Interpreting acoustic energy emission in SiC/SiC minicomposites through modeling of fracture surface areas

The relationship between acoustic emission (AE) and damage source areas in SiC/SiC minicomposites was modeled using insights from tensile testing in-scanning electron microscope (SEM). Damage up to matrix crack saturation was bounded by: (1) AE generated by matrix cracking (lower bound) and (2) AE generated by matrix cracking, and fiber debonding and sliding in crack wakes (upper bound). While fiber debonding and sliding exhibit lower strain energy release rates than matrix cracking and fiber breakage, they contribute significant damage area and likely produce AE. Fiber breaks beyond matrix crack saturation were modeled by two conditions: (i) only fiber breaks generated AE; and (ii) fiber breaks occurred simultaneously with fiber sliding to generate AE. While fiber breaks are considered the dominant late-stage mechanism, our modeling indicates that other mechanisms are active, a finding that is supported by experimental in-SEM observations of matrix cracking in conjunction with fiber failure at rupture.     

Matrix cracking onset stress and strain as a function of temperature,and characterisation of damage modes in SiCf/SiC ceramic matrix composites via acoustic emission

The complex damage mechanisms that accumulate within SiC_f/SiC ceramic matrix composites (CMCs) subject to thermal and mechanical stress are being investigated in anticipation of the material’s introduction into high performance gas turbine engines. Acoustic emission (AE) is recognised as a leading non-destructive evaluation (NDE) tool to this end, and was used in this study to determine the so-called matrix cracking onset stress under tensile load as a function of temperature up to a maximum of 1100 °C. Onset stress was interpreted using three traditional measurements based on AE energy characteristics during monotonic tests to failure. Pattern recognition (PR) analysis was performed on the AE data, revealing a specific cluster of signals that correlated closely with the initial matrix cracking region of the stress-strain curve. Taken in isolation, the onset stress of this activity was significantly lower than the conventional value. PR results were investigated further, and isolated clusters were linked to damage modes anticipated at other specific regions of the stress history. A secondary series of experiments was performed on specimens representing the individual constituents of the CMC (single-phase SiC flexural bars, Hi-Nicalon? fibre bundles and SiC_f/SiC mini-composites) in attempts to further validate the corresponding AE signal characteristics. Matrix cracking and interphase debonding/sliding damage modes could be identified consistently, while fibre breaks remained difficult to isolate under the current experimental conditions.     

Study on interfacial debonding stress and damage mechanisms of C/SiC composites using acoustic emission

Among ceramic matrix composites (CMCs), carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are widely used in numerous high-temperature structural applications because of their superior properties. The fiber–matrix (FM) interface is a decisive constituent to ensure material integrity and efficient crack deflection. Therefore, there is a critical need to study the mechanical properties of the FM interface in applications of C/SiC composites. In this study, tensile tests were conducted to evaluate the interfacial debonding stress on unidirectional C/SiC composites with fibers oriented perpendicularly to the loading direction in order to perfectly open the interfaces. The characteristics of the material damage behaviors in the tensile tests were successfully detected and distinguished using the acoustic emission (AE) technique. The relationships between the damage behaviors and features of AE signals were investigated. The results showed that there were obviously three damage stages, including the initiation and growth of cracks, FM interfacial debonding, and large-scale development and bridging of cracks, which finally resulted in material failure in the transverse tensile tests of unidirectional C/SiC composites. The frequency components distributed around 92.5 kHz were dominated by matrix damage and failure, and the high-frequency components distributed around 175.5 kHz were dominated by FM interfacial debonding. Based on the stress and strain versus time curves, the average interfacial debonding stress of the unidirectional C/SiC composites was approximately 1.91 MPa. Furthermore, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) were used to observe the morphologies and analyze the chemical compositions of the fractured surfaces. The results confirmed that the fiber was completely debonded from a matrix on the fractured surface. The damage behaviors of the C/SiC composites were mainly the syntheses of matrix cracking, fiber breakage, and FM interfacial debonding.     

Correlation between failure mechanism and rupture lifetime of 2D-C/SiC under stress oxidation condition based on acoustic emission pattern recognition

Creep tests of 2D-C/SiC in a wet oxidizing atmosphere were implemented for six samples. The loading process was monitored by acoustic emission (AE). Principal component analysis and a fuzzy clustering algorithm were used to perform pattern recognition of the AE data. All of the AE events were divided into four clusters and labelled as matrix cracking, interfacial damage, fiber breakage and fiber-bundle breakage respectively, according to their physical origin. It was found C/SiC has very scattered rupture lifetimes even under the same test conditions, and the evolution of AE events corresponding to fiber failure is quite different. With increasing rupture lifetime, the AE energy of fiber-bundle breakage is higher, while the number of these events is less. Thus, it is concluded that local oxidation and damage development is the controlling failure mechanism for short-lived specimens and uniform oxidation and damage development is the controlling failure mechanism for long-lived specimens.     

Fatigue behaviour and residual strength evolution of 2D C/SiC Z-pinned joints prepared by chemical vapour infiltration

Z-pinned joints prepared by chemical vapour infiltration are widely used in ceramic matrix composite components. Excellent fatigue behaviour is important for structural safety. In this study, 2D C/SiC Z-pinned joints were loaded in axial direction of the pins under static and cyclic loading. Internal damage was monitored in situ by an acoustic emission system. The binding force between pin and hole is relatively strong. Meanwhile, the joints exhibite promising resistance to fatigue. The residual strength increased first with the fatigue cycles then decreased after 10⁵ cycles. Microstructural analysis indicated that full-developed cracks and local stress redistribution resultes in the increase in the strength of the joints. The acoustic emission analysis also provides a supplementary understanding of the damage mechanism. The results show that damage fully develops at the early stage of fatigue. When the specimen is reloaded, less AE events are collected before the fatigue maximum stress.     

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.     

Failure analysis of 2D C/SiC composite z-pinned/bonded hybrid single-lap joints

Z-pinned/bonded hybrid joints are widely used in the 2D C/SiC composite structures, whose mechanical behavior and failure mechanisms are directly related with the structure integrity. The hybrid joints for 2D C/SiC composite structures are formed by depositing SiC into the gap of z-pinned joints. To evaluate the SiC bonding effect, the tensile experiments for two kinds of z-pinned joints with and without bonded layer were conducted. It was proved that the failure modes of the pin were the tensile and shear failure, and the bonded layer was the interlaminar shear failure of the substrate plate instead of the shear failure of SiC bonded layer due to the smaller interlaminar shear strength. The bonded zone states of specimens were also examined, which were reproduced in the shell-fastener numerical model with surface-based cohesive behavior. Combining with shear strength theory, a numerical model is developed to study the failure processes of the hybrid joints. The SiC bonded zone areas can also greatly influence the failure response of the hybrid joints. The large SiC bonded layer zone can improve the shear strength of the joints. The failure of the joints is determined by the SiC bonded layer when the SiC bonded zone is large enough. This study can be helpful to evaluate and design the SiC bonded zone states of the hybrid joints in the engineering application.     

Stress rupture of SiC/SiC composite tubes under high-temperature steam

SiC-fiber–reinforced SiC matrix composite cladding for light water reactor fuel elements must withstand high-temperature steam oxidation in a loss-of-coolant accident scenario (LOCA). Current composite designs include an outer monolithic SiC layer, in part, to increase steam oxidation resistance. However, it is not clear how such a structure would behave under high-temperature steam in the case when the monolithic layer cracks and carbon interphases and SiC fibers are exposed to the environment. To fill this knowledge gap, stress-rupture tests of prototypic SiC composite cladding at 1000°C under steam and inert environments were conducted. The applied stress was ∼120 MPa, which was beyond the initial cracking stress. The failure lifetime under steam was 400–1300 s, while 75% of the composite specimens did not fail after 3 h of total exposure under inert gases. Microstructural observations suggest that steam oxidation activated slow crack growth in the fibers, which led to failure of the composite. The results from this study suggest that stress rupture in steam environments could be a limiting factor of the cladding under reactor LOCA conditions.     

基于深度学习的平纹Cf/SiC复合材料原位拉伸损伤演化与断裂分析

为揭示平纹C_f/SiC复合材料的拉伸损伤演化及失效机理,开展了X射线CT原位拉伸试验,获得材料的三维重构图像,利用深度学习的图像分割方法,准确识别出拉伸裂纹并实现其三维可视化。分析了平纹C_f/SiC复合材料损伤演化与失效机理,基于裂纹的三维可视化结果对材料损伤进行了定量表征。结果表明:平纹C_f/SiC复合材料的拉伸力学行为呈现非线性,拉伸过程中主要出现基体开裂、界面脱黏、纤维断裂及纤维拔出等损伤;初始缺陷易引起材料损伤,孔隙多的部位裂纹数量也多;纤维束外基体裂纹可扩展至纤维束内部,并发生裂纹偏转。基于深度学习的智能图像分割方法为定量评估陶瓷基复合材料损伤演化与失效机理提供了有效分析手段。     

Effect of multiple loading sequence on time-dependent stress rupture of fiber-reinforced ceramic-matrix composites

In this paper, the effect of multiple loading sequence on time-dependent stress rupture of fiber-reinforced ceramic-matrix composites (CMCs) at intermediate temperatures in oxidative environment is investigated. Considering multiple damage mechanisms, a micromechanical constitutive model for time-dependent stress rupture is developed to determine damage evolution of matrix crack spacing, interface debonding and oxidation length, and fiber failure probability under single and multiple loading sequences. The relationships between multiple loading sequence, composite strain evolution, time, matrix cracking, interface debonding and oxidation, and fiber fracture are established. The effects of fiber volume, matrix crack spacing, interface shear stress in the slip and oxidation region, and environment temperature on the stress/time-dependent strain, interface debonding and oxidation fraction, and fiber broken fraction of SiC/SiC composite are analyzed. The experimental stress rupture of SiC/SiC composite under single and multiple loading sequences at 950°C in air atmosphere is predicted. Compared with single loading stress, multiple loading sequence affects the interface debonding and oxidation fraction in the debonding region, leading to the higher fiber broken fraction and shorter stress-rupture lifetime.     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.     

Microstructural characterization and failure analysis of 2D C/SiC two-layer beam with pin-bonded hybrid joints

In this paper, the microstructural characteristics and mechanical behavior of 2D C/SiC two-layer beams with pin-bonded hybrid joints were investigated. The pin joints were incorporated into the two-layer beam during the assembling stage, and the adhesively bonded joints can be introduced by subsequent chemical vapor infiltration (CVI) process. For 2D C/SiC two-layer beam it was confirmed that the distribution of the adhesively bonded joints was random. Based on the partial interaction composite beam theory, a simplified method was proposed to characterize the effect of adhesively bonded joints on the bending stiffness of the 2D C/SiC two-layer beam. The stress distribution around the pin joints was described by finite element modeling. It was found that the radial stress, the hoop stress and the in-plane shear stress around the pin joints were unique in their distributions. Under the action of these stresses, two kinds of failure modes can be initiated: one is the debonding of the pin–hole interface, and the other is the fracture of the 2D C/SiC plate. Based on the stress results, an empirical failure criterion was present to predict the ultimate failure of the 2D C/SiC two-layer beam.     

Modeling deformation of a melt-infiltrated SiC/SiC composite under fatigue loading

Results from fatigue experiments done on a SiC/SiC composite are presented. A micromechanics-based model is used to study the observed behavior under cyclic loading. The model includes consideration of progressive damage, creep and oxidation of the fiber and matrix. Comparison of model predictions with test data showed that the deformation during fatigue in this material is explained primarily by damage in the form of matrix microcracking and interface debonding, in combination with creep under the cyclic load. Stiffness of the material was observed to not change significantly during fatigue indicating that the contribution of fiber fracture to deformation is limited. Fiber fracture however was found to determine final failure of the composite. Failure under cyclic fatigue loading was found to be affected by load transfer from the matrix to the fiber due to damage and creep, and by progressive degradation of the load-carrying fibers due to the combined effect of oxidation and load cycling.     

Damage development of a woven SiCf/SiC composite during multi-step fatigue tests at room temperature

The monotonic tensile and multi-step fatigue tests of 2D woven SiC_f/SiC composite were performed to explore the damage development, respectively. The acoustic emission-based technique was used to analyze the damage state and select the peak stresses for fatigue tests. The damage evolution after specific mechanical tests was characterized by optical microscopy and scanning electron microscopy. Cracks are prone to occur in the vicinity of flaws and boundaries of different matrix components under relatively low fatigue stress. The cyclic fatigue stress can do much harm to the interfaces and facilitate the interfacial debonding. The damage characteristics of five types of cracking, fiber breakage and pull-out, and interfacial debonding of the composite after specific mechanical tests are concluded and discussed in detail, which can offer help for deeper analysis of the oxidation mechanism in service and more reasonable design of SiC_f/SiC composite.     

Electrical Resistance of SiC Fiber Reinforced SiC/Si Matrix Composites at Room Temperature during Tensile Testing

The implementation of Ceramic Matrix Composites necessitates the understanding of stress‐dependent damage evolution. Toward this goal, two liquid silicon infiltrated SiCf reinforced SiC composites were tensile tested with electrical resistance (ER) monitoring as well as acoustic emission to quantify matrix cracking. ER was modeled using a combination of resistors in series and parallel to model transverse matrix cracks and fiber/matrix segments between matrix cracks. It is shown that resistance change is sensitive to transverse matrix crack formation and stress‐dependent debonding length. The model appears to be accurate up to the stress for matrix crack saturation.     

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.     

Failure mechanism and tensile constitutive model of the unidirectional-laminated Cf/SiC–Al composites

The unidirectional-laminated C_f/SiC–Al composites were prepared by using precursor infiltration and pyrolysis (PIP) and vacuum pressure infiltration processes. Bulk density and open porosity of as-prepared C_f/SiC–Al composites were characterized which showed a large number of pores in the unidirectional-laminated carbon fiber preform were filled with SiC and Aluminum alloy matrix. The uniaxial tensile tests were conducted to study the mechanical properties. The fracture surface and cross-section of tensile specimens were characterized to clarify the failure mechanism. The results showed that under the action of load, the propagation of microcracks in matrix led to interface debonding, fiber fracture and pull-out. According to the stress-displacement behavior and analysis of damage process, the prediction formulas of the linear proportional limit stress value and the tensile strength value were proposed. A bilinear constitutive model was established based on the assumption of the damage process which well characterized constitutive response of the composites.     

Removal mechanism of SiC/SiC composites by underwater femtosecond laser ablation

Silicon carbide Ceramic matrix composites (SiC matrix with SiC fibers, abbreviated as SiC/SiC composites) are widely used in aerospace and energy applications due to their excellent resistance to high temperatures, corrosion, wear, and low density. However, the difficult machinability and surface oxidation of SiC/SiC composites are the main factors restricting their further application. To address these issues, this paper explores a novel method for underwater femtosecond laser ablation of SiC/SiC composites to obtain high cleanliness, low-oxidation microporous surfaces. This paper systematically analyses the changes in hole depth, material removal rate (MRR), surface morphology, and material components during underwater femtosecond laser ablation of SiC/SiC composites, and explains the formation of typical features such as induced cones, small banded pits, fiber debonding and shedding. Our work provides new research ideas for understanding the removal mechanism and surface oxidation resistance of SiC/SiC composites.     

SiC/SiC minicomposites with structure-graded BN interphases

BN interphases in SiC/SiC minicomposites were produced by infiltration of fibre tows from BF₃–NH₃–H₂ gaseous system. During interphase one-step processing, the tow travels through a reactor containing a succession of different hot areas. By TEM characterization, the BN interphases were found to be made of a structural gradient: from isotropic to highly anisotropic. The very first coating is poorly organised and allows to protect the fibre from a further chemical attack by the reactant mixture. The minicomposites were tensile tested at room temperature with unloading-reloading cycles. The BN interphases act as mechanical fuses; the fibre/matrix bonding intensity ranges from weak to rather strong depending on the tow travelling rate during interphase infiltration. The specimen lifetimes at 700°C under a constant tensile loading were measured in dry and moist air. Compared to a pyrocarbon reference interphase, the BN interphases significantly improve the oxidation resistance of the SiC/SiC minicomposites.     

Modeling matrix multicracking development of fiber-reinforced ceramic-matrix composites considering fiber debonding

In this paper, the effect of fiber debonding on matrix multicracking development of different fiber-reinforced CMCs is investigated using the micromechanical approach. The Budiansky–Hutchinson–Evans shear-lag model is adopted to analyze the fiber and matrix stress distributions of the damaged composite. The fracture mechanics approach is used to determine the fiber/matrix interface debonding length. Combining the critical matrix strain energy criterion and fracture mechanics fiber/matrix interface debonding criterion, the stress-dependent matrix multicracking development is analyzed for different fiber volume fraction, fiber/matrix interface properties and matrix cracking characteristic stress. The experimental matrix multicracking development of unidirectional C/Si₃N₄, SiC/Si₃N₄, SiC/CAS, SiC/CAS-II, SiC/SiC, SiC/Borosilicate and mini-SiC/SiC composites are predicted.