Acoustics based assessment of a composite material under very high cycle fatigue loading

Abstract

Carbon fibre reinforced polymers are increasingly used for high performance applications, especially in aerospace because of their excellent strength and stiffness. CFRP structures with a request of lifetimes of 30 years and more have still to be designed seemingly very conservative for long endurance (‘infinite life’) because their Very High Cycle Fatigue behaviour is not known in detail so far. The objective of the project being reported and performed within the scope of SPP 1466 has been the characterization of the fatigue behaviour of CFRP by means of non-destructive testing techniques under 3-point bending loads at cycle numbers beyond 108 cycles and hence in the VHCF regime. Therefore, an ultrasonic testing facility for bending fatigue tests operating at a frequency of 20 kHz developed at the Institute of Materials Science and Engineering at the University of Kaiserslautern was used to get these VHCF experiments realized within reasonable period of time. The fatigue damage processes were characterized by different analytic means to understand the failure mechanisms in CFRP. During the fatigue experiments, the vibration spectra of the specimens and their sound irradiation were recorded by means of a laser vibrometer and a microphone. The time domain signals have been used to characterize the 3-point bending fatigue behaviour and as input parameters for the simulation of damage progression. Phenomena observed from those signals have been interpreted by some supporting validations based on ultrasonic techniques and measurements by means of computed tomography applied offline before fatigue, during load-free interruptions and finally after failure.

This paper looks into the aspect on how damage incubators in composite materials and structures including matrix fracture, delamination, fibre-matrix debonding, fibre fracture and any progression of those damage incubators can be detected by taking advantage of the material’s inherit mechanical properties. To explain and understand the effect of the non-linear behaviour observed a 2D-FEM model using COMSOL with the presence of delamination damage has been generated which induced breathing of the delaminations under the fatigue load applied with higher harmonic eigenfrequencies transforming in dependence of delamination sizes and locations within the specimen considered.