Analysis of the deformation of aramid fibres and composites using Raman spectroscopy

Abstract

It is demonstrated that Raman spectroscopy can be used to study both the deformation micromechanics of highperformance aramid polymer fibres and of these fibres in model, single-fibre epoxy resin matrix composites. It is shown that the peak position of the 1610 cm-' aramid Raman band shifts to lower frequency under the action of tensile stress or strain due to the macroscopic deformation leading to direct stretching of the polymer molecules. These strain-induced band shifts can be used to map the distribution of stress or strain along a discontinuous, aramid fibre inside an epoxy resin matrix from which the interfacial shear stress can be calculated. It is shown that the behaviour is consistent with that predicted by the classical shear-lag analysis. The technique was also used to study the fragmentation process for an aramid fibre in an epoxy resin matrix in which the matrix strain exceeds the failure strain of the fibre. It is further demontrated that the technique can be used to compare the interfacial properties of aramid fibres where the interfacial shear stress is found to be higher for sized fibres than for those which have been de-sized.

Graphene/elastomer nanocomposites

“…It has been found that the rate of band shift per unit strain can be associated with the Young modulus of carbon fibres [89,90] and other high-performance fibres [89,91]. In a similar way it has been shown that the deformation of graphene leads to lattice distortion and bond stretching, which ultimately affect its properties [45,90,[92][93][94][95].…”

A strength based criterion for the prediction of stable fibre crack-bridging

“…Thus in a system with a high interfacial shear strength, fibre fracture will be more likely then for a system with low interfacial shear strength. The maximum recorded value of interfacial shear strength for the systems studied here, 43 MPa, is close to the shear yield stress of the matrix [8] and could represent interfacial failure by either matrix yielding or fibre/matrix debonding. 6…”

“…For many high-performance fibres the peak positions of certain Raman bands have been demonstrated to shift to lower wavenumber by the application of tensile stress [5][6][7][8][9]. For all the fibres used in this study there is an approximately linear shift of the peak wavenumber, , with axial fibre strain, e f .…”

“…The interfacial shear stress in the debonded regions varies but is generally between -5 and 5 MPa (the sign indicates direction of stress). The shear stress at the debonded-bonded transition, max , on both sides of the crack plane is found to be 30 -33 MPa indicating that for the Zylon/epoxy system debonding occurs well below the shear yield stress of the matrix which is 43 MPa [8]; the interfacial shear stress decays from these points with distance x along the fibre. Figure 6 shows the micromechanical strain distribution along a T50u carbon fibre where matrix crack propagation has caused the fibre to break; the COD was maintained at COD = 5 m. The strain distribution indicates that the fibre is subjected to a slight residual compressive strain over an approximate distance of 0.35 mm.…”

“…This is done by establishing a link between conventional strength based theory [6][7][8][9], micromechanical considerations and experimental data obtained using Raman 3 spectroscopy to follow the deformation micromechanics of the fibre/matrix interface in model composites where a matrix crack has been propagated.…”