A study on the fracturing behavior and scaling of carbon/glass hybrid composites

Abstract

One of the factors hindering the broad application of composites is their relative brittleness leading to sudden failure with no significant damage precursors. This makes designing with composites very challenging and frequent health monitoring becomes an expensive need. In the past ten years, significant work has been focused on introducing a certain level of ductility in composite structures through techniques such as adding nano-particles or leveraging a complex heterogeneous mesostructure such as in discontinuous fiber composites (DFCs). Hybridization is another such alternative and studies on carbon/glass hybrid composites have shown promising results. While extensive work has been done to characterize the behavior of smooth (un-notched or un-cracked) hybrid specimens, very little is known about the fracturing behavior in the presence of sharp notches. This lack of knowledge is an impediment to the widespread use of such materials. This work aims at filling this knowledge gap by means of a thorough experimental and computational investigation. Fracture in quasi brittle materials such as composites involve a number of non-linear phenomenon which makes it impossible to characterize them using Linear Elastic fracture Mechanics (LEFM) techniques. Moreover, a wide class of materials such as nanocomposites, concrete, composites, polymers show fracture behavior which can be characterized using a bi-linear cohesive law shape. This study develops a modeling technique which incorporatesa bi-linear cohesive law in ABAQUS models to simulate fracture. A MATLAB subroutine was devloped to generate Damage displacement data for bi-linear cohesive crack laws. This data was an input to the material model in ABAQUS which was used for the simulations. These simulation results were used to obtain size effect plots which characterize the specific material system and cohesive crack law. Once the modeling technique was established, a dimensional analysis was carried out to obtain the dimensionless parameters which are critical to the cohesive zone simulations. These dimensionsionless paramters were used to obtain size effect curves in the dimensionless space. Subsequently, the parameters were swept over a wide range of values to obtain master curves spanning different cohesive law shpaes and material properties. Finally a universal fitting equation was generated which can be used to accurately capture these size effect curves. The second part of the work involved experiments on Carbon/glass hybrid composites. Experiments were performed on Single Edge Notch Tension (SENT) samples over three different sizes to obtain experimental size effect curves. Finally an attempt was made to use the modeling technique developed previously to explain the results obtained through these experiments in order to explain the fracture of the hybrid composites