Micro-Raman study of the fatigue fracture and tensile behaviour of polyamide (PA 66) fibres

Abstract

The micro/nano structural evolution of two different types of ultra-high-performance polyamide (PA) 66 fibres, before and after tensile fatigue loading and failure, was studied using Raman spectroscopy. The nanostructure of these PA fibres consists of amorphous and crystalline domains. Two probes were considered: (i) low-wavenumber collective modes at ∼100 and ∼70 cm-1 as representative of the crystalline and amorphous domains, respectively; and (ii) N-H stretching mode at ∼3300 cm -1, as representative of the inter-chain behaviour. The analysis of the wavenumber distribution revealed that the skin/core heterogeneity is a maximum for the amorphous domains. The residual stress in the skin was evaluated at about 200 and 400 MPa from calibrations plots for crystalline and amorphous domains, for the two types of fibres. This stress disappears after thermal treatment around Tg. In situ analysis at different strain levels shows that the amorphous regions accommodate the stress up to ∼350 MPa, from which point elastic behaviour is observed. The analysis of a series of fibres failed in fatigue showed that amorphous domains are highly stressed during the failure and remnant compression can be measured. The extent of the stressed region extends to typically 100-400 μm from the fracture. Examination of the N-H wavenumber shift shows that the fracture squeezes the polymer chains together. Fatigue failure is explained by the loss of compliance in the amorphous region. Copyright © 2004 John Wiley & Sons, Ltd.