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.

FIBER MATRIX INTERFACE

• 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
-Delamination
• Previous studies show rate dependence of interfacial shear strength of fiber matrix interface

OBJECTIVE AND APPROACH

• 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

TENSILE HOPKINSON BAR

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

MICRODROPLET TEST SPECIMEN SIMULATION

• 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

HIGH RATE MICRODROPLET TEST

• 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

CONCLUSIONS

• 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

ACKNOWLEDGEMENTS

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

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