A semi-automatic measurement system based on digital image analysis for the application to the single fiber fragmentation test

Abstract

The computational prediction of the effective macroscopic material behavior of fiber reinforced composites is a goal of research to exploit the potential of these materials. Besides the mechanical characteristics of the material components, an extensive knowledge of the mechanical interaction between these components is necessary in order to set-up suitable models of the local material structure. For example, an experimental investigation of the micromechanical damage behavior of simplified composite specimens can help to understand the mechanisms, which causes matrix and interface damage in the vicinity of a fiber fracture. To realize an appropriate experimental setup, a novel semi-automatic measurement system based on the analysis of digital images using photoelasticity and image correlation was developed. Applied to specimens with a birefringent matrix material, it is able to provide global and local information of the damage evolution and the stress and strain state at the same time. The image acquisition is accomplished using a long distance microscopic optic with an effective resolution of two micrometer per pixel. While the system is moved along the domain of interest of the specimen, the acquired images are assembled online and used to interpret optically extracted information in combination with global force-displacement curves provided by the load frame. The illumination of the specimen with circularly polarized light and the projection of the transmitted light through different configurations of polarizer and quarterwave-plates enables the synchronous capturing of four images at the quadrants of a four megapixel image sensor. The fifth image is decoupled from the same optical path and is projected to a second camera chip, to get a non-polarized image of the same scene at the same time. The benefit of this optical setup is the opportunity to extract a wide range of information locally, without influence on the progress of the experiment. The four images are used to obtain information on the stress distribution based on photoelasticity, while the fifth image delivers the local strain as outcome of an image correlation algorithm and enables the observation and documentation of the visible damage phenomena. The acquisition of five different images at a time allows for the application to materials with time-dependent mechanical behavior which is an important added value of the developed measurement optics. The experimental setup is applied to the so-called single fiber fragmentation test, which defines a common test procedure to study the damage phenomena of single long-fiber reinforced specimens in transparent matrix material. When a tension load is applied to the specimen at low strain rate, damage of the fiber arises without a complete failure of the matrix material. As a result of the local failure of the fiber, a load transfer to the surrounding matrix material and the appearance of a characteristic stress distribution as well as evolving matrix and interface cracks can be observed. Using the described measurement system, it is possible to estimate the stress and strain distribution of the matrix material in the vicinity of the fractured fiber. In combination with the documentation and classification of the damage phenomena this enables the interpretation of the stress redistribution process inside the composite. This knowledge can be used to analyze the correlation between micromechanical phenomena and the effective macroscopic material behavior as well as to identify parameters of constitutive models for interface failure. The article demonstrates the potential of the measurement system and presents the results of its application to the single fiber fragmentation test. To point out the concluded facts, the results of differently manipulated specimen of epoxy matrix material with an embedded glass fiber are compared.