Pull-out Length Research Articles

Tensile Strength Deterioration of Short-Glass-Fiber Reinforced Thermoplastics by Addition of a Slight Amount of Inorganic Agent

This paper concerns a micromechanical study of the tensile strength deterioration of short-glass-fiber reinforced thermoplastics by addition of a slight amount of inorganic agent. Tensile tests were conducted using short E-glass fiber reinforced polyamide 6 with a slight amount of the inorganic addition agents TiO2, ZnO and ZnS. It is found by the tensile tests that the tensile strength decreases with increasing the hardness of the inorganic addition agents, and scarcely depends on the amount of the inorganic addition agents. After measuring the pull-out length of glass fibers on the fracture surfaces of test specimens, the variation of the scale parameter of Weibull moduli is estimated from the cumulative probability of pull-out length. Finally, the tensile strength deterioration is numerically predicted using the data. The predicted values of tensile strength are consistent with the experimental ones.

High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.

Earthquake Resistant Design of Reinforced Soil Structures Using Pseudo Static Method

Problem statement: This study presented a method to evaluate the internal stability of reinforced soil structures against tension and pullout modes of failure using pseudo-static method for earthquake conditions. Approach: Using limit equilibrium method and assuming the failure surface to be logarithmic spiral, analysis was conducted to maintain internal stability against both tensile and pullout failure of the reinforcements. For the seismic conditions, factors of safety of all the geosynthetic layers in relation to tension and pullout failure modes were determined for different magnitudes of friction angle of backfill, horizontal seismic accelerations and surcharge load acting on the wall. Results: The efforts had been made to obtain the number of layers, pullout length and total length of the reinforcement at each layer level for the desired safety level against tension and pullout modes of failure. The influence of friction angle of the backfill, horizontal earthquake acceleration and surcharge load on number of layers, pullout length and total length of the reinforcement needed for the stability at each level was discussed. Conclusion/Recommendations: The developed method provided a closed form solution for the active earth pressure acting on the reinforced soil structures using rotational log-spiral failure mechanism under earthquake loading ensuring safety against tension and pullout modes of failure.

Tensile properties of carbon nanotube reinforced aluminum nanocomposite fabricated by plasma spray forming

Uniaxial tensile tests were performed on plasma spray formed (PSF) Al–Si alloy reinforced with multiwalled carbon nanotubes (MWCNTs). The addition of CNTs leads to 78% increase in the elastic modulus of the composite. There was a marginal increase in the tensile strength of CNT reinforced composite with degradation in strain to failure by 46%. The computed critical pullout length of CNTs ranges from 2.1 to 19.7 μm which is higher than the experimental length of CNT, leading to relatively poor load transfer and low tensile strength of PSF nanocomposites. Fracture surface validates that tensile fracture is governed strongly by the constitutive hierarchical microstructure of the plasma sprayed Al–CNT nanocomposite. The fracture path in Al–CNT nanocomposite occurs in Al–Si matrix adjacent to SiC layer on CNT surface.

Stress-Strain Response in SiC/SiC Composites under Cyclic Loading

The tension-tension fatigue tests for SiC/SiC composites were performed under the conditions that the maximum load Pmax was 80-90% to the fracture load of the tensile tests and the stress ratio was Rσ = 0.5. The composites exhibited a width in stress-strain hysteresis loop under one load cycling. In some cases the mean strain εmean gradually increase with increasing in number of cycles. These variations would reflect the developments of the fatigue damage at the fiber/matrix interface during the cyclic loading process. The pull-out lengths of the fibers for the fatigued- and not fatigued-specimens were measured through the SEM observations after the tensile test. In all materials, the average pull-out length of fibers in fatigued material was larger than in not fatigued material because the cyclic loading affected on the fiber/matrix interfacial strength.

Crack Bridging in Polymer Nanocomposites

Carbon nanotube reinforced composites offer enhancements in fracture properties since the reinforcing nanotubes provide a bridging mechanism to resist crack growth. In this paper, a study of crack bridging by nanotubes in a nanotube-reinforced polymer composite is presented. The process of crack bridging is idealized as normal pullout of the participating nanotubes from the polymer matrix. The resistance to crack growth due to bridging is taken as the aggregate of the resistance offered by all the nanotubes, ignoring any interaction among the nanotubes themselves. The pullout of a single nanotube from the polymer matrix is modeled as an axisymmetric, nearly one-dimensional problem. This is done by assuming that fracture along the nanotube–polymer interface is dominated by shear openings, and that the nanotube behaves as a rigid body. When the polymer is a linear elastic material, the force–displacement relation for pullout is obtained as a function of dimensionless variables representing the interfacial fracture energy and the pullout length scale. Applying the correspondence principle, the elastic results are extended to the case where the polymer is a linear viscoelastic material with a single relaxation time. The force–displacement relation is then a function of the viscoelastic properties of the polymer and the pullout velocity as well. Using these results, the apparent enhancement in the fracture energy of the composite is obtained. This provides a guideline to design these composites for desired fracture properties in terms of the interfacial strength of the nanotube–polymer interface and the volume fraction of the nanotubes. Results of numerical simulations of nanotube pullout are compared to the predictions of the analytical model.

Distribution of fibre pullout length and interface shear strength within a single fibre bundle for an orthogonal 3-D woven Si–Ti–C–O fibre/Si–Ti–C–O matrix composite tested at 1100 °C in air

The distributions of fibre strength, pullout length, and fibre/matrix interface shear strength within a single fibre bundle were investigated for a 3-D woven SiC/SiC composite tensile tested at 1100 °C in air. Fibre pullout lengths were largest at the fibre bundle centre with an embrittled region of approximate width 15 μm at the perimeter. Whereas the fibre strength varied by less than a factor of 2 across the fibre bundle, the fibre/matrix interface shear strength varied by a factor of ∼23 with a minimum (100±16 MPa) at the centre and a maximum (2.25±0.21 GPa) close to the embrittled region. The minimum fibre/matrix interface shear strength required for the transition between pseudo-ductile and brittle behaviour was thus estimated to be 2.25±0.21 GPa for this composite system.

Degradation of SiC/SiC composite due to exposure at high temperatures in vacuum in comparison with that in air

Abstract Room temperature residual strength of the SiC/SiC composite exposed in vacuum at high temperatures (823–1673 K) was studied and compared with that exposed in air. The vacuum-exposed composite showed only the fiber-pullout type fracture, and the pullout length increased with increasing exposure temperature and time, while the fractured mode of the air-exposed one changed with progressing oxidation; from the fiber-pullout type to the nonfiber-pullout one characterized by the overall fracture perpendicular to the tensile axis without fiber-pullout. The reduction in residual strength in the case of vacuum exposure was attributed mainly to the extension of the decomposition-induced defects on the fiber surface into fiber, while that in the case of air exposure mainly to the extension of the crack made by premature fracture of the SiO 2 layer into the fiber. A simple model based on the kinetics of the growth of the defects and fracture mechanics was presented to describe the variation of composite strength as a function of exposure temperature and time for the vacuum exposure, which could describe the experimental results.

Aspects of alkali treatment of sponge gourd (Luffa cylindrica) fibers on the flexural properties of polyester matrix composites

AbstractSponge gourd (Luffa cylindrica) forms a natural mat that deviates the crack path in brittle thermoset resin matrix composites, leading to a controlled fracture mode and increasing the toughness of the composite. The use of luffa as reinforcement is, however, restricted by a very weak fiber–matrix interface. In this work, luffa fibers were alkali‐treated at two temperatures, with varying alkali concentrations. Although the surface analysis shows that the treatments promote a clear removal of the outer surface layer of the fibers with the exposition of the inner fibrillar structure and the consequent increase of the fiber surface area, only a secondary increase on the mechanical properties was obtained. The slight increase observed was attributed only to mechanical interlock. Long pullout lengths and neat fiber beads were the main features observed at the fracture surface of the treated and untreated fiber composites. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1927–1932, 2003

Fatigue Behaviour of Unidirectional Single-Walled Carbon Nanotube Reinforced Epoxy Composite under Tensile Load

Tension-tension fatigue behaviour of unidirectional, aligned single-walled carbon nanotube (SWNT) rope reinforced epoxy composites were studied. While the slope of the stress-life (S-N) curve of the SWNTs in SWNT/epoxy composites obtained is flat, similar to those of carbon fibre reinforced epoxy composites, the fatigue strength of the former is at least twice that of the latter. Morphology of the fatigue fracture surface of SWNT reinforced epoxy involves matrix plastic deformation and SWNT-bridged matrix cracks. Evidence of good adhesion between SWNTs and epoxy was seen, and pullout length of SWNTs from the matrix is about 30 μm. Results suggest that carbon nanotubes can be used for fatigue resistant, high fracture toughness composites.

938 SiC/SiC 複合材料の引張強さと引抜け長さの評価

938 SiC/SiC 複合材料の引張強さと引抜け長さの評価

Notched strength of satin-woven Si-Ti-C-O fiber-bonded ceramic composite

The fracture behavior of Si-Ti-C-O fiber-bonded ceramic composite produced by hot-pressing oxidized 8 harness-satin-woven Si-Ti-C-O fibers was investigated by using unnotched and double edge notched tensile test specimens with different width (8 and 40 mm). The main results are summarized as follows. (i) The tensile strength of unnotched specimens for 8 mm width was higher than that for 40 mm width. Such a width-dependence of the unnotched strength could be described fairly well from the viewpoint of effective volume by application of the experimentally estimated Weibull's shape parameter. (ii) The applicability of the fracture toughness criterion (fracture arises when the stress intensity factor reaches the critical value) and net section stress criterion (fracture arises when the strength of the ligament reaches the unnotched strength) to the present composite was examined. The fracture strength of a notched specimen for 8 mm width was described by the net stress criterion. On the other hand, the strength for 40 mm width obeyed the net stress criterion for a small notch length but it shifted toward the fracture toughness criterion for large one. The shift of the fracture criterion from net strength- to fracture toughness-criterion arose around at the relative notch length 0.2 (notch length 8 mm), corresponding to periodical spacing of fiber strands (8 harness). (iii) The fiber pull-out length (0.4 mm on an average) was nearly the same as the half length of the fiber strand whose deformation is not constricted by the other strands in the satin-weave. (iv) The present fiber-bonded ceramic composite is insensitive to notch under the condition where the width of specimen is narrow and the notch length is smaller than 8 mm. This composite could be therefore applicable to industrial objects safely when the objects are designed as to satisfy the notch-insensitive condition.

Temperature dependence of crack wake bridging stresses in a SiC-whisker-reinforced alumina

The crack face bridging behavior of a SiC-whisker-reinforced alumina composite was characterized between room temperature and 1400°C in air. The bridging relations of the material were deconvoluted from the chevron-notched specimen R-curves by employing the fracture mechanics weight function method and assuming that pullout bridging was the dominant bridging mechanism. The results indicated that the maximum bridging stress decreased linearly with increasing temperature while the distribution of the bridging stresses appeared to shift towards larger crack opening displacements. The former trend was found to be in good agreement with literature data on the temperature dependence of residual stresses measured in similar composites, while the latter agreed with the observation of an increasing whisker pullout length in such composites with increasing temperature. In agreement with previous studies of this material, some crack-tip toughening due to a microcrack damage zone formed around the crack-tip by diffusional cavitation could be observed at T>1000°C. Although the material exhibited toughening by both a microcrack zone and a bridging zone between 1000°C and 1300°C, no influence from the microcrack zone on the bridging stresses was detected.

Mechanical Behavior for 3-D Orthogonal Woven E-Glass/Epoxy Composites

A testing program is conducted to investigate the mechanical properties and failure mechanism for 3-D orthogonal woven E-glass/epoxy composite materials, which were manufactured using E-glass for fiber and Ciba-Geigy GY260 epoxy for matrix. For the mechanical property, the testing results indicate that at the loading range of 0 to 4 KN, the inplane Young’s modulus and Poisson’s ratio obtained using the strains recorded on the matrix surfaces are close to those recorded on the filler yarn surfaces. However, for the in-plane shear modulus, the difference between data from these two surfaces is obvious. The average elastic constants from the tests compare favorably with the predicted results using the laminate block model developed previously [1]. For the tensile failure strength, there is a reasonably good agreement between the measured and predicted results using the rule of mixture. Observation of the specimen fracture surfaces reveals that for a specimen cut along the x or y direction, its fracture surface is generally perpendicular to the applied loading direction. However, for a specimen cut along the 45° with respect to the stuffer yarn direction, the fracture surface is generally parallel to the stuffer yarn. The larger pullout lengths for the failure yarns are obviously observed from the specimen fracture surfaces, but yarn pullouts were not observed for the 3-D orthogonal woven CFRP composites [2].

In‐Plane Fracture Resistance of a Crossply Fibrous Monolith

The in‐plane fracture resistance of a crossply Si3N4/BN fibrous monolith in the 0°/90° and ±45° orientations is examined through tests on notched flexure specimens. The measurements and observations demonstrate the importance of fiber pullout following fiber fracture. The mechanical response is modeled using a crack‐bridging approach. Two complementary approaches to evaluating the bridging law are developed: one based on a micromechanical model of fiber pullout and the other based on the load versus crack mouth opening displacement response of the flexure specimens following fracture of all fibers. Both approaches indicate that the bridging law follows an exponential form, characterized by a bridging strength and an effective pullout length. An assessment of the bridging model is made through comparisons of simulations of the load–displacement response with those measured experimentally.

Mechanical Behavior for 3-D Orthogonal Woven E-Glass/Epoxy Composites

A testing program is conducted to investigate the mechanical properties and failure mechanism for 3-D orthogonal woven E-glass/epoxy composite materials, which were manufactured using E-glass for fiber and Ciba-Geigy GY260 epoxy for matrix. For the mechanical property, the testing results indicate that at the loading range of 0 to 4 KN, the inplane Young’s modulus and Poisson’s ratio obtained using the strains recorded on the matrix surfaces are close to those recorded on the filler yarn surfaces. However, for the in-plane shear modulus, the difference between data from these two surfaces is obvious. The average elastic constants from the tests compare favorably with the predicted results using the laminate block model developed previously [1]. For the tensile failure strength, there is a reasonably good agreement between the measured and predicted results using the rule of mixture. Observation of the specimen fracture surfaces reveals that for a specimen cut along the x or y direction, its fracture surface is generally perpendicular to the applied loading direction. However, for a specimen cut along the 45° with respect to the stuffer yarn direction, the fracture surface is generally parallel to the stuffer yarn. The larger pullout lengths for the failure yarns are obviously observed from the specimen fracture surfaces, but yarn pullouts were not observed for the 3-D orthogonal woven CFRP composites [2].

Temperature-Dependence of Mechanical Properties of Si-Ti-C-O Fiber-Bonded Ceramic.

Temperature-dependence of various mechanical properties of Si-Ti-C-O fiber-bonded ceramic is investigated. The material was produced by hot-pressing the laminae of oxidized satin-woven Si-Ti-C-O fibers, and then showed very high fiber-volume fraction (around 0.85) and very little porosity, compared with ordinary ceramic matrix composites. The in-plain tensile, compressive and inter-laminar shear strengths of the material at room temperature were maintained up to 1400°C, whereas the Young's modulus and the compressive strength of out-plain direction (transverse direction) decreased at 1400°C. The behavior toward the tensile and inter-laminar shear strengths were in good agreement with the changes in fiber pull-out length and delaminating position, respectively. A buckling of the fibers parallel to the stress axis and a large deformation of the specimen were observed in the fracture surface after compression tests of in- and out-plain directions at 1500°C, respectively. From the differences in the mechanical properties and fracture morphologies, it was concluded that the change in the fracture strength at the high temperature was caused by softening of the amorphous SiO2 phase.

Fiber/matrix interface shear strength estimated from fiber pullout length data for Tyranno® Si-Zr-C-O fiber composites with different SiC-based matrices and interfaces

Fiber/matrix interface shear strength estimated from fiber pullout length data for Tyranno® Si-Zr-C-O fiber composites with different SiC-based matrices and interfaces

Mechanical Behavior of Advanced Organic Composites in Space Environmental Condition(Student Poster Session)

We investigate the mechanical behavior of the fiber reinforced organic composites in high vacuum and temperature condition. The material used in this study is short carbon fiber reinforced thermoplastic polyimide composites (SCFR-PI) with various fiber contents. Tension tests were carried out with the aid of AE method. After the tests, we observed the damage propagation in SCFR-PI specimens by scanning electron microscope photographs and optical microscope photographs. The main results are summarized, as follows: (1) Even though tested at the same temperature 298 K (25℃), the SCFR-PI composite specimen kept in high vacuum condition has higher tensile strength and higher equivalent Young's modulus than that in the atmospheric condition. (2) The fiber pull-out length of the specimen in high vacuum condition is shorter than that in the atmospheric condition and the fracture surfaces are not flat. The strength of the interfacial bonding between carbon fibers and the thermoplastic polyimide resin matrix becomes better in high vacuum condition. The degas and/or moisture desorption may change the performance of each materials and microfractures of SCFR-PI composites. (3) It is found that there exists the effect of vacuum condition on the tensile strength and Young's modulus of SCFR-PI composites and the microfractures.

Dynamic effect on strength in SiC f/SiC m composite

Loading rate dependence of mechanical properties of SiC fibre-reinforced SiC composites (SiC f/SiC m) has been experimentally investigated as to the fibre volume fraction and coating materials for SiC fibre. The composites consisting of monolithic SiC and SiC fibre (Hi-Nicalon) coated with Boron-Nitride (BN) or Carbon (C) with fibre volume fractions of 20, 30 and 40% were fabricated by polymer infiltration–pyrolysis (PIP) process. The stress–strain response and strength were measured in tension over a wide range of strain rate,10 −4∼200 s −1. It was shown that the higher volume fraction, the larger tensile strength regardless of the kind of coating and strain rate. The interface friction stress evaluated by the fibre pullout length that is measured through microscopic observations of fractured specimens is larger in dynamic loading than in static loading. The BN-coated fibre gave the composite superior tensile strength to the C-coated fibre. This trend results from the variety of the interface friction stress associated with the coating thickness.