Hydrogen bonds, interfacial stiffness moduli, and the interlaminar shear strength of carbon fiber-epoxy matrix composites

Abstract

The chemical treatment of carbon fibers used in carbon fiber-epoxy matrix composites greatly affects the fraction of hydrogen bonds (H-bonds) formed at the fiber-matrix interface. The H-bonds are major contributors to the fiber-matrix interfacial shear strength and play a direct role in the interlaminar shear strength (ILSS) of the composite. The H-bond contributions τ to the ILSS and magnitudes KN of the fiber-matrix interfacial stiffness moduli of seven carbon fiber-epoxy matrix composites, subjected to different fiber surface treatments, are calculated from the Morse potential for the interactions of hydroxyl and carboxyl acid groups formed on the carbon fiber surfaces with epoxy receptors. The τ calculations range from 7.7 MPa to 18.4 MPa in magnitude, depending on fiber treatment. The KN calculations fall in the range (2.01 – 4.67) ×1017 N m−3. The average ratio KN/|τ| is calculated to be (2.59 ± 0.043) × 1010 m−1 for the seven composites, suggesting a nearly linear connection between ILSS and H-bonding at the fiber-matrix interfaces. The linear connection indicates that τ may be assessable nondestructively from measurements of KN via a technique such as angle beam ultrasonic spectroscopy.

Assessment of Adhesive Bond Strength from Ultrasonic Tone-Bursts

“…The physico-chemical model [10,11] of adhesive bond strength treats the adhesive interface as a finite array of interatomic bonds with a bond density of N bonds per unit interface area.…”

“…Realistically, the finite dimensions of atoms prevent the occurrence of an infinite array of bond sites. Adhesive bonding is more accurately described by the physico-chemical model of adhesion [10,11]. To obtain a truly quantitative assessment of adhesive bond strength a scaling equation is introduced in the present work to link ultrasonic measurements to the physico-chemical model.…”

“…The maximum average interatomic binding force per bond 〈F〉 max is calculated from Eqs. (31), (34), and the relation 〈F〉 = −〈du/dr〉, using the value δ = r ′ = r max = 3.63 x 10 −10 m, to obtain 〈F〉 max = 〈F(δ = r ′ )〉 = −3.74x10 −11 N.The maximum number of bond per unit area N 0 can be assessed from the relation[11] = 3200 is the molecular weight of epoxy molecules, M rep = 340 is the molecular weight of the repeat unit of the epoxy molecule, ρ epox = 1.3. x 10 3 kg m −3 is the mass density of epoxy, N A = 6.02 x 10 23 molecule/mole is Avogadro's number, and α is a factor to account for the interpenetration of epoxy molecular chains. Assuming that α = 0.05 (corresponding to a moderate to high degree of molecular interpenetration) leads to the maximum bond density value N 0 = 7.4 x 10 17 m −2 .…”

“…by the finite dimensions of the bonded atomic pairs. The limitation led previously to the development of the physico-chemical model of adhesive bonding[10,11]. To relate the present ultrasonic model to the physico-chemical model of adhesive strength a scaling equation is introduced that links the interfacial spring stiffness constant K ab in the ultrasonic model to the fraction of intact bonds f = N/N 0 (N = number of intact bonds) in the physicochemical model and thus to the interfacial spring stiffness constant K P-C = fN 0 k (k = interatomic force constant) in in the physico-chemical model.…”

Electron work function – a probe for interfacial diagnosis

“…This is also applied to interfacial zone with disordered structure but the interatomic potential is the same. In general, the higher the interfacial modulus, the stronger is the interface 36 , 37 . As demonstrated, the interface with smaller dφ / dx at location 1 is stronger than the one at location 2.…”