Characterization of Nicalon fibres with varying diameters: Part I Strength and fracture studies

Experimental studies have been conducted to examine the strength and fracture behaviour of monofilament Nicalon SiC fibres with diameters ranging from 8 to 22 m. The effects of varying fibre diameter, flaw location and flaw population on the mechanical response of individual fibres were investigated by recourse to extensive fractographic analysis performed on fibres fractured under tensile loading. Results indicate that variations in fibre diameter influence the apparent fibre fracture toughness (K1c), with higher K1c values observed for decreasing fibre diameters. Observations also suggest that the location of the critical flaw may play a role in the fracture of Nicalon fibres. Tensile strength values are shown to increase as the normalized distance of the critical flaw from the fibre centre increases, while critical flaw population appears to be strongly dependent on location. The ratio of K1c to geometry factor (Y) is observed to remain constant with varying flaw location. In addition to surface flaws, three distinct internal flaw populations are seen to cause fracture in Nicalon fibres. Based on these experimental findings, a statistical characterization of the strength of Nicalon fibres with varying diameters is presented in Part II of this paper. © 1998 Chapman & Hall     

Fracture behaviour of hybrid glass matrix composites: thermal ageing effects

The thermal aging of a glass matrix composite reinforced by short carbon fibres as well as by ZrO₂ particles (hybrid composite) was investigated at temperatures in the range 500–700 °C for exposure durations of 24 h in air. The mechanical properties of as-received and aged samples were evaluated at room temperature by using the three-point flexure chevron notch technique. The fracture toughness values of as-received specimens were in the range 2.6–6.4 MPa m^1/2. Fracture toughness was affected by the thermal aging conditions. For thermal aging at temperatures <700 °C, degradation of fibre–matrix interfaces occurred and therefore the apparent fracture toughness and flaw tolerant resistance decreased. For the most severe ageing conditions tested (700 °C/24 h), fracture toughness values dropped to 0.4 MPa m^1/2. Significant degradation of the material was detected for this aging condition, mainly characterised by porosity formation in the matrix as a result of softening of the glass and oxidation of the carbon fibres.     

Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Fibre-reinforced glasses: influence of thermal expansion of the glass matrix on strength and fracture toughness of the composites

The reinforcement of glasses by incorporation of fibres was considered to depend on the force transfer from the matrix on the fibres in order to obtain optimum strength and fracture toughness. This may occur by thermal shrinking of the matrix on the fibres after the hot-pressing procedure. It is shown that an optimum exists for strain and stress transfer from the matrix to the fibres if this shrinkage process is neither so strong that no pull-out and no bend-over effect is produced nor so weak that no stress transfer is possible. Therefore, experiments were performed with Nicalon-SiC fibres and with selected glasses which show different thermal expansion coefficients. In this way it was possible to produce fibre-reinforced glass composites with well-tailored special properties. Estimations of tensile stresses within the glass matrix led to values which are partly above those of the bulk glass. Because no cracks occurred during cooling and during heat shock treatment fromT _g, it was concluded that the strength of the thin glass layers between the very smooth surfaces of the Nicalon-SiC fibres cannot be compared with that of bulk glass but with that of protected (coated) glass fibres or thin sheet glass.     

Toughening mechanisms for a zirconia-lithium aluminosilicate glass-ceramic

The mechanical properties of a lithium aluminosilicate glass-ceramic and the same glass-ceramic containing 5 and 15 wt% zirconia were investigated. The aim of the study was to assess the contributions to toughening from various toughening mechanisms. For the zirconia-containing compositions, zirconia initially precipitated, upon heat treatment of the glass, as tetragonal zirconia (t-ZrO₂), and upon further heat treatment, transformed to monoclinic zirconia (m-ZrO₂). This transformation could also be induced by grinding samples containing t-ZrO₂. By heat treating, the fracture toughness of all compositions increased with increasing matrix grain size until the matrix grain size exceeded ∼ 1 μm, whereupon both the fracture toughness and strength decreased sharply. The matrix phases, lithium metasilicate and β-eucryptite, have either high thermal expansion mismatch or high thermal expansion anisotropy resulting in large thermal stresses. The initial toughness increases observed in each composition were attributed to the formation of a microcrack zone around the propagating crack. At larger grain sizes, thermal stresses caused spontaneous cracking and loss of strength. Zirconia additions also contributed to the fracture toughness improvement; however, the predominant toughening mechanism was not by transformation but due to crack deflection by the stress fields around the transformed, i.e. m-ZrO₂, particles.     

Comparison of fibres for creep strengthening of zinc-aluminium foundry alloys

A comparative evaluation is made of a variety of possible fibrous reinforcements for strengthening zinc-aluminium foundry alloys. The composites are processed by squeeze casting, using preforms of alumina, carbon, stainless steel or low carbon steel fibres. A drastic improvement of the creep strength is achieved with the use of alumina or steel fibres. However, an acceptable level of fracture toughness is maintained only in the composites reinforced with steel fibres. This property results from the low interface adhesion which allows bridging of the crack by the fibres. Low carbon steel fibres do not exhibit more interface reaction than stainless steel fibres. It is concluded that low carbon steel fibres provide a better compromise when taking into account the creep strength, the fracture toughness and the cost of the composite.     

Compositional effects on the fracture behaviour of alkali–silicate glasses

The two-point bend strength and the fracture toughness of a series of soda–potassia–silicate and soda–potassia–calcia–silicate glass fibres have been measured. There is a clear variation of mean strength with composition for the soda–potassia–silicate glasses, however, there is much less variation of mean strength with composition for the soda–potassia–calcia–silicate glasses. There is also a greater variation of fracture toughness with composition for the soda–potassia–silicate glasses than for the soda–potassia–calcia–silicate glasses. The mean strength, fracture toughness and inferred flaw sizes for the soda–potassia–calcia–silicate glasses are all less than the equivalent values for the soda–potassia–silicate glasses. These results are related to the structural models and durability of the glasses tested.     

Niobium coating effects on the tensile strength of sapphire fibres

Using a modified Weibull analysis to incorporate censored fracture data from fibres containing multiple flaw populations, this study has examined the effect of niobium coatings on the room temperature tensile strength of sapphire fibres. Fibre strengths were limited by failures which occurred predominantly from a combination of surface flaws, which are abrasion-induced, and internal voids, which form during the fibre growth process. Heat treatment of as-sputtered coated fibres at 1375°C caused a significant (∼36%) strength degradation. Unexpectedly, the cause of the strength degradation was traced to internal void growth which occurred only in the coated specimens.     

Development of mechanical properties in AA 8090 alloy produced by extrusion processing

Abstract

The effects of extrusion processing parameters on the mechanical properties of an AA 8090 alloy were monitored using a combination of hardness, tensile, andfracture toughness tests, and using light, transmission electron microscopy, and scanning electron microscopy. It was found that variations in the processing parameters affect the tensile properties to a greater extent in the as extruded condition than in the heat treated condition. In the former, the property changes occur as a result of both variation of grain structure and the solutionising effect during the process. In the latter, the tensile properties are controlled by the precipitation processes that occur, and the toughness remains essentially unaffected by changes in the processing conditions. Improved combinations of strength, ductility, and toughness are achieved when the material is subjected to suitable preaging treatments, which modify the precipitate morphology within the microstructure; the fracture surface characteristics of both tensile and fracture toughness test specimens reflect the microstructural changes.

MST/1115     

The strength and fracture toughness of polycrystalline magnesium oxide containing metallic particles and fibres

The transverse rupture strength of hot-pressed and annealed composites of magnesium oxide and dispersed metallic phases (nickel, iron, cobalt) increases with increasing volume fraction of metal and annealing temperature. The strengthening effect of the metal is attributed to an inhibition of grain growth while flaw healing occurs during the annealing of the composites. The strength of magnesium oxide hot-pressed with nickel fibres is not affected by the volume fraction of fibre or the annealing temperature, and is comparable to the strength of porous magnesia. However, the work of fracture, though insensitive to heat-treatment, increases by at least two orders of magnitude for a moderate volume fraction of randomly oriented fibres. Mechanisms of energy absorption during the fracture of composites containing weakly bonded, non-aligned fibres are discussed. They include the work done in plastically deforming the fibre as it is withdrawn from its socket. It is concluded that this mechanism may be of importance in composites containing very weakly bonded ductile fibres.     

Effects of testing temperature and thermal treatments on some mechanical properties of a Si3N4–TiN composite

Strength and fracture toughness of an electroconductive hot-pressed Si₃N₄–35vol.% TiN ceramic composite were evaluated in air as a function of testing temperature up to 1200 °C. The toughness already shows a clear decrease at 800 °C and then remains almost constant, and the flexural strength steadily decreases with increasing testing temperature. At 1200 °C, the strength value is about 40% of that measured at room temperature. After thermal treatments in air (800, 1000 and 1200 °C) and argon (1200 °C) for 100 h, the Young's modulus, hardness, fracture toughness and flexural strength were measured at room temperature and compared to the baseline material. Young's modulus and hardness remain unchanged. The fracture toughness does not show any clear trend with the treatment temperature, while the strength, which is unaffected by the thermal treatment in argon, decreases with increasing treatment temperature in air. The long-term oxidation involves microstructural changes at the surface and in the bulk, such as the formation of a surface oxide layer and a porous sub-layer. In the bulk, the main modification is the partial crystallization of the grain boundary phase.     

Mixed resin and carbon fibres surface treatment for preparation of carbon fibres composites with good interfacial bonding strength

The objective of this work is to improve the interlaminar shear strength of composites by mixing epoxy resin and modifying carbon fibres. The effect of mixed resin matrix’s structure on carbon fibres composites was studied. Anodic oxidation treatment was used to modify the surface of carbon fibres. The tensile strength of multifilament and interlaminar shear strength of composites were investigated respectively. The morphologies of untreated and treated carbon fibres were characterized by scanning electron microscope and X-ray photoelectron spectroscopy. Surface analysis indicates that the amount of carbon fibres chemisorbed oxygen-containing groups, active carbon atom, the surface roughness, and wetting ability increases after treatment. The tensile strength of carbon fibres decreased little after treatment by anodic oxidation. The results show that the treated carbon fibres composites could possess excellent interfacial properties with mixed resins, and interlaminar shear strength of the composites is up to 85.41 MPa. The mechanism of mixed resins and treated carbon fibres to improve the interfacial property of composites is obtained.     

电热作用对碳纤维树脂基复合材料力学性能的影响

采用复合材料电热实验平台,测试碳纤维树脂基复合材料（Carbon Fiber Reinforced Polymer,CFRP）电热作用下温度场变化规律,同时从单丝拉伸断裂界面剪切强度、短梁剪切性能变化和剪切断口等多方面揭示电热作用对CFRP力学性能的影响机制。结果表明:电热作用会使CFRP整体温度迅速升高,在约4 min时达到稳态温度,随着电流强度的增大,CFRP层板表面温度越高,当电流强度为8 A（0.44 A/mm2）时,CFRP的表面温度达到151℃;单丝拉伸和短梁剪切界面强度都随着电流强度增加呈现先增加后降低的趋势;小电流时,电热作用产生较少的焦耳热,优化界面性能,提高界面剪切强度,大电流时,电热作用产生的焦耳热过大,对界面产生烧蚀等不可逆损伤,降低了界面结合性能。      

Mechanical properties of carbon fibre-reinforced glasses

Carbon fibre-reinforced glasses exhibit very high values of flexural strength but usually a much less controlled fracture behaviour than SiC fibre-reinforced glasses. Some carbon fibre/glass composite combinations show a well controlled fracture, others a brittle fracture behaviour. The former combinations occasionally exhibit an increase in strength after an abrupt breakdown from the maximum strength. No correlation exists between the strength of the composites and the stresses in the glass matrix due to the thermal expansion mismatch between carbon fibres and glasses in contrast to the SiC fibre composites. The reason for that is seen in the structure of the surface and mainly in the anisotropic properties of the fibres, such as the large differences in the Young's moduli and thermal expansion coefficients parallel and perpendicular to the fibre axis. In particular, no radial compressive stress on the fibres can be built up at the fibre/glass interface because the thermal expansion coefficient of the fibres in the radial direction is much larger than that of the glass matrices used. Thus, the mechanism of load transfer from the matrix to the fibres is a complicated one, and cannot easily be predicted as in the case of the isotropic SiC fibres. A possible mechanism is described in order to interpret the experimental results.     

Can thermally degraded glass fibre be regenerated for closed-loop recycling of thermosetting composites?

Commercially manufactured E-glass fibres were heat-conditioned to mimic the effects of thermal recycling of glass fibre thermosetting composites. Degradation in the strength and surface functionality of heat-treated fibres was identified as a key barrier to reusing the fibres as valuable reinforcement in composite applications. A chemical approach has been developed to address these issues and this included two individual chemical treatments, namely chemical etching and post-silanisation. The effectiveness of the treatments was evaluated for both thermal degraded fibres and corresponding composites. Drastic reduction was observed in the properties of the composites with the heat-conditioned preforms indicating thermally degraded glass fibres have no value for second-life reinforcement without further fibre regeneration. However, significant regeneration to the above properties was successfully obtained through the approach developed in this work and the results strongly demonstrated the feasibility of regeneration of thermally degraded glass fibres for potential closed-loop recycling of thermosetting composites.     

The fracture toughness of composites reinforced with weakened fibres

Fibre fractures which occur near, but not at, the plane of matrix failure in a composite, lead to fibre pull-out during fracture. Energy absorbed in this process contributes directly to the work of fracture and hence to the toughness of the composite.Factors which determine the mean length of fibre pulled out during fracture are discussed for the case of composites reinforced with continuous fibres having variously spaced points of weakness. The presence of such weak points also affects the strength of the composite, but not all composites of the same strength have the same toughness. The greatest toughness for a given strength is always found in composites reinforced with discontinuous fibres.     

Fracture toughness behaviour of a magnesium alloy metal-matrix composite produced by the infiltration technique

Metal-matrix composites comprising short δ-alumina fibres embedded in an Mg 10Al0.4Zn alloy were produced by the squeeze infiltration process, with a fibre volume fraction of 20%. Tensile, hardness and fracture toughness tests were performed at room temperature on both the alloy matrix and the composite in the as-cast as well as in the T6 heat-treated condition. In the as-cast condition it was found that the composite had a markedly increased stiffness, tensile strength and hardness but slightly lower ductility and fracture toughness than the alloy matrix. Whilst a T6 heat treatment improved the mechanical properties of the magnesium alloy matrix, it adversely affected the tensile properties and fracture toughness of the composite.     

Mikromechanisches stochastisches Versagensmodell uniaxial faserverstärkter Verbundwerkstoffe

Micromechanical stochastic failure model of uniaxial fibre-reinforced composites A theoretical model of stress transfer between a transversal isotropic fibre and the surrounding matrix material in a uniaxially fibre-reinforced composite near a single matrix flaw is discussed including friction controlled fibre-matrix interface debonding. The rise of fracture toughness due to frictional fibre sliding is studied accounting for Weibull strength distribution of fibres. The total dissipative work may be used as figure of merit regarding the damage tolerance. A critical evaluation is presented concerning some previous models of local failure probabilities. Numerical results are demonstrated. Conditions for an optimized C/Al-composite are presented.