Fiber Strength Degradation Research Articles

Synergistic Effects of Temperature and Oxidation on Matrix Cracking in Fiber-Reinforced Ceramic-Matrix Composites

In this paper, the synergistic effects of temperatrue and oxidation on matrix cracking in fiber-reinforced ceramic-matrix composites (CMCs) has been investigated using energy balance approach. The shear-lag model cooperated with damage models, i.e., the interface oxidation model, interface debonding model, fiber strength degradation model and fiber failure model, has been adopted to analyze microstress field in the composite. The relationships between matrix cracking stress, interface debonding and slipping, fiber fracture, oxidation temperatures and time have been established. The effects of fiber volume fraction, interface properties, fiber strength and oxidation temperatures on the evolution of matrix cracking stress versus oxidation time have been analyzed. The matrix cracking stresses of C/SiC composite with strong and weak interface bonding after unstressed oxidation at an elevated temperature of 700 °C in air condition have been predicted for different oxidation time.

Effect of heat treatment on the microstructure and tensile strength of KD-II SiC fibers

KD-II SiC fibers, a new type of SiC fibers, were heat treated at elevated temperatures for 1h in an Ar atmosphere. The microstructure and mechanical properties of the fibers before and after annealing were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and tensile tests, et al. The results show that the as-received fibers consisted of β-SiC nano-crystals, free carbon and a small amount of amorphous intergranular SiCxOy phase. With increasing heat treatment temperature, significant growth of β-SiC grains, ordering of free carbon and decomposition of SiCxOy phase occurred and the fiber surfaces became coarser with development of new defects, such as pores and large SiC particles. It was also found that KD-II SiC fibers maintained high strength with heat treatment at temperatures up to 1500°C, exceeding which a sharp strength degradation was observed. The strength degradation and microstructural evolution of the fibers due to heat treatment were correlated, and it can be concluded that the β-SiC grains growth, residual tensile stresses as well as surface flaws were the dominating factors for the fiber strength degradation after exposure at high temperatures.

Influence of Fiber Orientation on Bridging Performance of Polyvinyl Alcohol Fiber-Reinforced Cementitious Composite

Crack bridging performance of fibers strongly affects the tensile characteristics of fiber-reinforced cementitious composites (FRCCs) after first cracking. The fiber orientation distribution is likely to be affected by factors that include fresh-state properties, casting method, formwork geometry, and others. The objective of this study is to investigate the influence of the fiber orientation on the bridging performance in polyvinyl alcohol (PVA) FRCCs through a visualization simulation using a water glass solution and a calculation of the bridging law. The main parameter of the investigations in the present study is the casting direction. To evaluate the fiber orientation distribution quantitatively, an approximation methodology using an elliptic function is newly introduced. The bridging stress versus crack width relationship is calculated considering the elliptic distribution, the snubbing effect, and the fiber strength degradation. The calculated stress-crack width curves can express the uniaxial tension test results after first cracking well.

High-temperature behavior and degradation mechanism of SiC fibers annealed in Ar and N2 atmospheres

The thermal and mechanical stability of SiC fibers at elevated temperature is an important property for the practical application of SiC fiber-reinforced ceramic matrix composites and is related to the heat-treating atmosphere. In this study, the high-temperature behavior of KD SiC fibers with low oxygen content was investigated in both Ar and N2 at temperatures from 1400 to 1800 °C through scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, resistivity measurements, and tensile tests in order to understand the effects of atmospheres on the degradation of the fibers. The results show that high-temperature treatment caused more severe strength degradation in Ar than in N2. In particular, the fibers heat treated in N2 at 1700 °C retained a relatively high strength of 1.52 GPa, 60 % of their original strength, while the fiber strength was completely lost after heat treatment in Ar. Fiber strength degradation was mainly caused by a combination of crystal growth and surface flaws. The formation of huge grains and porosity in the fiber surfaces, owing to the thermal decomposition of the SiC x O y N z and SiC x O y phases, significantly degraded the strength for fibers heat treated in Ar. However, the suppressing effect of N2 on the decomposition of the SiC x O y N z phase in the fiber surfaces and nitrided case on the decomposition of the SiC x O y phase in the fiber cores, led to higher SiC fiber temperature stability in N2 rather than Ar.

Formation mechanism of large SiC grains on SiC fiber surfaces during heat treatment

Three low oxygen-containing SiC fibers with different surface compositions were heat-treated at high temperature in argon, and large SiC particles were observed on the surfaces of SiC fibers with oxygen-containing surface layers. The SiC particles were identified as single crystal β-SiC. The growth process, formation mechanism of these large SiC grains and their influence on fiber strength were investigated. Nano-sized SiC particles were precipitated from the decomposition of SiCxOy in the fiber surface that acted as nuclei, and these nuclei grew gradually to form large grains because of the gaseous reaction of SiO and CO. The formation of large SiC grains was related closely to the SiC fiber surface composition, which affected SiC nucleation and SiO diffusion from the core to the fiber surface. The large grains influenced the fiber strength degradation significantly and should be avoided to retain high fiber strength.

Influence of carbon nanotube extending length on pyrocarbon microstructure and mechanical behavior of carbon/carbon composites

We present an in-depth study of the effect of carbon nanotube (CNT) extending length on microstructural and mechanical behavior of carbon/carbon (C/C) composite. High-purity CNTs with controlled extending length were in situ grown on the surface of carbon cloths using injection chemical vapor deposition (ICVD) by varying the reaction time. Microstructure analysis shows that compared with the CNTs of short extending length which only change the pyrocarbon (PyC) close to fiber surface, CNTs with long extending length can strongly affect the deposition behavior of PyC during chemical vapor infiltration and modify the whole matrix PyC. Mechanical tests reveal that CNTs with long extending length are more beneficial to enhance the interlaminar shear strength and in-plane compressive strength of the composites, while the reactive conditions during ICVD degrade the carbon fibers and lead to the decrease of flexural strength. Our work demonstrates that it is necessary to make CNTs long enough as well as to prevent strength degradation of fibers, if we want to largely increase delamination resistance and through-thickness properties without compromising in-plane performance of C/C composites.

Modeling the effect of oxidation on fatigue life of carbon fiber-reinforced ceramic-matrix composites at elevated temperature

An analytical method has been developed to investigate the effect of oxidation on fatigue life of fiber-reinforced ceramic-matrix composites (CMCs) at elevated temperature under air. The Budiansky–Hutchinson–Evans shear-lag model was used to describe the micro-stress field of the damaged composite considering fibers failure. The statistical matrix multicracking model and fracture mechanics interface debonding criterion were used to determine the matrix crack spacing and interface debonded length. The interface shear stress and fiber strength degradation model and oxidation region propagation model have been adopted to analyze the fatigue and oxidation effects on fatigue life of the composite, which is controlled by interface frictional slip and diffusion of oxygen gas through matrix multicrackings. Under fatigue loading, the fibers failure probabilities were determined by combining the oxidation model, interface wear model and fiber statistical failure model based on the assumption that the fiber strength is subjected to two-parameter Weibull distribution and the loads carried by broken and intact fibers satisfy the global load sharing criterion. The fatigue life S–N curves of unidirectional, cross-ply and 2.5D C/SiC composites at 800°C under air have been predicted.

Tyranno ZMI fiber/TiSi2–Si matrix composites for high-temperature structural applications

A Tyranno ZMI fiber/TiSi2–Si matrix composite was fabricated via melt infiltration (MI) of a Si–16at%Ti alloy at 1375°C under vacuum. The Si–Ti alloy was used as an infiltrant to conduct MI processing below 1400°C and inhibit the strength degradation of the amorphous SiC fibers. The alloy matrix formed was dense and comprised primarily of TiSi2–Si eutectic structures. The TiSi2–Si matrix composite melt-infiltrated at 1375°C showed a pseudo-plastic tensile stress–strain behavior followed by final fracture at ∼290MPa and ∼0.9% strain. When the MI temperature was increased to 1450°C, however, substantial reduction in the stiffness and ultimate strength occurred under tensile loading. Microstructural observations revealed that these degradations were attributed to the damages that occurred on the reinforcing fibers and pyrolytic carbon interfaces during the MI process. The present experimental results clearly demonstrated the effectiveness of the low-temperature MI process in strengthening Tyranno ZMI fiber composites and reducing the processing cost.

Modeling the Effect of Oxidation on Tensile Strength of Carbon Fiber−Reinforced Ceramic−Matrix Composites

An analytical method has been developed to investigate the effect of oxidation on the tensile strength of carbon fiber − reinforced ceramic − matrix composites (CMCs). The Budiansky − Hutchinson − Evans shear − lag model was used to describe the micro stress field of the damaged composite considering fibers failure. The statistical matrix multicracking model and fracture mechanics interface debonding criterion were used to determine the matrix crack spacing and interface debonded length. The fiber strength degradation model and oxidation region propagation model have been adopted to analyze the oxidation effect on tensile strength of the composite, which is controlled by diffusion of oxygen gas through matrix cracks. Under tensile loading, the fibers failure probabilities were determined by combining oxidation model and fiber statistical failure model based on the assumption that fiber strength is subjected to two-parameter Weibull distribution and the loads carried by broken and intact fibers statisfy the global load sharing criterion. The composite can no longer support the applied load when the total loads supported by broken and intact fibers approach its maximum value. The conditions of a single matrix crack and matrix multicrackings for tensile strength considering oxidation time and temperature have been analyzed.

Fiber strength measurement for KD-I(f)/SiC composites and correlation to tensile mechanical behavior at room and elevated temperatures

Mechanical properties of KD-I-fiber-reinforced silicon carbide matrix composites were evaluated under monotonic tension at room and elevated temperatures. The measured ultimate strength showed a big decline for uncoated specimens from a room temperature value of 350MPa, to 220 and 200MPa at 1100°C and 1300°C, respectively. The decrease in ultimate strength at elevated temperatures was believed to be partly due to the oxidation embrittlement and fiber strength degradation. Specimens with anti-oxidation coatings tested at 1300°C exhibited the ultimate strength of 320MPa, which was comparable to that tested at room temperature. Scanning electron microscopy was employed to evaluate the in-situ KD-I fiber strengths via fracture mirror size measurements. The analysis indicated that oxidation embrittlement was the dominated damage mechanism leading to the decrease of ultimate strength. Measured values of ultimate strengths of the CMCs were compared with predictions made on the basis of in-situ fiber strength characteristics and available model.

A multiaxial fatigue damage model for fibre reinforced polymer composites

A multiaxial fatigue damage model for fibre reinforced polymer composite materials is presented. The model combines (i) fatigue-induced fibre strength and modulus degradation, (ii) irrecoverable cyclic strain effects and (iii) inter-fibre fatigue. The inter-fibre fatigue aspect is based on a fatigue-modified version of the Puck multiaxial failure criterion for static failure. The model is implemented in a user material finite element subroutine and calibrated against fatigue test data for unidirectional glass fibre epoxy. A programme of uniaxial fatigue tests on quasi-isotropic glass fibre epoxy laminates is presented for validation of the novel fatigue damage methodology. The latter is successfully validated across a range of stress levels.

A study on the failure mode and load of the composite key joint

The joining of composite materials has traditionally been achieved by adhesive bonding or mechanical fastening. Mechanical fastenings require little or no surface preparation, and are easy to inspect for joint quality. However, they require machining of holes that interrupt the fiber continuity and may reduce the strength of the adherend. We proposed the new composite key joint to minimize the fiber discontinuity and strength degradation of adherend. Mechanical key joints with three different d/ t (slot depth to thickness) ratios and two different e/ w (edge length to width) ratios were manufactured and tested. Carbon-epoxy unidirectional prepreg (USN125) and plain weave prepreg (HPW193) were used for the composite key joints. Tensile tests were performed on the composite key joints and their failure modes were evaluated. Finite element analyses for the mechanical key joints were performed and the stresses around the key slot were calculated. From the tests, it could be concluded that the failure load of the key joints was about two times larger than those of a mechanical joint for the investigated composite joints.

Experimental and analytical approach for the size effect on the fiber strength of CFRP

AbstractIn this article, experimental test and an analytical approach were conducted to verify the fiber tensile strength of filament wound pressure vessel. As a test method, improved hoop ring test method was considered. From a series of fully scaled hoop ring tests, the size effect showed the clearly observed tendency toward fiber strength degradation with increasing stressed volume. As an analytical method, Weibull weakest link model and the sequential multistep failure model (SMFM) were considered and compared with the test data from hoop ring test and unidirectional specimens test. It was found that the Weibull weakest link model and the SMFM significantly overestimate the size effect on fiber strength. Thus, the fiber strength showed much lower values than those of test data. In this study, an improved SMFM was proposed by considering a modification of the length size effect. Because this model enhances the accuracy of predicting the fiber strength, the fiber strengths from the improved SMFM showed good agreement with the test data.POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers

Application of continuously-monitored single fiber fragmentation tests to carbon nanotube/carbon microfiber hybrid composites

Abstract To assess the effect of carbon nanotube (CNT) grafting on interfacial stress transfer in fiber composites, CNTs were grown upon individual carbon T-300 fibers by chemical vapor deposition. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on both pristine and CNT-decorated fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Despite the fiber strength degradation resulting from the harsh CNT growth conditions, the CNT-modified fibers lead to a twofold increase in interfacial shear strength which correlates with the nearly threefold increase in apparent fiber diameter resulting from CNT grafting. These observations corroborate recently published studies with other CNT-grafted fibers. An analysis of the relative contributions to the interfacial strength of the fiber diameter and strength due to surface treatment is presented. It is concluded that the common view whereby an experimentally observed shorter average fragment length leads to a stronger interfacial adhesion is not necessarily correct, if the treatment has changed the fiber tensile strength or its diameter.

기계 가공된 복합재료 키 조인트의 강도 연구

복합재료가 기계부품, 항공기 구조물에 폭 넓게 적용됨에 따라, 복합재료 구조물에서 가장 취약한 복합재료 체결부의 설계는 매우 중요한 연구 분야로 대두되고 있다. 본 논문에서는 기계적 체결방법의 문제점으로 발생하는 원공주위의 높은 응력집중현상을 감소시키기 위하여, 복합재료 키 조인트(composite key joint)를 제안하였고 파손강도를 평가하였다. 제안된 복합재료키 조인트 체결부의 파손 판정을 위해서 파손지수(failure index)와 파손영역법(damage area theory)이 각각 적용되었다. 실험 결과로부터 복합재료 키 조인트는 기계적 체결부의 파손강도보다 93% 높은 값을 가짐을 볼 수 있었고, 복합재료 키 홈 깊이(key slot depth)가 0.88 mm이고 끝단 길이(edge length)가 20 mm일 때 가장 높은 파손하중 나타내었다. The comparison of the numerical results with those measured by the experiment showed good agreement. The design of composite joint which is the weakest part in the composite structures has become a very important research area since the composite materials are widely used in the aircraft and machine structure. In this paper, the new composite key joints that minimize the fiber discontinuity and strength degradation of adherend were proposed and their failure loads were evaluated. The failure index and damage area method were used for the failure prediction of the composite key joint. From the tests, the failure load of the composite key joint was 93% larger than that of a mechanical joint and the key joint whose slot depth and edge length were 0.88mm and 20mm had the largest failure load. Also, the analytic failure modes by the failure index and damage area were compared with experimental failure modes.

Modeling oxidation damage of continuous fiber reinforced ceramic matrix composites

For fiber reinforced ceramic matrix composites (CMCs), oxidation of the constituents is a very important damage type for high temperature applications. During the oxidizing process, the pyrolytic carbon interphase gradually recesses from the crack site in the axial direction of the fiber into the interior of the material. Carbon fiber usually presents notch-like or local neck-shrink oxidation phenomenon, causing strength degradation. But, the reason for SiC fiber degradation is the flaw growth mechanism on its surface. A micromechanical model based on the above mechanisms was established to simulate the mechanical properties of CMCs after high temperature oxidation. The statistic and shearlag theory were applied and the calculation expressions for retained tensile modulus and strength were deduced, respectively. Meanwhile, the interphase recession and fiber strength degradation were considered. And then, the model was validated by application to a C/SiC composite.

Influence of fiber failure on fatigue hysteresis loops of ceramic matrix composites

An analytical methodology has been developed to investigate the influence of fiber failure on fatigue hysteresis loops of ceramic matrix composites in this article. During fatigue loading, matrix cracking, interface debonding and fiber failure occur upon first loading to the fatigue maximum stress. Matrix cracking space and interface debonding length are obtained by matrix statistical cracking model and fracture mechanics interface debonding criterion. Based on the assumption of global load-sharing criterion for the load distribution between the unbroken and broken fibers, an approach to determine fiber failure probability during fatigue loading for the degradation of interface shear stress and fiber strength is developed in this analysis. Upon unloading and subsequent reloading, stress—strain hysteresis loops develop as fiber sliding relative to matrix in the interface debonded region. The unloading interface counter slip length and reloading new slip length are obtained by the fracture mechanics interface debonding criterion. The effects of characteristic fiber strength, fiber Weibull modulus, and fatigue maximum stress on fiber failure during fatigue loading, and then on the shape, location, and area of the fatigue hysteresis loops are investigated. The fatigue hysteresis loops of three different ceramic composites corresponding to different cycles are predicted and agree well with the experimental data.

Strength degradation of glass fibers at high temperatures

This article presents an experimental investigation into the effects of temperature and heating time on the tensile strength and failure mechanisms of glass fibers. The loss in strength of two glass fiber types (E-glass and Advantex®, a boron-free version of E-glass) was investigated at temperatures up to 650 °C and heating times up to 2 h. The tensile properties were measured by fiber bundle testing, and the maximum strength was found to be temperature and time dependent. The higher softening point of the Advantex® fibers is reflected in superior high-temperature performance. A phenomenological model is presented for calculating the residual strength of glass fiber bundles as functions of temperature and time. The strength reduction mechanism was determined by single-fiber testing. Fracture mirror sizes on the E-glass fibers were related to the fiber strength after high-temperature treatment. Based on fracture mirror measurements, it was established that (1) the mirror constant of the glass, which reflects the network structure, does not change during heat treatment and (2) the strength degradation is a result of larger surface flaws present after heat treatment.

Thermal and mechanical stabilities of Hi-Nicalon SiC fiber under annealing and creep in various oxygen partial pressures

Thermal and mechanical stabilities were investigated on Hi-Nicalon SiC fibers under annealing and creep in various oxygen partial pressures by mass change, mechanical properties as well as microstructural features. In the case of mechanical stability, the tensile strength of fiber is strongly dependent on the oxygen partial pressure of testing environment, but a weak dependence of BSR creep resistance on oxygen partial pressure is appeared. The analyses of surface morphology and mass change indicated the thermal stability of fiber under annealing in different environments was different. At different exposure temperatures and oxygen partial pressure levels, the different oxidation regimes are responsible for the strength degradation and microstructure change of fibers. Furthermore, the microstructure change is also affected by the stress applied through BSR test. This means the thermal stability of fibers is related to not only the exposed environments, but also the mechanical state of fibers.

Influence of Precursor Densification on Strength Retention of Zirconia‐Coated Nextel™ 610 Fibers

Strength degradation of Nextel™ 610 fibers by continuous liquid phase coating was investigated for four different zirconia precursors. The precursors differed regarding their chemical composition (with or without yttrium), phase composition (amorphous or crystalline), and decomposition behavior. Phase transformation and densification of the films were characterized and found to depend on the kind of precursor. Single fiber Weibull's strength was measured for calcination temperatures between 250° and 1150°C for all precursors. Each precursor had an individual degradation behavior. For an annealing temperature of 1150°C highly damaged (∼1600 MPa) and undamaged (>3300 MPa) fibers were obtained depending on the kind of precursor. Fiber degradation could be correlated to mechanical stresses. Stress concentration due to inhomogeneous film thickness distribution is proposed as the cause of fiber strength degradation. Full strength could be retained for porous coatings or coatings where stresses were reduced by phase transformation.