Fiber Strength Degradation Research Articles

Influence of moisture-induced stress on in situ fiber strength degradation of unidirectional polymer composite

Abstract A three-phase concentric cylinder model is used to estimate moisture-induced internal stress in unidirectional polymer composite. It is found that moisture-induced tensile stress on the fiber is substantial when absorbed moisture level in the polymer matrix is high. It is also shown that the stress-state of the fiber is not significantly changed by inclusion of an interphase layer with properties similar to the matrix. The consequence of moisture-induced internal stress is that glass fibers are subjected to stress corrosion, even under no externally applied stress. It is found, by measuring the size of the fracture mirrors of failed glass fibers in model composites, that the in situ tensile strength of E-glass fibers in an epoxy matrix composite was degraded at a faster rate compared to the case of stress-free aging of glass fibers. Thus it is concluded that moisture-induced tensile stress in glass fibers plays an important role in strength degradation of unidirectional composite under environmental aging.

Strength Loss in Thermally Bonded Polypropylene Fibers

Single fiber experiments to analyze the relationships between fiber structure, fiber properties and bonding conditions were performed with three structurally different polypropylene (PP) fibers. Bonds were formed between pairs of fibers at different temperatures and bond strengths were measured. Fiber strengths were measured before and after bonding to estimate changes caused by the bonding process. Bonded fiber strength was also compared with the strength of single fibers which experienced the same thermal conditions to assess the effect of mechanical damage from pressing fibers together with steel rolls while creating bonds. In all fiber types, significant bonding occurred simultaneously with degradation of fiber strength. The observed reduction in fiber strength was caused mainly by thermal, rather than mechanical, damage during bonding. Fibers with low birefringence skins formed strong interfiber bonds at lower temperatures with smaller losses in fiber strength.

228 単繊維モデル試験片を用いたGFRP応力腐食割れに関する研究 : 樹脂中への拡散現象の定量的評価(OS 高分子および高分子複合材料)

Single fiber fragmentation test have been conducted to examine the degradation of inner fiber strength surrounded by the resin using E-glass/Vinylestr model composite. Stressed and unstressed specimens under the environmental attack by water and acid have been prepared. The effect of diffusion phenomenon into the resin on inner fiber strength is investigated as a function of immersion time. As a result, the strength of fiber in stressed resin was lower than that in unstressed resin. This result indicates that stressed polymer have the diffusion phenomenon accelerated. In reference to each environment conditions (water and acid), the fiber degradation behavior was unchanged. It seemed that the water would only diffuse into the resin, and the acid would not. These experimental results were explained analytically based on Fickian diffusion analysis. It is expected that water diffusion affect the mechanism of the fiber degradation in the earlier stage than acid diffusion.

Fabrication and Testing of Oxide/Oxide Microcomposites with Monazite and Hibonite as Interlayers

Oxide/oxide microcomposites were fabricated and tested to evaluate the effectiveness of monazite (LaPO4) and hibonite (CaAl12O19) as interlayers in sapphire‐reinforced Al2O3‐matrix composites. For interlayer thicknesses of 0.3‐0.5 µm, both interlayers showed evidence of crack deflection; however, debond lengths in hibonite‐coated specimens were limited to just a small fraction of the fiber diameter. Monazite‐coated specimens showed multiple matrix cracks and extensive debonding at the coating/matrix interface. Composite strengths were relatively high for both coatings, considering the fiber strength degradation during processing. The strengths were greater than the calculated matrix cracking stresses. However, the mean strengths were not significantly different from those of the control specimens, although coated composites had higher Weibull moduli. The lack of difference in strength is attributed to porosity in the matrix. The results imply that matrix density needs to be >85% to evaluate novel interface strategies reliably.

Influence of Fiber Structure on Properties of Thermally Point Bonded Polypropylene Nonwovens

Carded webs with a basis weight of 30 g/m2 are prepared from Hercules T-196 and Hercules T-101 polypropylene fibers, then thermally point bonded with "Novonette" rolls at various temperatures and pressures. A subset of samples is studied in relative detail using various methods. In particular, the distribution of bridging fiber strengths after bonding is measured directly. Bonding at higher temperatures reduces single fiber strength and increases its variability while increasing the effectiveness (strength) of the bonds. These competing effects appear to explain the maximum fabric-strength-versus-bonding temperature relationship. Bonding performance differences between these two fibers appear to depend on the fact that nonfailing bonds can be produced at lower temperatures in the T-196, where fiber strength degradation is less, and possibly to a lesser extent on the higher elongation and concave-down curvature of the stress-strain relationship.

Behavior of several interfaces during fatigue crack growth in SiC/Ti–6Al–4V composites

The fatigue crack growth behavior of a large number of continuous fiber-reinforced titanium matrix composite systems with significantly different interfacial characteristics were examined in this study using single-ply specimens. Crack deflection, debonding, and bridging were observed in most composites, even for the ones with strongly bonded fibers. Experimental results supported a stress-based criterion for predicting the debonding behavior. Crack kinking and penetration as well as fiber strength degradation and in situ fiber strength issues have been addressed in the context of preserving the bridging characteristics. General trends in the crack growth response with variations in the interface bond strength have been observed, wherein the normalized bridging stress decreases gradually with an increase in the bond strength. This gradual decrease suggests the possibility of obtaining improved transverse properties without substantial degradation to the longitudinal fatigue crack growth properties.

In-situ Strength Degradation of Glass Fibers in a Pultruded Composite by Environmental Aging

The problem of long-term environmental degradation of ®ber-reinforced composites has been receiving more attention in recent years because of the need to repair and retro®t rapidly deteriorating infrastructures using such materials. It has long been known that the residual tensile strength of glass ®bers decreases after stress corrosion and zero-stress aging [1±4], and in situ ®ber strength degrades in polymerbased composites under environmental aging [5]. Establishing a relationship between the aging process and the statistical ®ber-strength distribution will enable one to more accurately predict the long-term performance of the composite. Using a sub-critical crack growth model, the rate of crack growth on glass ®bers can be described by [1, 6]

Strength degradation of sapphire fibers during pressure casting of a sapphire-reinforced Ni-base superalloy

Copyright 1998 ASM International. This paper was published in Metallurgical and Materials Transactions A: PhysicalMetallurgy and Materials Science, Vol. 29.5, 1527-1530 and is made available as an electronic reprint with the permissionof ASM International. One print or electronic copy may be made for personal use only. Systematic or multiplereproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paperfor a fee or for commercial purposes, or modification of the content of this paper are prohibited.Available on publisher's site at:http://www.asminternational.org/portal/site/www/AsmStore/ProductDetails/?vgnextoid=743cc777b3426210VgnVCM100000621e010aRCRD.

Oxidation Induced Stress-Rupture of Fiber Bundles

The effect of oxidation on the stress-rupture behavior of fiber bundles was modeled. It is shown that oxidation-induced fiber strength degradation results in the delayed failure of the associated fiber bundle and that the fiber bundle strength decreases with time as t-1/4. It is also shown that the temperature dependence of the bundle loss of strength reflects the thermal dependence of the mechanism controlling the oxidation of the fibers. The effect of gauge length on the fiber bundle strength was also analyzed. Numerical examples are presented for the special case of Nicalon™ fibers.

Optical Fiber Strength and Its Relationship to Dynamic Mechanical Properties of Fiber Coatings by Direct Measurements on Fibers

AbstractWhile the telecommunications optical fiber technology is in its third decade of growth, and fibers that are aging-resistant and fatigue-resistant are highly desirable, basic physico-chemical aspects of fiber aging and mechanical strength degradation are not well understood. A key difficulty to understanding fiber strength degradation has been the unavailability of tools for direct assessments of the condition of fiber coatings on aged fibers. In this presentation, we report on our studies of the effects of DI-water aging on two experimental high n-value (fatigue-resistant) fibers and their coatings. We have found that the coatings of these high n value fibers are highly resistant to DI-water aging.

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized.

Effect of silicon additions on characteristics of carbon fiber reinforced aluminum composites during thermal exposure

The effects of Si additions on the behavior of high modulus carbon fiber reinforced aluminum matrix (CF/Al) composites during thermal exposure at 773 K for different times have been investigated. The composites were fabricated via hybridization with a small volume fraction of SiC particles using a pressure-casting process. The change of longitudinal tensile strength, the strength degradation of carbon fibers, and the microstructural observations on the interfaces of CF/pure Al composites and CF/Al-Si composites after thermal exposure undoubtedly indicate that the alloying element Si in an aluminum matrix can effectively prohibit the interfacial reactions at the fiber/aluminum interface and has positive effects on the characteristics of CF/Al composites.

Strength degradation of lightguide fibres in roomtemperature water

A fused silica optical fibre which shows 25% strength degradation in 2.3 days at 100°C, shows similar degradation after 9 years at 25°C. This is predictable using an activation energy of 90 kJ.mol-1.

Mechanical Behavior of SiC(f)/SiC Composites and Correlation to in situ Fiber Strength at Room and Elevated Temperatures

Mechanical properties of Nicalon‐fiber‐reinforced silicon carbide matrix composites were evaluated, in flexure, at various temperatures ranging from ambient to 1300°C. First matrix cracking stress ranged from 250 to 280 MPa and was relatively insensitive to test temperature. The measured ultimate strength showed a small increase from a room‐temperature value of 370 to 460 MPa at 800°C. Beyond 800°C, however, strength dropped to as low as 280 MPa at 1300°C. This decrease in ultimate strength at elevated temperatures is believed to be partly due to degradation of in situ Nicalon fiber strength. Scanning electron microscopy was employed to evaluate the in situ Nicalon fiber strengths via fracture mirror size measurements. Degradation of Nicalon fiber strength is attributed to thermal damage and to structural changes to the fiber at elevated temperatures. Measured values of ultimate strength of the composites were compared with predictions made on the basis of in situ fiber strength characteristics and an available analytical model.

Tensile behavior of AI2O3/FeAI + B and AI2O3/FeCrAIY composites

The feasibility of Al2O3/FeAl + B and Al2O3/FeCrAlY composites for high-temperature applications was assessed. The major emphasis was on tensile behavior of both the monolithics and composites from 298 to 1100 K. However, the study also included determining the chemical compatibility of the composites, measuring the interfacial shear strengths, and investigating the effect of processing on the strength of the single-crystal A12O3 fibers. The interfacial shear strengths were low for Al2O3/FeAl + B and moderate to high for Al2O3/FeCrAlY. The difference in interfacial bond strengths between the two systems affected the tensile behavior of the composites. The strength of the A12O3 fiber was significantly degraded after composite processing for both composite systems and resulted in poor composite tensile properties. The ultimate tensile strength (UTS) values of the composites could generally be predicted with either rule of mixtures (ROM) calculations or existing models when using the strength of the etched-out fiber. The Al2O3/FeAl + B composite system was determined to be unfeasible due to poor interfacial shear strengths and a large mismatch in coefficient of thermal expansion (CTE). Development of the Al2O3/FeCrAlY system would require an effective diffusion barrier to minimize the fiber strength degradation during processing and elevated temperature service.

Effects of Composite Processing on the Strength of Sapphire Fiber-Reinforced Composites

The current interest in tough, high-temperature materials has motivated fiber coating development for sapphire fiber-reinforced alumina composites. For this system, it has been demonstrated that the interfacial properties can be controlled with coatings which can be eliminated from the interface subsequent to composite consilidation. However, these fugitive coatings can contribute to the high temperature strength degradation of sapphire fibers. Such degredation,which compromises the composite strength and toughness, is the focus of the current investigation. It has been observed that, in some cases, by selecting appropriate composite processing conditions, such effects can be minimized. But overcoming fiber strength loss remains an important issue.

Effects of composite processing on the strength of sapphire fiber-reinforced composites

The current interest in tough, high-temperature materials has motivated fiber coating development for sapphire fiber-reinforced alumina composites. For this system, it has been demonstrated that the interfacial properties can be controlled with coatings which can be eliminated from the interface subsequent to composite consilidation. However, these fugitive coatings can contribute to the high temperature strength degradation of sapphire fibers. Such degredation,which compromises the composite strength and toughness, is the focus of the current investigation. It has been observed that, in some cases, by selecting appropriate composite processing conditions, such effects can be minimized. But overcoming fiber strength loss remains an important issue.

Interface formation in carbon fibre reinforced magnesium alloys (AZ91)

In metal matrix composites the adhesion between a ceramic fibre and the metal matrix should be submitted on the atomic scale to physical or chemical forces, without forming a reaction zone [1, 2]. Only in this case will fibres remain undamaged and a load transformation from the fibre to the matrix take place during composite application. For different metal matrix composites different processing methods have been developed to achieve this interface structure. In SiC-fibre reinforced titanium alloys the fibres are protected by a carbon coating [3, 4] and the mechanical properties of the composites remain unchanged as long as the protective coating prevents the reaction of titanium with the fibre surface [4]. In alumina fibre reinforced aluminium alloys fibre wetting and adhesion is submitted to atomic forces and can be supported by magnesium segregation in the interface [5, 6]. Magnesium atoms take up vacancy positions in the alumina surface, reducing the wetting angle during melt infiltration [6]. Thermal loading of these composites results in a fibre-matrix reaction that leads to a fibre and composite strength degradation [5, 6]. In carbon fibre reinforced magnesium alloys processed by hot pressing of fibre layers and matrix foils only unsufficient wetting and low adhesion between fibre and matrix result. At positions with moderate adhesion a thick magnesia layer was observed in these composites [7-9]. The magnesia formation was interpreted to arise from the oxidation of the matrix during processing, which was performed in an oxygen rich environment. This letter reports the results of transmission electron microscopy (TEM) studies on the interfaces of carbon fibre reinforced (Toray 300) magnesium alloys (AZ91:9 at% Zn, 1 at% A1, Mn, Si and Cu impurities) processed by squeeze casting of chopped carbon fibre preform bonded by an SiO2 binder (4 wt %). The fibre preform was heated to 600 °C before melt infiltration, which took place at 812 °C. Composites with a fibre volume fraction of 20% were obtained [10]. In the preform the fibres have a preferential orientation in a plane perpendicular to the infiltration direction. For TEM investigations sample preparation was performed parallel and perpendicular to the preferential fibre orientation plane. Preparation was performed in the usual way by grinding,

Effect of Cleaning and Abrasion‐Induced Damage on the Weibull Strength Distribution of Sapphire Fiber

Fractographic analysis revealed the presence of concurrent flaw populations in sapphire fibers which were tensile tested in the as‐received condition (sized and unsized) and after various cleaning procedures. The following flaw populations were identified: surface flaws attributed to handling and abrasion damage (type A), volume or internal flaws attributed to shrinkage voids which form during the manufacturing process (type B), localized fiber surface reaction flaws introduced during the flame‐cleaning procedure (type C), and self‐abrasion surface flaws intentionally introduced on unsized fibers (type D). The strength distribution associated with each flaw type was characterized using a censored data Weibull analysis for both the least‐squares and maximum‐likelihood estimation methods. The strength distribution for type C (flame‐cleaning) flaws exhibited an approximately 20% degradation in strength compared to the distribution for type A flaws. The strength distribution for type D (self‐abrasion) flaws exhibited an approximately 35% degradation in strength compared to the strength distribution for type A flaws. This result underscores the need for fiber sizings to prevent damage during shipping and handling. However, higher purity sizings and/or improved procedures for sizing removal are required to mitigate cleaning‐induced fiber strength degradation during composite fabrication.

Characteristics of several carbon fibrereinforced aluminium composites prepared by a hybridization method

The properties and microstructures of several high-strength and high-modulus carbon fibrereinforced aluminium or aluminium alloy matrix composites (abbreviated as HSCF/Al and HMCF/Al, respectively, for the two types of fibre) have been characterized. The composites evaluated were fabricated by pressure casting based on a hybridization method. It was found that the strength degradation of high-modulus carbon fibres after infiltration of aluminium matrices was not marked and depended upon the type of aluminium matrix. However, the strength of high-strength carbon fibres was greatly degraded by aluminium infiltration and the degradation seemed to be independent of the type of aluminium matrix. The longitudinal tensile strength (LTS) of CF/Al composites was very different between HMCF/Al and HSCF/Al composites. The HMCF/Al composites had LTS values above 800 MPa, but the HSCF/Al composites had only about 400 MPa. In contrast, the transverse tensile strength of the HSCF/Al composites, above 60 MPa, was much higher than that of the HMCF/Al composites, about 16 MPa. Chemical reactions were evident to the interface of high-strength carbon fibres and aluminium matrices. There was no evidence of chemical products arising between high-modulus carbon fibres and Al-Si alloy and 6061 alloy matrices. However, it was considered that some interfacial reactions took place in pure aluminium matrix composites. Fracture morphology observation indicated that the good LTS of CF/Al composites corresponded to an intermediate fibre pull-out, whereas a planar fracture pattern related to a very poor LTS and fibre strength transfer. The results obtained suggested that interfacial bonding between carbon fibres and aluminium matrices had an important bearing on the mechanical properties of CF/Al composites. An intermediate interfacial bonding is expected to achieve good longitudinal and transverse tensile strengths of CF/Al composites.