The review outlines modern trends in theoretical developments, novel designs and modern applications of sandwich structures. The most recent work published at the time of writing of this review is considered, older sources are listed only on as-needed basis. The review begins with the discussion on the analytical models and methods of analysis of sandwich structures as well as representative problems utilizing or comparing these models. Novel designs of sandwich structures is further elucidated concentrating on miscellaneous cores, introduction of nanotubes and smart materials in the elements of a sandwich structure as well as using functionally graded designs. Examples of problems experienced by developers and designers of sandwich structures, including typical damage, response under miscellaneous loads, environmental effects and fire are considered. Sample applications of sandwich structures included in the review concentrate on aerospace, civil and marine engineering, electronics and biomedical areas. Finally, the authors suggest a list of areas where they envision a pressing need in further research.

Review of current trends in research and applications of sandwich structures

Review of air-coupled ultrasonic materials characterization.

The propagation of low intensity ultrasound in a curing resin, acting as a high frequency oscillatory excitation, has been recently proposed as an ultrasonic dynamic mechanical analysis (UDMA) for cure monitoring. The technique measures sound velocity and attenuation, which are very sensitive to changes in the viscoelastic characteristics of the curing resin, since the velocity is related to the resin storage modulus and density, while the attenuation is related to the energy dissipation and scattering in the curing resin. The paper reviews the results obtained by the authors' research group in the last decade by means of in-house made ultrasonic set-ups for both contact and air-coupled ultrasonic experiments. The basics of the ultrasonic wave propagation in polymers and examples of measurements of the time-evolution of ultrasonic longitudinal modulus and chemical conversion of different thermosetting resins are presented. The effect of temperature on the cure kinetics, the comparison with rheological, low frequency dynamic mechanical and calorimetric results, and the correlation between ultrasonic modulus and crosslinking density will be also discussed. The paper highlights the reliability of ultrasonic wave propagation for monitoring the physical changes taking place during curing and the potential for online monitoring during polymer and polymer matrix composite processing.

https://www.mdpi.com/1996-1944/6/9/3783/pdf

Monitoring the Cure State of Thermosetting Resins by Ultrasound

https://hal.archives-ouvertes.fr/hal-02126019/document

On the use of in-situ piezoelectric sensors for the manufacturing and structural health monitoring of polymer-matrix composites: A literature review

This study concentrates on the reflection of Lamb waves at the free end of a plate. The conversion phenomena are examined in detail over a large frequency range and the energy conversion coefficients are obtained by three different ways: theoretically, numerically (finite element method) and experimentally. The experimental energy determination is obtained from the measurement of the plate normal displacements, by mean of a laser interferometer. All results are in relatively good agreement and the energy balance between incident and reflected waves is discussed in each case.

Lamb wave reflection at the free edge of a plate.

https://hal.archives-ouvertes.fr/hal-01917107/document

Ultrasound monitoring of the cure kinetics of an epoxy resin: Identification, frequency and temperature dependence

Ultrasonic characterization of a fluid layer using a broadband transducer.

Thermo-oxidative ageing of carbon-epoxy plates has been done for up to 3500 h at two temperatures. Non-destructive evaluation by means of non-contact ultrasonic methods has been tested to characterize the aged plates. Experimental results obtained using Lamb waves propagation in the plates are presented here. Experimental and theoretical dispersion curves are compared. It is shown that the phase velocity of the A0 and S0 Lamb modes is decreasing when ageing duration increases. Assuming the plate to be homogeneous and orthotropic, we demonstrate that this phenomenon is related to an important decrease of the C55 elastic parameter of the plate.

Air coupled Lamb waves evaluation of the long-term thermo-oxidative ageing of carbon-epoxy plates

To carry out numerical simulations able to describe the behavior of a structural bond during a mechanical test, it is common to use a theory of damage. This approach consists in modeling the bonded zone by a surface distribution of springs with or without mass. The elasto-plastic with damage model and the numerical simulations are carried out with a finite element code. The use of this model requires two types of data: the critical energies in modes I and II measured during mechanical tests and the stiffnesses of the springs. These ones cannot be identified by mechanical measurements and, in this paper we propose an ultrasonic method to measure them. The ultrasonic approach and its experimental validation are first presented. Then, the mechanical model is detailed. The whole identification strategy is applied on aluminum/epoxy/aluminum samples.

Hugues Duflo

Papers

Lamb wave reflection at the free edge of a plate.

Ultrasound monitoring of the cure kinetics of an epoxy resin: Identification, frequency and temperature dependence

Ultrasonic characterization of a fluid layer using a broadband transducer.

Air coupled Lamb waves evaluation of the long-term thermo-oxidative ageing of carbon-epoxy plates

Mechanical and acoustical study of a structural bond: comparison theory/numerical simulations/experiment