Band 18, Heft 4 - Special Issue: Durability of Composites

Due to their excellent mechanical properties and easy application carbon fibre reinforced polymers (CFRP) are being successfully used to repair and renew old and damaged civil engineering infrastructures. In particular, the method of strengthening concrete members with externally bonding of CFRP laminates has become increasingly popular. In fact, extensive research has shown that this technique can lead to a substantial improvement in the flexural and shear strength capacities of the member repaired. In such applications, materials involved will be exposed to outdoor environmental conditions, including humidity, water from rain, saline-water, ground water and high alkalinity solutions due to chemical composition of the concrete itself, as well as solar ultraviolet (UV) radiation. Thus, it is of great relevancy to study the durability of the materials involved, namely epoxy adhesives and CFRP laminates, as well as, the integrity of the entire system, when subjected to those environmental factors. This work is part of a much larger research study to predict the long-term behaviour of several commercial CFRP laminate/adhesive systems used in the rehabilitation of concrete structures. In the whole project it is recognised that the durability of these solutions depends not only on the durability of the composite materials but also on the performance of the systems used to bond them to the concrete. This present document provides details about the durability of CFRP laminates and the study carried out to investigate the environmental and accelerated ageing effects on three types of commercially available CFRP laminates. The changes in CFRP laminates properties after exposure up to 18 months under different environmental conditions – immersion in demineralised water, saline and alkaline solutions at room temperature, 40°C and 60°C, as well as under constant humidity condensation atmosphere at 40°C – are reported and discussed. The results of tests performed in accelerated weathering chambers – with UV radiation – are also included. Results obtained offer valuable information about the progression of material degradation under different ageing conditions (some of them accelerated), which can be used, not only to characterise the behaviour of CFRP laminates, but also for a better understanding of the degradation mechanisms involved, which is a base for the design of service life-prediction models.

The present study describes the results of a detailed investigation of the fracture toughness and fatigue crack growth behaviour of epoxy resins used for aeronautic applications. More specifically, the influence of incorporating thermoplastic or triblock copolymers to neat resin in order to increase the toughness is evaluated. The experimental results deduced from static as well as dynamic loading conditions highlight the varying degree of effectiveness of the chosen toughening approaches. In combination with electron microscopic analysis of the fracture surfaces, the basic structure-property relationships and deformation mechanisms are presented and the different strategies to improve the fatigue behaviour are discussed.

In deepwater offshore production, thermal insulation of pipelines and risers is now commonly made with glass syntactic foams. These composite materials offer both good insulation properties and pressure resistance, and also provide buoyancy to the systems. But service conditions are increasingly severe, including pressure (up to 30 MPa at 3000 m water depth), water environment and temperature (up to 130°C in contact with the steel tube). These coupled loadings induce complex effects on the material and make the prediction of its long-term behaviour difficult. Over the past 10 years, many studies have been performed to better understand the behaviour of the glass syntactic foams used as thermal insulation of pipes for deepwater production. Ageing tests in hot water revealed some specific behaviour of these materials with coupled chemical and mechanical phenomena of degradation. The experimental techniques included gravimetry, mechanical and physicochemical characterisations, impedance spectroscopy, X-ray tomography. As the rupture of glass microballoons is relatively emissive, acoustic emission technique has also been used. The contribution of this latter technique to the understanding of the syntactic foam behaviour is presented in this paper. Some ageing tests in water at several temperatures and on several syntactic foams have been conducted. Mechanical characterisation has been performed using a confined compression set-up instrumented with an acoustic emission sensor to allow the monitoring of glass microballoons degradation vs. time and temperature of ageing. These results are analysed for syntactic foam materials having various compositions. The comparison to the information given by other techniques is also discussed.

Damage, such as delamination and fiber breakages, to the carbon/epoxy laminates can be detected by measuring electrical resistance changes caused by the damage. The resistance changes result from the electrical current path being impeded and deformation of the composites relating to piezoresistivity. In the present study, the effect of the residual strain relief due to delamination is investigated analytically and experimentally for two different stacking sequences and it is confirmed that piezoresistivity has a significant effect on the measured electrical resistance change in the electrical resistance change method.

A computational simulation method is presented to assure life in composite systems by using dynamic buckling of smart composite shells as an example. The combined use of composite mechanics, finite element computer codes, and probabilistic analysis enable the effective assessment of the dynamic buckling load of smart composite shells. A universal plot is generated to estimate the dynamic buckling load of composite shells at various load rates and probabilities. The shell structure is also evaluated with smart fibers embedded in the plies right below the outer plies. The results show that, on the average, the use of smart fibers improved the shell buckling resistance by about 10% at different probabilities and delayed the buckling occurrence time. The probabilistic sensitivities results indicate that uncertainties in the fiber volume ratio and ply thickness have major effects on the buckling load. The uncertainties in the electric field strength and smart material volume fraction have moderate effects and thereby in the assured life of the shell.

This paper reports research about the durability of non-woven geotextiles when exposed to artificial weathering conditions [ultraviolet (UV) radiation, rain and dew] on a laboratory weatherometer. The geotextiles were specially manufactured from polypropylene fibers stabilized with different amounts of the additive Chimassorb 944 and the pigment carbon black. The degradation suffered by the geotextiles was evaluated by tensile tests and by scanning electron microscopy.

This work for prediction of damage and life is based on a new micromechanics of failure theory (MMF) and was started a few years ago. Instead of using the unidirectional ply as the building block, it was found that starting with the fiber, matrix and their interface would lead to a much simpler theory. The constituent properties will include two tensile and compressive strengths of matrix and fiber, plus normal and shear strengths covering the interface. The measured storage modulus of the viscoelastic matrix can be linked to the stiffness and strengths of plies and laminates from which creep rupture and fatigue strengths can be derived. One advantage of this approach is that the failure process of plies and laminates can be explicitly defined in terms of fiber, matrix and interfacial failures. No empirical factor is needed. The effect of moderately high temperature is fully integrated in the basic viscoelastic formulation, not accounted for by a knockdown factor. In addition, macro level properties of idealized square, hexagonal and random fiber arrays can be modeled and the resulting medium and scatter of predicted stiffness and strength are in reasonable agreement with available data. This modeling process is generic and can be applied to different material systems at room and elevated temperatures. Examples of metal matrix composites and hybrid composites like Tigr and Glare can be systematically described with no additional constants except the six basic strengths (four for fiber and matrix and two for the interface). First and last ply failures of laminates (FPF and LPF) can be similarly described with explicit definition of the failure modes. The failure process of matrix does not necessarily define the ultimate strengths of plies and laminates. Sophisticated modeling of the local degradation of matrix by micro cracking, ply delamination and fiber micro buckling will be explored with appropriate software packages developed for use by design engineers. In the mean time master curves covering the constituent stiffness and strength as functions of time and temperatures will be developed, measured and analyzed. In the end, failure envelopes of composite laminates in creep rupture and fatigue strength will be illustrated and their use for design be explained. In particular, the failure and life of a composite wind turbine blade is a good test of the predictive power of MMF.

The objective of this work is to study the influence of the loading path on the lifetime of self-healing ceramic matrix composites. The loading can be of different types: mechanical, thermal or chemical. The self-healing process consists of filling some cracks with an oxide plug, thus limiting the diffusion of oxygen toward the fibers. The three phenomena with the greatest influence on the lifetime are (a) crack opening, governed by the mechanical part of the loading, (b) volatilization of the oxide plug, and (c) diffusion governed by both the chemical part of the loading and the activation temperature of the chemical reactions. The examples given show the influence of each part of the loading path.

Hydroxyapatite (HAp) has many applications in the medical field. The objective of this study is to produce HAp/Ti composite coating with Supersonic Free-Jet PVD (SFJ-PVD). The SFJ-PVD is a technique to deposit nanoparticles with supersonic gas flow and to form a thick coating film. In a gas evaporation chamber, a source material is evaporated to form nanoparticles in an inert gas atmosphere. The nanoparticles are then carried to a substrate in a deposition chamber with an inert gas flow through a transfer pipe. The gas flow is generated by the pressure difference between the chambers and accelerated to the supersonic flow of 4.2 Mach through a specially designed supersonic nozzle. With SFJ-PVD, we obtain a uniform high-density HAp/Ti composite coating. XRD analysis reveals that the composite coating is composed of Ti and HAp. An in vitro study was carried out to investigate the bioactivity of the HAp/Ti composite coating under simulated body fluid.

Carbon fiber is a highly heat resistive material in an oxygen free atmosphere. Nickel is also a heat resistive metal and an important element for a superalloy. We can expect an excellent heat durable composite when they are combined. Nickel was electroplated on a carbon yarn to form a monofilament composite. When the high strength fiber was tested, the composite started to lose its strength at 600°C. The deterioration was caused by graphitization of the carbon in the fiber. This is caused by the difference of the graphitization degree between crystallites in a fiber. A high modulus carbon fiber has little difference of the graphitization degree in a fiber. The high modulus fiber did not react when tested and the fiber strength was maintained after heat treatment at 1100°C for 9 h. From the results, the high modulus carbon fiber composite is promising to be durable at a high temperature.