Fiber Push-out Tests Research Articles

Shear debonding behavior of a carbon-coated interface in a tungsten fiber-reinforced tungsten matrix composite

One of the crucial issues related to structural application of tungsten for fusion reactor components is its brittleness. To improve tungsten toughness we explored a novel toughening method based on W fiber reinforcement. The idea is to utilize the effective energy dissipation caused by controlled cracking and friction at fiber/matrix interfaces. To realize this, the interfaces need to be engineered by means of adequate coating. In this work we investigated fracture behavior of a carbon-coated (0.6μm) interface in a single-filament mini-composite using fiber push-out test. The composite was fabricated by CVD process. Mechanical parameters were determined by fitting the related theoretical models with the experimental data. Calibrated fracture energy and debonding strength was 7.4J/m2 and 285MPa, respectively. This fracture energy value satisfied the theoretical criterion of controlled crack deflection. The result of the carbon coating was compared to the case of uncoated interface which exhibited stronger friction.

The effect of heat treatment temperature on the interfacial shear strength of C/C composites

This paper investigates the effect of heat treatment temperature on the interfacial shear strength (IFSS) of carbon/carbon composites reinforced with polyacrylonitrile-based fibers. A series of single fiber push-out tests were performed on specimens heat treated at 1800, 2100, and 2400 °C, using a nanoindenter with a flat ended conical tip. The microstructure was characterized using polarized light and transmission electron microscopy and the debonded fiber/matrix interface was examined using scanning electron microscopy. Wavelet analysis of the load–displacement data was used as an additional tool to investigate the initiation and progression of debonding. Compared to 1800 °C, heat treatment at 2400 °C was associated with a decrease in IFSS, from 12 to 7 MPa. Transmission electron microscopy study showed that the microstructure of the fiber/matrix interphase remained amorphous even with heat treatment at 2400 °C. The decrease in the IFSS can be partly attributed to the reorganization of the graphene sheets in the matrix in the vicinity of fiber/matrix interphase. The thermal expansion mismatch between fiber and pyrocarbon matrix is another possible reason for the observed decrease in the IFSS.

Feasibility study of a tungsten wire-reinforced tungsten matrix composite with ZrO x interfacial coatings

Brittleness problem imposes a severe restriction on the potential application of tungsten as high-temperature structural material. In this paper, a novel toughening method for tungsten is proposed based on reinforcement by tungsten wires. The underlying toughening mechanism is analogous to that of fiber-reinforced ceramic matrix composites. Strain energy is dissipated by debonding and frictional sliding at engineered fiber/matrix interfaces. To achieve maximum composite toughness fracture mechanical properties have to be optimized by interface coating. In this work, we evaluated six kinds of ZrO x -based interface coatings. Interfacial parameters such as shear strength and fracture energy were determined by means of fiber push-out tests. The parameter values of the six coatings were comparable to each other and satisfied the criterion for crack deflection. Microscopic analysis showed that debonding occurred mostly between the W filament and the ZrO x coating. Feasibility of interfacial crack deflection was also demonstrated by a three-point bending test.

Interfacial fracture behavior of tungsten wire/tungsten matrix composites with copper-coated interfaces

The potential application of tungsten as a structural material has been strongly restricted by inherent brittleness. The hitherto metallurgical efforts to improve tungsten toughness seem to be still less matured. The authors have been exploring a novel toughening technique based on reinforcement by tungsten wires. Toughness is supposed to be enhanced through the energy dissipation at the wire/matrix interfaces which is caused by the controlled crack deflection and friction. In this work, we focus on two kinds of copper coatings for interface engineering, namely, copper single-layer and copper/tungsten multi-layer. Single-filament composites were fabricated using magnetron sputtering and CVD process. The interfacial parameters were identified by means of fiber push-out test and microscopic fracture features were investigated. In this paper the results from the extensive push-out experiments are presented together with the fractographs. Finite element simulation was also carried out to estimate the plastic strain of the copper layer. Essential role of the significant plastic deformation in the overall failure behavior is highlighted.

Fiber push-out study of a copper matrix composite with an engineered interface: Experiments and cohesive element simulation

The fiber push-out test is a basic method to probe the mechanical properties of the fiber/matrix interface of fiber-reinforced metal matrix composites. In order to estimate the interfacial properties, parameters should be calibrated using the measured load–displacement data and theoretical models. In the case of a soft matrix composite, the possible plastic yield of the matrix has to be considered for the calibration. Since the conventional shear lag models are based on elastic behavior, a detailed assessment of the plastic effect is needed for accurate calibration. In this paper, experimental and simulation studies are presented regarding the effect of matrix plasticity on the push-out behavior of a copper matrix composite with strong interface bonding. Microscopic images exhibited significant local plastic deformation near the fibers leading to salient nonlinear response in the global load–displacement curve. For comparison, uncoated interface with no chemical bonding was also examined where the nonlinearity was not observed. A progressive FEM simulation was conducted for a complete push-out process using the cohesive zone model and inverse fitting. Excellent coincidence was achieved with the measured push-out curve. The predicted results confirmed the experimental observations.

The effect of neutron irradiation on the fiber/matrix interphase of silicon carbide composites

Given the good stability of mechanical properties of silicon carbide (SiC) under neutron irradiation, the ultimate irradiation tolerance of SiC composite materials may be limited by the fiber/matrix interphase, which is critically important to the performance of these composites. This study investigates the irradiation stability of pyrolytic carbon (PyC) monolayer and PyC/SiC multilayer interphases by tensile and single fiber push-out test techniques. Neutron irradiation was performed to doses of 0.7–7.7 dpa at temperatures from 380 to 1080 °C. Both interfacial debond shear strength and interfacial friction stress apparently decrease by irradiation, although this is not so dramatic when T irr < 1000 °C. In contrast, the interfacial shear stresses are most affected by the higher temperature irradiation (>1000 °C). Noteworthy, these irradiation effects depend on the type of interphase material, i.e., for the pyrolytic carbon or multilayer SiC variants studied. In the range of irradiation temperature and dose, the degradation in interfacial shear properties, while measurable, is not of a magnitude to degrade the mechanical performance of the composites. This was observed for both interphase types studied. In particular, the proportional limit tensile stress decreases slightly by irradiation while the tensile fracture strength undergoes very minor change.

Fiber push-out tests on Al 2O 3 fiber-reinforced NiAl-composites with and without hBN-interlayer at room and elevated temperatures

Fiber push-out tests were performed on continuous Al 2O 3 fiber-reinforced NiAl-composites with and without hBN-interlayer at room temperature (RT) and elevated temperature (900 °C). The average interfacial shear stress was 248 ± 42 MPa and 72 ± 28 MPa for NiAl/Al 2O 3 and NiAl/BN/Al 2O 3 composites at room temperature, respectively. At 900 °C, the push-out tests were aborted for NiAl/Al 2O 3 composites because of fiber fracture and cracking owing to mechanical twinning on rhombohedral { 1 0 1 ¯ 2 } planes in single crystalline Al 2O 3 fibers, whereas the average interfacial shear stress was 13 ± 7 MPa for NiAl/BN/Al 2O 3 composites. The debonding position along the interface as well as the initiation of debonding on the top surface of NiAl/Al 2O 3 and NiAl/BN/Al 2O 3 composites were determined and discussed. Frictional coefficients of 0.29 and 0.11 for fiber sliding after interface debonding were obtained for NiAl/Al 2O 3 and NiAl/BN/Al 2O 3 composites at room temperature, respectively.

Shear properties at the PyC/SiC interface of a TRISO-coating

The failure behavior of TRISO-coated fuel particles depends significantly on the shear strength at the interface between the inner pyrolytic carbon (PyC) and silicon carbide (SiC) coatings. In this study, a micro-indentation fiber push-out test was applied to measure the interfacial shear properties of a model TRISO-coated tube. Of particular emphasis is that this study developed a non-linear shear-lag model for a transversely isotropic composite material due to insufficiency in the existing isotropic models as applied to layered TRISO-coating systems. In the model, the effects of thermal residual stresses and the roughness-induced clamping stress were identified as particularly important. The rigorous model proposed in this study provides more reasonable data on two important interfacial parameters: the interfacial debond shear strength and the interfacial friction stress. The modified model coupled with experiments yields an interfacial debond shear strength of 240 ± 40 MPa. This high interfacial strength, though slightly lower than that obtained by the existing isotropic model (∼280 MPa), allows significant loads to be transferred between inner PyC and SiC in application. Additionally, an interfacial friction stress of 120 ± 30 MPa was determined. This high friction stress is attributed primarily to the roughness at the cracked interface rather than clamping effects due to differing coefficients of thermal expansion.

The effects of neutron irradiation on shear properties of monolayered PyC and multilayered PyC/SiC interfaces of SiC/SiC composites

The effect of neutron irradiation on mechanical properties at the fiber/matrix interface of SiC/SiC composites was evaluated. The materials investigated were Hi-Nicalon™ Type-S fiber reinforced chemically vapor infiltrated SiC matrix composites with varied interphases: monolayered pyrolytic carbon (PyC) or multilayered PyC/SiC. The neutron fluence was 7.7 × 10 25 n/m 2 ( E > 0.1 MeV), and the irradiation temperature was 800 °C. Interfacial shear properties were evaluated by the fiber push-out test method. A modified shear-lag model was applied to analyze the interfacial shear parameters. Test results indicate that the interfacial debond shear strength and the interfacial friction stress for the multilayer composites were significantly degraded by irradiation. Nevertheless, the multilayer composites retained sufficient interfacial shear properties so that overall composite strength after neutron irradiation was unaffected. The actual mechanism of interphase property decrease for the multilayer composites is unknown. The interfacial shear properties of the irradiated monolayer composites appear unaffected.

Microstructure and mechanical properties of continuous Al2O3 fibre reinforced Ni45Al45Cr7.5Ta2.5 alloy (IP75) matrix composites

Single crystalline Al2O3 fibres (sapphire), coated with the NiAl alloy IP75 by physical vapour deposition (PVD), were assembled to fabricate composites by means of diffusion bonding. The microstructure and chemistry of both as-coated fibre and as-diffusion bonded composites were investigated by electron microscopy and microanalysis. The interface shear stress for complete debonding was measured by fibre push-out tests at room temperature, and the composite tensile strength was measured at 900°C and 1100°C. An amorphous layer with a thickness of about 400 nm formed between the fibre and the matrix during the PVD process and was maintained during diffusion bonding. A Laves phase precipitated along NiAl grain boundaries in the IP75 matrix. This caused a lower tensile strength of the IP75/Al2O3 composite at high temperatures compared to as-cast monolithic IP75 and rendered the composite useless for structural applications.

Effects of fiber-types on braking behavior of carbon–carbon composites

The effects of fiber-types (PAN-T300 (CF), PAN-oxidized fiber (OF)) on the friction and wear behavior of C/C composites were investigated. The C/C composites were fabricated by densifying porous needle-punched felts with isothermal chemical vapor infiltration to 1.40 g/cm 3 followed by phenolic resin impregnation and carbonization till the ultimate density of about 1.75 g/cm 3. Carbon fiber/pyrocarbon interfacial strength was measured by single fiber push-out test. And the flexural strength compressive strength of the composites was tested. The braking behavior was investigated by modeling normal braking conditions of aircraft. The fiber/matrix interfacial strength of the OF-derived composite is higher and the flexural strength and compressive strength is lower than that of the CF-derived composite. The needle-punched felt made from OF, result in C/C composite with higher and much more stable friction coefficient, but higher mass wear, than the composites with the porous preform made from CF. Combining fracture of composite and carbon fiber/matrix interface, a modified friction mechanism is suggested that a suitable flexural strength of the composite and carbon fiber/matrix interfacial strength favors formation of friction films. The friction film can result in higher and more friction coefficient.

Interface structure, chemistry and properties of NiAl composites fabricated from matrix-coated single-crystalline Al 2O 3 fibres (sapphire) with and without an hBN interlayer

NiAl composites with and without an hBN interlayer were produced from matrix-coated single-crystalline Al 2O 3 fibres (sapphire) by diffusion bonding. The evolution of interface structure and chemistry during the fabrication processes (fibre coating, diffusion bonding and embedded casting) was characterized by electron microscopy. The interface shear stress for complete debonding was measured by fibre push-out tests at room temperature. Interface structural analysis by transmission electron microscopy demonstrates that a high interface shear strength (about 230–250 MPa) in the composites without hBN interlayers is achieved by direct contact of NiAl with aluminium oxide (intrinsic coherence). In the composites with hBN interlayers the boron nitride was partially (as-diffusion-bonded composite) or completely (as-cast composite) transformed to AlN owing to a chemical reaction with NiAl at high temperatures. The low interface shear strength (about 70 MPa) of the as-diffusion-bonded composites was caused by sliding of textured hBN basal planes. The low interface shear strength (about 75 MPa) of the as-cast composites was attributed to segregation of aluminium boride to triple junctions and grain boundaries of AlN. The interfacial reactions in the composites with hBN interlayers can be rationalized from thermodynamic calculations.

A theoretical model for electroelastic analysis in piezoelectric fibre push-out test

A theoretical model is presented to study the elastic deformation process and frictional sliding behavior in single piezoelectric fibre push-out tests. Based on the theoretical model and some necessary simplifications, stress and electric fields are obtained for push-out tests of a circular piezoelectric fibre embedded in an elastic matrix. Numerical results of a piezoelectric fibre/expoxy matrix system are presented to verify the proposed formulation. The study shows that there is a significant effect of the piezoelectric parameter and embedded fibre length on stress transfer, electric field distribution and load-displacement curve of the frictional sliding process. This study also indicates that the piezoelectric effect has a distinct influence on the mechanical behavior and properties of the interface in a fibre/matrix system.

2729 一方向SiC/SiC複合材料の組織による引張特性への影響(S28-1 セラミックスおよびセラミックス系複合材料(1),S28 セラミックスおよびセラミックス系複合材料)

The influence of each constituent of SiC/SiC composite, i.e. fiber, interface and matrix, on the fracture mechanism and mechanical properties were experimentally studied. As main factors in manufacturing, the carbon thickness of interface, and the kind of precursor polymer of the matrix, the blending rate of filler particle, and the number of densification processing were interested. As a basic structure of various reinforcing structures, systematic evaluation was carried out for the bundle composites which were composed of a fiber bundle densified with a matrix. The specimens were prepared by chemical vapor infiltration to form an interfacial carbon layer on Tyranno-ZMI fiber and polymer impregnation and pyrolysis to form a matrix in the fiber bundle. The specimens were evaluated by tensile test, fiber push-out test and electron microscopy etc.

Osteon interfacial strength and histomorphometry of equine cortical bone

The interfacial strength of secondary osteons from the diaphysis of the Thoroughbred equine third metacarpal was evaluated using the fiber pushout test. The pushout was performed on 300–500 μm sections of 4×4×15 mm bone blocks machined from four anatomic regions of the cortex. Pushout strength was evaluated from proximal to distal location within the diaphysis on four osteon types classified under polarized light on adjacent histologic sections from each block. The shear strength of the interfaces were estimated from shear lag theory. Differences were found in the interfacial strength of osteons based on appearance under polarized light with bright field having the highest interfacial strength (40.3 MPa). The lowest strength was found in the dark field osteons (22.8 MPa). The dorsal region had the highest shear strength and toughness compared to all other regions. The cement line and interlamellar interfaces are similar in strength, but exhibit regional dependence—specifically, the palmar region strength is less (17.5 MPa) than the osteon interlamellar interfaces (30.4 MPa) and osteon type dependent (alternating significantly weaker than other types). Histomorphometry revealed significant regional differences ( p < 0.0001 ) in osteon area fraction among the four osteon types as well as differences in the osteon diameter ( p = 0.01 ), with dorsal regions having larger osteons (170 μm) than the palmar region (151 μm). Fatigue life and fracture toughness of Haversian bone are reported in the literature to be regionally dependent and are known to be associated with osteon pullout—an osteon interfacial phenomenon. Therefore, the results presented in this study are important to further the understanding of the mechanisms of fragility and damage accumulation in cortical bone.

Free-edge Stress Singularities and Edge Modifications for Fiber Pushout Experiments

A comprehensive investigation of the fiber pushout test reveals that the existence of free-edge stress singularities hinder the accurate measurement of interfacial shear strengths. An integrated analytical, numerical, and experimental investigation is conducted to remove the free-edge stress singularities by modifying the edge design in a model fiber pushout test. The proposed interfacial joint angles are applicable to most composite material systems. The microdroplet test is recommended over the fiber pushout test since the convex shape of the microdroplet matrix makes it a natural choice.

Measurement of fiber/matrix interface properties of C/C composites by single fiber and fiber bundle push-out methods

Interfacial fracture stresses of carbon/carbon composites were measured by indentation methods. Two types of test methods, namely, single fiber push-out, and bundle fiber push-out tests were conducted. Both methods successfully gave fiber/matrix interface mechanical properties, especially debonding behavior. However, when the interface was strong, the single fiber push-out test encountered technical difficulty in processing the extremely thin specimen required to realize the fiber push out. On the other hand, the bundle fiber push-out test gave a good estimation of interfacial fracture stresses.

連続繊維強化ＳＰ７００／ＳＣＳ‐６チタン基複合材料の熱機械的疲労と高温低サイクル疲労の非類似性と類似性

Behavior of thermo-mechanical fatigue (TMF) failure of a new generation titanium alloy matrix composite, SP700/SCS-6, was studied, based on a series of investigations on ; (i) mechanical properties under monotonic loading of the SP-700/SCS-6 composite and the matrix alloy; (ii) fiber push-out tests to represent the fiber/matrix interfacial strength at elevated temperatures ; (iii) isothermal low cycle fatigue (ILCF) tests, (iv) interfacial reaction zone to characterize the microscopic damage evolution under the TMF test and the thermal cycle test without external load. A special focus was paid to understand both the mechanisms of TMF failure, and the dissimilarities and the similarities between the TMF and ILCF failures. From the results of these tests, it was shown that the differences of damage characteristics may be concerned with the degradation of the fiber/matrix interface by the crack propagation and reaction layer due to thermal cycles. These systematic investigations showed that the lives and the failure mechanisms of the TMF should be clearly distinguished from those of the isothermal ILCF, whereas there were a few similarities between them. It was also suggested the difference should be primarily originated from the cracking by the repeat of the thermal cycle and the resultant change in fiber/matrix adhesion strength.

Normal and Abnormal Fracture Behaviors in Low-Cycle Fatigue of a Unidirectionally Reinforced SCS-6/SP-700 Composite

Titanium alloy matrix composites (TMCs) have received considerable interest as structural materials for aeronautical applications, because of their higher specific strength and stiffness. When applying TMCs at elevated temperatures, high temperature isothermal low-cycle fatigue (LCF)failure is one of critical issues to be concerned. A unidirectionally reinforced SCS-6/ SP-700 composite is a tentative target in this work, where the matrix alloy, SP-700 is a new generation high strength Titanium alloy developed by NKK Inc., and the SCS-6 is a beta-SiC fiber developed by Textron Specially Materials, respectively. A merit to employ the SP-700 is that this alloy enables to reduce a fabrication temperature, because of its capability for superplasticity at relatively lower temperatures. The 7-plies composite specimen was produced by hot isostatic pressing (HIP) at 800°C for 0.5 hrs. in vacuum, alternating layers of thin-foils of the SP-700 and the green tapes of the SCS-6 fibers, so that the fibers were uniformly distributed as a hexagonal array in the matrix. The volume fraction of the fibers in the composite is about 28 %. In this work, the following articles in a unidirectionally reinforced SCS-6/SP-700 composite have been studied and evaluated: (i)mechanical properties of the SCS-6/SP-700 composite and the matrix alloy at temperatures ranged between room temperature and 450°C; (ii) LCF lives and the failure modes of the composite and the matrix alloy at room temperature and 450°C; (iii) fiber push-out tests at elevated temperatures ranged between room temperature and 600°C, to represent the fiber/matrix interfacial strength; and (iv) observation and the characterization of the interfacial reaction zone by means of a transmission electron microscope (TEM) and an energy dispersive X-ray spectrometer (EDS). Based on these experimental results, the effects of temperature and the loading frequency on LCF failure of the SCS-6/SP-700 composite were discussed.

Numerical study of the single fibre push-out test: Part III. Singularity of interface stresses

The singular behaviour at the free edges of the fibre-matrix interface is analysed for the fibre push-out test geometry based on the boundary element method. The fibre push-out test has been extensively used to measure the fibre-matrix interfacial properties in polymer, ceramic and metal matrix composites. There are two free edges in the fibre push-out specimen: one is at the loaded fibre end and the other at the supported fibre end. The singular stresses can be expressed as a function of singular exponent and singular stress intensity. It is shown that the singular exponents obtained at both fibre ends are characteristic of composite constituent properties, such as Young's moduli of fibre and matrix, and does not vary with specimen dimensions. The singular exponents are real and identical for the shear and radial stress components at fibre ends where the wedge angles are the same. The singular stress intensities are also implicit in material properties, and vary with specimen dimensions, such as fibre to matrix radius ratio, fibre aspect ratio and support hole size. An interfacial failure criterion is proposed here based on the average stress concept to determine the critical singular stress intensities in mode I and mode II loads.