Introduction

• S-2 Glass fibers exhibit a probabilistic distribution of strength which can be fitted into a 2-parameter Weibull distribution.
• This depends on the size and spatial distribution of critical defects in the fiber

State-of-the-Art Modeling

• Monte-Carlo simulations based on statistical fiber strength distribution.
• Strength values assigned randomly to fiber elements
• Not based on physical defects in fiber
• When fiber break occurs, stress concentration factors are applied to neighboring fiber elements
• High mesh dependence
• Stress concentration profiles determined from static FE simulations/shear lag theory
• Multiple breaks accounted for by linear superposition of stress concentrations
• Do not consider the dynamic effects of fiber break

Research Objectives

• Predict the HSR tensile failure of unidirectional composites while accounting for micro-mechanical
damage mechanisms
• Tailor interface and matrix to enhance tensile properties and energy absorption

Our Modeling Approach

• Model the fiber break as a dynamic process.
• Dynamic fiber fracture using cohesive surfaces
• Mode I fracture initiated based on maximum stress criterion
• Interface modeled using ‘zero-thickness’ cohesive surfaces
• Traction-separation behavior determined from micro-droplet experiments

Dynamic Effects of a Fiber Break

• Even under static tensile loading, fiber break is a dynamic event!
• Local strain rates in the order of 10 6/s

Dynamic Progression of Events After Fiber Fracture

Dynamic Interfacial Debonding

• Ineffective length in dynamic solution increases by the debond length

Qualitative Comparison with Experiments

• Quasi-static solution with pre-broken fiber over-predicted strain to failure in the composite
• Formation of large clusters during single strain increment observed in experiments
• More gradual development of clusters predicted by Quasi-static FE model
• Increase in probability of failure in the neighboring fiber due to dynamic effects
• Zone of influence is 5-6 times larger in dynamic solution
• Significant proportion (30% of all clusters) of non-co-planar clusters observed experimentally
• Dynamic unstable debonding of the interface at break stresses > 2.4 Gpa
• Axial splitting observed in composite tensile tests

Path Forward

• Dynamic model with fracture planes representing characteristic defect distributions in fiber

Acknowledgements

Research was sponsored by the Army Research Laboratory and was accomplished under
Cooperative Agreement Number W911NF-12-2-0022. The views and conclusions contained in this
document are those of the authors and should not be interpreted as representing the official policies,
either expressed or implied, of the Army Research Laboratory or the U.S. Government.