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.
Owing to their unique properties such as damage tolerance, fracture toughness, wear and corrosion resistance with respect to monolithic ceramics, and crack growth resistance, CMCs can withstand severe thermomechanical loading conditions [
The importance of monitoring the structural safety of aerospace structures is imperative. Prevention of catastrophic failure as well as safe and economical management of the structures can be achieved by early assessment of material conditions before the appearance of large-scale fracture. Regular observation of the structures for signs of damage or deterioration will enable the realization of proper repair actions which, in turn, will help extend the useful life span of the component. Among the highly sought-after nondestructive methods capable of monitoring the structural integrity of aerospace structures in an efficient and economical manner, infrared thermography and acoustic emission stand out for being fast, straightforward, and highly reliable. Today both the National Aeronautics and Space Administration (NASA) and Astrium, the European space company, use IRT and AE to detect defects in shuttle wings, rudders and tails, thruster chamber assemblies, and other composite components [
While IRT and AE have been successfully applied to detect flaws in CMCs, little information is available on their potential to capture and follow the formation of subsurface cracks. Moreover, it is extremely interesting to investigate the advantages of combined application IRT and AE and to evaluate complementary input that these two techniques can give about CMC damage.
In the present work, IRT and AE are combined to monitor the formation and development of damage during cyclic and fatigue loading of SiC-fiber reinforced barium osumilite (barium, magnesium, aluminium, and silicate (BMAS)) glass-ceramic matrix composites. IRT was used to identify the most critical, with respect to fracture, damage mechanisms as well as to monitor crack propagation under cyclic and dynamic loads and to predict the composite’s residual life. State-of-the-art IR lock-in thermography was used in a unique manner to rapidly and precisely assess the fatigue limit of the CMC, using data from a single specimen test. AE parameters were very powerful in identifying and quantifying real time damage in the CMC. The significance of a large number of IRT and AE indices with respect to mechanical performance and damage evaluation is discussed and explained in the text.
SiC/BMAS laminates were provided as 3 mm thick plates. The BMAS glass matrix consisted of 50 wt% SiO2, 28 wt% Al2O3, 7 wt% MgO, and 15 wt% BaO and was reinforced by SiC Tyranno fibers stacked and hot-pressed at 1200°C for 10 min in a symmetric
Double-edge-notch and dogbone specimen configurations with marked AE monitoring locations (grey circles).
All mechanical testing was performed at ambient temperature on an Instron 8800 servohydraulic frame equipped with a ±100 kN load cell. Specimens were gripped with a pressure of 4 MPa and were tested without end tabs at a nominal gauge length of 50 mm. Static tensile testing, both monotonic and cyclic, was performed under crosshead displacement control with a rate of 0.2 mm/min corresponding to an initial strain rate of 4.0 × 10−3 min−1 within the 25 mm gauge length of the external, knife-edge-mounted axial extensometer. In cyclic tension experiments with unloading/reloading loops, unloading commenced at 10−3 strain and repetitions occurred with a step of 1.5 × 10−3 strain. The composites were unloaded to full relaxation before reloading.
Fatigue step loading until fracture was conducted on dogbone specimens. The first loading step was set to 10%
Schematic of fatigue loading protocol.
Acoustic emission activity was monitored throughout mechanical testing on all specimens using two “Pico” microminiature AE sensors tape-mounted at a separation of 40 mm on the central part of the specimen (Figure
Throughout testing, temperature variations due to the applied loading, on the AE/extensometer-free face of the specimen, were monitored by an infrared thermography camera (CEDIP, MIW). The camera featured a cooled indium antimonide (InSb) detector (3–5
Throughout cyclic loading of DEN specimens of the SiC/BMAS composite, thermographs were recorded from the high-stress concentration area between the notches. For lock-in thermography measurements during fatigue loading, specimens were spray-coated with a matte black varnish in order to achieve uniform high-level surface emissivity. Optimal field of view (FOV) conditions were achieved by positioning the camera at approximately 40 cm in front of the gripped specimen. The IR camera was connected to the lock-in amplifier which, in turn, was connected to the servohydraulic controller. This enabled the synchronization of the lock-in amplifier and the testing machine frequencies and capturing of lock-in images and data during fatigue loading. The IR camera was used to measure the amount of energy emitted as infrared radiation, which is a function of the temperature and emissivity of the specimen. According to a previous study, the measured energy corresponds to the intrinsically dissipated energy while the fatigue limit is located at the break of the intrinsic dissipation regime of the loaded specimen [
The stress-strain response of notched and dogbone SiC/BMAS specimens under cyclic and monotonic tension, respectively, is presented in Figure
Stress-strain response of SiC/BMAS under monotonic loading (dotted line in (a)) and cyclic tension with unloading/reloading loops for (a) un-notched specimens and (b) notched specimens with various notch lengths.
Most importantly, the stress-strain curves of unnotched specimens are defined with unique precision, a common intersection point (CIP) of unloading-reloading curves in the first quadrant of the stress-strain curve in the tension domain. The coordinates of the CIP, 0.001 strain and 90 MPa stress, are directly related to the axial residual stress state of the composite [
Comparing the monotonic and cyclic tension curves for the SiC/BMAS composite (Figure
Composite strength and modulus appeared to increase with decreasing notch length. Unnotched specimens enjoyed average strengths and moduli of 355 MPa and 151 GPa, respectively. The corresponding values for the 0.2 and 0.35 notched-to-width length were 280 MPa/119 GPa and 270 MPa/108 GPa, respectively.
Temperature variation as measured by IRT,
(a)
In Figure
The thermographic behaviour within the ultimate loading cycle, of the same 0.35 notch-to-width ratio specimen, is demonstrated in Figure
Final loading cycle of 0.35 notch-to-width ratio DEN specimen. (a) Thermographs showing crack propagation and (b) load versus time curve.
Similar trends were observed for DEN composites with smaller notches, as in specimens with 0.2 notch-to-width ratios; the thermographic behaviour within the ultimate loading cycle of such a specimen is demonstrated in Figure
((a), (b)) Thermographs of crack propagation and diagram of load versus time of final loading cycle (0.2 notch-to-width ratio specimen).
Peak
Peak
Lock-in thermography was applied during fatigue loading of SiC/BMAS dogbone specimen. The intrinsically dissipated energy as monitored by the IR camera for 10 different stress levels ranging from 30% to 90%
Dissipated energy versus %
The curve exhibits two distinct slopes, visualized by the two linear regressions seen in Figure
An examination of the thermographic pattern of cross-ply SiC/BMAS at different stress levels is of particular interest in view of the established fatigue limit value. This information is presented in Figures
Thermographic pattern during fatigue loading of a SiC/BMAS composite.
Cumulative AE signal history collected during cyclic loading of a DEN specimen with 0.35 notch-to-width ratio is shown in Figure
Strain and AE cumulative history for specimen B.
Apart from the cumulative activity, which counts the separate acquisitions of the sensors, different AE descriptors help to distinguish the severity of the condition according to loading level. Two of them are the ASL and RMS. ASL is the average signal level defined as the average amplitude of samples of the rectified waveform while RMS is the square root of the average of the squares of all points of a waveform (root mean square) [
They are given by
Therefore, both of them are indicative of the AE signal emitted by the fracture. Figure
Strain history and AE amplitude parameters for a double-edge notched specimen with notch-to-width ratio of 0.35.
Apart from the amplitude or energy-related parameters, significant information can be derived by the frequency content of the emitted waves. In mechanical materials it has been shown that a drop of frequency indicates increase of damage accumulation and is linked with the shift between fracture modes (e.g., initial tensile matrix cracking to ultimate shearing) [
Strain history and AE peak frequency for a double-edge notched specimen with notch-to-width ratio of 0.35. The solid line is the sliding average of recent 20 points.
The observed behavior can be due to the increasing number of interfacial F that is expected to happen at the higher strain levels of the cycles. This allegation is supported by visual evidence of fiber bundle sliding and existence of off-axis layers seen in the postmortem side view of a specimen’s notched ligament, microphotograph in Figure
Stereoscope image of the side of a fractured specimen.
During fatigue loading continuous monitoring by AE was applied, as mentioned earlier. Apart from the activity (number of emissions) all different AE parameters were recorded. As the fatigue life proceeds and damage is being accumulated, apart from the larger number of signals which are emitted, the nature (waveform shape) of the AE incidents starts to change. One of the indicative AE descriptors is the RA value which is the inverse of the rising angle of the waveform and is given by
Cumulative RA value for the different fatigue loading stages.
The rate of RA accumulation is shown in Figure
RA increasing rate for different fatigue loading amplitudes.
Thermography and acoustic emission were used to capture the initiation and evolution of damage in SiC-fiber reinforced glass-ceramic matrix composites under static and fatigue loading. Infrared thermography results helped identify the intact fiber population as the mechanism that control ultimate material failure and that under the presence of notches the composite fails shortly after the attainment of a saturated matrix cracking state. Infrared thermography (IRT) was also used to monitor, both in location and in time, the crack propagation path during mechanical testing in cyclic tension and fatigue. The technique also enabled early prediction of the residual life of the material, as early as at 73% of the duration of the final loading cycle. Successful application of the technique under such dynamic conditions where the surface changes with usage is close to real-life scenarios found in aerospace applications.
A novel infrared lock-in thermographic methodology was used for the determination of the fatigue limit of the ceramic matrix composites (CMCs). The limit was unconventionally rapidly assessed by the thermographic technique at 70%
Furthermore, acoustic emission (AE) monitoring enables monitoring fracture behavior in real time. Apart from the increase of AE acquisition for higher load and damage accumulation, energy- and frequency-related parameters help discern the moments higher stress. Descriptors like the root mean square (RMS) and average signal level (ASL) increase their values at high stresses, while peak frequency shows the inverse trend, being continuously downgraded for the successive loading steps. Concerning fatigue, AE showed the capability of detecting the different intensity of the fracture incidents. Waveform shape parameters like the RA exhibit changes as the load increases above the fatigue level of the material. The results are benchmarked by the heat dissipation curves offered by thermography and allow the determination of the fatigue limit of the material by using only one specimen.