Through-transmission Mode Research Articles

Spectral analysis of acousto-ultrasonic waves for defect sizing

A study of the feasibility of employing acousto-ultrasonic (AU) testing to size defects has been carried out using a model system consisting of a single defect in a glass matrix. A through-transmission mode was used for the AU testing and the output signals were analysed by a counting technique (cumulative ringdown count, CRDC) and spectral analysis. The CRDC decreased with increasing defect size but the scatter in the data did not allow accurate sizing. The frequency spectra exhibited multiple peaks which occurred at constant frequency intervals. The frequency interval decreased with increasing defect size and this could be attributed to either multiple time delay or a resonance effect or to both phenomena. Most evidence supported the resonance effect and good agreement was found between defect size calculated from AU data using a resonance theory and that measured optically.

Wave propagation in composite materials with fibre waviness

A discrete ray tracing model based on elastodynamic ray theory was developed and applied to a composite material with fibre waviness. The varying anisotropy of this composite causes the ultrasonic waves to travel along curved paths. Particular attention was paid to the comparison with experimental measurements in order to check the applicability of the method for non-destructive characterization of fibre waviness, especially in the high frequency regime. A specimen with intentional fibre waviness was fabricated by winding prepreg tape around a specially designed mandrel. The specimen was tested in the through-transmission mode using special ultrasonic techniques for thick composites. The theoretical model and experimental results agree, at least with regard to the period of fibre waviness.

Identification and characterization of reclaimed and recycled polymers by ultrasound attenuation

Ultrasound attenuation analysis has been demonstrated as a cheap, reliable method of identifying and characterizing polymeric materials and components, which can assist in the separation and reclamation of thermoplastics from polymer waste streams. Material-component broad band ultrasonic attenuation measurement and the measurement of the time of flight of an ultrasonic pulse, in either pulse echo or pulse through-transmission mode, can be combined to attain a signature of the material under investigation. The HL (Hull-Langton) signature is independent of component dimensions and hence is a powerful tool for the identification of components of complex shape where access to all surfaces may be restricted. The results presented demonstrate the ability of the technique to identify a range of different polymers, including reinforced materials.

Comparative ultrasonic analysis of damage in CFRP under static indentation and low-velocity impact

Non-destructive testing plays an important role in analyzing composite damage by supplying useful information on damage initiation and propagation. However, for the scientist, the main problem to be solved is the choice of how best to detect the damage. For impact damage to carbon/epoxy plates, widespread methods such as X-ray inspection are not easy to use because the damage does not always penetrate completely through the thickness. The choice of an ultrasonic facility designed to dimension the damage correctly and give access to interface delaminations has thus proved to be necessary. The ultrasonic method has been applied here for a comparative analysis of the damage detected after static indentation and after low-velocity impact on a composite with quasi-isotropic lay-up and a comparative analysis of the damage caused by static indentation for two different lay-ups (quasi-isotropic and cross-ply) on T300/914 material. The results show the damage process. A crack initiation threshold, crack propagation, delamination initiation threshold and delamination propagation are observed. The propagation of the delaminated area is similar for static and impact damage versus the maximum load. Similarly, propagation of the delaminated areas obtained in through-transmission mode is similar for the two lay-ups analyzed. By using a ply-by-ply ultrasonic analysis the authors observed a slightly different internal delamination process comparing cross-ply and quasi-isotropic laminates under static indentations. Two other methods, fractography and accelerometry, were used in association with the ultrasonics to complete the results. The influence of parameters, such as the indenter diameter and the velocity and form of loading, was also evaluated under local bending loads.

Quantitative porosity characterization of composite materials by means of ultrasonic attenuation measurements

Ultrasonic techniques utilizing a pair of transducers in a combination of pulse-echo and through-transmission modes were developed. Two types of methods were discussed for determination of material attenuation in composites: direct or absolute methods for materials with low signal loss, and indirect or relative methods for materials with higher signal loss. In all cases, transfer functions of the transducers and specimen surfaces were taken into consideration so that the measurement system is self-calibrated. Void content was measured by means of microscopic image analysis of photomicrographs of the specimen cross-sections and a correlation of material attenuation with porosity was established.

The chirp-Z transform applied to adhesively bonded structures

A signal-processing technique based on the chirp-Z transform is presented to evaluate the echo signals of longitudinal ultrasonic transducers in contact with bonded materials. Both a simulated glass-glass interface of variable thickness and a realistic double lap bonded aluminum sample were tested. The observed frequency dips and peaks in the transducer spectrum from pulse-echo and through-transmission modes were recorded and related to the condition of zero reflection coefficient at the interfacial layer. Resulting thickness predictions for different transducer center frequencies ranging from 5-20 MHz are in excellent agreement with experimental measurements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>