Development of a composite with an inherent function of visualization of a mechanical action

A concept of a composite possessing a biomimetic function of visual response in the form of a "bruise" to an external mechanical action has been developed. The concept is implemented by creating a model layered epoxy composite. Methods for creating a sensitive layer, containing microcapsules with a leuco dye and particles of a developer, and a composite including this layer are described. The threshold of a visual response, i.e., the sensitivity of the composite to a local indentation load is controlled by means of a protective epoxy coating of various thickness. Series of mechanical tests with a gradually increasing load and subsequent digital image analyses were carried out. The data obtained were used to determine the required thickness of the epoxy coating in relation to the desirable threshold of visual response to an indentation load. The Hertz theory was used to analyze the resulting stresses in the coating in the elastic approximation. The experimental results allowed us to determine the normal threshold pressure causing the action of a sensitive layer embedded into the epoxy composite.