Mourad Bentahar

Monitoring progressive damage in polymer-based composite using nonlinear dynamics and acoustic emission

In this work quantitative results of applying nonlinear acoustic dynamics to study progressive damage in a polymer-based composite SMC (sheet molding compound) are presented. Via carefully controlled resonant plate experiment, nonlinear slow dynamics (SNLD) response of SMC in terms of relaxation time and frequency shift has been shown to be very sensitive to gradual damage induced using three-point bending tests. Besides, acoustic emission monitoring is used to characterize damage through the elastic energy released by SMC at every damage step. Interesting logarithmlike changes of the SNLD parameters as a function of the acoustic emission cumulated energy are found.

Influence of noise on the threshold for detection of elastic nonlinearity

Nonlinear effects on the propagation of elastic waves in hysteretic media are of great importance, with applications in imaging and monitoring evolution of damage/phase transitions/biological features. However, excitation amplitudes are an issue in order to improve performances of the techniques. Noise effects in the signals can be stronger than the generated nonlinear effects, thus rendering difficult the nonlinear analysis. Here, we analyze the link between the amplitude threshold for detection of nonlinear effects and different kinds of noises, which might be present in experiments. We also discuss the implications in the intrpretation of experiments.

Hysteretic nonlinearity analysis in damaged composite plates using guided waves

Sensitivity of nonlinear acoustic methods to the presence and the evolution of micro-damage has been proven in various studies on a wide range of materials. In this contribution, a guided wave approach is proposed to characterize polymer based composite plates taken at intact as well as damaged states. The changes in the nonlinear hysteretic parameters are observed by changing the order of the excited flexural resonances. The analysis is based on the evolution of the velocity frequency dispersion of the generated guided waves, where flexural modes are considered separately and together as a function of the dynamic strain.

Nonlinear acoustic fast and slow dynamics of damaged composite materials: correlation with acoustic emission

Slow and fast dynamics in the elastic response of damaged materials to external excitations show evidence of an anomalous hysteretic elastic behavior. Experimental observations may be used to detect and eventually characterize damage existing in structural components. Here we analyze the evolution of nonlinear dynamic behavior of a polymer-based composite (SMC) and concrete. More particularly, nonlinear slow dynamics parameters have been found to be very sensitive to damage evolution for both materials. Besides, acoustic emission monitoring is used to calculate the elastic energy released by SMC during a gradual increasing damage procedure. Interesting logarithm-like evolution of the followed nonlinear parameters as a function of the calculated energy is found. A classification of the calculated acoustic emission signal is proposed to understand the contribution of the different damage mechanisms to the evolution of the nonlinear behavior of SMC.

Acoustic emission to probe slow dynamics in complex materials

Acoustic emission has already been applied to give complementary information on the evolution of nonlinear behavior microcracked materials. In particular, energies of elastic waves emitted during the creation and propagation of microcracks in composites revealed to be in good correlation with relaxation time [1]. However, it remains important to study the relaxation (and/or conditioning) of complex materials as a function of mechanisms that lie behind the experimental observations. In this contribution, composite and concrete samples taken at microcracked states are submitted to slow dynamics experiments. Acoustic emission hits recorded during conditioning are first presented. During the high level excitation, the pump signal is at a very low frequency (few hundreds of hertz) in order to separate clearly the acoustic emission activity (> 50 kHz) and the low frequency excitation. Relaxation of complex materials has been realized with and without a probing ultrasonic wave. Results revealed that despite the ...

Detection and localization of microcracking in polymer concrete using coda wave interferometry (CWI) at resonance

Nonlinear resonance and Coda Wave Interferometry (CWI) proved their potential to detect the evolution of a structure, namely, in the case of damage. Nevertheless, these techniques do not allow localizing defects in a structure. CWI may allow this localization through heavy statistical algorithms. In this case, the main problem remains the real time monitoring on one hand. On the other hand, in the case of highly scattering media, the amplitude of the acoustic pulse may cause the Coda signal to be affected by conditioning/relaxation effects. Moreover, Coda signal is very sensitive to experimental conditions, namely, to temperature. In order to overcome these inconveniences, we propose a method based on a comparative study of Coda signal contents in rest state and under a weak (linear) vibration. In this communication, we report the results obtained on a polymer concrete specimen excited under different resonance modes and for different dispositions of the sample which defect is not isotropic on which we ap...

Characterization of damage evolution in non linear media by means of power law exponent: Modeling and experiments

Non linear Mesoscopic Elastic materials are characterized by the presence of a distribution of microscopic features like grain boundaries, phase transition and micro-cracks which result in a heterogeneous structure at the mesoscopic scale. It comes out from experimental observations that these materials are described by a non classical nonlinear Equation Of State (EOS) which has the particularity of being multivalued (presents hysteresis in the quasi static stress-strain relation) and amplitude dependent (due to nonlinearity in dynamic experiments). To evaluate the material’s integrity, one needs to learn about microscopic defects which are at the origin of fracture process in materials. Thus non linear indicators y could be extracted from quantification of the amplitude dependence of non linear phenomena where power laws y = axb are found to be well fitting this kind of behavior. Several experiments on damage evolution show a wide range of variation of the exponent b of the power law going from 1 up to 3...

Fast and Slow Dynamics in Damaged Materials: Correlation to Acoustic Emission

Results are reported of the study of anomalous nonlinear fast dynamics (FD) and slow dynamics (SD) in damaged solids. Experimental observations characterizing FD and SD may be used to detect and eventually characterize induced damage of structural components. Here, we analyze the evolution of the non‐linear dynamic behaviour of two polymer‐based composites of different reinforcing elements as Sheet Moulding Compound (SMC) and Polymer Concrete (PC) as a function of their gradual damage. More particularly, non‐linear FD and SD parameters (time and frequency shift) have been found to be very sensitive to damage creation and evolution for both materials. Besides,acoustic emission monitoring was performed and allowed to calculate the elastic energy released by SMC and PC at every damage stage. Interesting logarithm‐like evolution of the followed non‐linear parameters as a function of the calculated energy is found. A classification of the collected acoustic emission signals is proposed to understand the contri...

Ultrasound elasticity assessment of in vivo human achilles tendons during a quick‐release exercise

An ultrasound (US) axial transmission technique has been developed to assess in vivo real time tendon biomechanical properties during calibrated exercises. The US propagation velocity changes are indeed related to the tendon visco-elasticity variations. We propose to monitor the propagation velocity changes during a very short period of time corresponding to the tendon/muscle complex release just after a sub-Maximal Voluntary isometric Contraction (MVC). This measurement is repeated for several sub-MVC amplitudes. The propagation velocity variations, linearly related to the torque measurements, allow to estimate a stiness index, essentially due to the Series Elastic Component of the Achilles tendon. This study has been performed in collaboration with the UMR 6600 (UTC, France), and we have used the Ergometer they developed a few years ago. The US device is composed of a 1.8 MHz US probe (1 emitter, 20 receivers), and an electronic