EFFECT OF Z-PINS’ DIAMETER,SPACING AND OVERLAP LENGTH ON CONNECTING PERFORMANCE OF CMC SINGLE LAP JOINT

The effect of through-thickness reinforcement by composite pins （Z-pins） on the static tensile strength and failure mechanisms of the joints made from ceramic matrix composite （CMC） is investigated. Overlap length of the single lap joint is 15 mm, 20 mm, 23 mm, 37 mm, and 60 mm, respectively. The experimental results indicate that the final failure modes of the joints can be divided into two groups, （a） the bond-line stops debonding until crack encounters Z-pins; and then the adherends break at the location of Z-pins, when overlap length is more than 20 mm; （b） the bond-line detaches entirely and Z-pins are drawn from adherends, when overlap length is equal to 15 mm. A simple efficient computational approach is presented for analyzing the benefit of through-thickness pins for restricting failure in the single lap joints. Here, the mechanics problem is simplified by representing the effect of the pins by tractions acting on the fracture surfaces of the cracked bond-line. The tractions are prescribed as functions of the crack displacement, which are available in simple forms that summarize the complex deformations to a reasonable accuracy. The resulting model can be used to track the evolution of complete failure mechanisms, for example, bond-line initial delamination and ultimate failure associated with Z-pin pullout, ultimate failure of the adherends. The paper simulates connecting performance of the single lap joints with different Z-pins＇ diameter, spacing and overlap length; the numerical results agree with the experimental results; the numerical results indicate enlarging diameter and decreasing spacing of Z-pins are in favor of improving the connecting performance of the joints. By numerical analysis method, the critical overlap length that lies between two final failure modes is between 18 mm and 19 mm, when Z-pins＇ diameter and spacing are 0.4 mm, 5 mm, respectively.

SIMULATION OF COMPOSITE NON-LINEAR MECHANICAL BEHAVIOR OF CMCS BY FEM-BASED MULTI-SCALE APPROACH

The non-linear behavior of continuous fiber reinforced C/SiC ceramic matrix composites(CMCs)under tensile loading is modeled by three-dimensional representative volume element(RVE)models of the composite. The theoretical background of the multi-scale approach solved by the finite element method(FEM)is recalled firstly.Then the geometric characters of three kinds of damage mechanisms,i.e.micro matrix cracks,fiber/matrix interface debonding and fiber fracture,are studied.Three kinds of RVE are proposed to model the microstructure of C/SiC with above damage mechanisms respectively.The matrix cracking is modeled by critical matrix strain energy(CMSE)principle while a maximum shear stress criterion is used for modeling fiber/matrix interface debonding. The behavior of fiber fracture is modeled by the famous Weibull statistic theory.A numerical example of continuous fiber reinforced C/SiC composite under tensile loading is performed.The results show that the stress/strain curve predicted by the developed model agrees with experimental data.

High-temperature oxidation resistance of Si-coated C/SiC composites

In this paper,Si coatings were sprayed onto C/SiC composite substrates by atmospheric plasma spraying(APS).The high-temperature oxidation behavior of the substrate and coating at temperatures of 1100 and 1300℃was also studied.The C/SiC ceramic matrix composite will be damaged seriously and even failed due to the oxidation of carbon fibers in matrix.The Si coating effectively improved the oxidation resistance of the C/SiC substrate in the high-temperature oxidation test.The effect of the thickness of the Si coatings on the oxidation resistance was investigated.The 150-μm coating is proved to enable the substrate to have the lowest oxidation weight loss and the best oxidation resistance after static oxidation for 5 h.

Rare earth monosilicates as oxidation resistant interphase for SiCf/SiC CMC:Investigation of SiC_(f)/Yb_(2)SiO_(5)model composites

Model composites consisting of SiC fiber and Yb_(2)SiO_(5)were processed by the spark plasma sintering(SPS)method.The mechanical compatibility and chemical stability between Yb_(2)SiO_(5)and SiC fiber were studied to evaluate the potential application of Yb monosilicate as the interphase of silicon carbide fiber reinforced silicon carbide ceramic matrix composite(SiC_(f)/SiC CMC).Two kinds of interfaces,namely mechanical and chemical bonding interfaces,were achieved by adjusting sintering temperature.SiC_(f)/Yb_(2)SiO_(5)interfaces prepared at 1450 and 1500℃exhibit high interface strength and debond energy,which do not satisfy the crack deflection criteria based on He-Hutchison diagram.Raman spectrum analyzation indicates that the thermal expansion mismatch between Yb_(2)SiO_(5)and SiC contributes to high compressive thermal stress at interface,and leads to high interfacial parameters.Amorphous layer at interface in model composite sintered at 1550℃is related to the diffusion promoted by high temperature and DC electric filed during SPS.It is inspired that the interfacial parameters could be adjusted by introducing Yb_(2)Si_(2)O_(7)-Yb_(2)SiO_(5)interphase with controlled composition to optimize the mechanical fuse mechanism in SiC_(f)/SiC CMC.

A novel approach for manufacturing of layered,ultra-refractory composites using pliable,short fibre-reinforced ceramic sheets

A new additive technique for manufacturing of short fibre-reinforced ultra-refractory ceramics is presented.This technique allows the fabrication of solvent-free,thin(~100µm),flexible,and easy-to-handle sheets suitable for fabricating homogeneous or layered structures.A large range of compositions,in terms of matrix and fibre volumetric contents,from 0%to 100%is possible.The amount of short carbon fibres incorporated in the sheets ranged from 20 to 50 vol%,whereas the fibre length ranged from 3 to 5 mm.The matrix composition investigated with this technique consisted of ZrB_(2)/SiC/Y_(2)O_(3).By increasing the fibre amount from 35 to 50 vol%,an improvement of mechanical properties was observed.Four-point flexural strength(σ)ranged from 107 to 140 MPa,depending on the amount of carbon fibres(Cf).The same holds true for the work of fracture,ranging from 108 to 253 J/m^(2).Functionally graded composites were fabricated by overlapping sheets with a fibre gradient(0%-50%).

Propulsion tests on ultra-high-temperature ceramic matrix composites for reusable rocket nozzles

Ultra-high-temperature ceramic matrix composites(UHTCMCs)based on a ZrB_(2)/SiC matrix have been investigated for the fabrication of reusable nozzles for propulsion.Three de Laval nozzle prototypes,obtained by sintering with either hot pressing(HP)or spark plasma sintering(SPS),were tested 2-3 times in a hybrid rocket motor for proving reusability.Sections were extracted after oxidation tests to study the microstructural changes and oxidative and thermomechanical stresses induced by the repeated tests.Compared to a reference graphite nozzle,no measurable erosion was observed for the UHTCMC-based nozzles.The oxidation mechanism consisted in the formation of a ZrO_(2)intermediate layer,with a liquid silicon oxide(SiO_(2))layer on the surface that was displaced by the action of the gas flux towards the divergent part of the nozzle,protecting it from further oxidation.Both specimens obtained by HP and SPS displayed similar performance,with very slight differences,which were attributed to small changes in porosity.These tests demonstrated the capability of complex-shaped prototypes made of the developed UHTCMCs to survive repeated exposure to environments representative of a realistic space propulsion application,for overall operating time up to 30 s,without any failure nor measurable erosion,making a promising step towards the development of reusable rocket components.

Design, Preparation and Properties of Carbon Fiber Reinforced Ultra-High Temperature Ceramic Composites for Aerospace Applications： A Review

Carbon fiber reinforced ultra-high temperature ceramic （UHTC） composites, consisting of carbon fibers embedded in a UHTC-matrix or a C-SiC-UHTC-matrix, are deemed as the most viable class of materials that can overcome the poor fracture toughness and thermal shock resistance of monolithic UHTC ma- terials, and also improve the oxidation resistance and ablation resistance of C/C and C/SiC composites at ultra-high temperatures. In this review, we summarize the different processing routes of the compos- ites based on the UHTC introducing methods, including chemical vapor infiltration/deposition （CVI/D）, precursor infiltration and pyrolysis （PIP）, reactive melt infiltration （RMI）, slurry infiltration （SI）. in-sito reaction, hot pressing （HP）, etc; and the advantages and drawbacks of each method are briefly dis- cussed. The carbon fiber reinforced UHTC composites can be highly tailorable materials in terms of fiber. interface, and matrix. From the perspective of service environmental applications for engine propul- sions anti hypersonic vehicles, the material designs （mainly focusing on the composition, quantity, structure of matrix, as well as the architecture of carbon fibers, UHTCs and pores）, their relevant processing routes and properties （emphasizing on the mechanical and ablation properties） are discussed in this paper. In addition, we propose a material architecture to realize the multi-function through changing the distri- bution of carbon fibers, UHTCs and pores, which will be an important issue for future development of carbon fiber reinforced UHTC composites.

Frequency-dependent random fatigue of panel-type structures made of ceramic matrix composites

The panel-type structures used in aerospace engineering can be subjected to severe highfrequency acoustic loadings in service. This paper evaluates the frequency-dependent random fatigue of panel-type structures made of ceramic matrix composites（CMCs） under acoustic loadings. Firstly, the high-frequency random responses from the broadband random excitation will result in more stress cycles in a deinite period of time. The probability density distributions of stress amplitudes will be different in different frequency bandwidths, though the peak stress estimations are identical. Secondly, the fatigue properties of CMCs can be highly frequency-dependent. The fatigue evaluation method for the random vibration case is adopted to evaluate the fatigue damage of a representative stiffened panel structure. The frequency effect through S-N curves on random fatigue damage is numerically veriied. Finally, a parameter is demonstrated to characterize the mean vibration frequency of a random process, and hence this parameter can further be considered as a reasonable loading frequency in the fatigue tests of CMCs to obtain more reliable S-N curves.Therefore, the inluence of vibration frequency can be incorporated in the random fatigue model from the two perspectives.

Effect of interface debonding on matrix multicracking evolution of fiber-reinfroced ceramic-matrix composites

An analytical methodology was developed to investigate the effect of fiber/matrix interface debonding on matrix multicracking evolution of fiber-reinforced CMCs(ceramic-matrix composites).The Budiansky-Hutchinson-Evans shear-lag model was adopted to analyse the micro-stress field of the damaged composites.The critical matrix strain energy criterion,which presupposes the existence of an ultimate or critical matrix strain energy with matrix,was obtained to simulate the matrix multicracking evolution of CMCs.With the increase of the applied stress,the matrix multicracking and fiber/matrix interface debonding occurred to dissipate the additional energy entered into the composites.The fiber/matrix interface debonded length under matrix multicracking evolution was obtained by treating the interface debonding as a particular crack propagation problem.The conditions for no-debonding and debonding during the evolution of matrix multicracking were discussed in terms of two interfacial properties,i.e.,the interface shear stress and interface debonded toughness.When the fiber/matrix interface was bonded,the matrix multicracking evolution was much more intense compared with the interface debonding;when the fiber/matrix interface was debonded,the matrix crack density increased with the increasing of interface shear stress and interface debonded energy.The theoretical results were compared with experimental data of unidirectional SiC/CAS(calcium alumina silicate),SiC/CAS-Ⅱand SiC/borosilicate composites.

Microstructure and high-temperature oxidation behavior of plasma-sprayed Si/Yb2SiO5 environmental barrier coatings

An environmental barrier coating(EBC) consisting of a silicon bond coat and an Yb2-SiO5 top-coat was sprayed on a carbon fibers reinforced SiC ceramic matrix composite(CMC) by atmospheric plasma spray(APS). The microstructure of the coating annealed at 1300 ℃ and its high-temperature oxidation behavior at 1350 ℃ were investigated. The significant mass loss of silica during the plasma spray process led to the formation of Yb2SiO5 and Yb2O3 binary phases in the top-coat. Eutectics of Yb2SiO5 and Yb2O3 were precipitated in the top-coat, and channel cracks were formed in the top-coat after 20 h annealing because of the mismatch between the coefficients of thermal expansion(CTEs) of Yb2SiO5 and the SiC substrate. The EBC effectively improved the oxidation resistance of the CMC substrate. The channel cracks in the Yb2SiO5 top-coat provided inward diffusion channels for oxygen and led to the formation of oxidation delamination cracks in the bond coat, finally resulting in spallation failure of the coating after 80 h oxidation.

Fatigue Life of a 2.5D C/SiC Composite Under Tension–Tension Cyclic Loading:Experimental Investigation and Sensitivity Analysis

Engineering structures made of ceramic matrix composites(CMCs)usually suffer from cyclic loads during service,which could lead to disastrous failures.This work focuses on the fatigue behavior of a 2.5D C/SiC composite under tension–tension cyclic loading.Experiments of the 2.5D C/SiC composite are firstly carried out to determine the fatigue lifetime of the material at different stress levels.The fracture surfaces examined by a scanning electronic microscope indicate that the damage mechanisms under cyclic loading are closely related to crack propagation,fiber/matrix interfacial degradation,and fiber breakage.Considering the damage evolution of fibers and interfacial resistance,a micromechanical model is adopted to describe the fatigue behavior of 2.5D C/SiC composite,and the numerical results are compared with the experimental results.Further,a sensitivity analysis is performed as a function of the interfacial shear stress,fiber Weibull modulus,and fiber strength.The calculation of sensitivity factors shows that the variations of the fiber Weibull modulus and fiber strength have the most significant influence and,thereafter,the variation of interfacial shear stress.