Choc Detection by Innovative Reversible Piezochromic COatings for COMPosite Parts – CHOCOCOMP

Because of their very light weight, excellent in-plane properties and high specific strength, Carbon Fiber-Reinforced Polymers has found many uses in structural applications. Nevertheless they are particularly prone to damage. These low energy shocks can occur during assembly phases in FAL (Final Assembly Line). Although damage is usually localized at the impact site, internal damages (delaminations, fibers to resin decohesion, transverse cracks) can be more widespread. Their propagation, under fatigue loading, can lead to serious issues like significantly compressive strength reduction. Moreover, impact damages can be subsurface or barely visible, necessitating expensive and time-consuming non-destructive inspection. Thus, a “visible” detection system will be very useful to focus ultrasonic inspection only where needed.
In this context, CHOCOCOMP aims at developing, characterizing and assessing innovative impact sensitive and reversible coatings to detect and quantify damages on composite substrates. Piezochromic probes (ie: pigments that lead to color change under pressure) dispersed in hybrid polymeric/sol-gel matrix are promising candidates. Quantification of damage in composite will be used to calibrate impact energy/coating answer/substrate. Mechanical and chemical parameters will be correlated to ensure appropriate coating use with industrial application (manufacturing in FAL). I addition coating application process (spray), studied in parallel, fits with the current eco-efficiency trends strongly linked to production costs and cycles reduction.

The strategy of this project will focus on 6 axes:
-Relevant impact energy threshold definition to apply to the assembly “composite +coating” for creating damage in the composite part (calibration)
-Design and optimization of piezochromic impact probes
-Elaboration of impact sensitive and reversible coatings by the incorporation of the probes in a “home-made” hybrid polymeric/sol-gel matrix: combination of piezochromic properties, mechanical toughness and adhesion to substrates of the coatings
-Impact probes and impact sensitive coatings characterization, led at macro- and microscopic scales
-Performance assessment: mechanical behavior and shock sensitivity of the impact probes and sensitive coatings. Correlation with damages occurred in composite parts Quality evaluation of the interface coating/composite
-Process optimization at pilot-scale of the best performing piezochromic probes. Spray-deposition method (appropriate viscosity range, pot life…) will be evaluated
To date, this global scientific approach (chemical+mechanical+process) has not been investigated for damage indicating.
CHOCOCOMP is a consortium, which draws together academic laboratories (LCMCP, ICMCB, P’UP, ENSMA-P’) to perform piezochromic pigments and the required coatings; and industrial companies including two SMEs (MAPAERO, OliKrom, EADS IW) to solve the specific industrial requirements. The complementarity of the expertise of each partner will allow delivering a composite substrate (250x250 cm2) coated with an impact-sensitive layer, at the end of the project. This coating will allow not only to detect shock, but also to quantify damages occurring in the composite substrate. Moreover, it will be possible to restore the coating at the initial stage by thermal constrain after US inspection of the impacted area.
CHOCOCOMP is the 1st phase of pressure-sensitive coating development. The structure of the project sets to focus work on material and calibration for manufacturing applications. Results obtained will provide Go/NoGo criteria for a 2nd step dedicated to up-scale and industrialization. Moreover, results on such smart coatings could be extended, not only to aircrafts (launchers, helicopters, planes…) in service life, but also to other various applications as wind turbine blades, yaching, and many other industries and other substrates where impact detection is required (public works, motorcycle, sport)