Research On Flaw Characterization By Chirplet And Strength Damage Of Unidirectional Cf/al Composites

Continuous carbon fiber reinforced aluminum matrix composite (Cf/Al) becomes outstanding alternative material for space devices due to its good space characteristics,such as high specific strength and high stiffness. Thus, it is necessary to detect the anomaly and evaluate the quality of composite products. However, anomalies exist in component are inevitable, so the design of reliability and service damage tolerance becomes highly important. In this paper, by using parametric Chirplet decomposition to Ultrasonic signal based on ultrasonic detection method, anomalies of composite were identificated and characterized legibly. The effect of anomalies (e.g. Al-rich, inclusion, delamination and holes) on the tensile and compressive mechanical properties, damage failure mechanisms of Cf/Al composite was systematically studied through by electronic tensile machine and SEM in situ observation technique. Furthermore, residual strength of Cf/Al composite with layer anomalies and hole anomalies was obtained by finite element method and residual strength prediction model.The artificial anomalies were introduced into Cf/Al composite, which were able to simulate the layer anomalies of Al-rich, inclusion and delamination. The Chirplet parameters (center frequency and phase) variance was obtained through the Chirplet decomposition for ultrasonic signal of the above anomalies, which could be used as the characteristic parameters for the anomaly characterization. Furthermore, the effect of anomaly type, depth, thickness and anomaly boundaries on Chirplet parameters was deeply studied, the results show that the ultrasonic signal characteristic parameters of three types of anomalies were obviously different; the Chirplet parameters of the layer anomaly were essentially invariable with the anomaly depth, but they increased with increasing thickness of aluminum accumulation layer in less than the critical thickness (0.27mm); the Chirplet parameters could significantly improve the identification of anomaly boundary.The C scan imaging of Chirplet range, depth, center frequency and phase of anomalies was gained through the Chirplet decomposition of the anomaly signals. The results show that the C scan imaging of range could improve the contrast of C scan images; the C scan imaging of depth could show the depth distribution of anomalies in the samples; the center frequency and phase imaging made the anomaly border clear and distinct, which could improve the measurement accuracy of the anomaly size; the C scan imaging of phase could improve the resolution of the details and characteristics of the anomalies, so it made the characterization of delamination growth process more accuratly.The effect of Al-rich, inclusion, delamination and holes on the tensile and compressive strength of Cf/Al composite material was studied on the basis of the ultrasonic testing. The follwing results were obtained: the tensile and compressive strength respectively decreased by 15.8% and 23.1% for the composite containing Al-rich; the tensile and compressive strength respectively decreased by 35.2% and 50.5% for the composite containing inclusion anomaly; the compressive strength decreased by 47.9% for the the composite containing delamination; the tensile and compressive strength both decreased exponentially with the pore diameter.The finite element analysis results of the Cf/Al composite with the local layer anomalies show that the axial and shear stress concentration in the edge transition zone of the inclusions is more serious than that of composite with the Al-rich anomalies under the same tensile load conditions. Stress distribution of the composite with layered anomalies is changeless with the increase of transverse dimensions, the residual strength of the Cf/Al composite with the local layer anomalies were calculated under the axial maximum stress criterion, the obtained results have a good agree with the experimental results. Under the compressive load, the transverse stress concentration of inclusions in the edge transition area is the most serious, the delamination takes the second place, and Al-rich affects the minimum. The compression residual strength of Cf/Al composites with Al-rich, delamination or inclusions was estimated under transverse and axial maximum stress criteria, the estimated results of compression residual strength was slightly less than experimental results.The tensile residual strength of the Cf/Al composite with holes was predicted by using point strength criterion (PSC), average strength criterion (AVC), damaged zone criterion (DZC) and effective crack growth model (ECGM). Results show that in the whole test range of 0.05~0.5 (D/W), high predicted precision can be obtain from the ECGM and the DZC; when D/W > 0.2, the predicted result from the PSC and the AVC can't predict accurately the tensile residual strength because of the greater error. DZC and ECGM are not suitable for residual strength prediction of the Cf/Al composite with holes because of the different failure mechanisms between the tensile load and the compressive load.The failure process of notch contained Cf/Al composite with Al-rich layer and inclusion was observed by the SEM in suit observation technique. The results show that the Al-rich with good toughness and strength can release stress concentration in the front of the internal main crack of Cf/Al composite and make the crack spread around, which prevents the crack propagation and delays the damage failure of Cf/Al composite. However, the inclusion, which has lower strength, can make the cracks spread vertically, which results in the instantaneous destruction of composite materials and accelerats the crack propagation and failure of composite. Compression failure process of Cf/Al composite with layer anomalies (including Al-rich, inclusions and local delamination) was observed with the method of in-situ observation composite by scanning electron microscopy (SEM).The results show that the compression failure mechanism of Cf/Al composite mainly includes fiber micro-buckling and the interface delamination. And the compression failure mode of the composites containing inclusions or local delamination is mainly buckling delamination. The compression failure mode for the composite with the Al-rich is the combination of fiber micro- buckling failure and buckling delamination.The characterization of the damage extension of Cf/Al composite with anomalies (e.g. Al-rich, inclusions and local delamination) under the compressive load was realized by Ultrasonic Chirplet Parameter C Scan. It is more significant for Phase C Scans to obtain the characterization comparing with Ultrasonic Chirplet Parameter C Scan.