Interfacial bond strength determination

Caldwell, D.L., Babbington, D.A. and Johnson, C.F., Interfacial bond strength determination in manufactured composites. In Jones, F.R. (Ed.), Interfadal Phenomena in Composite Materials. Butterworths, London, 1989, pp. 44-52. 

The shape of the stress-strain curves of GRC composites, such as those shown in Figure 8.12, can be predicted by the ACK model (Chapter 4), as it clearly shows the three zones predicted by this model elastic, multiple cracking and post-multiple cracking. Thus, the concepts of this model, and their various modifications, have been used to analyse the mechanics of GRC when loaded in tension [46,53-56]. The tensile stress-stress relation was used to predict the flexural behaviour, based upon the concepts discussed in Section 4.7 [57]. An essential element in an analysis of this kind is the determination of the fibre-matrix interfacial bond strength. 

The method adopted for the comparison of silane treatment effects on fiber-matrix bond strength was the determination of interfacial shear strength (IFSS) in single-filament composite (SFC) specimens [13, 14], which we have used extensively in investigating fiber-matrix interactions. The conditions of silane treatment of single fibers as well as the corresponding effects on IFSS could thus be carefully controlled, measured, and compared. 

The adhesion of metal layers deposited onto polymer surfaces is determined by the concentration and the bond strength of the chemical and physical interactions between the metal atoms and the functional (polar) groups at the polymer surfaces. Each type of functional group produces individual metal-polymer interactions, and makes a specific contribution depending on its concentration to the interfacial adhesion and consequently to the related shear or peel strength of metal-polymer systems (see Fig. 18.1). Thus for each type x of metal-functional group interaction a, the work of adhesion is calculated with Eq. (1), with A=area, I = Loschmidfs constant, and C = concentration. 

The ideal SiC whisker for reinforcement of ceramic composites would have several characteristics that would also depend on the matrix phase. In every case, the ideal whisker would have no internal structural imperfections, be relatively smooth and possess high strength. For alumina matrices where the difference in thermal expansion is great between the matrix and the whisker, the whisker diameter would be in the range of 1.5-2.0 p.m [9]. For alumina-SiC composites, it was determined that the presence of excess surface carbon on the whiskers resulted in a weak interfacial bond and produced materials with high toughness and strength [9]. 

Continuous carbon fiber reinforced SiC composites were prepared by Xu and Zhang [211] using CVI, in which the preforms were fabricated by the three dimensional braid method. For the composites with no interfadal layer, flexural strength and fracture toughness increased with the density of the composites and the maximum values were 520 MPa and 16.5 MPam , respectively. The fracture behavior was dependent on the interfacial bonding between fiber /matrix and fiber bundle/bundle, which was determined by the density of the composites. Heat treatment had a significant influence on the mechanical properties and fracture behavior. The composites with pyrolysis interfacial layers exhibited characteristic fracture and relatively low strength (300 MPa). 

At the substrate-adhesive interface both the shear and tensile stresses reach a maximum at the free edge of the adhesive bond. Harrison and Harrison used a finite element analysis to determine the effect of varying Poisson s ratios on interfacial shear strengths.Rubbery materials can distribute the stress over larger areas while materials with lower Poisson s ratios produce greater interfacial shear stresses. 

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