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Title: Evaluation of Stress Intensity Factors in Metal Matrix Composites
Author: Contreras, Gerardo Plata
Awarding Body: Oxford Brookes University
Current Institution: Oxford Brookes University
Date of Award: 2007
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The drive of the aerospace industry to use better materials to save weight and increase the performance of the components has been the principal motive to create new materials like Titanium Metal Matrix Composites (Ti-MMC). The Ti-MMCs were designed to combine high stiffness with low weight. Most aerospace components are exposed to fatigue loads during their operation life, hence extensive research has been carried out to investigate the behaviour of Ti-MMC under fatigue loading. During fatigue of Ti-MMC in four point bend specimens, two cracks grow quazisymmetrical at an angle relative to the normal direction of the notch. It is clear therefore that Mode I and Mode II are present in the specimen. Solutions for predicting the Stress Intensity Factor (SIF) for TI-MMC under these load conditions have not been carried out before and presents difficulties, both analytically and experimentally. Some experimental technique like Stereo-Imaging Technique (Sin, Scanning electron microscope (SEM), Laser interferometry displacement gage system and mechanical extensometer have been used to calculate the SIF in Ti-MMC specimens by measuring the Crack Open Displacement COD during the fatigue crack propagation, but these technique have limited measuring capabilities. However, the Moire interferometry technique gives very sensitive and detailed data, due to its ability to measure the full field displacements near the crack tip. Moire Interferometry was applied in this thesis to calculate KI and KII using different analytical methods, for fatigue crack growth in unidirectional Metal Matrix Composite. The composite considered was Textron SCS-6ffi-6-4 and the bend was carried out in four point bending at room temperature. Zero load ratio and four different load ranges were considered. Two finite element models were also developed to predict the crack path direction and to calculate the SIFs. The results obtained from FE models and from the experimental technique show good correlation. The experimental results though show that the SIFs do not reduce in a linear manner but have several stages of crack retardation which is believed to be due to fibres becoming active in bridging behind the crack tip. The results presented are unique in that very little data on SIFs in Ti-MMC material is available and the effect of off-axis cracks on the KI and KII SIFs has not been addressed until now.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available