospace and marine industries, for example. During their lifetime impact events are inevitable, which can result in extensive internal damage. Once damaged, the only options to date are manual repair or replacement. If self-sensing and self-healing properties could be imparted to the next generation of structural composites, this would bring greater safety and reduced maintenance costs for aircraft, as well as an extended operational lifetime for equipment such as satellites. The concept of the new material is to embed three additional components into a fibre-reinforced composite material: (i) microcapsules containing a liquid healing agent, together with a solid catalyst in the matrix, (ii) optical fibre Bragg grating (FBG) strain sensors and (iii) woven shape memory alloy (SMA) wire actuators. An impact event causes a crack to propagate at the damage site, rupturing the microcapsules in its path. This releases the liquid healing agent into the crack, where it comes into contact with the catalyst and begins to polymerise. The strain shock pulse radiating out from the impact site is detected by a sparse array of FBG sensors, which locate the impact position by time-of-flight. With this information, the SMA wires in the impact region are thermally activated using resistive heating, causing them to contract and exert a compressive force, closing the crack. The SMA wires then remain activated during the polymerisation period of the healing agent. The new material is developed in several stages. In the first stage, SMA wires and microcapsules are combined in an epoxy matrix. The addition of SMA wires results in a significant improvement in healing efficiency; the healed fracture toughness approaches that of the virgin material. The improvement results both from crack-closure and also from the heating effect of the wires, which increases the degree of polymerisation of the healing agent. In the second stage, a liquid composite moulding cure schedule that allows straightforward integration of the SMA wires during composite fabrication is developed. With this process, the SMA wires do not need to be maintained in place with an external frame, even though the peak post-cure temperature exceeds the activation temperature of the wires. The wires remain bonded to the matrix both during processing and subsequent activation. In the third stage, impact sites are localised on composite plates to a precision of a few centimetres using three FBG sensors spaced several tens of centimetres apart. In the final stage, a prototype self-healing carbon-fibre composite with embedded microcapsules, Grubbs' catalyst and woven SMA wires is fabricated and tested. While the results presented in this thesis represent only a first step towards a fully-functional Active Sensing and Repair Composite, they successfully demonstrate the advantages of combining the three components in a single material, as well as validate the scientific concept of the system.

EPFL2009

A combined experimental/numerical study of the scaling effects on mode I delamination of GFRP

John Botsis, Behzad Dehghan Manshadi, Anastasios Vassilopoulos

Bridging by intact fibers is an important toughening mechanism in composite materials. However, a direct experimental evaluation of its contribution is difficult to achieve and in the case of large scale bridging the thickness, h, may have an important influence on the energy release rate (ERR). In this work a semi-experimental method is adopted to quantify the effects of thickness to fracture of unidirectional glass fiber reinforced polymer (GFRP) double cantilever beam specimens in mode I fracture under monotonic loads. Several specimens with thickness in the range of h = 3.5-19 mm are tested. In two selected specimens, embedded optical fibers with an array of eight or ten wavelength division multiplexed fiber Bragg gratings were are to measure local strains close to the crack plane and employed in an inverse identification procedure to determine the bridging tractions. The results suggest that the stress at the start of the bridging zone and the crack opening displacement at its end are independent of the specimen thickness. However, the rate of change of the tractions with crack length depends on the specimen thickness. While the initiation value of the ERR is independent of h, at the steady state it varies from 900 (h = 3.5 mm) to 21 00 J/m(2) (h = 19 mm). Using the identified tractions, the total stress intensity factor (SIF) and the corresponding ERR are calculated. The results show that the SIP approach gives similar results. Thus thickness, via the bridging fibers, is responsible for the effects on steady state ERR observed in the specimens used herein. (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier2013

An experimental-numerical investigation of hydrothermal response in adhesively bonded composite structures

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

Water absorption and thermal response of adhesive composite joints were investigated by measurements and numerical simulations. Water diffusivity, saturation, swelling, and thermal expansion of the constituent materials and the joint were obtained from gravimetric experiments and strain measurements using embedded fiber Bragg grating (FBG) sensors. The mechanical response of these materials at different temperatures and water content was characterized by dynamic mechanical analysis. Thermal loading and water absorption in joint specimens were detected by monitoring the FBG wavelength shift caused by thermal expansion or water swelling. The measured parameters were used in finite element models to simulate the response of the embedded sensor. The good correlation of experimental data and simulations confirmed that the change in FBG wavelength could be accurately related to the thermal load or water absorption process. The suitability of the embedded FBG sensors for monitoring of water uptake in adhesive composite joints was demonstrated. (C) 2015 Elsevier Ltd. All rights reserved.

Elsevier2015

Active sensing and repair composites

Eva Kirkby

EPFL2009