Fracture behavior and creep-fatigue response of fiber reinforced glass-ceramic matrix composites

The mechanical behavior of a continuous silicon carbide fiber reinforced Barium Magnesium Aluminosilicate (BMAS) glass-ceramic matrix composite is investigated at room and high temperatures. The materials were heat-treated in an oxidizing environment for 1h at 1100°C previous to mechanical testing. The static fracture behavior at room and high temperatures was analyzed in direct tension tests, while the creep-fatigue (cyclic-creep) behavior was estimated in four-point flexural tests at 1100°C. The experimental results of tensile tests have highlighted the importance of the carbon-rich layer at the fiber/matrix interface for obtaining “graceful” failures. At high temperatures (1100°C), oxidative degradation of the interface results in significant strength reduction and a transition to the brittle fracture mode. The creep-fatigue results at different stress levels are analyzed in terms of the creep-recovery behavior. Extensive viscous strain recovery was found upon the unloading period. The crept composites retained their “graceful” fracture behavior after testing, indicating that no (or limited) damage in the matrix was induced during cyclic creep at the conditions tested. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 1, pp. 81–90, January–February, 1996.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Fatigue behaviour of continuous glass fibre reinforced composites

A study has been undertaken of fatigue in glass fibre reinforced composites. Two matrix resins were tested: a standard polyester and a polyurethane-vinyl-ester, which was designed to have a higher toughness. Three different types of glass fibre fabrics were used for reinforcement: a conventional woven roving and two stitch-bonded cloths. The glass cloths were combined into various lay-ups, in order to consider the effects of matrix, cloth and lay-up on the fatigue strength. Additionally, a study was undertaken to evaluate the micromechanisms that occurred during fatigue and how damage accumulated throughout the sample lifetime. This involved measuring stiffness changes during fatigue cycling, followed by microscopic study of the samples. It was found that similar damage micromechanisms occurred in each lay-up regardless of resin and cloth type, and these included matrix cracking, delamination and fibre breakage. However, differences were observed in the extent, location and rate of damage, and these were consistent with the variations seen in the fatigue strengths.     

Creep behaviour of unreinforced and short fibre reinforced AlSi12CuMgNi piston alloy

The isothermal creep resistance of AlSi12CuMgNi alloys produced by squeeze casting, unreinforced and reinforced with 10, 15 and 20 vol% of short alumina fibres is investigated by means of long-term tensile creep tests at 300 °C. Dislocation creep-mechanisms associated with a Norton exponent n = 3 are attributed to both the unreinforced and the short fibre reinforced materials (SFRMs) in the stress range between 10 and 50% of the yield strength at 300 °C. The embedding of short fibres reduces the creep rate of the alloy by more than one order of magnitude. The SFRM with 15 vol% of reinforcement is the most creep resistant, while the 20 vol% SFRM is less creep resistant than the 10 and 15 vol% SFRMs due to its higher defect density and larger interface area.

Load changes are imposed during creep tests to study the influence of periods of overloading. Returning to the initial load causes the stationary creep rates of the SFRMs to be further reduced down to one third of their initial values. Furthermore, a small increase in the creep exponent n of the SFRMs is observed after an overloading cycle. The increase of the connectivity of the Si/Al₂O₃-short fibre network in the direction of loading enhances the load transfer effect from the matrix to the reinforcement decreasing thus the external stress acting on the matrix. The yielding of highly stressed zones in the matrix around the ceramic reinforcement during the overload is able to produce a more uniform distribution of internal stresses when returned to a lower stress level. Both effects are considered to be responsible for the observed reduction of the stationary creep rate.

The unexpected low creep resistance of the 20 vol% SFRM in comparison with the two other composites can be explained by the larger number of micro-structural defects (≤1 vol% porosity) found in this material after the infiltration process. Although the small portion of these defects does not affect the high temperature tensile properties of the composite, it accelerates the diffusion controlled creep mechanisms. The larger interface area present in the materials with higher reinforcement volume fraction increases the diffusivity as well. The connectivity of the Si/Al₂O₃-short fibre (SF) network is higher for the 20 vol% SFRM than for the other SFRMs already before creep exposure. Thus the training effect by overloading periods is less efficient in the 20 vol% SFRM than in SFRMs with less fibre content. Those effects are counteracted by the fact that the higher the reinforcement volume fraction the smaller the load acting on the matrix. Yet for the investigated 20 vol% SFRM samples, the negative contributions prevail resulting in a lower creep resistance than for the other two composites.     

Transverse tensile behaviour of fibre reinforced titanium metal matrix composites

Four Ti MMCs have been tested in transverse tension, at ambient temperature and 600 °C. Generally, mechanical properties are reduced compared to monolithic Ti alloys. Transverse Young's modulus is, however, higher than in monolithic alloys, as a result of constraint of the matrix by the fibres.MMC proportional limits are associated with the onset of interfacial failure. Fibre coating cracking and longitudinal fibre splitting may also contribute to MMC yield and the associated acoustic emission peak. The fibre/matrix interface in IMI 834/SM1140+ appears to be weaker than in the other MMCs, resulting in a lower proportional limit and less acoustic emission. Final failure of the MMCs is generally via ductile shearing of matrix ligaments. The exception to this is IMI 834/SM1140+ in which the matrix fails in a brittle manner. This causes poor transverse tensile strength and failure strain in this MMC.A model to predict the MMC proportional limit, previously proposed by Jansson et al., has been modified to take account of the tensile strength of the fibre/matrix interface. The model previously used by Jansson et al. to predict the transverse tensile strength is acceptably accurate provided that the area fraction of matrix appearing on fracture surfaces is accurately determined.     

Fatigue behaviour of borosilicate glass-ceramic matrix,nicalon (silicon carbide) fibre composites

Uniaxial fatigue damage analyses were performed on borosilicate glass-ceramic matrix, Nicalon (silicon carbide) fibre reinforced unidirectional composites. The fibre volume fraction varied from about 0.25 to 0.60. Load-controlled tension-tension fatigue tests (R ratio = 0.1) were conducted at room temperature and 540°C (1000°F). The fatigue life was found to decrease with increasing cyclic stress level and a power-law relationship of the form _app = _uts(2N _f)^b was established where _app is the applied maximum stress, _uts the monotonic tensile strength, N _f is the number of cycles to failure and b is the fatigue strength exponent. The fatigue damage evolution manifested itself as a decrease in stiffness of the composite with fatigue cycles. This stiffness drop was associated with matrix cracking followed by fibre-matrix debonding and fibre sliding breakage/pull-out, and final failure, respectively at 540°C. The damage evolution at room temperature was associated with degradation of the matrix followed by steady breakage of fibres with no debonding/pull-out, leading to eventual failure of the net section of the composite. In general, quantitative microscopic observations of debonded and pulled-out fibres showed a good correlation with the observed reduction in stiffness. A predictive model to interpret the drop in stiffness is presented and validated using experimental results from the current study.     

Deformation and fracture behaviour of flax fibre reinforced thermosetting polymer matrix composites

The mechanical properties of unidirectional flax fibre reinforced unsaturated polyester resin composites were studied with particular emphasis on their tensile deformation behaviour. These materials displayed characteristic non-linear behaviour when loaded parallel to the axis of the fibre, with a distinct knee preceding a drop in stiffness. Further deformation resulted in strain hardening behaviour. Load cycling and acoustic emissions analysis were used to investigate the nature of the knee and it was found that this corresponded with yielding behaviour in the composite. A well-defined yield point could be identified, which in composites of around 60% fibre volume fraction, occurred at a strain of some 0.12% and a tensile stress of 32 MPa. Varying the interfacial properties, through chemical modification of the fibre prior to lamination, was found to have a marked effect upon the onset of yielding and the yield point itself, as well as the deformation and fracture behaviour of the laminate. It is considered that this behaviour is intimately linked to the straining behaviour of the fibre as well as the fibre–matrix interaction and hypotheses to explain the observed behaviour are presented.     

Discontinuous metallic-glass ribbon reinforced glass-ceramic matrix composites

Discontinuous metallic-glass ribbons of varying lengths and widths were used to reinforce a brittle glass-ceramic matrix. The fracture strength and toughness of such composites as a function of ribbon volume fraction and geometry were measured by three-point bending. The mechanical properties were found to be relatively isotropic in the plane of compaction (without significant loss of strengthening achieved with unidirectional reinforcement). The higher composite strength exhibited in a direction perpendicular to the plane of compaction was attributed to the higher percentage of ribbons oriented with their short transverse faces perpendicular to the opening crack front.     

Creep and rupture behaviour in fibre bundles

The time dependent rupture of a fibre bundle under constant tensile load and with scatter in fibre stiffness and fibre strength is analysed using a general formulation of the creep law. The total rupture time is divided into an incubation time, with all fibres intact, and a failure propagation time where fibres fail progressively. Governing equations are given for both time intervals. It is shown that the failure propagation time interval is independent of the load if no spatial stress redistribution occurs and all fibres remain intact upon loading. For a bundle consisting of originally uniform fibres where a fraction is afflicted with a certain defect, the time intervals are calculated assuming Norton's non linear creep law.     

Fatigue behaviour of aluminium matrix composites reinforced with continuous alumina fibres

Abstract

Alumina (N610) reinforced pure Al (A9) or Al–2 wt-%Cu alloy (AU2) unidirectional (UD) composites which combine a highly ductile matrix with a strong interface bonding, present high static and dynamic mechanical performances. In the present paper, the fatigue behaviour of quasi­UD N610+A9 and N610+AU2 composites is investigated. Notwithstanding the presence of the transverse bundles, the longitudinal behaviour in tension and fatigue of these quasi-UD composites is nearly equivalent to that of the pure UD. Only in the case of the N610+AU2 quasi-UD composite, the fatigue limit is about 30% lower than that of the pure UD. Acoustic emission (AE) monitoring correlated with microfractography and microstuctural examinations has enabled identification of different stages in the evolution of damage and the association of these with damage and failure mechanisms. In fatigue, three main damage mechanisms are activated in a sequential and/or superposed mode during a three stage evolution.     

Effects of fibre content on mechanical properties and fracture behaviour of short carbon fibre reinforced geopolymer matrix composites

Geopolymer matrix composites reinforced with different volume fractions of short carbon fibres (C_f/geopolymer composites) were prepared and the mechanical properties, fracture behaviour and microstructure of as-prepared composites were studied and correlated with fibre content. The results show that short carbon fibres have a great strengthening and toughening effect at low volume percentages of fibres (3·5 and 4·5 vol.%). With the increase of fibre content, the strengthening and toughening effect of short carbon fibres reduce, possibly due to fibre damage, formation of high shear stresses at intersect between fibres and strong interface cohesion of fibre/matrix under higher forming pressure. The property improvements are primarily based on the network structure of short carbon fibre preform and the predominant strengthening and toughening mechanisms are attributed to the apparent fibre bridging and pulling-out effect.     

Influence of microstructure on axial strength of unidirectional continuous fibre reinforced aluminium matrix composites

Abstract

Systematic empirical investigations on the relationship between microstructural features and mechanical performance of unidirectionally reinforced continuous fibre Al matrix composites (CFAMCs) carried out by the present authors in recent years are summarised. The employment of a high strength matrix alloy and the development of a strong fibre/matrix interface are beneficial to maximise the strengthening effect of the fibre reinforcement. Processing defects, such as second brittle phases in the matrix, non-infiltration defects, matrix solidification shrinkage voids, excessive interfacial reactions, the presence of reaction products on the interface, weak interfacial binding, and excessively high fibre volume fraction reduce composite strength to different extents via a number of different mechanisms. Criteria for the microstructure design of CFAMCs for optimum fibre strengthening efficiency are proposed.     

Post-cracking behaviour of steel fibre reinforced concrete

Recently, RILEM TC 162-TDF has proposed equivalent,f _eq , and residual,f _R , flexural tensile strength parameters to characterize and simulate the post-cracking behaviour of steel fibre reinforced concrete (SFRC) structures. In the current work, more than two hundred flexural tests are carried out according to the RILEM TC 162-TDF recommendations and the corresponding values off _eq andf _R parameters are evaluated. In series of specimens reinforced with fibres of a distinct length/diameter ratio, similar values off _eq andf _R parameters were obtained in these series. Although a strong correlation betweenf _eq andf _R was determined, a larger scatter off _R values was observed thereby revealingf _eq to be more appropriate for design purposes. A numerical strategy involving a cross sectional layered model and an inverse analysis was developed to evaluate the post-cracking stress-strain and the stress-crack opening diagrams for the tested SFRC. This strategy was also used to determine a relation between the post-cracking strain, ɛ ^pcr , and the crack opening displacement,w, (ɛ ^pcr =w/L _p ) which is useful for evaluating the crack opening when numerical strategies based on a stress-strain approach are used. The obtainedL _p values range from half the specimen cross section height to half the distance between the tip of the notch and the top of the cross section.

---

Résumé Récemment, pour caractériser et simuler le comportement post-fissuration en traction du béton renforcé des fibres d'acier, la Commission Technique 162-TDF de la RILEM a proposé des paramètres désignés par résistance équivalente, f_eq, et résistance résiduelle, f_R, à la contrainte en flexion. Dans le travail présent, des valeurs de ces paramètres sont obtenues sur plus de deux cents essais de flexion effectués en accord avec les recommandations du TC 162-TDF de la RILEM. Des valeurs semblables de f_eq et f_R ont été obtenues dans des séries d'éprouvettes renforcées avec des fibres d'un rapport longueur/diamètre distinct. Bien qu'une forte corrélation entre f_eq et f_R ait été déterminée, une plus grande dispersion de valeurs du f_R a été observée, en démontrant que f_eq est plus approprié pour les buts du projet. Pour évaluer les diagrammes contrainte-déformation et contrainte-ouverture après fissuration, une stratégie numérique a été développée, en utilisant un modèle de section et en effectuant une analyse inverse. Cette stratégie a aussi été utilisée pour déterminer une relation entre la contrainte après fissuration, ɛ ^pcr , et l'ouverture de fissure, w, (ɛ ^pcr =w/L _p ) utile pour évaluer l'ouverture de la fissure quand les stratégies numériques sont basées sur une approche contrainte-déformation. Les valeurs de L_p obtenues ont varié entre la demi-hauteur de la section de l'éprouvette et la demi-distance entre l'extrémité de l'entaille et le sommet de la section.

     

Fracture behaviour of long fibre reinforced thermoplastics

This paper deals with the fracture performance of injection moulded long glass fibre composites based on polybutylene terephthalate (PBT) and polypropylene (PP) matrices. The tensile behaviour of these composites is analysed using the shear lag theory taking into consideration the interfacial shear strength, fibre length distribution and fibre orientation in the mouldings. The fracture performance is investigated using the post yield fracture mechanics approach. The crack growth resistance of the PP and PBT long fibre composite was found to increase with increasing fibre volume content up to 35%. Above 35% a plateau in the fracture performance was observed. A combination of high fibre degradation and a change in the fibre orientation pattern of the moulded pieces is found to be responsible for the plateau region in the performance of the high concentration system. In fact, the dependence of the maximum crack growth resistance of the composites on fibre length and fibre orientation is also controlled by testing temperature. The competition between fibre-induced matrix deformation and the fibre pull-out determines the ability of the composites to resist crack propagation.     

Creep behaviour of glass reinforced plastic box beams

Three 6m long box beams were made from glass-reinforced plastic material and subjected to various loading procedures to study their creep behaviour. The first beam carried a constant load, equivalent to one-third its ultimate load, the second was alternately loaded and unloaded, with a similar load, for various time intervals and the third was left to support its own weight. Measurements were taken at specified time intervals over a period of about 20 months. It was found that glass-reinforced plastic material is more liable to creep than other conventional construction materials. Initial deflections increase by over 100% over the whole time period, but most of this occurs in the first 1000 hours. A set of best fit equations were found which can be used to predict the creep behaviour.     

Interfacial effects in a metallic-glass ribbon reinforced glass-ceramic matrix composite

The effect of temperature on the interfacial effects in a metallic-glass ribbon reinforced glass-ceramic matrix composite was investigated. The metallic-glass ribbon present in this composite was found to be significantly affected at elevated temperatures, owing to the diffusion of lead and zinc from the matrix. The presence of the matrix in the vicinity of the ribbon enhanced the formation of an oxide layer on the ribbon surface. The oxide layer decreased the interfacial bond strength between the ribbon and the matrix, affecting the failure mode of such composites at elevated temperatures.     

Effect of thermal cycling on the microstructure of continuous carbon fibre reinforced copper matrix composites

Copper reinforced by continuous carbon fibres is a candidate material for heat sinks of electronic modules. The thermal expansion can be matched to the adjacent ceramics and the thermal conductivity is higher than that of alternative materials. Because the material is expected to work in cyclic thermal conditions during application its structure must withstand such a load without any severe damage. The structure of unidirectionally and cross-ply fibre reinforced samples was observed before and after thermal cycling by optical microscopy and Scanning Electron Microscopy (SEM). SEM was used for examining the fibre-matrix interface before and after thermal cycling. Optical microscopy showed the arrangement of individual monolayers. The influence of adding copper foils between individual monolayers on the structure of the material is reflected in a reduction in the number of cracks in monolayers.