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This study extends the use of both the amplitude and time-of-flight based pulsed ultrasonic polar scan (P-UPS) towards the inspection of damaged composite materials, more specifically fatigued carbon fabric reinforced thermoplastics (CETEX). The results demonstrate the excellent capability of the P-UPS method for nondestructively assessing and quantifying both shear-dominated and tension-tension fatigue damage in fabric reinforced plastics. The P-UPS results indicate that shear-dominated fatigued CETEX goes with a reduction of shear properties combined with large fiber distortions up to 13°. The P-UPS results of the tension-tension fatigued CETEX samples on the other hand reveal a directional degradation of the stiffness properties, reaching a maximum reduction of 12.8% along the loading direction. The P-UPS extracted damage characteristics are fully supported by simulations, conventional destructive tests as well as visual inspection.

International audience; A method is introduced for the measurements of strain exploiting the interaction between ultrasound waves and characteristics of the insonified specimen. First, the response of obliquely incident harmonic waves to a deterministic surface roughness is utilized. Analysis of backscattered amplitudes in Bragg diffraction geometry then yields a measure for the in-plane strain field by mapping any shift in angular dependency. Secondly, the analysis of the reflection characteristics of normal incidence pulsed waves in frequency domain provides the out-of-plane normal strain field component, simply by tracking any change in the stimulation condition for a thickness resonance. As such, the developed method yields an absolute, contactless and single-sided mapping of a local 3D strain field, in which both sample preparation and alignment procedure are ubiquitous. The ultrasonic strain gauge is applicable to any material, though under the restriction that a deterministic surface roughness is present. Results are presented for cold-rolled DC06 steel samples onto which skin passing of the work rolls is applied. The samples have been mechanically loaded, introducing plastic strain levels ranging from 2% to 35%. The ultrasonically measured strains have been validated with other strain measurement techniques, yielding good agreement. As the ultrasonic strain gauge provides all three strain field components, we extracted Lankford ratios for the DC06 steel sheet at different applied plastic strain levels revealing a strain dependent plastic anisotropy of the DC06 steel.

The conventional ultrasonic polar scan (UPS) records the transmitted amplitude of an acoustic pulse for a certain solid angle, resulting into a unique fingerprint of the local critical bulk wave angles, which relate to the local elasticity tensor at the insonified material spot. Numerical plane wave computations reveal that the use of harmonic sound, instead of sound pulses, provides a picture that exposes the leaky Lamb wave angles. For the first time, UPS experiments have been obtained with the use of quasi-harmonic pulses to check this premise. To achieve good correspondence with the experimental recordings, a more advanced numerical model (based on the partial wave technique and the stiffness matrix method) has been implemented. Both temporal and spatial beam dependency is accounted for by application of the Fourier transform. Besides a traditional amplitude measurement, the analysis has also been extended, both numerically and experimentally, to the phase of the transmitted sound, hence introducing the concept of a 'phase ultrasonic polar scan'. The results clearly indicate that a phase UPS offers very defined and detailed results, with high sensitivity to small disturbances. Even more, combination of both the amplitude and phase UPS yields complete knowledge about the complex transmission coefficient. To demonstrate the potential of the quasi-harmonic UPS for NDT and material characterization, it has been applied to a unidirectional (UD) carbon/epoxy laminate which contains an artificially generated delamination. It is demonstrated that, contrary to the pulsed UPS, the quasi-harmonic UPS show great sensitivity to the presence of the delamination.

The fatigue behavior of composite materials under time-dependent loading is investigated. Fatigue is induced in test beams using an electro-magnetic shaker. The test beams made of prepreg composite materials with a layup [(±45)2]S before and after fatigue loading are used as a case study for the proposed method. The averaged stiffness properties of the test specimen are monitored during the fatigue cycles by measuring the first natural frequency. The first bending mode shapes of the test beams are measured using a laser scanning vibrometer at selected fatigue stages. Based on this modal information, the (reduced) bending stiffness distribution of the test beam is identified using an inverse method.

Ultrasonic non-destructive testing is a well known technique in present days, in which the C-scan is the most wide spread. Though, because of the inherent limitations of most methods to quantitatively characterize (damaged) composite materials, the quest for more sophisticated methods is put forward. This study reports experimentally registered polar scans using an in house developed ultrasonic test setup for some typical composite materials. Both pulsed and harmonic ultrasonic beams are considered, which impinge onto the immersed anisotropic layer under investigation. Numerical computations are shown and compared with the experimental results. The experimental polar scan of a carbon fabric/PPS laminate with an artificially added delamination shows a drastic change in the observed patterns, compared to the one of the undamaged carbon fabric/PPS laminate. Combined with the sensitivity to the local stiffness tensor, a polar scan can become a great tool to quantitatively evaluate (damaged) composite materials.

The pulsed ultrasonic polar scan (UPS) technique provides an amplitude map with characteristic contours reflecting the critical bulk wave angles, which relate to the mechanical stiffness of the insonified material spot. Besides these contours, additional information is captured in a UPS experiment which has particular importance for the inspection of anisotropic materials. By extensive post-processing on a single UPS experiment, we successfully extracted (i) the critical bulk wave angles, (ii) the phase velocity profile, (iii) the slowness vector, (iv) the walk-off angles and (v) the energy velocity profile, and this for the three different polarization states of bulk waves. The procedure is demonstrated and discussed for an aluminum sample as well as an autoclave manufactured orthotropic [0]8 carbon/epoxy laminate, both having arbitrary dimensions. Comparison with numerical computations yield good agreement. It is worth noting that the carbon/epoxy laminate shows an extraordinary large walk-off angle of morethan 70° for the longitudinally polarized wave.