Effect of surface treatment and sizing on composite properties

Carbon fibers are surface treated by the manufacturers using proprietary processes to improve the bond between resin and fiber. For control purposes, it is usual to measure the three point interlaminar shear strength and the choice of resin, volume fraction, span/depth ratio, mode of failure and time of treatment are important parameters. As the fiber modulus increases, longer treatment times will be required to obtain the same level of ILSS and overtreatment can result in loss of properties, particularly impact strength.

The effect of surface finishes on the properties of carbon fiber composites has been discussed by Wright and Peebles.

Work by Drzal and co-workers has attempted to determine the interfacial shear strength length of Hercules AU-1 and AU-4 fibers, as well as when surface treated together with the effect of a 100-200nm coating of DER330 epoxy size. A single fiber was encapsulated in an 828/m- PDA epoxy matrix cured for 2h at 75C and 2h at 125C and subsequently, tension applied. The tensile forces are transferred to the fiber through shear forces at the fiber resin interface and as the maximum strain to failure of the fiber is much less than the matrix, the fiber fractures into small fragments within the matrix. The process is continued until fracture no longer occurs and the minimum length of the fiber fragments can be obtained, allowing the calculation of the interfacial shear strength.

When dealing with a brittle matrix, the composite failed at the first or second break. The surface treatment process increased the interfacial shear strength by about 6%. It was found that the atomic surface concentrations of oxygen did not account for the improved shear strength of the AS-1 fiber. The surface oxygen groups only accounted for some 10% of the improvement. They concluded that the finish contained less than the stoichiometric quantity of curing agent, if any, and thereby created a layer with a higher modulus and lower fracture toughness than the bulk resin, hence promoting better stress transfer and consequently, a higher interfacial shear strength. Absorption of water makes the interlayer more compliant than the matrix and acts to reduce the amount of damage occurring at the interface.

A more comprehensive investigation was undertaken by Herrera-Franco and Drzal and the authors suggested that the differences observed in the tests were due to the manner in which each test reacted to the conditions that prevailed at the interface region. The short beam shear is only an estimate of the interlaminar shear, whereas the iosipescu does produce a state of pure shear.

Drzal and Madhukar undertook the detailed measurement of the properties of AU-4, AS-4 and AS-4C carbon fiber/epoxy composites to evaluate the effect of fiber/resin adhesion.

If the interfacial shear strength is weak, this results in early failure of the composite. Conversely, if the treatment is too strong, then the composite becomes brittle. Increasing the fiber matrix adhesion does enhance the composite compressive strength.

When considering off-axis properties, the fiber matrix adhesion has a greater effect on the transverse flexural strength than the transverse tensile strength.

A number of workers have published results on cfrp using the Iosipescu test, 4- point shear test, effect of specimen size, effect of strain rate, in-plane shear test, shear strength of pultruded product, rail shear test and torsion test.

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