Graduate Thesis Or Dissertation


The Applicability of Linear Elastic Fracture Mechanics to Compressive Damage of the Carbon Fiber Reinforced Plastic Matrix Public Deposited

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  • With the widespread replacement of aluminum with composite materials, particularly in aircraft structures, the lack of comprehensive understanding of these materials is unsettling. The complex nature of these materials, with respect to isotropic aluminum, makes the prediction of their mechanical response equally complex. The failure of carbon fiber reinforced plastic composites has been categorized into fiber tension, fiber compression, matrix tension, and matrix compression. The first three of these failure modes have been explored rigorously, and the behavior is fairly well established. Matrix compression, however, has not been investigated as thoroughly and assumptions have been made throughout research and industry that had not been validated. This research served to identify, through experimental techniques, a test specimen suitable for isolating the matrix compression damage response. After the development of a suitable specimen, the investigation to the validity of the assumptions commonly made regarding matrix compression loading was conducted. It was often assumed that the matrix under compressive loads followed the relationships established by linear elastic fracture mechanics, which has never been experimentally demonstrated; the work presented here does so. Through specimens of variable notch lengths, the failure load was related to the corresponding notch length, and the relationship observed was compared to the relationship defined by linear elastic fracture mechanics. The defined relationship would have a linear slope of -0.5, and the data reported here showed a linear trend with a slope of -0.54 with error bars that included the linear relationship having a slope of -0.5, thus confirming the applicability of linear elastic fracture mechanics to the matrix in compression. A method was also proposed to calculate the critical strain energy release rate for matrix compression—a value that is often considered negligible or approximated from other parameters for implementation into damage models. Using a displacement-controlled definition of the critical strain energy release rate, a value of 65.51-lb/in with a standard deviation of about 18.66-lb/in, comparing well with a reported value. A final observation that further supported the need for matrix compression investigations was the post-damage initiation behavior in the matrix; after damage initiated, the fractured surfaces continued to support load—in continuum damage mechanics models currently implemented, the load-carrying ability is linearly degraded to zero. Matrix compression must be researched further.
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