The weight reduction of structures is still a challenge for reducing fuel consumption. For example, in the automotive sector, EU regulations on CO2 emissions from passenger cars set a threshold of 130 g/km and lowered the threshold to 95 g/km by 2020. Vehicle weight reduction with the introduction of composite materials is one of the areas of improvement necessary for achieving this objective. For aircraft manufacturers, the current challenge is also to reduce the fuel consumption of aircrafts and/or increase their efficiency, either by increasing the temperature of the engines or by reducing the weight of the parts. Metallic superalloys, currently used in commercial aircraft engines, have a high density, which penalizes the weight/power ratio. In the field of aeronautics, composite materials have been used since the 1970s for secondary parts and are now increasingly used for primary parts. Organic matrix composites (OMCs) reinforced with glass or carbon fibers are now widely used. For example, in the latest generation of commercial aircraft, they are the most used type of material (50% for the Boeing 787 Dreamliner and 52% for the Airbus A350). Thanks to the realized weight gain, the fuel consumption was reduced by about 20%. SAFRAN has developed blades and a fan casing made of 3D woven composite material produced by the RTM (Resin Transfer Molding) process for the LEAP engine. These blades allow a significant weight benefit. In addition, as part of the ARCOCE program, an A320 airbus has been flown with a CFM-56-5B engine equipped with the ARCOCE plug (rear ceramic composite bodies), an ejection nozzle produced entirely in ceramic matrix composites (CMCs).

In this context, damage of structural composite materials is a key factor in mastering the durability and reliability of parts in service. It is necessary not only to quantify the damage but also to identify the different mechanisms responsible for it. It is therefore essential to characterize the composite materials, define the most suitable damage indicators and use or develop models to estimate the remaining lifetime from analysis of precursor events resulting from damage. To address this issue, acoustic emission (AE) is a well-adapted technique. Indeed, the damage mechanisms are accompanied by energy release in the form of acoustic waves. Sensors on the surface of the structure allow the recording of AE signals. Acoustic emission refers to a non-destructive technique that allows the detection of material damage in real time. This technique has the advantage of being totally passive and therefore having no influence on the observed damage.

In 1964, the control of the casings of the Polaris rocket gave rise to the industrial applications of AE. In 1982, AE received its first administrative recognition with the publication of CARP (Committee on Acoustic Emission from Reinforced Plastics) codes [CAR 93]. Since the rise in the use of the method in the 1980s, its applications on structures in composite materials have been constantly increasing. An example of an industrial application is the control of glass fiber reinforced storage tanks according to ASTM E1067 [AST 11]. AE and composite material characterization have been the most active fields since the 1990s, especially for OMCs, accounting for more than 50% of publications.

One of the stakes in structural health monitoring is the early detection of damage before the latter leads to disaster. The classical integrated health monitoring process is based on the characterization of the damage and the prognosis of the remaining useful life. A diagnosis phase and a prognostic phase can thus be distinguished. The diagnosis phase must be able to detect the damage, locate it, identify it and assess its severity. The prognosis phase, based on the results of the diagnosis and on models, makes it possible to evaluate the residual lifetime. Acoustic emission allows this objective to be met, as this technique allows the health of an entire structure regardless of its size to be monitored while reducing maintenance costs.

The analysis of the collected data can be used to discriminate the sources of damage (matrix cracks, fiber breaks, fiber/matrix decohesion, delamination, etc.) and to measure the kinetics of the various degradation mechanisms. In addition, an estimate of the composite material’s remaining lifetime can be considered based on a real-time tracking of the damage recorded by AE. Indeed, the initial damage is often diffused and uncorrelated before it becomes organized. The events occurring at the beginning of the commissioning of the part strongly influence the lifetime; they can be described as precursor events. Knowledge of the latter is then indispensable to the evaluation of the remaining lifetime. The greater the heterogeneity, the greater the number of precursor events. This leads to a somewhat paradoxical conclusion: the more complex the system, the easier the prediction.

presents acoustic emission and its main characteristics. is devoted to the identification of the acoustic signature of the various damage mechanisms occurring in OMCs and also in CMCs. The contributions of this approach as well as its main difficulties and limitations are presented. is devoted to lifetime prediction using AE for long-term tests.