Investigation of the micromechanics of the microbond test

Abstract

The microbond test is a method which is sometimes used for measuring interfacial shear strength. In the analysis of the data it is often assumed that the interfacial shear stress is constant and thus, by implication, that the strain in the fibre along the embedded fibre decreases linearly from the point of entry to the point of exit. In this paper the results of conventional microbond tests and simulated microbond tests performed under a Raman microscope on a Kevlar-49/epoxy system are reported. The conventionally performed tests show that the calculated interfacial shear strength for this system is approximately 16 MPa regardless of the position of the supporting knife edges. The strain distribution along the fibre during simulated microbond tests was studied as a function of knife edge position, interfacial area and level of load by means of Raman spectroscopy. It was found that the interfacial shear stress was not constant, as is frequently assumed, but was strongly dependent upon distance through the droplet, knife-edge position and applied load. At low loads the strain was a maximum at the point where the fibre entered the droplet and then dropped off sharply through the embedded length. This effect was enhanced when the knife-edge separation was reduced. The variation of the shape of the stress distribution was similar to that predicted by a linear finite element analysis. At higher load levels the onset of failure in the region closest to the point where the fibre entered the droplet could be observed. M1 7HS AbstractThe microbond test is a method which is sometimes used for measuring interfacial shear strength. In the analysis of the data it is often assumed that the interfacial shear stress is constant and thus, by implication, that the strain in the fibre along the embedded fibre decreases linearly from the point of entry to the point of exit. In this paper the results of conventional microbond tests and simulated microbond tests performed under a Raman microscope on a Kevlar 49/epoxy system are reported. The conventionally performed tests show that the calculated interfacial shear strength for this system is approximately 16 MPa regardless of the position of the supporting knife edges. The strain distribution along the fibre during simulated microbond tests was studied as a function of knife edge position, interfacial area and level of load using Raman spectroscopy. It was found that the interfacial shear stress was not constant, as is frequently assumed, but was strongly dependent upon distance through the droplet, knife edge position and applied load. At low loads the strain was a maximum at the point where the fibre entered the droplet and then dropped off sharply through the embedded length. This effect was enhanced when the knife edge separation was reduced. The variation of the shape of the stress distribution was similar to that predicted by a linear finite element analysis. At higher load levels the onset of failure in the region closest to the point where the fibre entered the drople...

Ballistic impact velocity response of carbon fibre reinforced aluminium alloy laminates for aero-engine

“…This situation agrees with a previous work in Ref. [29]. The flat cylindrical bullet has penetrated all composites during the impact, with increasing hole diameter from the front face to the rear face.…”

Development and application of micromechanical techniques for characterising interfacial shear strength in fibre-thermoplastic composites

“…Consequently, individual laboratories tend to create their own methods with essentially the same idea but different procedures. Although the application of MBT to thermosetting composites has attracted a great deal of attention over the years [4,[12][13][14][15], it has been, to a lesser extent, considered for thermoplastic systems [16][17][18]. As a result, the procedure for using this method with thermoplastic polymers needs to be further improved.…”

Investigation of fiber/matrix adhesion: test speed and specimen shape effects in the cylinder test

“…value was determined according to as the ratio of deboning force and embedded surface of fiber [15,17]:…”

“…1), can be considered as a variant of the microdroplet test [12], that eliminates basic disadvantages of the original method and diminishes some related disturbing effects related to test configuration. Thus, errors due to uncertainties of droplet shape and size, major stress variations arising around the supporting points of the fixing blades and concomitant deformations [13,15,19] could be eliminated with specific cylinder geometry. With the cylinder configuration the sensitivity of the method, influenced significantly by the skill of the personnel, could be markedly reduced.…”

“…Some results are presented in Table 1. Based on the literature data [13][14][15] it can be concluded that only few authors investigated the effects of specimen size/test configuration and pulling speed. The main reason for this is probably linked with the first contact between the blade and droplet, which may result in off-axis loading, especially at higher deformation rates.…”