Creep-rupture behaviour of notched oxide/oxide ceramic matrix composite in combustion environment

ABSTRACT

Creep-rupture tests were performed in the combustion environment on double-edge notch and centre hole oxide/oxide ceramic matrix composite specimens. The specimens were exposed to the maximum temperature of 1250?±?50°C in the notch region where the combustion flame directly impinged. Specimens were loaded to the desired creep load levels and the loads were sustained till either the specimens ruptured or a run-out time of 25?h was achieved. Optical and scanning electron microscopes were used to characterise specimen damage. The test results were compared to its counterparts in 1200°C (isothermal) laboratory air environment. At a given creep life, the applied creep stress for both the notch types was generally lower in the combustion environment than the laboratory air environment. Finite element simulations attributed lower applied creep stress in the combustion environment to the presence of thermal gradient stresses, which were not present in the isothermal laboratory air environment.     

Tension-tension fatigue behavior of a melt-infiltrated SiC/SiC ceramic matrix composites in a combustion environment

An experimental thermo-mechanical facility was developed with conditions towards that of the combustion environments experienced by the hot section components of a jet engine. Two different melt-infiltrated (MI) ceramic matrix composites (CMCs) were evaluated, one containing Hi-Nicalon Type S fibers and the other Tyranno SA fibers. Specimens considered in this study were subjected to fatigue loading with a stress ratio of 0.1, frequency of 1 Hz and a specimen surface temperature of 1200 ± 20 °C. Results indicate that fatigue life in the combustion environment was an order of magnitude lower compared to the furnace environment and is attributed to the hostile environment present with the burner rig. Post-test microscopy was conducted in order to understand the damage mechanisms and oxidation behavior. Polished longitudinal sections of the burner rig specimens revealed longitudinal cracking which could be attributed to the presence of thermal gradient stress. Electrical resistance (ER) was implemented to monitor the damage.     

Investigation of long-term thermal aging-induced damage in oxide/oxide ceramic matrix composites

Long-term thermal aging is a typical factor affecting the thermo-mechanical fatigue life for hot-end components in the gas turbine. The present work focuses on the development of thermal aging-induced damage in 2-D woven oxide/oxide ceramic matrix composites from micro-mechanism and macroscopic mechanical performance. The porosity evolution and mechanical performance after long-term thermal aging were characterized through mercury intrusion measurements and uniaxial compressive tests, respectively. The results show that the decrease of micro-porosity directly reflects the irreversible evolution of material microstructure in the thermal aging process, and the decrease of compressive strength after aging is the macroscopic reflection of the microstructure variation. The porosity increment of matrix was thus used to characterize the thermal aging-induced damage, establishing a unique analysis model between the increment of micro-porosity under thermal aging and the corresponding degradation of material compressive strength. The experimental results are in good agreement with the established model.     

Erosion characteristics of N720/alumina oxide/oxide ceramic matrix composites in a combustion environment

The erosion behavior of N720/Alumina oxide/oxide composite was investigated under a combustion environment to better represent particle ingestion of a jet engine. The effect of particle velocity, particle size, temperature and impingement angle were investigated. In addition, room temperature studies were also conducted for comparison. Eroded sites were investigated using optical and scanning electron microscopy to understand the extent of erosion and erosion mechanisms. The results indicate that erosion rate increased with an increase in particle velocity and particle size. Also, erosion rate increased from room temperature to 815 °C and then decreased from 815 °C to 1200 °C. Brittle fracture is the predominant mode under normal impacts and as the impact angle is decreased increased ploughing/wear is evident.     

Crack opening behavior in ceramic matrix composites

The evolution of matrix cracks in a melt‐infiltrated SiC/SiC ceramic matrix composite (CMC) under uniaxial tension was examined using scanning electron microscopy (SEM) combined with digital image correlation (DIC) and manual crack opening displacement (COD) measurements. CMC modeling and life prediction strongly depend a thorough understanding of when matrix cracks occur, the extent of cracking for given conditions (time‐temperature‐environment‐stress), and the interactions of matrix cracks with fibers and interfaces. In this work, strain relaxation due to matrix cracking, the relationship between CODs and applied stress, and damage evolution at stresses below the proportional limit were assessed. Direct experimental observation of strain relaxation adjacent to regions of matrix cracking is presented and discussed. Additionally, crack openings were found to increase linearly with increasing applied stress, and no crack was found to pass fully through the gage cross‐section. This calls into question the modeling assumption of through‐cracks for all loading conditions and fiber architectures, which can obscure oxidation mechanisms that are active in realistic cracking conditions. Finally, the combination of SEM with DIC is demonstrated throughout to be a powerful means for damage identification and quantification in CMCs at stresses well below the proportional limit.     

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.     

A micromechanical damage model for oxide/oxide ceramic matrix composites with hierarchical porosity under thermomechanical loading

This work presents a micromechanical damage model to describe the microstructural damage behaviors of ceramic matrix composites with hierarchical porosity during thermomechanical loading. The microstructure evolution may cause the nonlinear constitutive behavior, and a hierarchical porosity-based elasto-plastic constitutive model was developed. Damage mechanisms of matrix-crack, hierarchical pore nucleation and fiber-breaking are incorporated into the formulation of the damage model to describe various micromechanical damage modes of ceramic matrix composites accurately. Two damage variables are proposed for the damage evolution of matrix and fiber bundles. The main damage mechanisms in the matrix are matrix-cracking, and fibers breaking in the fiber bundles. The performance of the proposed damage model is verified by comparing with the existing experimental data. The proposed damage model outperforms the existing counterparts by capturing the microstructural damage mechanism and integrated into the damage model, and the contribution of different damage mechanisms can be quantified. The present work will provide a robust tool for describing the damage behaviors of matrix and fiber bundles in the ceramic matrix composites under thermomechanical loading, as well as allow a more accurate characterization of microstructural damage for a large extent of ceramic matrix composites.     

Direct ink writing of ceramic matrix composite structures

We present the development of an ink containing chopped fibers that is suitable for direct ink writing (DIW), enabling to obtain ceramic matrix composite (CMC) structures with complex shape. We take advantage of the unique formability opportunities provided by the use of a preceramic polymer as both polymeric binder and ceramic source. Inks suitable for the extrusion of fine filaments (<1 mm diameter) and containing a relatively high amount of fibers (>30 vol% for a nozzle diameter of 840 μm) were formulated. Despite some optimization of ink rheology still being needed, complex CMC structures with porosity of ~75% and compressive strength of ~4 MPa were successfully printed. The process is of particular interest for its ability to orient the fibers in the extrusion direction due to the shear stresses generated at the nozzle tip. This phenomenon was observed in the production of polymer matrix composites, but it is here employed for the first time for the production of ceramic matrix ones. The possibility to align high aspect ratio fillers using DIW opens the path to layer‐by‐layer design for optimizing the mechanical and microstructural properties within a printed object, and could potentially be extended to other types of fillers.     

Joining oxide ceramic matrix composites by ionotropic gelation

The production of complex-shaped all-oxide ceramic matrix composites (Ox-CMC) is somewhat restricted by their current processing methods, as well as by the lack of applicable joining techniques. Thus, we present a new method for joining Ox-CMCs based on the gelation of slurries with the polysaccharide polymer alginate. For this investigation, Nextel 610/alumina-zirconia composites were produced using alginate as binder and aluminum acetate as gelling agent. The joining capabilities of this technique were investigated with microstructural analyses and single-lap compression shear tests. For that, a slurry-containing alginate was used to join two composite plates at different stages of the processing: gel state, dried green body and after sintering. Joining composites plates in their gel or green state was successful as the joints showed shear strength values similar to the interlaminar shear strength of the composites plates. The quality of the joints was attributed to the interactions between the alginate chains of the composite plates and the joint. We also show that even the joining of already sintered Ox-CMCs is feasible. However, densification cracks and lower shear strength are observed for such cases.     

陶瓷基复合材料疲劳损伤模拟与界面优化设计

本文从界面损伤模拟出发研究了陶瓷基复合材料(CMCs)的抗疲劳设计方法.以CMCs微观结构演变为切入点,在微观尺度建立复合材料各组分损伤机制的物理模型,然后集成到细观尺度的有限元分析之中,从而建立CMCs疲劳损伤的数值模拟方法,并对界面相组成、结构等因素影响疲劳性能的作用机制进行探究,以实现界面的抗疲劳设计.通过多尺度...     

Fatigue behavior and damage analysis of PIP C/SiC composite

In this work, we study the fatigue behavior of a C/SiC composite produced by several cycles of polymer infiltration and pyrolysis (PIP). Fatigue tests were performed with maximum stresses corresponding to 60–90% of the tensile strength of the composite. During the fatigue tests, acoustic emission (AE) monitoring was performed and the measured AE energy was utilized to quantify the damage and distinguish possible damage mechanisms. Most of the fatigue damage in the form of matrix cracking, interface damage and fiber breakage occurs in the first cycle. As loading cycles proceeded, damage in form of matrix crack re-opening and interfacial friction constantly accumulates. Nevertheless, all samples survived the run-out of 1,000,000 cycles. After the fatigue tests, an increase of the tensile strength is observed. This phenomenon is associated with the relief of process-induced internal thermal stresses and the weakening of the fiber-matrix interface. In general, the studied material shows very high relative fatigue limit of 90% of its tensile strength.     

Increasing the tensile strength of oxide ceramic matrix mini-composites by two-step sintering

Adapting conventional sintering (CS) techniques of monolithic ceramics for the production of oxide ceramic matrix composites (Ox-CMCs) comes along with a few drawbacks, such as fiber degradation. Thus, the applicability of two-step sintering (TSS) for the production of Ox-CMCs based on Nextel™ 610 fibers and porous alumina matrix is investigated in this study for the first time. Uniaxial tensile tests were performed to evaluate the performance of mini-composites produced by TSS and compared with those produced by CS. Parameters known for influencing the mechanical behavior of the mini-composites, such as grain size, porosity, shrinkage, as well as matrix properties, were analyzed. Both sintering techniques resulted in similar grain size distributions, whereas TSS showed higher total porosity and lower amount of sintering-induced cracks. As a result, TSS samples showed a higher tensile strength of 230±27 MPa when compared to 133±8 MPa for CS. In general, it was observed that most of the densification happens during the first phase of TSS, while the matrix is slowly strengthened during the second step. Therefore, the reported TSS process is a very promising and easy-to-apply heat treatment for producing Ox-CMCs with controlled microstructure.     

Damage analysis of a SiCf/SiC ceramic matrix composite under stepwise fatigue loading with acoustic emission

Continuous fiber-reinforced ceramic matrix composites (CMCs) exhibit different damage mechanisms at multiple scales under cyclic loading. In this paper, the tension-tension fatigue behavior of a plain woven SiC_f/SiC CMC was investigated, and damage accumulation and evolution process were studied in detail via acoustic emission (AE) method. With the increase of cycles, the material exhibits obvious hysteresis behavior affected by interfacial slip and wear mechanisms. Most of the fibers with radial fracture characteristic have relatively high strength, showing excellent toughening property. In the stepwise cyclic loading process, the Kaiser effect of AE determines the initiation of AE activities at each initial loading moment, which shows obvious nonlinear damage accumulation behavior of the material. High-energy events are related to significant matrix cracking and fiber fracture, and the evolution process of material damage initiation and propagation is monitored in real time.     

Statistical analysis of the influence of microstructure on damage in fibrous ceramic matrix composites

The effect of microstructure on cracking was analyzed in a CMC using statistical methods. It was determined that the amounts of coating surrounding fibers and their dispersion within the matrix influenced where cracks evolved in transverse plies. Linear models predicted that maximum principal strains in transverse fiber coatings increased as (i) the fiber coating area increased and (ii) the length of matrix ligament between fibers decreased. Logistic models indicated that the likelihood of transverse fibers residing on a matrix crack increased as the (i) ratio of coating to filament decreased, (ii) distance between fibers decreased, or (iii) coating area increased.     

Environmental effects on the strength and impact damage resistance of alumina based oxide/oxide ceramic matrix composites

In this study the effects of high temperature and moisture on the impact damage resistance and mechanical strength of Nextel 610/alumina silicate ceramic matrix composites were experimentally evaluated. Composite laminates were exposed to either a 1050°C isothermal furnace-based environment for 30 consecutive days at 6 h a day, or 95% relative humidity environment for 13 consecutive days at 67°C. Low velocity impact, tensile and short beam strength tests were performed on both ambient and environmentally conditioned laminates and damage was characterized using a combination of non-destructive and destructive techniques. High temperature and humidity environmental exposure adversely affected the impact resistance of the composite laminates. For all the environments, planar internal damage area was greater than the back side dent area, which in turn was greater than the impactor side dent area. Evidence of environmental embrittlement through a stiffer tensile response was noted for the high temperature exposed laminates while the short beam strength tests showed greater propensity for interlaminar shear failure in the moisture exposed laminates. Destructive evaluations exposed larger, more pronounced delaminations in the environmentally conditioned laminates in comparison to the ambient ones. External damage metrics of the impactor side dent depth and area directly influenced the post-impact tensile strength of the laminates while no such trend between internal damage area and residual strength could be ascertained.     

碳纤维增强SiC陶瓷复合材料的研究进展

碳纤维增强SiC陶瓷基复合材料具有良好的高温力学性能，是航空航天和能源等领域新的高温结构材料研究的热点之一．本文回顾了增强体碳纤维的发展，对材料的成型制备工艺，材料的抗氧化涂层研究进展和现有的一些应用做了综述，并展望了碳纤维增强SiC陶瓷基复合材料以后的研究重点及发展前景．     

Preparation and properties of an oxide fiber‐reinforced silica matrix composite

Oxide (Nextel? 440) fiber‐reinforced silica composites, with the density and porosity of 1.97 g/cm³ and 21.8%, were prepared through sol‐gel. Their average flexure strength, elastic modulus, shear strength, and fracture toughness at room temperature were 119.7 MPa, 25.6 GPa, 10.8 MPa, and 4.0 MPa·m^1/2, respectively. The composites showed typical toughened fracture behavior, and distinct pullout fibers were observed at the fracture surface. Their mechanical properties were performant up to 1000°C, with the maximum flexural strength of 132.2 MPa at 900°C. Moreover, the composites showed good thermal stability, even after thermal aging and thermal shock at elevated temperatures.     

Mechanical hysteresis and damage evolution in C/SiC composites under fatigue loading at room and elevated temperatures

In this paper, the mechanical hysteresis and damage evolution in C/SiC ceramic matrix composites (CMCs) under cyclic tension-tension fatigue loading at room and elevated temperatures in air and in inert atmosphere and different loading frequencies are investigated. The fatigue hysteresis loops models considering multiple matrix cracking modes are developed to establish the relationships between fatigue hysteresis loops, fatigue hysteresis dissipated energy, and fiber/matrix interface shear stress. The evolution of fatigue hysteresis dissipated energy and interface shear stress vs applied cycles is analyzed. It was found that the interface shear stress degradation rate increases with fatigue peak stress, and loading frequency from 40 to 375 Hz.     

Temperature‐dependent tensile strength model for 2D woven fiber reinforced ceramic matrix composites

This paper presents a temperature‐dependent model for predicting the tensile strength of 2D woven fiber reinforced ceramic matrix composites. The model takes into account the combined effects of temperature, temperature‐dependent residual thermal stress, temperature‐dependent matrix strength, and fibers strength on the tensile strength of composites. To verify the model, the tensile strengths of 2D woven fiber reinforced ceramic matrix composites available are predicted at different temperatures. The model predictions agree well with the experimental data. This work could provide a practical technical means for predicting the temperature‐dependent tensile strength of 2D woven fiber reinforced ceramic matrix composites and uncovering the dominated mechanisms leading to the change of tensile strength and their evolution with temperature.     

Fatigue of three advanced SiC/SiC ceramic matrix composites at 1200°C in air and in steam

High‐temperature mechanical properties and tension‐tension fatigue behavior of three advanced SiC/SiC composites are discussed. The effects of steam on high‐temperature fatigue performance of the ceramic‐matrix composites are evaluated. The three composites consist of a SiC matrix reinforced with laminated, woven SiC (Hi‐Nicalon?) fibers. Composite 1 was processed by chemical vapor infiltration (CVI) of SiC into the Hi‐Nicalon? fiber preforms coated with boron nitride (BN) fiber coating. Composite 2 had an oxidation inhibited matrix consisting of alternating layers of silicon carbide and boron carbide and was also processed by CVI. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Composite 3 had a melt‐infiltrated (MI) matrix consolidated by combining CVI‐SiC with SiC particulate slurry and molten silicon infiltration. Fiber preforms had a CVI BN fiber coating applied. Tensile stress‐strain behavior of the three composites was investigated and the tensile properties measured at 1200°C. Tension‐tension fatigue behavior was studied for fatigue stresses ranging from 80 to 160 MPa in air and from 60 to 140 MPa in steam. Fatigue run‐out was defined as 2 × 10⁵ cycles. Presence of steam significantly degraded the fatigue performance of the CVI SiC/SiC composite 1 and of the MI SiC/SiC composite 3, but had little influence on the fatigue performance of the SiC/SiC composite 2 with the oxidation inhibited matrix. The retained tensile properties of all specimens that achieved fatigue run‐out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.