Experimental and numerical study on creep behaviors of 2D twill woven quartz fiber/silica matrix composites

This study investigates the creep deformation, damage, and rupture behaviors of 2D woven SiO₂/SiO₂ composites via experimental and numerical methods. In situ monotonic tensile tests and creep tests were conducted at 900 °C using a self-designed experimental system and digital image correlation. The tested specimens were characterized by X-ray computed tomography and scanning electron microscopy to conduct quantitative analyses and fracture observations. The obtained creep strain–time curves consist of primary and secondary stages, similar to the creep strain–time curves of most ceramic matrix composites. The matrix at the intersection of fiber bundles cracked under tensile loading. During subsequent creep loading, the propagation of matrix cracks, interfacial debonding, and fiber breakage in longitudinal fiber bundles were observed. At the mesoscale, the creep rupture entails a mechanism analogous to that observed in the monotonic tensile tests. Overall, the SiO₂/SiO₂ composites employed in this study exhibit excellent potential for long-term operation under mechanical loads at high temperatures. Next, a micromechanics-based creep model was proposed to simulate the creep behavior of the composites. In this model, the primary creep law and rule of mixtures were combined to describe the stress redistribution of various constituents and predict the deformation of the composites. In addition, the rupture life was predicted based on the global load-sharing model, two-parameter Weibull model, and shear lag model. The degradation of the matrix modulus and fiber strength was also considered to improve the accuracy of the simulation. The predicted results were in good agreement with the experimental data.     

Strain field evolution of 2D needled C/SiC composites under tension

In this study, the digital image correlation (DIC) technique was applied to the tensile test of 2D needled C/SiC composites as a full-field measuring tool with the purpose of characterizing the tensile behavior of the material. The non-linear macroscopic tensile stress-strain curve was obtained. The relationship between the local non-linearity and the macroscopic non-linearity was investigated. The spot- and band-type strain field distributions were observed, and the evolution of the non-uniform strain field distribution was studied. The correlation between the strain field distribution and the structure of the needled preform of the material was also discussed.     

Volumetric assessment of fatigue damage in a SiCf/SiC ceramic matrix composite via in situ X-ray computed tomography

To enhance the understanding of matrix cracking and damage progression on the macroscopic scale, within a 0/90° fibre reinforced SiC_f/SiC ceramic matrix composite (CMC), X-ray computed tomography (XCT) imaging and analysis have been performed in conjunction with a commercially available in-situ mechanical loading device. CMC test coupons were subjected to tensile cyclic loads and inspected using XCT without removal from the tensile loading device. Attempts to measure and quantify the resulting damage using volumetric image analysis techniques are presented, by characterising the crack network from XCT images acquired at both the maximum and minimum load condition during selected fatigue cycles. The XCT detection of significant crack development within the first loading half-cycle shows good agreement with cumulative acoustic emission energy data recorded under similar test conditions. The results are seen as an important step towards correlating the damage behaviour detected via different NDE and health monitoring techniques.     

Microstructure evolution in densification of SiC ceramics by aluminium vapour infiltration and investigation of mechanical properties

The densification and phase formation of 6?wt% Y₂O₃ containing SiC compacts infiltrated by aluminium vapour were investigated. The densification occurred through infiltration of aluminium vapour that formed through a reaction between alumina and carbon powders at 2000?°C. Infiltrated specimens were evaluated concerning the density, phase, microstructure and mechanical properties including hardness and fracture toughness. X-ray diffraction studies showed the presence of yttrium aluminium garnet (YAG) and Al₂O₃ as the secondary phases along with other minor phases. Sectioning of the infiltrated specimen showed two regions: a dense layer starting from the surface of about 1?mm thickness followed by a relatively porous structure at the core. The effect of infiltration depth on densification and evolution of microstructure are studied. Also, the changes in Vickers’ hardness and fracture toughness with the increase in specimen depth are discussed.     

Damage accumulation mechanisms during dynamic compressive failure of boron carbide

This paper explores the compressive response of two grades of boron carbide across a range of strain rates between 10² to 10³ s^?1. Using ultra-high-speed photography to perform digital image correlation in conjunction with strain gauges, the evolution of apparent elastic moduli was tracked through the failure process. Analysis of damage metrics shows that the two most important measures of damage accumulation in these materials are apparent changes to shear modulus and Poisson’s ratio, rather than only apparent changes to Young’s modulus as is commonly assumed in the literature. For one grade of the boron carbide the range of strain-rates studied also included a transition from quasi-static-type failure behavior where bulking was not a significant factor, to dynamic-type failure where bulking became a significant influence on failure; this transition was linked to material-specific fracture mechanisms. For the other grade of boron carbide, only dynamic-type failure was seen in this strain-rate range, suggesting that the transition point was at a lower strain-rate.     

Damage evolution analysis in mortar, during compressive loading using acoustic emission and X-ray tomography: Effects of the sand/cement ratio

This paper explores the use of acoustic emission (AE) and X-ray tomography to identify the mechanisms of damage and the fracture process during compressive loading on concrete specimens. Three-dimensional (3D) X-ray tomography image analysis was used to observe defects of virgin mortar specimen under different compressive loads. Cumulative AE events were used to evaluate damage process in real time according to the sand/cement ratio. This work shows that AE and X-ray tomography are complementary nondestructive methods to measure, characterise and locate damage sites in mortar. The effect of the sand proportion on damage and fracture behaviour is studied, in relation with the microstructure of the material.     

R-curves determination of ordinary refractory ceramics assisted by digital image correlation method

As a figure-of-merit, the rising ratio of crack propagation resistance to fracture initiation resistance indicates a reduction of the brittleness and enhances the thermal shock resistance of ordinary refractory ceramics. The significant nonlinear fracture behaviour is related to the development of a fracture process zone (FPZ). The universal dimensionless load–displacement diagram method is applied as a promising graphical method for the determination of R-curves for magnesia refractories showing different brittleness. By applying digital image correlation (DIC) together with the graphical method, the problems arisen with accurate determination of the fracture initiation resistance and the crack length are overcome. Meanwhile, the R-curve is subdivided with respect to the fracture processes, viz the fracture initiation, the development of FPZ and the onset of traction free macro-crack. With the simultaneous crack lengths evaluated from DIC, the contribution of each fracture process to the crack propagation resistance at certain loading stage is quantitatively presented.     

Damage and failure analysis of a SiCf/SiC ceramic matrix composite using digital image correlation and acoustic emission

The analysis of failure behaviors of continuous fiber-reinforced ceramic matrix composites (CMCs) requires the characterization of the damage evolution process. In service environments, CMCs exhibit complex damage mechanisms and failure modes, which are affected by constituent materials, meso architecture, inherent defects, and loading conditions. In this paper, the in-plane tensile mechanical behavior of a plain woven SiC_f/SiC CMC was investigated, and damage evolution and failure process were studied in detail by digital image correlation (DIC) and acoustic emission (AE) methods. The results show that: the initiation of macro-matrix cracks have obvious local characteristic, and the propagation paths are periodically distributed on the material surface; different damage modes (matrix cracking and fiber fracture) would affect the AE energy signal and can be observed in real-time; the significant increase of AE accumulated energy indicates that serious damage occurs inside the material, and the macroscopic mechanical behavior exhibits nonlinear characteristic, which corresponds to the proportional limit stress (PLS) of the material.     

In situ tensile damage characterization of C/C composites through X-ray computed tomography and digital volume correlation

Carbon fiber reinforced carbon matrix (C/C) composites have been used in aerospace applications due to their excellent performance. Exploring their failure mechanisms is a subject of extensive research. Nowadays, to obtain information about changes in the failure processes, a technology known as in situ X-ray computed tomography is used. In this paper, tensile loads were applied to 3D fine-woven punctured and needle-punched C/C composites perpendicular to the punctured and needle-punched directions. In situ X-ray computed tomography was employed to observe damage development, and digital volume correlation was used to assess the laboratory X-ray computed tomographs to measure local strains. Assimilation of pores is observed in C/C composites, with cracks evolving from original micro-pores. While fine-woven punctured C/C composites present an elegant linear failure, needle-punched C/C composites present a traditional non-linear failure. This difference is due to the different structures of the preforms. Furthermore, the C/C composites are weak at the sites where they are punctured or needle-punched.     

Improvements in microstructural and mechanical properties of ZrO2 ceramics after addition of BaO

The effects of the addition of BaO on the sinterability, phase balance, microstructure, and mechanical properties of 8 mol% yttria-stabilized cubic zirconia (8YSZ) were investigated using scanning electron microscopy, X-ray diffraction (XRD) analyses, and micro-hardness testing. The 8YSZ powder was doped with 0–15 wt% BaO using a colloidal process. The undoped and BaO-doped 8YSZ specimens were sintered at 1550 °C for 1 h. The XRD analyses results showed that the specimens doped with up to 1 wt% BaO did not exhibit BaO-related peaks, indicating that BaO was completely solubilized in the 8YSZ matrix. However, when more than 1 wt% BaO was added, BaZrO₃-related peaks appeared, suggesting that the overdoped BaO did not dissolve in the 8YSZ matrix but formed a secondary phase of BaZrO₃ at high temperatures. Grain size measurements showed that the grain size of 8YSZ decreased with an increase in the amount of BaO added. The decrease in the grain size was owing to the fact that the grains of BaZrO₃, which precipitated at the grain boundaries and grain junctions of 8YSZ, increased the grain boundary cohesive resistance because of the pinning effect. This resulted in a decrease in the grain boundary mobility, and an increase in the grain boundary energy. Furthermore, while the addition of BaO to 8YSZ caused a slight decrease in the hardness of 8YSZ, the fracture toughness of 8YSZ increased from 1.64 MPa m^1/2 to 2.08 MPa m^1/2, owing to the resulting decrease in the grain size.     

Development and evaluation of sub-element testing of SiC/SiC ceramic matrix composites at elevated temperatures

SiC/SiC ceramic matrix composites (CMCs) are being developed for use in aero-engines to replace nickel superalloy components. Sub-element testing acts as the key stepping stone in bridging understanding derived from basic coupon testing and more complex component testing. This study presents the development of high temperature C-shape sub-element testing with the use of digital image correlation to study damage progression. The specimen is designed with a bias towards a mixed mode-stress state more similar to what a CMC component may see in service. Both monotonic and fatigue tests were completed on C specimens and compared with predicted behaviour from modelling. Test data from both test types suggested that specimens were failing once they reached a critical radial stress level. However evidence from fractography of specimens showed that in both monotonic and fatigue tests radial cracks (driven by hoop stresses) are initiating prior to circumferential cracks.     

Thermal expansion and mechanical properties of self-reinforced aluminum titanate ceramics with elongated grains

To fabricate aluminum titanate ceramics that possess both low thermal expansion coefficients and excellent mechanical properties, the co-doping of MgO with Y₂O₃, La₂O₃ and Nb₂O₅ was examined. Doping with MgO lowered the formation reaction temperature of aluminum titanate and prevented the formation of oriented grain regions. Liquid-phase sintering at 1500 °C of the MgO-La₂O₃-doped ceramic resulted in the formation of a minor amount of elongated grains with lengths of approximately 130 μm. This microstructure resulted in a high resistance against crack propagation during the bend test. Grain pull-out and grain bridging mechanisms as well as crack deflection and branching resulted in the high resistance. A low thermal expansion coefficient of 0.7 × 10⁻⁶/deg was observed for this ceramic. The co-doping of MgOY₂O₃ led to high bending strength and moderate low thermal expansion coefficient. The co-doping of MgO-Nb₂O₅ resulted in an extended grain growth by liquid-phase sintering at 1500 °C and poor mechanical properties.     

Microstructural evolution and texture analysis of magnesium phosphate cement

Three-dimensional quantitative image analysis from synchrotron X-ray microcomputed tomography indicated a coarsening of the microstructure of magnesium potassium phosphate cements driven by crystallization of K-struvite from the first amorphous product. Porosity and pore surface area increased because of the progressive build-up of a network of elongated/tabular crystal domains, with density higher than the amorphous. The known increase in strength with time is thought to occur thanks to the overwhelming contribution of a developing interlocked lath-shaped microstructure. Combined X-ray and neutron diffraction texture analysis indicated that at least a fraction of K-struvite nucleates at the surface of MgO grains, suggesting the intervention of more than one crystallization mechanism. The detected weak texture, compatible with a nearly random orientation of crystallites, and the isotropic pore fabric, are beneficial with respect to crack propagation.     

Interface controlled micro- and macro- mechanical properties of aluminosilicate fiber reinforced SiC matrix composites

Novel Nextel™ 440 aluminosilicate fiber reinforced SiC matrix composites, with/without chemical vapor deposited carbon interphase were fabricated by polymer derived ceramic process, and they were studied by a combination of micro- and macro- mechanical techniques such as nanoindentation, micropillar splitting, fiber push-in, digital image correction and high temperature three point bend tests. Specifically, micropillar splitting test was firstly employed to measure in-situ the localized fracture toughness. The results revealed that the carbon interphase can effectively hinder the interfacial reactions between Nextel™ 440 fiber and SiC matrix, thus remarkably weakening the composite interfacial shear strength from ∼293 MPa to ∼42 MPa, and enhance the composite fracture toughness from ∼1.8 MPa√m to ∼6.3 MPa√m, respectively. This is mainly a consequence of weak interface that triggers crack deflection at the fiber/interphase interface. Finally, this novel composite showed stable mechanical properties in vacuum at temperature range from 25 °C to 1000 °C.     

The effect of alkaline earth carbonates on the microstructure and mechanical properties of impermeable and lightweight ceramics

Lightweight impermeable ceramic bodies were designed by combining pore templating and controlled viscous sintering through in-situ crystallization. Various amounts of limestone were added to a glass-fluxed low-temperature stoneware tile formulation. Closed porosity was created by decomposition of carbonates prior to sintering, thus leaving voids that were not completely filled by the viscous melt. The resulting oxides chemically modified the liquid phase and promoted the crystallization of β-wollastonite, diopside and anorthite. Hence, viscous sintering was affected. The addition of limestone brought on several advantages: the temperature of maximum sintering rate was decreased (<900?°C); the dimensional stability range was extended; the matrix was reinforced by newly-formed crystals that compensated for the global structure weakening evoked by increased porosity; an increase in whiteness was observed in concomitance to crystallization, reaching values only obtained when using zircon as opacifier (L*=87).     

Effects of debinding condition on microstructure and densification of alumina ceramics shaped with photopolymerization-based additive manufacturing technology

Alumina green bodies shaped with digital light processing (DLP) technology were debinded in four atmospheres, including air, argon (Ar), mixture of 95% argon and 5% hydrogen (95% Ar +5% H₂) and vacuum, at different heating rates (0.5 °C/min, 1 °C/min and 3 °C/min), followed by sintering in air at 1650 °C for 3 h. The effects of debinding atmosphere and heating rate on microstructure and densification of the brown bodies and sintered bodies were evaluated via morphology characterization, X-ray computed tomography (CT) reconstructions and other tests. Focusing on the quantitative analysis of the geometric characteristics and distribution of defects in the brown bodies and sintered bodies, it is found that the type (holes, cracks and delamination), content and volume distribution of defects during the debinding in aerobic and anaerobic environments are different. Our study indicates that low-rate vacuum debinding should be adopted as a promising debinding approach for ceramics shaped with photopolymerization-based additive manufacturing technology.     

Damage evolution in SiC/SiC unidirectional composites by X-ray tomography

Melt infiltrated SiC/SiC ceramic matrix composite unidirectional (UD) composite specimens were imaged under load using X-ray microtomography techniques in order to visualize the evolution of damage accumulation and to quantify damage mechanisms within the composite such as matrix cracking and fiber breaking. The data obtained from these in situ tensile tests were used in comparison with current models and literature results. Three-dimensional (3D) tomography images were used to measure the location and spacing of matrix cracking that occurred at increasing stress increments during testing within two UD composite specimens. The number of broken fibers and the location of each fiber break gap that occurred within the volume of both specimens were also quantified. The 3D locations of fiber breaks were correlated with the location of each matrix crack within the volume of the specimen and it was found that at the stress scanned directly before failure, most of the fiber breaks occur within 100 microns of a matrix crack.     

Microstructure and damage evolution of SiCf/PyC/SiC and SiCf/BN/SiC mini-composites: A synchrotron X-ray computed microtomography study

SiC_f/PyC/SiC and SiC_f/BN/SiC mini-composites comprising single tow SiC fibre-reinforced SiC with chemical vapor deposited PyC or BN interface layers are fabricated. The microstructure evolutions of the mini-composite samples as the oxidation temperature increases (oxidation at 1000, 1200, 1400, and 1600?°C in air for 2?h) are observed by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction characterization methods. The damage evolution for each component of the as-fabricated SiC_f/SiC composites (SiC fibre, PyC/BN interface, SiC matrix, and mesophase) is mapped as a three-dimensional (3D) image and quantified with X-ray computed tomography. The mechanical performance of the composites is investigated via tensile tests.The results reveal that tensile failure occurs after the delamination and fibre pull-out in the SiC_f/PyC/SiC composites due to the volatilization of the PyC interface at high temperatures in the air environment. Meanwhile, the gaps between the fibres and matrix lead to rapid oxidation and crack propagation from the SiC matrix to SiC fibre, resulting in the failure of the SiC_f/PyC/SiC composites as the oxidation temperature increases to 1600?°C. On the other hand, the oxidation products of B₂O₃ molten compounds (reacted from the BN interface) fill up the fracture, cracks, and voids in the SiC matrix, providing excellent strength retention at elevated oxidation temperatures. Moreover, under the protection of B₂O₃, the SiC_f/BN/SiC mini-composites show a nearly intact microstructure of the SiC fibre, a low void growth rate from the matrix to fibre, and inhibition of new void formation and the SiO₂ grain growth from room to high temperatures. This work provides guidance for predicting the service life of SiC_f/PyC/SiC and SiC_f/BN/SiC composite materials, and is fundamental for establishing multiscale damage models on a local scale.     

Damage characterization model of ceramic coating systems based on energy analysis and bending tests

A quantitative damage model of ceramic coating systems was developed based on their load-displacement curves obtained from three-point bending tests. According to the energy mechanism of damage, the normalized damage rate of such systems can be simply expressed using the load and the tangent slope of their load-displacement curves. The experimental results demonstrated the thickness dependence of fracture and damage. In thin coating systems, tensile failure was found to be predominant and multiple transverse cracks appeared in the coatings. In contrast, thick coating systems showed a predominance of interface shear failure and the occurrence of interface delamination. These observations are consistent with previous experimental results. The damage of the systems displayed catastrophic characteristics when the load tended to reach the failure point, i.e., the damage increased rapidly, and the damage rate displayed a power-law singularity at the failure point. These results are consistent with the damage characteristics predicted using the mathematic model. The damage evolution in the case of interface delamination in the thick coating systems was faster than that for transverse cracking in the thin coatings because of the difference in the degree of damage localization. The present model provides an effective method to elucidate the damage behavior of brittle ceramic coating systems, and hence, it is expected to greatly aid the coating design.     

Microstructure and mechanical properties of C/C composites modified by single-source precursor derived ceramics

To improve the mechanical properties of C/C composites, the SiC(N)/TiC ceramic derived from single-source precursors (SSPs) were introduced into the C/C by precursor infiltration and pyrolysis (PIP) at 1100 °C. The shear strength of all modified composites were improved by nearly 2 times compared with the pristine C/C composites (37.4 MPa) due to the increased interfaces by multiphases incorporation. After further annealing at 1500 °C, the SiC(N)/TiC ceramic in C/C composite transferred into the nanocomposites, SiC(N)/TiC-NCs, in which nano-scale TiC particles distributed into the SiC(N) matrix. The modified samples still exhibited about 39% improvement on shear strength. Large numbers of uniform micro-cracks occurred in both above SSPs derived ceramic cases, reducing stress concentration during the shear testing. Moreover, some ring-shaped cracks were observed in the fracture region which can consume large amounts of energy in crack propagation process and then benefit to improve the shear strength.