Matrix Microcracking Research Articles

Multi-method characterization of 50-year-old mass concrete from the nuclear power plant Unterweser in Germany: A forensic approach

Concrete core samples extracted from the base of the 50-year-old reactor building at the Unterweser nuclear power plant in Germany were analyzed using contemporary and complementary experimental techniques. The analysis focused on qualitative and quantitative assessments of the mineralogical composition of ingredients, mixture composition, aggregate size distribution, presence of cracks, porosity, and compressive strength. Optical microscopy revealed insignificant micro-cracking in the concrete matrix, while the presence of blast furnace slag was clearly visible. Petrographic analysis of polished and thin sections has enabled the estimation of the concrete mixture composition and the grain size characteristics of the aggregates. The aggregates consist of quartz, silt, and volcanic rock with minor amounts of fossil-bearing grains. Pore size distribution obtained from mercury intrusion porosimetry (MIP) reveals that the concrete has a relatively refined pore structure. Hydration products in the matrix, i.e., hardened paste, were analyzed using powder X-ray diffraction (p-XRD) and thermogravimetry (TGA). Furthermore, backscattered electron (BSE SEM) image analysis was conducted to quantify the original mixture composition, including cement and slag contents, and water/binder ratio. This study emphasizes strategic and practical aspects of conducting in-depth analysis on hardened concrete samples that are several decades old and may serve as a general guideline for similar inquiries in diverse concrete structures.

Multiscale modelling of nanoparticle toughening in epoxy: Effects of particle-matrix interface, particle size, and volume fraction

Toughening matrix materials by incorporating rigid nanoparticles has received significant interest for mitigating matrix microcracking in fibre-reinforced polymer-matrix composites, particularly at cryogenic temperatures. Despite recent experimental observations indicating the effectiveness of nanoparticle toughening, a notable gap remains in understanding the effects of nanoparticle size and volume fraction, and particle-matrix interfacial properties, on toughening efficacy. To address this gap, a multiscale model has been developed which employs a high-fidelity micromechanical model to quantify the deformation and energy dissipation caused by the rigid nanoparticles under a triaxial strain state determined from a macroscopic elastoplastic analysis. The interface between the nanoparticle and the matrix is characterized by a cohesive zone model (CZM), revealing a size-dependent phenomenon distinct to nanoparticles, sharply contrasting with micrometre-sized particles. Furthermore, the results reveal, for the first time, an optimum particle size that maximizes the toughening effect for a given particle-matrix combination. The existence of an optimum size is ascribed to the incomplete particle debonding from the matrix as the particle radius approaches the critical separation distance of the CZM. This revelation challenges the prevailing belief that, for a fixed volume fraction of nanoparticles, the enhancement in toughness rises as the particle size decreases as a result of the increase in total particle surface area. The implications for enhancing cryogenic performance are explored.

Effect of TiO2 and CaO Addition on the Crystallization and Flexural Strength of Novel Leucite Glass-Ceramics.

The aim of this study was to investigate the effects of TiO2/CaO addition on the crystallization and flexural strength of leucite glass-ceramics (GC). Synthesis of translucent and high strength GCs is important for the development of aesthetic and durable dental restorations. To achieve this, experimental aluminosilicate glasses (1-3 mol% TiO2 and CaO (B1, B2, B3)) were melted in a furnace to produce glasses. Glasses were ball milled, screened and heat treated via crystallization heat treatments, and characterized using XRD, differential scanning calorimetry, dilatometry, SEM and biaxial flexural strength (BFS). Increasing nucleation hold time (1-3 h) led to a reduction in crystallite number for B2 and B3 GC, and significant differences in leucite crystal size at differing nucleation holds within and across test groups (p < 0.05). A high area fraction of leucite crystals (55.1-60.8%) was found in the GC, with no matrix microcracking. Changes in the crystal morphology were found with higher TiO2/CaO addition. Mean BFS of the GC were 211.2-234.8 MPa, with significantly higher Weibull modulus (m = 18.9) for B3 GC. Novel glass compositions enriched with TiO2/CaO led to crystallization of leucite GC of high aspect ratio, with high BFS and reliability. The study's findings suggest a potential high performance translucent leucite GC for use in the construction of dental restorations.

Study on the workability and early mechanical properties of carbon nanotube-coated fly ash modified cement-based grouting materials

Carbon nanotube (CNTs) reinforced cementitious composites have obvious advantages over traditional building materials in strength and impermeable durability. However, the material in question has not been widely employed in grouting engineering due to the limitations posed by dispersion issues. This study introduces a novel method of using amino functional groups to coat carbon nanotubes on the surface of fly ash to improve dispersion. The objective is to produce high-quality industrial-grade slurry and facilitate the transition of carbon nanotubes from small-scale research to large-scale engineering applications. The results show that mixing 14.3 wt% fly ash and 0.023 wt% CNTs in cementitious composites can replace an equivalent proportion of cement. Compared to traditional cement-based grouting material, the modified composites exhibit a 21.6 % increase in fluidity, a 7.9 % increase in the 7-day consolidation strength, and an 11.9 % reduction in the porosity of the slurry consolidation. The CNTs with large specific surface area on the fly ash surface provide sites for cement hydration reaction, accelerate cement hydration, and form dense hydration products. Additionally, the interaction between hydration products and CNTs within the microcracks of the cement matrix forms a 'bridging role' that enhances mechanical properties. The effectiveness of incorporating CNTs in reducing micro-damage during the loading process of slurry consolidation, enhancing integrity, and improving the grouting material's resistance to loading was further proved by acoustic emission, scanning electron microscopy, and fractal analysis of the fracture surface.

Analytical simulation of the effects of local mechanisms and microstructure on the creep response of unidirectional ceramic matrix composites

A micromechanics-based method has been developed to analyze the creep response of uncoated ceramic matrix minicomposites. Although the global stress level at which the creep response is analyzed is lower than the composite proportional limit, local stresses are assumed to be high enough that localized damage is present in the composite in the form of matrix microcracks. To model the composite creep response including the effects of matrix cracking, a fiber shear lag-based methodology is employed. In this approach, stresses are assumed to vary in the fiber as a function of time and distance from the crack plane. The varying stresses are then used to compute the overall creep strain for the composite. Various assumptions regarding the level of matrix microcracking in the composite, and the level of creep in the fiber and matrix in various portions of the composite unit cell, are also examined. The creep response of the fiber is modeled using a linear Burgers model. The model is applied to a SiCf/SiC unidirectional minicomposite system. The computed creep results are compared to experimentally obtained values. The effects of the local fiber volume fraction on the overall creep response of the composite are also studied. This work will allow increased understanding of the key material damage mechanisms and load sharing that take place during creep conditions and can be expanded to provide improved analysis methods for full macrocomposites.

Damage and failure mechanisms of carbon fiber reinforced composite thin-walled bushings under hygrothermal-mechanical loadings

Numerous thin-walled bushings, injection-molded from AS4 short carbon fiber reinforced polyetherketone (AS4/PEK) composites, are employed in the chains of stenter heat-setting machines. These bushings rapidly deteriorate under the hygrothermal-mechanical loadings prevalent in their service conditions. This necessitates regular replacements, critically limiting machine efficiency. In this paper, considering hygrothermal-mechanical loadings, the macro–micro dual-scale damage model of composite bushings is established. The wet and thermal effects are introduced into constitutive models and damage criteria based on the macro–micro mechanics of composites and their constituents. The micro-damage and macro-destruction phenomena in the bushing are analyzed, and the related measures are proposed to delay its failure. Comparing the simulation results with the test, the macro–micro damage and failure behavior are discussed for the failed bushing. It reveals that stress concentrations near AS4 fibers initiate interfacial damage and microcracks in the PEK matrix, inciting interface debonding and matrix fragmentation, ultimately provoking composites failure. Increasing the interface thickness can delay the onset of damage. Fiber tension failure, matrix tensile, and compressive failure induce destruction at the component extremities. According to the influence of hygrothermal performance, within the glass transition temperature range of 160 °C to 240 °C, selecting composites with higher glass transition temperatures can enhance the failure strength of bushings by 19 %. Conversely, within a moisture absorption range of 0.5 % to 0.8 %, opting for materials with lower moisture absorption decreases the failure strength of bushings by 37 %. Optimal failure strength is achieved with composites having a 0.5 % moisture absorption rate and a 240 °C glass transition temperature.

Reinforcement induced microcracking during the conversion of polymer-derived ceramics

This study investigates the role of discontinuous reinforcement on the microcracking of a preceramic polymer matrix during polymer to ceramic conversion. The material system comprised a carbosilane-based, preceramic polymer reinforced with mullite particles. The preceramic resin slurry was formulated for photopolymerization on a digital light projection 3D printer. Micro X-ray computed tomography, using a synchrotron light source, monitored a printed composite part during pyrolysis. The conversion of the carbosilane matrix into Si(O)C generated microcracks in the matrix that radially extended from particles. The likelihood of microcrack formation positively correlated with particle size. The largest particles (>104 µm3 volume or >60 µm side length) always abutted microcracks, while the matrix surrounding smaller particles (<5 µm) was free of microcracks. Numerical calculations showed the particles resist the shrinking matrix during its conversion. This created tensile stresses within the matrix. The driving force for microcrack growth was also analyzed with regard to the influence of matrix material parameters, particle form-factor and crack length. These results reveal the need to consider the form factor of discontinuous reinforcements, with regard to the material properties of the polymer-derived ceramic, in order to reduce or eliminate defects in the final part.

Reinforcement effects on the tensile properties of seawater sea-sand engineered cementitious composites reinforced with multi-scale hybrid fibers

The preparation of seawater sea-sand engineered cementitious composites (SS-ECC) holds great potential for development in coastal and marine infrastructure. To improve its compressive strength and tensile properties, this study investigated the crack characteristics, micro-mechanics, fibers reinforcement index, and micro-structure of SS-ECC using a multi-scale hybrid fibers system consisting of CaCO3 whiskers, PE fibers, and stainless steel fibers (SSF). The aim was to gain an in-depth understanding of the reinforcement effects of multi-scale hybrid fibers. The results showed that CaCO3 whiskers increased the number of cracks and the stability of crack evolution in SS-ECC during the ultimate tensile state by bridging the matrix micro-cracks, improving the matrix micro-structure, and filling the interfacial transition zones between the macroscopic fibers (PE fibers and SSF) and the matrix. Additionally, it reduced the crack width and spacing. These combined effects increased the compressive strength, tensile strength, and tensile strain capacity of SS-ECC by 12.8%, 15.7%, and 11.9%, respectively. Finally, the reinforcement effects of multi-scale hybrid fibers were analyzed based on the pseudo-strain hardening performance indices, fibers reinforcement index, and hybrid coefficient. This analysis provides reference and insight for the design of multi-scale hybrid fibers SS-ECC with a high comprehensive performance (σcσtε/w) index.

Effects of temperature and moisture fluctuations for suitable use of raw-crushed wind-turbine blade in concrete

Raw-crushed wind-turbine blade (RCWTB), a waste from the recycling of wind-turbine blades, is used as a raw material in concrete in this research. It contains not only fiberglass-composite fibers that bridge the cementitious matrix but also polyurethane and balsa-wood particles. Therefore, concrete containing RCWTB can be notably affected by moisture and temperature fluctuations and by exposure to high temperatures. In this research, the performance of five concrete mixes with 0.0%, 1.5%, 3.0%, 4.5%, and 6.0% RCWTB, respectively, is studied under moist/dry, alternating-sign-temperature-shock, and high-temperature-shock tests. Two damage mechanisms of RCWTB within concrete were found through these tests: on the one hand, micro-cracking of the cementitious matrix, which was verified by microscopic analyses and was dependent on concrete porosity; on the other, damage and degradation of the RCWTB components, as the polyurethane melted, and the balsa-wood particles burned. Both phenomena led to larger remaining-strain levels and reduced concrete compressive strength by up to 25% under temperature and humidity variations, although the bridging effect of the fiberglass-composite fibers was effective when adding RCWTB amounts higher than 3.0%. The compressive-strength loss after the high-temperature-shock test increased with the RCWTB content, reaching maximum values of 8% after an exposure time of 7 days. Statistical analyses revealed that effect of the RCA amount in the concrete was conditioned by the exposure times in all the tests. The accurate definition of those times is therefore key to set an RCWTB content in concrete that ensures its suitable behavior under the environmental conditions analyzed.

Carbonation resistance of recycled fine aggregate concrete reinforced by calcium sulfate whiskers

This study investigates mechanical and carbonation resistance tests conducted on Recycled Fine Aggregate Concrete (RFAC) with three different Recycled Fine Aggregate (RFA) replacement rates and four calcium sulfate whisker (CSW) contents, and investigates the effects of RFA replacement rate and CSW content on the mechanical properties of RFAC and the carbonation resistance of RFAC with calcium sulfate whiskers at different ages of carbonation. The microstructure of CSW-modified RFAC was observed and analyzed by SEM electron microscopy to explore further enhancement and toughening mechanisms of CSW in RFAC, as well as the optimal content of CSW in RFAC. In addition, the carbonation depth of recycled concrete with calcium sulfate beard is modeled based on a quadratic fitting formulation. The test results show that the mechanical properties and carbonation resistance of RFAC would decrease with the increase of the RFA replacement rate. Incorporating CSW can effectively improve the mechanical properties and carbonation resistance of RFAC. At 60 % RFA substitution, CSW at 2 % content has the most significant effect on the mechanical properties and carbonation resistance of RFAC. Microscopic analysis shows that CSW has a bridging effect at the micro-cracks in the concrete matrix, preventing the creation and expansion of micro-cracks in the concrete matrix, thus improving RFAC mechanical properties of RFAC and resistance to carbonation. The predicted results of the carbonation depth model which are constructed from the test data coincide with the results. We can use them to predict and analyze the carbonation depth of the carbonation resistance of recycled concrete.

Toughening epoxy by nano-structured block copolymer to mitigate matrix microcracking of carbon fibre composites at cryogenic temperatures

The incorporation of rigid nanoparticles has proven to enhance microcracking resistance in carbon fibre reinforced polymer (CFRP) composites at cryogenic temperatures, enabling CFRP tanks to store cryogenic liquid like hydrogen without requiring liners. Herein, we investigate efficacy of low-modulus soft nanoparticles in addressing the microcracking challenges inherent in CFRP at cryogenic temperatures. By incorporating a tri-block copolymer (BCP) into an epoxy, nano-structured fillers with an average diameter of approximately 100 nm are formed. Experimental results reveal that, at a 2.5 wt% loading, the BCP significantly increase the fracture energy of the nanocomposite by 392% at −196 °C while maintaining stiffness and strength. More importantly, composite laminates made with the BCP-modified nanocomposite matrix can withstand cryogenic temperatures without matrix microcracking, even when they contain multiple plies with the same orientation, such as [04/904]s, which are known to be highly susceptible to matrix microcracking at cryogenic temperatures. An advanced high-fidelity micromechanical model revealed that the observed toughening effect of nanostructured block copolymer at cryogenic temperatures can be attributed to the increased fracture resistance of the nanocomposite matrix. The findings of this research demonstrate that low concentration of block copolymer can effectively mitigate the initiation and propagation of matrix microcracks in carbon fibre composites at ultra-cold temperatures.

Characterization, quantitative evaluation, and formation mechanism of surface damage in ultrasonic vibration assisted scratching of Cf/SiC composites

Carbon fiber reinforced silicon carbide ceramic matrix composites (Cf/SiC composites) have a wide range of applications in aerospace, nuclear energy, braking systems owing to its excellent mechanical performance. Nevertheless, the hardness, brittleness, heterogeneity, and anisotropy of Cf/SiC composites give rise to difficulties in machining them. In addition, the characterization and formation mechanism of surface damage in the grinding of Cf/SiC composites have not been fully elucidated. The purpose of this paper is to provide a characterization method for surface damage of Cf/SiC composites and an evaluation index for surface edge chipping damage (SECD) through conventional scratching (CS) / ultrasonic vibration assisted scratching (UVAS) tests with single abrasive. Towards revealing the surface damage behavior of Cf/SiC composites during scratching, as well as the impact of fiber orientation on surface damage. The findings indicate that main forms of surface damage of Cf/SiC composites in single abrasive scratching are fiber breakage, fiber fracture-shedding, fiber fracture, fiber-matrix interface debonding, interface fragmentation, matrix cracking, and matrix microcracks. Further, ultrasonic vibration could help to suppress the SECD, and the SECD factor was smallest when scratching along the perpendicular fiber. Furthermore, the fiber orientation can significantly affect the scratching force and cross-sectional area of scratches on Cf/SiC composites via single abrasive scratching. The tangential scratching force was usually smaller as compared to the normal scratching force, and the cross-sectional area of scratches in UVAS is smaller than that in CS. Based on the above findings, this study elucidates the formation and evolution of surface damage after scratching under different fiber orientations, filling the research gap in surface damage under ultrasonic assisted machining of Cf/SiC composites and providing technical guidance for the machining.

Enhanced thermal shock resistance of low‐carbon Al2O3‐C refractories via CNTs/MgAl2O4 whiskers composite reinforcement

AbstractThe CNTs/MgAl2O4 whiskers composite reinforcement was prepared by catalytic carbon‐bed sintering and added to low‐carbon Al2O3‐C refractories to enhance the thermal shock resistance of refractories. The effects of Ni(NO3)2·6H2O content on the phase and microstructure of CNTs/MgAl2O4 were investigated. The nano‐indentation technology was used to quantitatively evaluate the effect of CNTs/MgAl2O4 on the thermal shock resistance of low‐carbon Al2O3‐C refractories, and the toughening mechanism of CNTs/MgAl2O4 on refractories was further investigated. The results revealed that the inner and outer diameters of the CNTs were approximately 5.69 and 16.26 nm, respectively. The CNTs winded around interlocking structural MgAl2O4 whiskers with a high aspect ratio (&gt;100), which was beneficial to the dispersion of CNTs in the refractory matrix. The thermal shock resistance of low‐carbon Al2O3‐C refractories was significantly improved by adding 3.0 wt.% of CNTs/MgAl2O4 (named C3). The specific fracture energy of refractory matrix and residual strength ratio of C3 reached 141 N·m−1 and 39.2%, which were 29.4% and 97.0% higher than those of the blank sample (109 N·m−1 and 19.9%), respectively. This is because the refractory matrix was significantly reinforced by CNTs and MgAl2O4 whiskers, promoting the occurrence of “bridging” and “crack deflection” and the generation of microcracks in the refractory matrix.

Tensile and flexural mechanical attributes of hybrid carbon/basalt fiber metal laminates under various hybridization and stacking sequences

In this work, experimental measurements and theoretical validation are adopted to examine the tensile and flexural behaviors of titanium-based carbon/basalt fiber metal laminates under various hybridization ratios and stacking sequences. Firstly, the mechanical response and damage patterns of fiber metal laminates (FMLs) subjected to different tensile and flexural loads have been explored. By observing the fracture surfaces of FMLs with various hybridization ratios and stacking sequences, scanning electron microscopy (SEM) has been used to identify the related microscopic damage patterns. The principal damage mechanism were attributed to fiber/matrix debonding, matrix microcrack, fiber pull-out, and delamination. Subsequently, to analyze the discreteness of experimental results and evaluate the theoretical flexural strength of FMLs under different conditions, the two-parameter Weibull statistics model for engineering application of FMLs was established. These results indicate that the tensile and flexural strength of FMLs can be improved by altering the hybridization ratios and stacking sequences. The thorough understanding of the mechanical behavior and failure mechanism of FMLs under various hybridization conditions can provide the basis for the design and utilization of FMLs structures.

Experimental Study on Flexural Fatigue Resistance of Recycled Fine Aggregate Concrete Incorporating Calcium Sulfate Whiskers

In order to study the flexural fatigue resistance of calcium sulfate whisker-modified recycled fine aggregate concrete (RFAC), flexural fatigue cyclic loading tests at different stress levels (0.6, 0.7, and 0.9) considering a calcium sulfate whisker (CSW) admixture as the main influencing factor were designed. Furthermore, the fatigue life was analyzed, and fatigue equations were established using the three-parameter Weibull distribution function theory. In addition, the micro-morphology of CSW-modified recycled fine aggregate concrete was observed and analyzed through Scanning Electron Microscopy (SEM), and the strengthening and toughening mechanisms of CSW on recycled fine aggregate concrete were further explored. The test results demonstrate that the inclusion of recycled fine aggregate reduces the fatigue life of concrete, while the incorporation of CSW can effectively improve the fatigue life of the recycled fine aggregate concrete, where 1% of CSW modification can extend the fatigue life of recycled fine aggregate concrete by 56.5%. Furthermore, the fatigue life of concrete under cyclic loading decreases rapidly as the maximum stress level increases. Fatigue life equations were established with double logarithmic curves, and P-S-N curves considering different survival probabilities (p = 0.5, 0.95) were derived. Microscopic analyses demonstrate that the CSW has a “bridging” effect at micro-seams in the concrete matrix, delaying the generation and enlargement of micro-cracks in the concrete matrix, thus resulting in improved mechanical properties and flexural fatigue resistance of the recycled fine aggregate concrete.

Effects of one-sided thermal shock and hold on alumina-based oxide/oxide ceramic matrix composites for temperatures in excess of 1200 °C

Nextel™720/alumina based ceramic matrix composite plates were subjected to one-sided thermal shock and 30-minute hold at 1100 °C, 1250 °C, 1400 °C and 1550 °C. Post exposure, the plates were evaluated for physical, structural, and microstructural changes using state-of-the art characterization techniques as well as standard mechanical testing procedures. Physically, no significant mass or dimensional changes were noted, while the surface roughness decreased on the exposed side for all the cases. Structurally, both the tensile and flexural strengths decreased for the 1550 °C case, but not for the other temperatures. In addition, short-beam strength tests showed no effect on interlaminar strength as a result of the elevated temperature exposures for the 30-minute duration considered. Microstructurally, SEM/EDS evaluations showed no significant change in the microstructure or element composition on the exposed and back sides. However, mercury intrusion porosity measurements did show a change in pore size distribution due to the combined effect of matrix shrinkage and further matrix microcracking. Combined results show that the operational limit of the composite can be pushed significantly beyond 1200 °C for shorter (in minutes) durations provided it is structurally viable for the application sought.

Hydrogen permeability of thin-ply composites after mechanical loading

Hydrogen is a sustainable alternative to conventional fuels, and it may be obtained with near zero carbon footprint. However, hydrogen storage remains a key challenge, and the use of composite tanks has gained significant interest over the last few years. In addition, thin-ply composites promote fibre damage by delaying matrix microcracking and free edge delamination. In this work, the H2 permeation/diffusion performance of virgin and mechanically loaded thin cross-ply laminates is studied. In addition, Scanning Electron Microscopy (SEM) is used to identify defects and micro-damage in the laminates and explain the experimental values. The study shows that the hydrogen (H2) barrier performances of thin-ply composites are lower than conventional metallic systems. Obtained permeability values, however, resulted well below the allowable limits for most combinations of temperature and pressure and remain unaffected despite the application of high tensile strains showing that permeation is not accelerated.

Exploring the Iosipescu method to investigate interlaminar shear fatigue behavior and failure mechanisms of carbon fiber reinforced composites

Iosipescu method, commonly adopted to determine the static shear properties of composite material, is extended to analyze the shear fatigue properties of carbon fiber reinforced epoxy composites. Following the results of the ultimate shear strength (τ) and interlaminar shear modulus (G13) in the static tests, this work conducted interlaminar shear fatigue tests with two stress ratios and four stress levels to obtain the S-N and stiffness degradation curves. The S‐N curves of the stress level versus logarithmic fatigue life exhibit a good linear relationship, and the specimens display a better fatigue performance at the stress ratio of 0.1 than −1. A significant three-stage cumulative damage evolution characteristic was found in the fatigue stiffness degradation process. In addition, the damage evolution, dominated by the propagation of matrix microcracks along the fiber direction inside the material, that is, debonding and delamination of the interface, was captured and analyzed via microscopic observation of the damaged zone on the specimen surface. Understanding the revealed processes of shear fatigue failure is crucial for ensuring the safe utilization of composite structures.

Ply-Blocking Phenomenon and Hole Size Effects in Modeling Progressive Damage in Fiber-Reinforced Plastics Laminates

Abstract This work presents a 3D progressive damage model based on Puck’s failure theory and linear damage evolution in fiber-reinforced plastic (FRP) laminates. It includes shear nonlinearity, in situ strengths, equivalent stress–strain, and mixed-mode fracture energy, and is implemented in abaqus/explicitTM through VUMAT subroutine. Various test cases were performed to validate the model and demonstrate its applications. The shear nonlinearity test shows that transverse compression retards matrix microcracking while transverse tension accelerates it. The open hole tension (OHT) test of laminates shows that delamination initiates around the holes and free edges, spreads the most, and propagates in different directions at different interfaces. Later, interfiber damage in 45 deg or −45 deg plies initiates and spreads at a slight inclination to the tip of the hole. Finally, fiber damage in 0 deg plies initiates at the tip of the hole, spreads the least, and propagates perpendicular to the loading direction. The ply-blocked laminates show around 30% higher strength and fracture strain than non-ply-blocked laminate due to delay in damage propagation, and are less sensitive to the hole size. Accordingly, their OHT strength reduces by 14.3% as opposed to 21.14% in the non-ply-blocked laminates, when the hole size increases from 6 to 9 mm. The damage location, magnitude, and propagation were corroborated with experimental findings in the literature.

In-plane thermal expansion behavior of M55J carbon fiber reinforced SiC matrix composite

With the rapid development of high-precision ultra-stable spacecraft, the absolute values of coefficients of thermal expansion (CTE) of carbon fiber reinforced SiC matrix composite (C/SiC) are required to be closer to 10−7 K−1 or even lower. In this work, high-modulus and low-expansion M55J carbon fiber was selected to replace T300 carbon fiber as the reinforcement to decrease the CTE of C/SiC. The interfacial region, pore distribution and microstructure evolution during the densification process were investigated. With the replacement of T300 carbon fiber by M55J carbon fiber as the reinforcement, axial thermal residual stress in SiC matrix can increase to 378 ± 26 MPa, resulting in the generation of matrix microcracks. This can reduce the constraint of matrix to carbon fiber and provide expansion space for M55J-C/SiC. Therefore, the average in-plane technical CTE of C/SiC in the range of −20 to 50 °C can be reduced from 1.09 × 10−6 to 0.34 × 10−6 K−1.