[Paper Review] Effect of the Thickness on the Fracturing Behavior of Discontinuous Fiber Composite Structures

Seunghyun Ko, James Davey+4 more|arXiv (Cornell University)|Mar 25, 2019

Mechanical Behavior of Composites26 references17 citations

TL;DR

This study experimentally and numerically investigates the effect of thickness on the mode I intra-laminar fracture and size effect in discontinuous fiber composites (DFCs) using geometrically-scaled Single Edge Notch Tension (SENT) specimens. By integrating equivalent fracture mechanics with stochastic finite element modeling, it is found that fracture energy increases with thickness up to 2 mm, saturating at 57.77 N/mm—4.81× higher than aluminum—while the size effect diminishes with increasing thickness, shifting from pseudo-ductile to brittle behavior in larger specimens.

ABSTRACT

In this study, we investigate experimentally and numerically the mode I intra-laminar fracture and size effect of Discontinuous Fiber Composites (DFCs) as a function of the structure thicknesses. By testing geometrically-scaled Single Edge Notch Tension (SENT) specimens a notable structure size effect on the nominal strength of DFCs is identified. As the specimen size increases, the nominal strength decreases. For small specimens, we find a limited size effect with enhanced pseudo-ductility and a strong divergence from Linear Elastic Fracture Mechanics (LEFM). For sufficiently large specimen sizes, the scaling of the nominal strength follows closely LEFM with a strong brittle failure. As the thickness increases, the size effect decreases. We identify the fracture energy and the effective size of the fracture process zone as a function of the thickness of the structure. To do so, we integrate equivalent fracture mechanics and stochastic finite element modeling. Experimentally, we collect the nominal strength of geometrically-scaled Single Edge Notch Tension (SENT) specimens. The numerical stochastic model captures the complex, inhomogeneous mesostructure of DFCs by explicitly generating the platelets. From the integrated analysis, it is found that the fracture energy depends significantly on the structure thickness. It is shown to increase gradually up to 2 mm and saturates after 3 mm to a value of 57.77 N/mm, which is 4.81 times larger than a typical aluminum alloy.

Motivation & Objective

• To investigate how structural thickness influences the mode I intra-laminar fracture behavior and size effect in discontinuous fiber composites (DFCs).
• To quantify the fracture energy $G_f$ and effective fracture process zone length $c_f$ as functions of thickness using experimental and numerical methods.
• To determine the transition from pseudo-ductile to brittle fracture behavior with increasing specimen size and thickness.
• To develop and validate a stochastic meso-scale finite element model that captures the inhomogeneous platelet distribution and thickness variations in DFCs.

Proposed method

• Experimental testing of geometrically-scaled Single Edge Notch Tension (SENT) specimens across four thicknesses to measure nominal strength and fracture response.
• Stochastic finite element modeling that explicitly generates platelets in a 1 mm × 1 mm partition grid, with random orientation and spatial distribution based on uniform probability.
• Thickness adjustment algorithm to maintain uniform total thickness, with resin layers added where needed to match mean thickness and reduce CoV to 4–7%.
• Use of equivalent fracture mechanics to compute energy release rate $G(\delta,a)$ from potential energy $\Pi$ via $G = -\frac{1}{t} \left[ \frac{\partial \Pi}{\partial a} \right]_\delta$.
• Mesh refinement using 8-node S8R shell elements with crack increment size $\Delta\alpha \approx 0.008$ for accurate $g(\alpha)$ and $g_D(\alpha)$ computation.
• Linear regression on $g(\alpha)$ to extract $g_D$ at initial crack length $\alpha_0$, enabling estimation of $G_f$ and $c_f$.

Experimental results

Research questions

• RQ1 How does increasing thickness affect the nominal strength and fracture behavior of DFCs in mode I loading?
• RQ2 What is the scaling relationship between specimen size and nominal strength in DFCs, and how does thickness modulate this size effect?
• RQ3 How does fracture energy $G_f$ vary with thickness, and does it saturate at a certain thickness?
• RQ4 To what extent does the fracture process zone length $c_f$ change with thickness, and how does this influence the transition from pseudo-ductile to brittle behavior?

Key findings

• Fracture energy $G_f$ increases with thickness up to 2 mm and saturates at 57.77 N/mm for thicknesses ≥3 mm.
• The saturated fracture energy of 57.77 N/mm is 4.81 times higher than that of typical aluminum alloys.
• For small specimens, a limited size effect is observed with enhanced pseudo-ductility and strong deviation from Linear Elastic Fracture Mechanics (LEFM).
• For large specimens, the nominal strength scales closely with LEFM, indicating a transition to brittle behavior.
• The size effect diminishes with increasing thickness, indicating that thicker DFCs exhibit more stable, ductile fracture characteristics.
• The effective fracture process zone length $c_f$ is found to be thickness-dependent, with higher values in thicker specimens, contributing to increased energy absorption.