Compressive splitting failure of composites using modified shear lag theory

The shear lag model has been used in conjunction with the 3D elasticity equations to determine the stress state in a fiber/matrix system containing an interface crack. The use of a shear lag model to capture the stress state at the crack tip and the modelling of the region away from the crack tip by the elasticity equations leads to a simple analytical expression which can be used to determine the compliance changes for both unsteady crack growth as well as steady state crack propagation under compressive loading. Certain modifications to the assumptions used in the classical shear lag model have been made to increase the accuracy of the predictions for the rate of change of compliance with respect to crack length, dc / dl . The present approach leads to closed form expressions for the compressive strength of unidirectional fiber reinforced composites.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42758/1/10704_2004_Article_405549.pd

A mechanism based modeling approach to failure in fiber reinforced composites.

The increasing use of fiber reinforced polymer composites (FRPC) in civil and military applications has made it imperative that the behavior of FRPC be understood under a variety of loading conditions. It is known that the compressive load carrying capacity of FRPC is low compared to its tensile load carrying capacity. Thus, the compressive behavior of FRPC has been a limiting factor in the design of FRPC structures. In the current work, the compressive behavior of FRPC has been studied with an aim to identify and understand the important parameters affecting the compressive strength and failure mechanisms in FRPC, particularly under combined stress states. The purpose is to establish compressive strength degradation (or enhancement) in the presence of combined stress states. Results from experiments have led to the development of a mechanism based failure model, based on the principles of fracture mechanics for the splitting failure of FRPC. A mechanics model has been developed for both pure compression and combined compression-torsion loading. The predictions of the model were found to compare favorably with experimental data obtained from glass and carbon FRPC under pure compression and combined compression-torsion loading. A 3D finite element simulation of a representative cylindrical section of the composite was performed. The results indicated the importance of fiber diameter on the predicted compressive response of the composite. It also indicated the possibilities of fiber breakage under axial loading as a cause for the initiation of kinking in case of small diameter fiber reinforced composites. Pure compression tests were also conducted on hybrid (glass/carbon) composites under static and dynamic loading conditions. The static compressive strength of hybrid composites shows a non-monotonic behavior with respect to the hybrid ratio. The compressive strength first decreases and then increases as we approach either pure carbon or pure glass composites. The hybrid composites tested show an increase in strength with strain rate at all hybrid ratios. Based on the above experiments/analysis a non-dimensional number has been derived to a priori identify the composite failure mechanism, and thus aid the accurate prediction of composite compressive strength.Ph.D.Aerospace engineeringApplied SciencesMaterials scienceMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/123500/2/3079552.pd

Numerical investigation on the effect of specimen gripping arrangement on dynamic shear characterization using Torsion Split Hopkinson Bar

Torsion Split Hopkinson Bar (TSHB) is widely used in the dynamic shear characterization of material under pure shear loading. In TSHB, tubular specimens with either circular or hexagonal flanges are used. The specimens with circular flanges are generally bonded using adhesive to the incident and transmission bars. The specimens with hexagonal flanges are gripped into the hexagonal holders that are fixed onto incident and transmission bars. In the current study, numerical simulations are carried out to see the effect of gripping arrangements on the dynamic shear characterization quality. Numerical experiments with three gripping configurations are studied—the first gripping configuration with a direct bond (numerically-tie) between specimen and bars. The second configuration with the specimen gripped by hexagonal holders fixed to bars. The third configuration with specimen directly gripped into the incident and transmission bars having hexagonal slots

Compressive Behavior of Hybrid Composites

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76187/1/AIAA-2003-1509-167.pd

Wind Turbine Manufacturing Process Monitoring

To develop a practical inline inspection that could be used in combination with automated composite material placement equipment to economically manufacture high performance and reliable carbon composite wind turbine blade spar caps. The approach technical feasibility and cost benefit will be assessed to provide a solid basis for further development and implementation in the wind turbine industry. The program is focused on the following technology development: (1) Develop in-line monitoring methods, using optical metrology and ultrasound inspection, and perform a demonstration in the lab. This includes development of the approach and performing appropriate demonstration in the lab; (2) Develop methods to predict composite strength reduction due to defects; and (3) Develop process models to predict defects from leading indicators found in the uncured composites