Center for Composite Materials - University of Delaware

Research Summary

High Rate Test Method for Fiber-Matrix Interface Characterization

Authors: Sandeep Tamrakar, Bazle Z. (Gama) Haque and John W. Gillespie, Jr.


• It is a 3D region between fiber and matrix
• Determines the level of interaction between fiber and resin
• Mechanism of energy absorption in composite
-Fiber breakage
-Fiber-matrix debonding
-Frictional sliding
-Matrix cracking
• Previous studies show rate dependence of interfacial shear strength of fiber matrix interface


• Develop high strain rate test methodology for characterizing fiber-matrix interface using microdroplet test specimen
• Adopt tensile Hopkinson bar with a modified loading end
• Design test specimen geometry using LS-DYNA by investigating stress wave propagation in Hopkinson bar and test specimen


Aluminum bar (6061)
• ¼” diameter and 5’ length
• Hollow aluminum striker bar

Transmission bar replaced with a load cell
• Quartz - piezoelectric load cell (Kistler 9712 B5)
• Capacity: 22.24 N

Laser sensor to measure the displacement
• Laser diode: 110 mW, 660nm, circular beam
• Photodetector: DET100A – Si Detector
• 400 – 1100 nm
• Sensor area: 75.4 mm²


• Study stress wave propagation in fiber free length
• Study the evolution of interfacial shear stress as a function of time
• Effect of striker bar velocity and droplet size

• 2D Axisymmetric model
• Incident bar and striker bar material: Aluminum
• Linear elastic elements
• Perfect bond between fiber and matrix
• Knife edge rounded off with a radius of 10 μm
• Mesh size of 0.625 µm chosen along the droplet length based on the characteristic times of the fiber and resin
-Wave velocity in fiber: 5470 m/s
-Wave velocity in matrix: 1840 m/s
-Characteristic time of fiber: 1.1 µs
-Characteristic time for element size 0.625 µm = 0.00011µs
-Number of elements along the droplet length = 160
-Number of elements across the diameter of fiber = 16

• Fiber gage length: 2 mm & 6 mm
• Droplet diameter: 100 μm and 200 μm
-Maximum droplet size to avoid fiber failure: 200 µm
• Fiber diameter: 16 μm
• Loading rates:
-5m/s, 10 m/s, 20 m/s and 60 m/s

• Interfacial shear stress distribution and the stress wave propagation in the fiber gage length studied for different loading rates to identify dynamic effects
• Acceptable loading rate: 10 m/s
• Acceptable test specimen geometry:
-Fiber gage length: up to 6 mm
-Droplet size: up to 200 μm


• Tests conducted using modified tensile Hopkinson bar
• S2 Glass Fiber
• Matrix – DER353 epoxy resin and PACM – 20 curing agent
• Epoxy-silane sizing
• Rate of loading: ~ 1 m/s
• Average interfacial shear stress: 79.2 ± 13.1 MPa

• The difference between interfacial shear strength for high rate loading and quasi static loading was found to be statistically significant
• Interfacial shear strength increased by 1.54 times when the loading rate increased from 3 μm/s to 1 m/s
• Interfacial shear strength increased almost linearly with the loading rate


• A modified tensile Hopkinson bar was developed and employed to conduct dynamic microdroplet experiments
• Acceptable loading rate and specimen geometry determined through LS-DYNA simulation
• High rate tests were conducted at 1 m/s, which is 2-3 orders of magnitude higher compared to previous experiments
• Fiber matrix interface strength property was found to be sensitive to the rate of loading


This work is supported by the Army Research Laboratory through the Composite Materials Research program. 

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