M Surgeon

Modal analysis of acoustic emission signals from CFRP laminates

As a result of its continuous and in situ detection capabilities, the acoustic emission (AE) technique is the prime candidate for damage monitoring in loaded composite structures. None of the AE analysis techniques used in laboratory studies has, however, proven to be capable of consistently dealing with the difficulties encountered in larger structures: large amount of data, the elimination of noise sources and the influence of wave propagation effects (attenuation, dispersion). This work will use the modal acoustic emission (MAE) technique as a more intelligent and efficient way of analysing AE results. AE waveforms obtained during tensile and bending testing of CFRP laminates will be presented. It will be demonstrated how taking into account the modal nature of AE waves can in future lead to more quantitative and accurate results.

One sensor linear location of acoustic emission events using plate wave theories

Acoustic emissions (AE) are transient stress waves generated in a material under load upon damage formation. By detecting these waves and analysing their properties, information can be obtained about the damage initiation and propagation in loaded structures. The AE technique possesses a number of distinct advantages, one of which is the possibility to calculate the spatial source location based on arrival time differences between a number of sensors. In this way, the use of two sensors allows for a linear source location. This paper will demonstrate how taking into account the modal nature of AE signals can be used to reduce the number of sensors needed in AE source location. Using signals obtained during tensile and bending tests performed on a number of cross-ply and unidirectional carbon fibre reinforced polymer (CFRP) lay-ups, it will be shown how a linear source location can be calculated using one sensor. To achieve this goal two different plate wave theories will be used and the results will be compared to the ones obtained by a traditional two sensor linear location scheme.

Acoustic emission during tensile testing of SiC-fibre-reinforced BMAS glass-ceramic composites

One of the problems preventing the industrial application of ceramic-matrix composites is the lack of an efficient method to detect and discriminate among types of damage occurring during service. With this in mind, the mechanical response, damage development and acoustic emission activity during monotonic tensile testing of a BMAS glass-ceramic matrix reinforced with SiC fibres have been investigated. Damage initiation and propagation were easily detected and evaluated using the acoustic emission technique. Comparing the acoustic emission activity characteristics in simple lay-ups with those of more complex lay-ups allowed discrimination between matrix microcracking, matrix macrocracking accompanied by interface debonding, and delamination cracking. In this way, the paper contributes to the development of the acoustic emission technique for in situ monitoring of damage development in ceramic-matrix composites.

Static and dynamic testing of a quasi-isotropic composite with embedded optical fibres

An efficient use of composite materials in loaded structures requires NDT techniques that can reliably monitor the damage state of these materials in situ and continuously during service. A promising solution to this problem is the incorporation of optical fibres into the composite structure during manufacture. However, because optical fibres are always an order of magnitude bigger than material fibres, stress concentrations will inevitably be created which can lead to premature damage initiation and thus to a reduction in the mechanical properties. Therefore the first step in the development of a damage detection system based on optical fibre technology always has to be an investigation of the mechanical properties of the resulting structures. In this paper, optical fibres were incorporated in the different interfaces of a quasi-isotropic composite laminate. Both static and dynamic mechanical tests were carried out to determine the influence of the optical fibres on the mechanical properties of the resulting composite structures. Differences in behaviour between the different configurations were correlated with differences in damage propagation.

Transverse cracking and Poisson's ratio reduction in cross-ply carbon fibre-reinforced polymers

Gradual damage development in carbon fibre-reinforced polymers (CFRP) and its effect on the mechanical properties have been important subjects of investigation for many years. Most authors have studied transverse matrix cracking in cross-ply lay-ups and used the longitudinal Youngs modulus as an indicator of the extent of damage development. Reductions of typically only a few percent have been found at saturation crack spacing. Some authors have studied the effect of matrix cracking on Poissons ratio. The results show large reductions, but few data are available on the evolution of Poissons ratio throughout the process of gradual matrix cracking and on the influence of the 0°/90° ply thickness ratio. Moreover, none of the available models seems to accurately predict the quantitative evolution of Poissons ratio. In this work the degradation of the longitudinal and the transverse properties of a number of cross-ply CFRP laminates due to transverse matrix cracking under longitudinal tension was studied. The longitudinal Youngs modulus appeared to be less sensitive to damage development, in contrast to Poissons ratio which exhibited significant reductions in all lay-ups. A micromechanical model, based on the shear lag theory, was developed to predict the evolution of Poissons ratio and the effect of the 0°/90° ply thickness ratio. The correlation between experiment and theory was very satisfactory.

The influence of Embedded optical fibres on the fatigue damage progress in quasi-isotropic CFRP laminates

Embedded optical fibre sensors have in the last decade been introduced as possible candidates for continuous damage monitoring in loaded composite structures. However, incorporation of foreign bodies like optical fibres in laminated structures can lead to substantial microstructural disturbances, stress concentrations and degradations in mechanical properties. Optical fibres were embedded in the different interfaces of a quasi-isotropic CFRP laminate which was subsequently tested in interrupted tension-tension fatigue. During each interruption the damage state of the samples was quantified by using penetrant enhanced microfocus radiography. The results were compared to earlier results obtained during continuous fatigue testing is which the number of cycles to fracture and the stiffness degradation were monitored. The main conclusion is that some optical fibre configurations can affect the fatigue properties by weakening the 0°-plies or accelerating the damage evolution. These results are the first to demonstrate a faster damage evolution caused by embedded optical fibres.