Damage initiation and structural degradation through resonance vibration: Application to composite laminates in fatigue

Abstract

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. AbstractThe definition of failure is fundamental to the characterisation of fatigue strength of components and structures and is often expressed as a percentage of stiffness degradation. This article proposes a new method to capture damage initiation and structural degradation during a fatigue test, by exploiting resonance vibrations. The method entails monitoring dynamic parameters, including the phase angle between excitation and response, which suddenly changes as soon as the overall stiffness changes as a consequence of damage. In this paper, a theoretical approach based on equations of motion is presented in order to describe the possible scenarios in which a component undergoes structural degradation. In the proposed approach, an abrupt change in the time history of the stiffness of the component is assumed and analysed in both the quasi-static case at low excitation frequencies and the dynamic case, when the excitation frequency is close to the resonance frequency.The resulting analytical solution shows that any small stiffness variation is amplified by the dynamic response of the component as a large phase change, even when it is too small to be captured by the quasi-static response. The idea is successfully applied to capture the initiation of delamination in CFRP laminates, and the relative S-N curve to initiation is drawn. The experimental evidence of a critical event in the fatigue life of a component is found both in the dynamic phase response and in the base acceleration. The critical event is captured from a sudden change in the dynamic parameters of the structure as soon as a delamination is initiated. Thermography, C-scan and micrograph confirmed that the onset of delamination occurs at the moment of the critical event,after an initial stage of microcracking.

Criticality of the Self-Heating Effect in Polymers and Polymer Matrix Composites during Fatigue, and Their Application in Non-Destructive Testing

“…The self-heating temperature distribution depends directly on the applied stress, and is usually non-uniform due to the non-uniform stress distribution during cyclic loading (see Figure 2). Similar temperature distributions can be found, e.g., in [24,25], while a typical distribution during cyclic loading is presented in [20,26,27,28] for tensile, in [29] for compressive, and in [20] for shear loading, respectively. The dependency between these two quantities is as follows: the higher the stress concentration, the higher the self-heating temperature.…”

“…As was previously observed, the highest values of the self-heating temperature are noticeable in the region of the highest stress concentrations, which often indicate the locations of initiation of fracture. This property can be used for damage detection in polymeric and PMC structures, which were reported in numerous studies [1,24,128,129,130]. An overview on the self-heating based diagnosis of such structures is extended in further sections.…”

Variable Surface Temperature Distribution as a Criticality Indicator of the Self-Heating Effect in Composites

“…This phenomenon was highlighted and initially studied by the author in [13], where the relation between the self-heating temperature distribution and formation of a macrocrack was shown. Later, the authors of [14,15] confirmed the relation of damage formation and evolution with changes in the self-heating temperature distribution.…”

A Concept of Thermographic Method for Non-Destructive Testing of Polymeric Composite Structures Using Self-Heating Effect

“…In VT methods, the excitation is also possible by subjecting a tested structure to mechanical vibrations with a low-range frequencies. Such an approach was reported in several studies [ 13 , 14 ]. In this case, depending on the materials condition and properties, three mechanisms are possible: energy dissipation due to friction, micro-plasticization, and thermoviscoelasticity [ 15 , 16 , 17 ].…”

“…These distributions coincide with modal shapes of vibration, when the tested structures were loaded on their natural frequencies of vibration. The self-heating effect in polymeric structures was used for thermal excitation in several studies (see e.g., [ 13 , 20 , 21 ]), however, in the first case, the self-heating effect was just a tool for observation of an initiated crack, while, in the two latter cases, the excitation in the ultrasonic frequency range was performed.…”