In this study, a fretting fatigue test has been equipped with an acoustic emission (AE) device in order to identify the successive crack propagation mechanisms. The fretting fatigue crack nucleation and propagation is a complicated process. Cracks initiate and propagate firstly due to shearing (mode II) and then by tension (mode I). The crack propagation generates mechanical energy emission. Elastic waves appear and can be detected through AE. A complete analysis of the AE signals (multi-parameter analysis, location of the AE in the loading cycle and a statistical analysis) led to an identification of three different steps in the crack propagation process. The evolution of the shearing and the tension influences in the crack propagation process is recognizable separately. Therefore, the three crack propagation steps have been identified as (a) crack propagation in mode II, (b) mixed mode crack propagation and (c) pure mode I crack propagation.

A non-destructive testing technique based on the time reversal principle is applied to cementitious materials. Preliminary results are presented showing the reversibility of wave propagation and that focusing by time reversal is feasible in cementitious materials. Then, the technique is implemented on a mechanical test in order to monitor the cracking process of a concrete specimen loaded under three point bending. The acoustic mirror permits qualitative monitoring of the propagation of a crack by analyzing the focusing of a reference signal on a sensor placed near the expected crack path. A decrease of energy and of amplitude of the focused signal is observed upon cracking. Then, the specimen made of a material with high capabilities of self healing, is immersed in water, and healing processes are monitored. The results show a recovery of signal focusing upon healing. Comparisons with mechanical test data indicate that the method is sensitive to the variation of stiffness of the structure due to crack propagation and crack healing.

Cette contribution traite du role, au niveau des proprietes mecaniques, du phenomene d'auto-cicatrisation des fissures dans les betons ainsi que de la caracterisation de ce phenomene par une methode non destructive originale basee sur le retournement temporel. Un programme experimental est developpe sur un beton modele a ultra hautes performances. Des tests mecaniques sont effectues sur des eprouvettes prismatiques entaillees, comprenant des phases de fissuration et de vieillissement, suivies de la caracterisation du comportement mecanique des eprouvettes vieillies. Les resultats montrent une reprise de proprietes mecaniques. Les phases de propagation de fissure et de cicatrisation sont egalement analysees par la technique de retournement temporel. Les deux phenomenes sont relies a l'evolution de la focalisation de signaux sur un capteur place sur le trajet de fissure. (A). (Voir fiche generale du congres : F101134).

This paper presents the first results obtained from the coupling of acoustic emission, ultrasonic testing and electrochemical noise measurements for the real-time monitoring of metal–electrolyte interfacial processes of stochastic nature. In this first paper, this combined approach is applied to gas-evolving electrodes (hydrogen and oxygen) to verify the ability of acoustic–ultrasonic measurements to complement the purely electrochemical information brought by conventional potential and current electrochemical noise measurements. Results have been treated both in the time and frequency domains indicating that acoustic–ultrasonic measurements actually sense the dynamic bubble evolution behaviour. It is hence shown that this new approach is a promising efficient tool that can be potentially extended to the study of other complex multiphase phenomena.

Acoustic Emission (AE) is a non destructive testing method allowing to detect in real time growing defects. Many damaging processes can be in the scope of the technique such as crack propagation, delamination, corrosion of reinforced bar,... The AE signature of each phenomenon under interest can be characterised in order first to discriminate genuuine AE from noise but moreover to identify the damaging process and to evaluate the severity of the damage for the integrity of the structure, with often the help of the analysis of the AE signals with statistic tools and pattern recognition processing, to provide a good diagnosis. Intrinsic mechanical properties of concrete give raised high signal to noise emission when damage propagates. The high attenuation coefficient of elastic wave in concrete material was a limitation to apply AE on a large civil structure. The development of specific AE sensors has contributed to overcome this difficulty and nowadays AE can be applied on full scale structures. In real time, AF monitoring detects the occurring of growing damage versus the loading level, displays the trend of its AE activity and locates it. AE is applied during proof test in order to investigate the fitness for service or repair validation, during service to detect a problem of fatigue or corrosion. AE continuous monitoring allows to maintain a damaged structure in service. This paper covers applications from laboratory scale experiments to actual civil structures. For the covering abstract see ITRD E123168.

The crack resistance of sheet asbestos cement has been characterized in terms of anR-curve which can accomodate effects which often influence the measurement of the critical stress intensity factorKc. The detection and location of the acoustic emission (AE) obtained from the asbestos cement has shown that it originates from microcracks in a zone just in front of the crack. The size of this zone increases to a maximum during slow propagation of the major crack and afterwards remains of constant size during the final crack growth. The form of theR-curve has been explained in terms of the mechanisms of fracture with the aid of AE and fractography studies. An analytical study has related the experimentalR-curve to a theoreticalR-curve and, hence, to the volume fraction, fibre aspect ratio and the strength of the fibre—matrix interface. It has been shown that the microcracking zone can be considered as a theoretical extension, of about one third of the zone length, to the real crack length.

The effect of wall structure on the fatigue life of grp tubes has been studied. The tubes were repeatedly internally pressurised with water and the leakage of the water was taken as a failure criterion. Wall structure has been shown to influence greatly the behaviour under cyclic conditions. Structures giving the same limits in monotonic pressurisation tests did not necessarily show the same characteristics in fatigue. The winding angle of the innermost layer of the tube was found to be important as well as the properties of the inner lining.

This paper deals with the role, on mechanical properties, of the self healing of cracks in concrete and with the characterization of the phenomenon by means of an original non-destructive technique based on the time reversal principle. An experimental program is developed on an ultra high performance concrete, considered as a model material. Mechanical tests are carried out on prismatic notched specimens, including pre-cracking and ageing phases, followed by the characterization of the mechanical behaviour of healed specimens. The results show a recovery of mechanical properties. The pre-cracking and ageing phases are also analyzed with the time reversal technique. The propagation and the healing of the crack are related to the evolution of the focusing of signals on a transducer placed on the crack path.