Hygrothermal Ageing and Damage Characterization in Epoxies and in Epoxy-Glass Interfaces

Nowadays, composite materials are increasingly used in high technology fields such as aerospace and automotive because of the combination of excellent mechanical performance and lightweight. For these characteristics, classical metal alloys are progressively replaced in primary structural components by reinforced polymer composites. The service life of such structural components is more and more increased thus raising questions on the composites' durability since it is well known that they are susceptible to long term aggressive environments. In fact, UV radiations, high temperature, solvents, fuel, humidity and water synergistically act with mechanical loads to reduce the resistance of composite materials. To address this problem two main approaches are used. On one hand, the macromechanical approach is used for the long term reliability of composite materials that experimentally is characterized by means of accelerated ageing tests that reproduce the service environments. On the other hand, the micromechanical approach is aimed at understanding the intimate degradation mechanisms triggered by the environmental conditions. This objective is partially accomplished by reducing the geometrical complexity of the material microstructure subdividing it into elementary unit cells. In this work, the combination of moderate high temperature and moisture is chosen as the ageing environment. The effects of the hygrothermal ageing on the properties of an epoxy resin are studied using a single fiber composite (SFC) unit cell. The reinforcement of the SFC is an optical glass fiber that presents, along a portion of its core, a fiber Bragg grating (FBG) sensor capable of gathering information about the deformation field inside the unit cell. This particular configuration allows to study evolution of the internal stress state during cure, at high temperatures and during the ageing process. The residual deformation field in the SFC is investigated by using a mixed experimental-numerical technique. A series of radial cuts are introduced into the SFC geometry while the induced strain perturbation is recorded by the embedded FBG sensor. This knowledge is used in an identification scheme in order to determine a shrinkage function capable of reproducing the initial deformation state in the axysimmetric configuration. The calculated deformation field is in good agreement with similar studies in litterature. The study of the thermomechanical response of the SFC allows to determine the resin's coefficient of thermal expansion. The procedure is validated in the case of an epoxy resin reinforced with different weight percent of SiO2 nanoparticles and rubber microparticles. It is shown that the method provides more stable results if compared to the classical TMA analysis. The resin response to the wet environment at 50°C is characterized firstly by means of a gravimetric analysis that led to the determination of the water diffusion kinetic parameters. Secondly, the mechanical property evolution as function of humidity is determined by means of tensile, multiple relaxation tests and microindentations. Lastly, the resin coefficient of moisture expansion is determined and displays a non linear trend. A residual-thermo-hygro mechanical model is built on the basis of the determined parameters showing excellent agreement with the recorded experimental data. Moreover, the hygrothermally induced fiber fracture is analyzed by means of the linear elastic fracture mechanics and the shear lag theory. Finally, the fiber-matrix debonding, triggered by the reinforcement failure, is characterized by the use of cohesive elements whose mechanical properties are varied as function of the concentration at the interface. The calculated redistribution of stresses after the fiber failure and the debonding kinetic reproduces accurately the recorded experimental behaviour.

Tailored Capsule Based Self-Healing for Epoxy Matrix Composites

Sam Neuser

Self-healing materials are a novel class of materials able to repair structural or, more generally, functional damage to a part. They hence have the potential to improve reliability and cost effectiveness, because failure or loss of functionality may be postponed or completely avoided. This concept has been applied to all classes of materials, but is particularly suited to polymers and their composites. Indeed, it was first demonstrated for epoxies containing capsules of dicyclopentadiene and dispersed Grubbs’ catalyst. However, the high cost of the catalyst precludes commercial use of this self-healing system. In the present thesis, two alternative approaches to self-healing of an epoxy matrix were investigated: (1) a solvent based self-healing epoxy; (2) a two-component epoxy/amine self-healing system using a novel encapsulation technique. The solvent-based system showed efficient healing in an under-cured epoxy matrix. The solvent in question was ethyl phenylacetate (EPA) encapsulated with urea-formaldehyde (UF) or UF-polyurethane. It was demonstrated that during a crack event, the solvent diffused into the epoxy matrix following a non-Fickian case II profile, with a geometrical swelling of the surface layer of the crack. For a healing time of 24h, a critical crack gap of 30μm was identified, beyond which the swelling was insufficient to bridge the crack faces and the healing efficiency was limited to around 25%. For smaller crack gaps, solvent swelling led to intimate contact of the crack faces and reaction of residual functional groups through a reduction of the glass transition temperature Tg of the epoxy, resulting in improved healing efficiency. The use of shape memory alloys (SMA) that are strained during the fracture event and subsequently heated to reduce the crack opening was also investigated. High healing efficiencies were observed and it was found that longer SMA wire activation times further improved the healing efficiency up to almost 80% through improved crack closure and an increased degree of polymerization. The behavior of this material during aging was then investigated in partially cracked samples, guaranteeing small crack gaps. It was found that the premature diffusion of the solvent through the microcapsule membrane into the epoxy matrix led to a decrease in healing efficiency from around 75% in fresh samples, to about 15% in samples aged for 75 days. Increasing the thickness of the polymeric shell of the capsules did not significantly improve performance. Moreover, moisture uptake by the epoxy also contributed to the reduced healing efficiency. Because the healing agent is hydrophobic, a higher water content in the epoxy matrix limited solvent diffusion into the epoxy and hence the extent of healing. A potential solution to the problem of premature diffusion of solvent into the epoxy is suggested to be the use of other shell materials or additional coatings on the capsules. The response of the solvent based self-healing system to dynamic fatigue was also studied. While in static testing, matrix swelling governs crack healing, a different mechanism was observed in the case of dynamic loading. In contrast to the unmodified epoxy, which showed rapid crack propagation under all the conditions investigated, in the presence of the solvent, crack arrest was observed for stress intensities of up to 0.48 MPam1/2. This phenomenon was attributed to crack pinning owing to diffusion of the solvent into the matrix surrounding the crack tip. The elongation at break of the solvated epoxy increased threefold, and the Young’s modulus decreased by more than 50%. Using finite element analysis (FEM) to model the stress distribution around the solvent-infused crack tip, it was shown that the local crack tip stress was reduced by 43%, accounting for this novel toughening effect. While solvent healing performed well in an under-cured matrix and interesting phenomena were observed, a fully cured matrix with a two-component self-healing system may be preferable for real applications because it avoids issues pertaining to premature solvent diffusion. The possibility of encapsulating an amine curing agent in a core-shell structure was therefore investigated as an alternative. A microfluidic glass chip was used to encapsulate a hydrophilic amine/water based curing agent in an acrylate resin shell using ex-situ exposure to UV light. Low viscosity epoxy was encapsulated with UF using a modified agitated emulsion encapsulation technique. Successful self-healing was demonstrated for epoxy specimens containing these microcapsules, along with embedded SMA wires, with healing efficiencies of up to 100% and 40% for low and high temperature curing epoxy, respectively. Overall, this work represents a significant step towards affordable epoxy self-healing materials, the feasibility of solvent and epoxy/amine healing having been successfully demonstrated for an epoxy matrix. The epoxy/amine self-healing system is thought to be particularly promising, considering the early stage of development. Furthermore, the integration of SMA wires proved to improve healing efficiencies dramatically, especially with the epoxy/amine microcapsules. With further optimization and scale-up of the microfluidic encapsulation method, a new low cost self-healing system, and hence potentially compatible with a wide range of polymeric materials, including composites, is therefore within reach because it is based on a commonly used epoxy/amine adhesive.

EPFL2013

Comportement au jeune âge et différé d'un BFUP écrouissant sous les effets thermomécaniques

Aicha Kamen

This thesis is dedicated to study the early age behavior and time dependant as well as the thermo-mechanical response under restraint of a hardening Ultra High Performance Fiber Reinforced Concrete (UHPFRC). The knowledge of the UHPFRC behavior is indispensable to estimate the cracking risk in composites structures. The phenomena which develop during the hydration progress of the material as the capillary network development and the physico-mechanical properties were considered. The effect of temperature curing conditions on these phenomena was characterized. The test and predicted (issue from modeling and numerical simulations) results are presented and discussed in this report. The tested UHPFRC is characterized by high mechanical performances with a faster evolution of the Young modulus with regard to the other properties. The measured mechanical properties can be predicted very well with the CEB-FIB model. The tested UHPFRC is characterized by a low degree of hydration due to the low water-binder (w/b) ratio. This experimental result was confirmed by a numerical estimation by means of the adapted Powers and Waller models as well as a numerical estimation through the heat realized during the hydration process. A linear relationship between the mechanical performances and the degree of hydration was shown and the action of the microstructure evolution on the mechanical performances development was elucidated. It was attributed to the beneficial effect of the unhydrated particles of silica fume and cement because of the pore size refinement in the microstructure. Besides, the current UHPFRC is characterized by a fast autogenous shrinkage kinetics related to the dominant self-desiccation during the first days of the hydration. Thanks to a numerical estimation with Laplace and Kelvin laws, self desiccation measurement under various temperatures were used to distinguish and to put in evidence the effect of the temperature on the porous structure, the capillary pressures and consequently on the contraction of the solid matrix. The contribution and the beneficial effect of fibres towards the autogenous shrinkage were also showed. In fact, fibres act as internal restraint and can prevent the free matrix deformation. The high temperatures improve widely the rate of hydration at early age (7 days) and so lead to increase initially the autogenous shrinkage. In the long term, an inverse effect of the temperature was observed on the evolution of the degree of hydration and the measured physico-mechanical properties, which was attributed to the influence of the temperature on the ions haste and distribution during the hydration process. The influence of the temperature on the degree of hydration and the compressive strength was characterized and modeled and the corresponding parameters were used to estimate the energy of activation of the studied UHPFRC. The dominant role of the autogenous shrinkage at early age was clarified and the associated stresses, which develop at the early age under total restraint, were quantified by means of the TSTM. These stresses remain moderate. The UHPFRC creep potential is important. UHPFRC exhibits high creep. This was attributed to the high volume of paste (88 %), which is subjected to creep. Existing models in the literature were used; certain models predict the UHPFRC behavior in a satisfactory way and with a certain precision. One notes that the early age stress predictions are dependent on the software used and especially on the creep model. In our case, the generalized Maxwell model was used.

EPFL2007

On the relationship between the microstructure and the mechanical properties of an ODS ferritic/martensitic steel

Amuthan Ramar

In the quest of materials for the first wall of the future fusion reactor, it has been shown that oxide dispersion strengthened (ODS) ferritic / martensitic (F/M) steels appear to be promising candidates. The inherent good mechanical properties supported by a good thermal conductivity, swelling resistance and low radiation damage accumulation of the base material, such as EUROFER97, are further enhanced by the presence of a fine dispersion of nanometric oxide particles. They would allow in principle for a higher operating temperature of the fusion reactor, above 550°C, which improves its thermal efficiency. In effect, their strength remains higher than the base material at high temperatures. There are however limitations to their application in the reactor, which relate to the intimate link between the microstructure and the mechanical properties. These limitations, such as an increased ductile to brittle transition temperature above room temperature relative to the base material, come directly from the role played by the oxide particles but also from their indirect impact on the overall microstructure, stemming from the procedure of elaboration of these materials. In addition, there is a lack of knowledge on their response to irradiation, which, even though it appears to be promising with a reduced irradiation induced hardening, is not very well understood. In this work we have developed and evaluated ferritic/martensitic steel, EUROFER97, strengthened by a dispersion of yttria particles. (1) The powder metallurgical process used to obtain it was optimized, (2) the basic mechanisms of hardening were identified and (3) the impact of irradiation on its microstructure and mechanical properties were investigated. In order to understand the behaviour of this class of material other oxide dispersion strengthened (ODS) ferritic/martensitic steels based on EUROFER97 and model alloys were also investigated. For the elaboration of the ODS steel the powders of atomized EUROFER97 and yttria are ball milled in a planetary ball mill and the material is compacted by hot isostatic pressing (HIP). The mechanical properties are evaluated by uniaxial tensile test, Charpy test and micro-indentation while the microstructure is investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The focused ion beam (FIB) technique is also used. In the study of the ball milling it appears that the optimal time is 20 hours, as the X-ray peak broadening, crystallite size and hardness of the particles remain unchanged at longer times. Yttria nanoparticles clusters are no more detectable by X-ray after 2 hrs of milling, which is attributed here to their incorporation in the matrix, as opposed to dissolution in the matrix as is often suggested in the open literature. HR-TEM performed on the yttria particles before and after incorporation in the steel by ball milling shows that the yttria maintain its bcc equilibrium structure throughout the process. This could suggest that they are not dissolved during ball milling. We have clarified this point by a cross sectional examination of the EUROFER97 ball milled particles with 0.3wt% of yttria in an FIB. By X-ray EDS analysis it shows nanometric regions reach in yttrium and oxygen, which are the yttria particles embedded in the EUROFER97 particles, proving that they are not dissolved by ball milling. The mechanism at the origin of the alloy compaction during HIP was elaborated. The alloy compaction was done through HIPing at 1150°C for 2.5 hrs at a pressure of 190 MPa. This condition is shown to provide the highest bulk density, to a value close to 99.9%. In the optimization process, it is found that temperature plays a larger role than pressure for alloy compaction. The compaction appears to be mainly due to diffusion. Also, it appears that the ball milled particle size plays a major role in the alloy compaction. It is shown that smaller particles size induces better compaction, provided that the densification proceeds through diffusion flow. This further suggests to terminate the ball milling process by 20 hrs as the particles size reaches its minimum. The impact of the ball milling process on the mechanical properties was evaluated. EUROFER97 with and without ball milling, with and without oxide addition presents an average density around 99.8%. In the as received state, casted EUROFER97, atomized EUROFER97, ball milled EUROFER97 and ODS Yttria present microstructure morphology typical of tempered martensite, whereas ODS Ti presents a nano sized grain microstructure with an average grain size of 200 nm. Ball milling and HIPping does have a slight influence up to 400°C, due to the higher dislocation density induced by ball milling relative to casted EUROFER97. It becomes negligible above 400°C, since both casted and ball milled EUROFER97 present similar hardness and microstructure. In ODS Yttria, yttria presents an average particle size of 25 nm. In ODS Ti, Y-Ti-O presents an average particle size of 7 nm. In the following the identification of the hardening mechanisms in ODS F/M steels. A new model was developed to quantify the collective hardening due to different obstacles, assuming that they provide independent hardening mechanisms. The model helped identifying the dominating source of hardening in ODS F/M steels. The base of the model is the dispersion barrier strengthening model, which is given by Δσsparticle or defects = Mαμb(Nd)1/2 for the hardening due small defects or oxide particles and is given by Δσd-d = Mαμb(ρ)1/2 for the hardening due to dislocations. In this approximation, the total hardening is then given by Δσtotal = {(Δσd-d)2 + (Δσparticle or defects)2}1/2 . The model is used to explain the hardening due to oxides, dislocations and irradiation induced defects. It appears that dispersed oxides is dominating hardness up to 800°C, while following a heat treatment at 1000°C it is the dislocations that then dominate the hardness. Deformation mechanisms were investigated using differential strain rate experiments. The activation parameters, i.e. the activation volume and activation energy, were calculated. It appears that there is a decrease in the activation volume from room temperature to 400°C, which is less than 3%. At about 500°C there is a sudden drop in the activation volume that reaches a minimum at 600°C. With further increase in temperature, there is a significant raise in the activation volume. This abrupt decrease between 400°C and 600°C hints at a change in dislocation-rate controlling mechanism, marking two regimes, a low temperature one and a high temperature one. In situ TEM observations show that at low temperature the deformation is mainly due to the formation of the Orowan loops around the particle. At high temperature the activation energy is about 3.4 eV for ODS, whereas it is 1.3 eV for EUROFER97. The calculated activation energy corresponds well to the one of dislocation climb mechanism. This indicates that at high temperature the yttria particles could be overcome by climb, while at low temperature they are overcome by the Orowan mechanism. The peculiar response of the yield stress to irradiation for this class of material was rationalized using the total hardening model we developed. At a dose of 2 dpa, irradiation induced defects are higher in number density than the dispersed yttria particles, which although they are weaker than the yttria particles they can explain the significant hardening observed in the material. Conversely, the shallow impact of irradiation on ODS F/M steels at low doses, relative to base F/M steels, is explained by the fact that the amount of dispersoid is found to be higher than the density of irradiation induced defects.

EPFL2008