Damage In Ceramic Matrix Composites Research Articles

High-temperature transient-induced thermomechanical damage of fiber-reinforced ceramic-matrix composites in supersonic wind tunnel

This article is based on the supersonic directly connected wind tunnel. Through a specially designed experimental chamber, combined with infrared temperature measurement, high-speed camera, etc., in-situ monitoring of composite materials under airflow at Ma 3.0 with a total temperature of 950 ∼ 1473K was carried out. The dimensional analysis method was used to propose dimensionless parameters to characterize the thermal coupling caused by high-speed airflow thermal shock. Research has shown that the thermal coupling effect of supersonic airflow causes uneven temperature inside the material, and the thermal stress caused by temperature gradient changes (including increasing and decreasing processes) is the main reason for material damage. The damage of ceramic matrix composites under thermal shock mainly manifests as a decrease in surface roughness, surface fiber fracture and a decrease in elastic modulus. In addition, the study also found that there are damage thresholds for the thermal shock effect of airflow at different total temperatures, which helps to further understand the thermomechanical damage mechanism and degradation law of composite structure under high-temperature transient conditions.

A novel quantitative analysis approach for hole surface damage of ceramic matrix composites machined by abrasive waterjet

To realize low-damage hole-making machining of AWJ-machined CMC, it is necessary to achieve quantitative analysis of hole surface damage. Therefore, a novel approach to quantifying damage based on the principle of invariance of damage area is proposed in this study. The amount of total damage is attributed to the sum of the equivalent average damage lengths of the four common types of damage (peripheral compound damage, tearing damage, crack damage, and burrs damage), which are uniformly arranged inside and outside the hole in sequence according to the principle of constant area. The influence law and influence mechanism of common process parameters on the surface damage in Cf/SiC during hole making via AWJ are investigated based on the established quantitative damage analysis model. The total surface damage increases with the increase of the standoff distance, and decreases with the increase of waterjet pressure and abrasive flow rate. The most significant factor affecting the total surface damage is the standoff distance, followed by waterjet pressure and abrasive flow rate. However, the effect of traverse speed is not noteworthy. The approach of quantitative damage analysis will provide the necessary theoretical basis and technical guidance for achieving low damage hole-making surface utilizing AWJ.

Foreign object damage and post-impact tensile behavior of plain-woven SiC/SiC composites

Plain-woven SiC/SiC composite plates, fabricated using the chemical vapor infiltration (CVI) technology, were impact-tested at different velocities using 3-mm-diameter steel spheres. The states of the impacted targets were divided into three categories: surface damage, critical penetration, and full penetration. The microstructures of the foreign object damage (FOD) in the targets were carefully observed using microscope and X-ray computerized tomography (CT) scans. Based on the results of the observations, the forms of impact damage of ceramic matrix composites (CMCs), including cone cracking, yarn fracture and bending, and delamination, are revealed and summarized. The mechanisms of the penetration process were further analyzed by combining these damage modes of the CMCs. Subsequently, tensile tests of the impacted specimens were performed to investigate the effect of FOD on the tensile behavior of the CMCs. The digital image correlation (DIC) method was used to obtain the strain distribution of the impacted targets during tension. The tensile curves of the undamaged and impacted specimens were obtained and compared. The evolution of the tensile failure of the damaged beams was then analyzed. Finally, the residual strengths of CMC plates impacted at different velocities were compared.

Foreign Object Impact Damage in Ceramic Matrix Composites: Experiments and Computational Predictions

Abstract Foreign object impact of ceramic matrix composite (CMC) materials and components in a gas turbine engine environment could be detrimental to engine performance and hence must be accounted for in the design of such components. This paper is concerned with experiments and computational modeling of foreign object impact phenomenon in silicon carbide (SiC)-based CMC. Controlled impact experiments were conducted on the CMC material using a gas-gun apparatus with spherical hardened steel projectile. The internal damage state within the CMC specimens was assessed using X-ray computed tomography scan technique. The computational modeling involved explicit dynamic finite element (FE) simulation of the impact process wherein either delamination mechanism is modeled or both ply damage and delamination mechanisms are modeled in a coupled manner. The delamination mechanism is modeled explicitly using cohesive-zone (CZ) fracture mechanics approach, whereas, the ply-damage mechanisms are modeled implicitly using simplified continuum damage mechanics approach. The simulation results were found to be in reasonable qualitative and quantitative agreement with the experimental results. Furthermore, it is shown that modeling both the ply damage and delamination mechanisms are essential to predict the correct delamination pattern even for intermediate velocity impacts that leads to predominantly delamination damage. The predictive nature of the modeling approach is demonstrated and approaches to enhance the models are also discussed.

Advances in Damage Monitoring Techniques for the Detection of Failure in SiCf/SiC Ceramic Matrix Composites

From a disruptive perspective, silicon carbide (SiC)-based ceramic matrix composites (CMCs) provide a considerable temperature and weight advantage over existing material systems and are increasingly finding application in aerospace, power generation and high-end automotive industries. The complex structural architecture and inherent processing artefacts within CMCs combine to induce inhomogeneous deformation and damage prior to ultimate failure. Sophisticated mechanical characterisation is vital in support of a fundamental understanding of deformation in CMCs. On the component scale, “damage tolerant” design and lifing philosophies depend upon laboratory assessments of macro-scale specimens, incorporating typical fibre architectures and matrix under representative stress-strain states. This is important if CMCs are to be utilised to their full potential within industrial applications. Bulk measurements of strain via extensometry or even localised strain gauging would fail to characterise the ensuing inhomogeneity when performing conventional mechanical testing on laboratory scaled coupons. The current research has, therefore, applied digital image correlation (DIC), electrical resistance monitoring and acoustic emission techniques to the room and high-temperature assessment of ceramic matrix composites under axial tensile and fatigue loading, with particular attention afforded to a silicon carbide fibre-reinforced silicon carbide composite (SiCf/SiC) variant. Data from these separate monitoring techniques plus ancillary use of X-ray computed tomography, in-situ scanning electron microscopy and optical inspection were correlated to monitor the onset and progression of damage during mechanical loading. The benefits of employing a concurrent, multi-technique approach to monitoring damage in CMCs are demonstrated.

Damage Determination in Ceramic Composites Subject to Tensile Fatigue Using Acoustic Emission.

Acoustic emission (AE) has proven to be a very useful technique for determining damage in ceramic matrix composites (CMCs). CMCs rely on various cracking mechanisms which enable non-linear stress–strain behavior with ultimate failure of the composite due to fiber failure. Since these damage mechanisms are all microfracture mechanisms, they emit stress waves ideal for AE monitoring. These are typically plate waves since, for most specimens or applications, one dimension is significantly smaller than the wavelength of the sound waves emitted. By utilizing the information of the sound waveforms captured on multiple channels from individual events, the location and identity of the sources can often be elucidated. The keys to the technique are the use of wide-band frequency sensors, digitization of the waveforms (modal AE), strategic placement of sensors to sort the data and acquire important contents of the waveforms pertinent for identification, and familiarity with the material as to the damage mechanisms occurring at prescribed points of the stress history. The AE information informs the damage progression in a unique way, which adds to the understanding of the process of failure for these composites. The AE methodology was applied to woven SiC fiber-reinforced melt-infiltrated SiC matrix composites tested in fatigue (R = 0.1) at different frequencies. Identification of when and where AE occurred coupled with waveform analysis led to source identification and failure progression. For low frequency fatigue conditions, damage progression leading to failure appeared to be due to fiber failure at or near the peak stress of the stress cycle. For higher frequency fatigue conditions, significantly greater amounts of AE were detected compared to low frequency tests a few hours prior to failure. Damage progression leading to failure included AE detected events which occurred on the unloading part of the fatigue cycle near the valley of the stress cycle. These events were associated with 90 tow longitudinal split and shear cracks presumably due to local compressive stresses associated with mating crack surface interactions during unloading. The local region where these occurred was the eventual failure location and the “valley” events appeared to influence the formation of increased local transverse cracking based on AE.

Multi-Lead Direct Current Potential Drop Method for In Situ Health Monitoring of Ceramic Matrix Composites

The method of direct current potential drop (DCPD) can be utilized as an effective and convenient approach for in situ damage detection, and as a nondestructive evaluation technique. We present the results from use of a multiprobe DCPD technique for in situ damage detection in loading of a SiC/SiC composite. It is shown that in three different modes of loading (monotonic, fatigue, and cyclic load–unload), the sensing capabilities of DCPD technique compare well to the techniques of modal acoustic emission (AE) and digital image correlation (DIC). It was also found that DCPD technique provides a far earlier warning of failure under fatigue loading than the other two methods. In addition, we show that strategically placed multiple voltage leads on the specimen surface provide a promising way of qualitatively determining the crack initiation site. Therefore, the use of multiple lead DCPD method, together with other techniques, provides a viable option for sensing damage in ceramic matrix composites (CMCs) with complex geometries, and for applications at higher temperatures.

Application of Direct-Current Potential Difference Method to High-Temperature Fatigue Damage of Ceramic Matrix Composites

Application of Direct-Current Potential Difference Method to High-Temperature Fatigue Damage of Ceramic Matrix Composites

Experimental assessment of toughness in ceramic matrix composites using the J-integral with digital image correlation part I: methodology and validation

The complex nature of damage in ceramic matrix composites (CMCs) renders conventional methods of measuring quantitative fracture properties impractical. This paper is the first of two-part series that assesses the feasibility of using the J-integral with full-field deformation data from digital image correlation (DIC) to characterize toughness in continuous fiber SiC/SiC CMC laminates. It provides a resource for best practices when incorporating experimentally measured, full-field deformation data into J-integral evaluations of toughness. The techniques discussed are important for researchers attempting to measure fracture properties in advanced materials with damage mechanisms that have yet to be well characterized. Two methods for evaluating potential energy release rates J are presented: (1) numerical integration over a line contour and (2) Gaussian integration over an area contour. Accuracy and path independency for both methods were verified using analytically derived deformation fields for a center-cracked, infinite plate of isotropic material under equi-biaxial tension. Inherent noise in the deformation data and necessary contour truncation at the crack surfaces reduced accuracy and introduced path dependency. However, this was mitigated by careful noise filtering of the deformation data prior to the evaluation of J. Applying the line and area integrals to DIC data from tapered, double-cantilever beam, acrylic compact tension specimens resulted in the findings that (1) both integrals measured fracture toughness within a range of published values; (2) both integrals captured the constant stress intensity factor behavior that is characteristic of the tapered beam geometry; and (3) the area integral measurements were consistently larger than line integral measurements. The area integral is more accurate than the line integral as it samples more data points, reducing its sensitivity to experimental noise. Although variability in experimental data can be minimized, it cannot be eliminated. Measurement error is inevitable; thus, the line and area integrals must be used with caution when characterizing quantitative properties including fracture toughness.

Techniques for monitoring damage in ceramic matrix composites

This article offers a review of various nondestructive evaluation and structural health monitoring techniques that have been successfully utilized for assessing the damage state of woven ceramic matrix composites consisting of silicon carbide fibers and silicon carbide matrices. The techniques include acousto-ultrasonics, modal acoustic emissions, electrical resistance, impedance-based structural health monitoring, pulsed thermography, as well as thermoelastic stress analysis. The damage, in the form of distributed matrix cracks and delaminations, was introduced using multiple tactics. These included load/unload/reload uniaxial tensile tests, creep tests, and ballistic impact. Although other nondestructive evaluation techniques have been applied to this material system, the select nondestructive evaluation tools described here are limited to approaches that are of current research interest within programs at the NASA Glenn Research Center.

Nondestructive Damage Evaluation in Ceramic Matrix Composites for Aerospace Applications

Infrared thermography (IRT) and acoustic emission (AE) are the two major nondestructive methodologies for evaluating damage in ceramic matrix composites (CMCs) for aerospace applications. The two techniques are applied herein to assess and monitor damage formation and evolution in a SiC-fiber reinforced CMC loaded under cyclic and fatigue loading. The paper explains how IRT and AE can be used for the assessment of the material's performance under fatigue. IRT and AE parameters are specifically used for the characterization of the complex damage mechanisms that occur during CMC fracture, and they enable the identification of the micromechanical processes that control material failure, mainly crack formation and propagation. Additionally, these nondestructive parameters help in early prediction of the residual life of the material and in establishing the fatigue limit of materials rapidly and accurately.

A model for estimating nonlinear deformation and damage in ceramic matrix composites

The present paper addresses the estimation of global deformation of a continuous-fiber brittle matrix composite over a length much larger than the largest microstructural dimension of the composite, such as the fiber diameter and the fiber spacing. Initial microcracks and progressive microcracking of the matrix and interfaces under time-dependent load and temperature are considered. The model also includes consideration of inelastic deformation of one of the constituents. In this paper, the equations of the model are developed and applied to model the stress–strain behavior of several silicon carbide/silicon carbide ceramic matrix composites and the creep behavior of an oxide/oxide ceramic matrix composite. Comparison with test data shows that the model is able to capture the wide range of deformation behavior seen in these ceramic matrix composites.

A Study on Low-Velocity Impact Characteristics and Compression Strength after Impact of Ceramic/Resin Matrix Composites

An experimental study of low-velocity impact characteristics and strength after impact was carried out on both woven fiber-reinforced resin matrix composites and woven fiber-reinforced ceramic matrix composites. The test specimens were impacted using a dropped-weight impact test apparatus with an instrumented spherical tip. Ultrasonic C-scan was used in nondestructive testing to characterize and quantify the impact damage. Much more damage of ceramic matrix composites than that of resin matrix composites occur and process in loading stage. The peak load of resin matrix composites is higher than that of ceramic matrix composites. According to the results of observing optical photographs and C-scan images, the damage area of ceramic matrix composites is greater than that of resin matrix composites and the difference increases as the energy increases. Damage resistance of ceramic matrix composites is lower than that of resin matrix composites, but damage tolerance of ceramic matrix composites is higher than that of resin matrix composites.

The importance of damage and slow crack growth in the creep behavior of ceramic matrix composites

Three ceramic matrix composites (CMCs) were investigated after creep tests: a 2D SiCf-MLAS, a 2D SiCf-SiC and a 2.5D Cf-SiC. The damage features of each one are identified, using optical and scanning electron microscopies (SEM), and the complete damage sequences are presented. As a result of its time-dependence, damage in CMCs may be considered as slow crack growth in the temperature and stress fields investigated. But the quantification of damage, through the classical damage mechanics, appears as a complex issue, due to the architecture effects in composite materials.

Ultrasonic scattering and velocity methods for characterization of fibre-matrix interphases

Abstract This paper discusses two potential ultrasonic techniques for characterization of fibre-matrix interphases in composites: measurements of wave scattering (attenuation) and its phase velocity. The implementation procedures of these two methods are described using analytical or numerical models with an emphasis on their applicability. Examples of ultrasonic characterization of interphases are given for ceramic and intermetallic matrix composites reinforced with large diameter SiC fibres. Carbon-rich interphase coatings are added on these fibres to improve composite performance and chemical and thermal compatibility. The experimentally measured interphase moduli from ultrasonic data are compared with those determined from interphase microstructures showing good agreement. Finally, several applications of these techniques for interphasial damage assessment are demonstrated, including characterization of oxidation damage in ceramic matrix composites and of fatigue damage in metal matrix composites. These experimental results show that ultrasonic wave attenuation and velocity are significantly affected by degradation of the elastic properties of the interphasial layer; thus, both methods are very useful for non-destructive assessment of interphasial damage.

Ultrasonic Evaluation of Initiation and Development of Oxidation Damage in Ceramic-Matrix Composites

In this paper we report on the development of a method for ultrasonic nondestructive characterization of oxidation damage in ceramic-matrix composites. The method is based on ultrasonic measurement of elastic moduli of the composite, which are then used to determine the elastic moduli of the fiber-matrix interphase. Thus the interphasial damage may be estimated quantitatively. As a model system we used, to demonstrate applicability of the method, a unidirectional SiC-fiber-reinforced reaction-bonded silicon nitride matrix composite (SiC/RBSN). The composite samples were oxidized in flowing oxygen for 0.1, 1, 10, and 100 h at 600°, 900°, 1200°, and 1400°C. The ultrasonic phase velocity in the composite was measured at room temperature before and after oxidation; the data were then used to find the composite moduli, which quantify the induced damage. Significant changes in ultrasonic velocities and composite moduli were found as a result of oxidation. Fiber-matrix interphasial moduli were determined by multiphase micromechanical analysis. We found that oxidation of the carbon interphasial layer is the dominant mechanism in decreasing the elastic moduli of the composite. The critical exposure time for transition from the nondamaged to the damaged state at different oxidation temperatures has been determined.

Determination of fiber-matrix interphase effective elastic moduli from ultrasonic phase velocity and attenuation data

This paper discusses two potential ultrasonic methods for characterization of fiber-matrix interphases in composites: measurement of phase velocity and attenuation. The characterization procedures of these two methods, including different implementations using analytical or numerical models, are reviewed with emphasis on the applicability of these methods. Examples of ultrasonic characterization of interphases in ceramic and intermetallic matrix composites are given and the relation between the measured and actual interphase moduli is discussed. Application of these techniques for interfacial damage assessment is also demonstrated, including characterization of oxidation damage in ceramic matrix composites and fatigue damage in metal matrix composites. The experimental results show that degradation of the interphasial layer significantly affects ultrasonic wave attenuation and velocity. Thus both methods are very useful for assessment of interfacial damage.

Influence of matrix cracking on mechanical properties of a ceramic matrix composite

The first apparent evidence of damage in ceramic matrix composites occurs via matrix cracking when these composites are subjected to stresses beyond the elastic limit. The presence of these matrix cracks may have several consequences to the mechanical response of composites both at room and elevated temperatures. For example, composites containing matrix cracks may show reduced modulus and enhanced susceptibility to damage by fatigue and oxidation at elevated temperatures. In addition, many composites have inherrent matrix cracks/ porosity produced either because of processing limitations (chemical vapour infiltration (CVI), solgel) or as a result of a mismatch in fibre/matrix thermal expansion. Therefore, an understanding of the influence of matrix cracks on the mechanical response of composites is important for designing composites and for assessing their performance under service conditions. In this study, a fully dense and crack-free zircon composite reinforced with SiC whiskers and SiC continuous fibres was loaded in three-point flexure up to the matrix cracking stress, then unloaded. Subsequently, these samples were repeatedly loaded and unloaded below the original matrix cracking stress to study the influence of matrix cracking on the elastic modulus of the composite. The objective of this letter is to report some of the initial observations of such mechanical testing on the mechanical response of a zircon composite reinforced with SiC fibres and whiskers. Some of these observations on matrix cracking behaviour are related to predictions of the steady state and nonsteady state micromechanical models for matrix cracking. This study was performed on a zircon matrix composite reinforced with SiC whiskers and uniaxially aligned SiC continuous filaments. Silicon carbide whiskers were Tateho SCW-1 and SiC monofilaments were SCS-6 type obtained from Textron Corporation. All of the composites tested in this study were processed in a manner described in a previous study [1]. As-fabricated composites were fully dense with an insignificant (< 1%) porosity, and contained about 20 vol % SiC whiskers in the zircon matrix phase and about 25% SiC monofilaments in the entire composite volume (including the zircon matrix, SiC whiskers and SiC filaments). The matrix phase containing zircon and SiC whiskers had an average grain size of about 3/~m, and X-ray diffraction (XRD) analysis of the composite revealed zircon and SiC as major matrix phases with a small but detectable amount of the free-zirconia phase. The zirconia phase was not detected in the starting zircon powder. Therefore, it must have been produced because of the decomposition of zircon [2] at the high processing temperature of 1610 °C. Average mechanical and fibre-matrix interracial properties of as-fabricated composites were measured by three-point flexure and fibre pushout tests, respectively. The details of these tests were also given elsewhere [1]. Typically, composite samples with dimensions of 3.2 cm length, 0.8 cm width and 0.15 cm thickness were individually ground to a finish of 60/~m and then tested in three-point flexure with lower support pins 2.54 cm apart (span-tothickness ratio: 17). All of the tests were performed at a crosshead rate of 0.0127 cmmin -1 to generate load-deflection data. A number of average mechanical properties were determined from these loaddeflection data. These included elastic modulus (E), first matrix cracking strength (Ocr), ultimate strength (~ru) and work-of-fracture (WOF), as described previously [1]. At least four samples of each type were tested to measure the average mechanical properties, which are summarized in Table I. Fibrematrix interracial shear strength was measured by a fibre pushout test [3, 4], and at least 10 fibres in each sample were pushed to calculate an average value of the interfacial shear strength. Average interfacial shear strength was measured during the first push as well as upon re-push (second push) of the already pushed fibres. These results are also summarized in Table I. In this study, a zircon composite reinforced with SiC whiskers and uniaxially aligned SCS-6 monofila-

Ultrasonic Assessment of Interfacial Oxidation Damage in Ceramic Matrix Composites

A new approach to characterizing oxidation damage in ceramic matrix composites using ultrasonic techniques is proposed. In this approach the elastic constants of the composite are determined nondestructively by measuring the angular dependence of both longitudinal and transverse wave velocities. A micromechanical model for composites with anisotropic constituents is used to find the anisotropic properties of an effective fiber which is a combination of the fiber and the interface. Interfacial properties are extracted from the properties of this effective fiber by analyzing the difference between effective and actual fiber properties. Unidirectional [0]28 SiC/Si3N4 composites with 30 percent fiber volume fraction and 30 percent matrix porosity are used. The samples are exposed in a flowing oxygen environment at elevated temperatures, up to 1400°C, for 100 hours and then measured by ultrasonic methods at room temperature. The Young’s modulus in the fiber direction of the sample oxidized at 600° C decreased significantly but it was unchanged for samples oxidized at temperatures above 1200° C. The transverse moduli obtained from ultrasonic measurements decrease continuously up to 1200°C. The shear stiffnesses show behavior similar to the transverse moduli. The effective elastic moduli of the interfacial carbon coating are determined from the experimental data and their change due to thermal oxidation is discussed.

Errata

Journal of MicroscopyVolume 170, Issue 2 p. 190-190 Free Access Errata First published: May 1993 https://doi.org/10.1111/j.1365-2818.1993.tb03340.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Fullwood, N.J. & Meek, K.M. (1993) A synchrotron X-ray study of the changes occurring in the corneal stroma during processing for electron microscopy. J. Microsc. 169, 53– 60. Powell, K.L., Yeomans, J.A. & Smith, P.A. (1993) Characterization of subsurface damage in ceramic-matrix composites by confocal scanning laser microscopy. J. Microsc. 169, 189– 195. Volume170, Issue2May 1993Pages 190-190 ReferencesRelatedInformation