Mechanism of cumulative damage to short fiber type C/SiC under tension

Stress–strain relations at different degrees of peak stress were investigated using loading–unloading tests to elucidate cumulative damage mechanisms of short fiber type C/SiC under tension. Damage observations revealed their crack length, number, and angle characteristics. Furthermore, stress–strain relations were estimated by expanding Basista’s equations and by substituting measured damage characteristics into them, which revealed a nonlinear stress–strain relation. Cracks propagated in transverse fiber bundles without fiber fracture, connecting other cracks that had 75 ° – 90 ° orientation to the tensile axis. Stress–strain relations estimated qualitatively and quantitatively suggest that mixed mode I and mode II crack opening in transverse fiber bundles in the through-thickness plane caused the stress–strain nonlinear relations.     

Micromechanical testing of carbon fibers deposited by low-pressure laser-assisted chemical vapor deposition

Carbon fibers were deposited directly from ethylene by laser-assisted chemical vapor deposition. The precursor gas pressures and the incident laser powers were varied. Micro-mechanical testing was carried out using a high-precision micro-manipulator. During three-point bend testing the fibers showed an elastic response, with no residual strain upon unloading, until fracture. The fibers’ strength and Young’s modulus are reported. A model for fiber fracture is proposed based on fiber cross-section analysis. Scanning electron microscopy was used to study the fiber cross-sections and the fiber surface morphology. The mechanical properties are related to the characteristic fiber microstructure investigated by Raman spectroscopy.     

Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites

The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.     

金属基聚合物复合材料短纤维桥接界面强化机理

针对金属基聚合物复合材料易诱发界面剥离损伤失效的共性问题,研究了通过多层复合组装注射成型,在聚合物复合层与粘接层界面形成短纤维桥接,实现复合界面强化.基于内聚力剥离损伤模型,构建了短纤维桥接强化界面剥离裂纹扩展断裂失效过程的模拟仿真技术,模拟建立了界面剥离裂纹快速失稳扩展断裂损伤失效临界载荷—桥接纤维特性—界面剥离断裂...     

One‐step green treatment of hemp fiber used in polypropylene composites

In this article, an eco‐friendly and cost effective surface treatment method is proposed for hemp fiber, enabling fabrication of hemp fiber/polypropylene (PP) composites, which show better mechanical properties than the PP composites containing untreated or alkali treated hemp fiber. Various techniques, such as scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), moisture analysis, and differential scanning calorimetry (DSC), are employed for the characterization of hemp fiber and polymer composites interface. Based on these results, the conventional alkali treatment of hemp fiber results in the damage of elementary fiber by eliminating parts of lignin and hemicellulose, which could be the reason for the decline of mechanical properties for the resulted polymer composites. On the contrary, water treatment cleans the fiber surface by effectively removing the water‐soluble polysaccharides while ensures minimum degradation on elementary fiber structure, which contributes to the improved mechanical properties of final polymer composites: the Young's modulus, fracture stress and fracture strain were enhanced by 3.66, 7.86, and 14.6%, respectively, when compared with untreated fiber reinforced composites. POLYM. COMPOS., 37:385–390, 2016. © 2014 Society of Plastics Engineers     

In-situ tensile damage and fracture behavior of PIP SiC/SiC minicomposites at room temperature

In-situ tensile damage and fracture behavior of original SiC fiber bundles, processed and uncoated SiC fiber bundles, SiC fiber bundle with PyC interphase, SiC/SiC minicomposites without/with PyC interphase are analyzed. Relationships between load-displacement curves, stress-strain curves, and micro damage mechanisms are established. A micromechanical approach is developed to predict the stress-strain curves of SiC/SiC minicomposites for different damage stages. Experimental tensile stress-strain curves of two different SiC fiber reinforced SiC matrix without/with interphase are predicted. Evolution of composite’s tangent modulus, interface debonding fraction, and broken fiber fraction with increasing applied stress is analyzed. For the BX™ and Cansas-3303™ SiC/SiC minicomposite with interphase, the composite’s tangent modulus decreased with applied stress especially approaching tensile fracture; the interface debonding fraction increased with applied stress, and the composite’s tensile fracture occurred with partial interface debonding; and the broken fiber fraction increased with applied stress, and most of fiber’s failure occurred approaching final tensile fracture.     

Fracture properties of porous yttria-stabilized zirconia under micro-compression testing

Micro-compression tests were carried out on pillars of 60 μm in diameter, milled by plasma focused ion beam in porous Yttria-Stabilized Zirconia (YSZ) pellets. The fracture properties were determined over a wide range of porosities (33 %–63 %) for 8YSZ and at a given pore volume fraction of 63 % for 3YSZ. The mechanical properties determined from testing were reproducible thanks to the homogeneity of the microstructures. The Young’s modulus was estimated as a function of the porosity from the unloading curve of tests stopped before fracture. The experiments conducted until the total rupture allowed measuring the compressive fracture strength, which was found to decrease when increasing the porosity. Specimen tested and unloaded just before the total fracture were cross-sectioned by focused ion beam - scanning electron microscope. A transition was detected from a brittle behavior, with macro-cracks parallel to the direction of solicitation, to a diffuse damage with micro-cracks, when increasing the porosity.     

Microstructure and tensile behavior of (BN/SiC)n coated SiC fibers and SiC/SiC minicomposites

Interphase plays an important role in the mechanical behavior of SiC/SiC ceramic-matrix composites (CMCs). In this paper, the microstructure and tensile behavior of multilayered (BN/SiC)_n coated SiC fiber and SiC/SiC minicomposites were investigated. The surface roughness of the original SiC fiber and SiC fiber deposited with multilayered (BN/SiC), (BN/SiC)₂, and (BN/SiC)₄ (BN/SiC)₈ interphase was analyzed through the scanning electronic microscope (SEM) and atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. Monotonic tensile experiments were conducted for original SiC fiber, SiC fiber with different multilayered (BN/SiC)_n interfaces, and SiC/SiC minicomposites. Considering multiple damage mechanisms, e.g., matrix cracking, interface debonding, and fibers failure, a damage-based micromechanical constitutive model was developed to predict the tensile stress-strain response curves. Multiple damage parameters (e.g., matrix cracking stress, saturation matrix crack stress, tensile strength and failure strain, and composite’s tangent modulus) were used to characterize the tensile damage behavior in SiC/SiC minicomposites. Effects of multilayered interphase on the interface shear stress, fiber characteristic strength, tensile damage and fracture behavior, and strength distribution in SiC/SiC minicomposites were analyzed. The deposited multilayered (BN/SiC)_n interphase protected the SiC fiber and increased the interface shear stress, fiber characteristic strength, leading to the higher matrix cracking stress, saturation matrix cracking stress, tensile strength and fracture strain.     

Stress‐transfer micromechanics for fiber length with a photocure vinyl ester composite

The objective was to test how increasing fiber length above the critical length would influence mechanical properties and fracture crack propagation. Micromechanics considering fiber/matrix stress‐transfer was used to evaluate the results in addition to a shear debonding volume percent correction term necessary for the final analysis. Fiber lengths of 0.5, 1.0, 2.0, 3.0, and 6.0 mm with 9 μm diameters were added into a photocure vinyl ester particulate‐filled composite at a uniform 28.2 vol%. Mechanical flexural testing was performed using four‐point fully articulated fixtures for samples measuring 2 × 2 × 50 mm³ across a 40 mm span. Fiber length correlated with improved mechanical properties for flexural strength, modulus, yield strength, strain, work of fracture, and strain energy release, p < 0.001. In addition, sample fracture depth significantly decreased with increasing fiber lengths, p < 0.00001. All mechanical properties correlated significantly as predictors for fracture failure, p < 0.000001, and as estimators for each other, p < 0.0001. The stress‐transfer micromechanics for fiber length were improved upon for strength by including a simple correction factor to account for loss of fiber volume percent related to cracks deflecting around debonded fiber ends. In turn, the elastic property of modulus was shown to exhibit a tendency to follow stress‐transfer micromechanics. Polym. Compos. 27:153–169, 2006. © 2006 Society of Plastics Engineers.     

Effects of matrix structure on mechanical properties of carbon/carbon composites

Mechanical properties of carbon/carbon composites prepared by thermal-gradient CVD technique were investigated by three-point flexural tests. The mechanical properties are strongly influenced by the matrix structure, which in turn depends on the deposition conditions employed. The lower fracture stress and elastic modulus of isotropic structure and columnar structure are due to the low matrix density and to the presence of matrix cracks, respectively. The transition structure shows relatively high modulus and stress. The fracture strain remains almost constant over the whole ranges investigated. The role of the matrix crack during the fracture is discussed.     

Mechanical properties and prediction of fracture parameters of geopolymer/alkali-activated mortar modified with PVA fiber and nano-SiO2

Properties of fly ash (FA) and metakaolin (MK) based geopolymer/alkali-activated mortar modified with polyvinyl alcohol (PVA) fiber and nano-SiO₂, including workability, compressive strength, flexural performance, elastic modulus and fracture property were tested in this study. PVA fiber content varies from 0 to 1.2%. Nano-SiO₂ content is 0 and 1%. Adaptive neuro-fuzzy interfacial systems (ANFIS) method was used to establish the artificial intelligence (AI) model to predict the fracture parameters of geopolymer/alkali-activated mortars. The inputs of ANFIS models include PVA fiber content, nano-SiO₂ content, compressive strength, flexural strength, elastic modulus, critical crack mouth opening displacement, crack load and peak load. The outputs of ANFIS model include critical effective crack length, initiation fracture toughness, unstable fracture toughness, and fracture energy. Experiment results showed that PVA fiber addition enhanced the mechanical properties especially the compressive strength and fracture performance, but decreased the workability. 0.8%–1.0% was considered as the optimal content of PVA fiber. Addition of 1% nano-SiO₂ shows a slight improvement on both workability and mechanical properties of the mortar no matter how much fiber is added. Based on the ANFIS algorithm and 42 sets of experimental data, the trained models were proved to have high accuracy with root mean square error (RMSE) under 0.15, mean absolute error (MAE) under 0.01, and coefficient of determination (R²) over 0.85. The ANFIS model established in this study combined the fracture properties with the basic mechanical properties of geopolymer/alkali-activated composites, which can provide a new method to assess the fracture performance of geopolymer/alkali-activated mortars modified with PVA fiber and nano-SiO₂ in the future.     

Temperature Dependence of Tensile Strength for a Woven Boron-Nitride-Coated Hi-Nicalon™ SiC Fiber-Reinforced Silicon-Carbide-Matrix Composite

The temperature dependence of tensile fracture behavior and tensile strength of a two-dimensional woven BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composite fabricated by polymer infiltration pyrolysis (PIP) were studied. A tensile test of the composite was conducted in air at temperatures of 298 (room temperature), 1200, 1400, and 1600 K. The composite showed a nonlinear behavior for all the test temperatures; however, a large decrease in tensile strength was observed above 1200 K. Young's modulus was estimated from the initial linear regime of the tensile stress–strain curves at room and elevated temperatures, and a decrease in Young's modulus became significant above 1200 K. The multiple transverse cracking that occurred was independent of temperature, and the transverse crack density was measured from fractographic observations of the tested specimens at room and elevated temperatures. The temperature dependence of the effective interfacial shear stress was estimated from the measurements of the transverse crack density. The temperature dependence of in situ fiber strength properties was determined from fracture mirror size on the fracture surfaces of fibers. The decrease in the tensile strength of the composite up to 1400 K was attributed to the degradation in the strength properties of in situ fibers, and to the damage behavior exception of the fiber properties for 1600 K.     

Mechanical properties of oxynitride glasses

Oxynitride glasses combine a high refractoriness, with Tg typically >850°C, and remarkable mechanical properties in comparison with their parent oxide glasses. Their Young's modulus and fracture toughness reach 170 GPa and 1.4 MPa m^.5, respectively. Most reports show good linear relationships between glass property values and nitrogen content. There is a clear linear dependence of Young's modulus and microhardness on fractional glass compactness (atomic packing density). They also have a better resistance to surface damage induced by indentation or scratch loading. The improvements stem from the increase of the atomic network cross-linking—because of three-fold coordinated nitrogen—and of the atomic packing density, despite nitrogen being lighter than oxygen and the Si–N bond being weaker than the Si–O bond. For constant cation composition, viscosity increases by ∼3 orders of magnitude as ∼17 eq.% oxygen is replaced by nitrogen. For rare earth oxynitride glasses with constant N content, viscosity, Young's modulus, Tg, and other properties increase with increasing cation field strength (decreasing ionic radius). Research continues to find lighter, stiffer materials, including glasses, with superior mechanical properties. With higher elastic moduli, hardness, fracture toughness, strength, surface damage resistance, increased high temperature properties, oxynitride glasses offer advantages over their oxide counterparts.     

复合材料压力容器基体开裂损伤的研究

复合材料压力容器损伤破坏的主要形式之一是基体开裂,本文利用损伤力学方法中的细观力学对复合材料压力容器进行分析,采用等效夹杂法分析基体中任一点的应力情况,当应力达到临界值时基体开始产生裂纹,描述了基体开裂损伤的演化过程,建立裂纹密度和压力容器弹性模量之间的关系式,表现基体裂纹对压力容器刚度的影响。用压力容器的弹性模量定义损伤变量,进而建立了弹性模量和载荷循环次数之间的关系式。     

Characterization of cyclic loading/unloading damage behavior in fiber-reinforced ceramic-matrix composites using inverse tangent modulus

Under cyclic loading/unloading, the mechanical hysteresis appears in fiber-reinforced ceramic-matrix composites (CMCs) due to multiple micro damage mechanisms. In this paper, the cyclic loading/unloading damage evolution in different CMCs is analyzed using the inverse tangent modulus (ITMs). Experimental micro damage mechanisms are observed using the X-ray computed tomography (XCT) and scanning electron microscopy (SEM). Based on the damage mechanisms’ analysis, a damage-based micromechanical constitutive model is developed to predict the cyclic loading/unloading curves and related damage parameters. Effects of composite’s constitutive properties, peak stress, damage state and interface properties on the cyclic loading/unloading damage evolution are discussed. For the 1D and 2D SiC/SiC, and 3D C/SiC composites, the evolution curves of ITMs can be divided into two regions. In region I, the increasing rate of the ITMs is constant and depends on the composite’s constitutive properties; and in region II, the increasing rate of the ITMs decreases as the interface slip range approaches the interface debonding tip.     

Mechanical and fracture behavior of an artificially ultraviolet‐irradiated poly(ethylene‐carbon monoxide) copolymer

In this work, 1 wt % carbon monoxide (CO) poly(ethylene‐carbon monoxide) (ECO) copolymer sheets were artificially exposed to ultraviolet (UV) light with a power density of 3 mW/cm² for up to 130 h. A thorough mechanical characterization of the irradiated material was conducted, in which both the stress–strain data and the values of the quasistatic crack initiation and growth toughness were measured and correlated with companion uniaxial tensile tests and single‐edge‐notched fracture tests. Average values of the elastic modulus, failure strain, and failure stress were determined from the tensile tests. The full‐field optical technique of digital image correlation was used to quantify in‐plane deformation (displacements and displacement gradients) during the fracture experiments and to extract values of the crack initiation and growth fracture toughness. The elastic modulus increased monotonically with UV irradiation for the exposure times used in this investigation. In addition, for low irradiation times of less than 5 h, both the failure strain and failure stress of ECO decreased, and this caused a corresponding decrease in the crack initiation and growth toughness. However, for longer irradiation times, the failure strain remained almost invariable, whereas the failure stress increased by about 25% over that of unirradiated ECO. As a result, for longer irradiation times (>5 h), 1 wt % CO ECO became not only stiffer but also stronger and tougher, as quantified by companion fracture experiments. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 139–148, 2004     

Deformation mechanisms and evolution of mechanical properties in damaged advanced ceramics

We investigated the uniaxial compressive behavior of damaged and intact alumina using quantitative X-ray computed tomography (XCT) analysis coupled with digital image correlation (DIC) for mechanical characterization. Internal three-dimensional crack characteristics such as crack surface area and orientation were quantified using XCT to assess the level of damage. From the quasi-static and dynamic stress–strain results, the primary effects of crack damage are to reduce the initial stiffness and rate of lateral expansion in damaged alumina. With increasing axial strain, crack closure was found to lead to a recovery of elastic properties, in some cases to intact levels, in the damaged specimens. Localized deformation mechanisms related to the crack structure, including lateral crack closure, axial crack opening and closing, and inclined crack sliding, were visualized in-situ and connected to XCT reconstructions. High-speed imaging also revealed a mixed fracture mode for damaged alumina that included axial splitting and failure along pre-existing cracks.     

High-velocity impact behaviors and post-impact tension performance of 2D-C/SiC composite beams

2D-C/SiC composites have widely been used in aeronautical and aerospace engineering, but their mechanical behaviors under small-mass and high-speed impact have not been thoroughly studied yet. In this paper, 2D-C/SiC beam specimens were impacted by a single-stage light-gas gun and the fracture processes were captured by a high-speed camera. Post-impact internal and surface damage morphologies were scanned by a CT and a SEM, respectively. Similar damage modes were revealed by high-speed images. Subsequently, quasi-static post-impact tension tests were conducted to understand the residual mechanical properties. Acoustic emission (AE) signals of specimens were detected during the tests and then classified by the K-means algorithm. Therefore, evolutions of matrix damage, interfacial delamination and fiber fracture were recognized. At the same time, strain value was obtained by digital image correlation (DIC) method and main crack propagations were obvious in strain contours. A combination of the AE and DIC methods very well monitored the real-time damage during post-impact testing, which further revealed the damage during impact phase.     

Effects of high-temperature heat treatment on the mechanical properties of unidirectional carbon fiber reinforced geopolymer composites

Unidirectional carbon fiber reinforced geopolymer composite (C_uf/geopolymer) is prepared by a simple ultrasonic-assisted slurry infiltration method, and then heat treated at elevated temperatures. Effects of high-temperature heat treatment on the microstructure and mechanical properties of the composites are studied. Mechanical properties and fracture behavior are correlated with their microstructure evolution including fiber/matrix interface change. When the composites are heat treated in a temperature range from 1100 to 1300 °C, it is found that mechanical properties can be greatly improved. For the composite heat treated at 1100 °C, flexural strength, work of fracture and Young's modulus reach their highest values increasing by 76%, 15% and 75%, respectively, relative to their original state before heat treatment. The property improvement can be attributed to the densified and crystallized matrix, and the enhanced fiber/matrix interface bonding based on the fine-integrity of carbon fibers. In contrast, for composite heat treated at 1400 °C, the mechanical properties lower substantially and it tends to fracture in a very brittle manner owing to the seriously degraded carbon fibers together with matrix melting and crystal phases dissolve.     

Improvement of high-temperature stability of PIP SiCf/SiC material through in situ grown BNNTs

In this paper, the effect of in situ grown boron nitride nanotubes (BNNTs) and preparation temperature on mechanical behavior of PIP (Precursor Infiltration and Pyrolysis) SiC_f/SiC minicomposites under monotonic and compliance tensile is investigated. In situ BNNTs are grown on the surface of SiC fibers using ball milling–annealing process. Composite elastic modulus, tensile strength, fracture strain, tangent modulus, and loading/unloading inverse tangent modulus (ITM) are obtained and adopted to characterize the mechanical properties of the composites. Microstructures of in situ grown BNNTs and tensile fracture surfaces are observed under scanning electronic microscopic (SEM). For SiC_f/SiC minicomposites with BNNTs, the elastic modulus, tensile strength, and fracture strain are all lower than those of SiC_f/SiC minicomposites without BNNTs, mainly due to high preparation temperature and the oxidation of the PyC interphase during the annealing process. Tensile stress–strain curves of SiC_f/SiC minicomposites with and without BNNTs are predicted using the developed micromechanical constitutive model. The predicted results agreed with experimental data. This work will provide guidance for predicting the service life of SiC_f/SiC composite materials and may enable these materials to become a backbone for thermal structure systems in aerospace applications.