Degradation at the fiber-matrix interphase and effects on long-term performance of composites

Suny At Stony Brook, Stony Brook NY

Investigators

Abstract

Abstract: CMS-0626025 Long-term degradation of composites is important for the reliability and improvement of composite material life-times. In this proposal, mechanical performance of composites is assessed with respect to quantitative measurements of nanomechanical fiber-matrix interphase properties performed as a function of degradation. Carbon-fiber reinforced composites are governed by and have been shown to fail at the interface between the fiber-reinforcements and the matrix material. The region is characteristically called the interphase between the fiber and the matrix and contains unique micromechanical properties that have demonstrated effects on bulk composite properties. The ability to quantitatively measure the interphase properties is critical to the long-term performance of composite materials, but due to the typical length-scale of the interphase being on the order of micrometers, the quantitative measurement of the interphase mechanical properties of various composites has been difficult. The use of instrumented-indentation techniques to probe mechanical properties, which have been proven viable on coating materials with sub-micrometer thicknesses, does not garner the lateral spatial resolution necessary for quantitative interphase measurements. To achieve the lateral resolution necessary, Atomic Force Microscopy (AFM) has been utilized to measure quantitative and assess qualitative mechanical properties of the fiber-matrix interphase (namely stiffness), though quantitative AFM-based techniques so far used (i.e. nanoindentation) rely on discrete point analysis of the surface and cannot provide continuous stiffness mapping. The use of Atomic Force Acoustic Microscopy (AFAM) has gained attention in the past few years as a viable method for quantitative mechanical property measurement using AFM technology. The technique involves ultrasonic acoustic transduction of the sample and measurement of the sample-AFM tip contact stiffness by means of the AFM cantilever vibrations. Nevertheless, the use of any AFM-based techniques has not been employed to study the composite fiber-matrix interphase mechanical properties as a function of degradation to achieve resolution on the sub-micrometer scale. This will be accomplished here quantitatively by using AFAM. In addition, the fracture toughness of the interphase region is of interest to bulk composite properties since it is related to cracking that occurs along the fiber-matrix interface. Measurement using normal instrumented-indentation has been attempted, but does not provide the lateral resolution necessary for adequate analysis of the interphase region only. In this work, the fracture toughness will be measured as a function of degradation utilizing a novel instrumented-scratch testing technique that has resolution on the order of the interphase length. The micromechanical properties of stiffness and fracture toughness will be used in existing micromechanical models for bulk composite mechanical properties and compared with existing data as a function of degradation.