Design and use of a Fatigue Test Machine in Plane Bending for Composite Specimens and Bonded Joints

Polymeric and composites materials are increasingly used in industry as structural parts, so that specific information on their mechanical properties "at long term" (creep, relaxation, fatigue) are requested. The design and implementation of these components also requires that they are permanently connected together to perform certain functions. The use of adhesives in structural joints has increased over recent years thanks to the continuous development by companies which provide the market with adhesive having better and better mechanical properties. The main areas relate to the aerospace, aeronautical and automotive industries, where requirements are required for lightness and reliability. One of the main causes of out of order of the glue joints is due to the propagation of defects within the adhesive layer. Indeed, a microscopic defect could evolve because of fatigue loading and then propagate to the final break of the component. The defect may occur during the manufacturing process of the component, during the installation or for the incompatibility of the elements to connect. Delamination depends mainly on the extent of crack and on the mode of loading. Proper design and verification of structural elements cannot therefore leave apart from these aspects, so that in recent decades, in parallel with traditional methods, criteria based on fracture mechanics have been developed for the understanding of issues relating to the damage phenomena. Fatigue tests usually require several hours of application of cyclic stress to obtain an appreciable damage. These tests lead to results more or less uneven as to require the repetition of the test several times to obtain more accurate estimation of the endurance limit. All these considerations are economically incompatible with the use of particularly costly servo-hydraulic machines. Thus the development of a specific approach to fatigue tests based on the use of specific materials testing machines (FTM - Fatigue Testing Machine) becomes very important. Some ideal characteristics of such machines is described in the following: 1) adaptability to different geometries and rigidities of the specimens; 2) ease to perform various conditions of stress (alternate or pulsatory load); 3) possibility to develop studies of fatigue by the recording of the data obtained from different samples of different materials tested, to which any criterion for predicting the fatigue life can be applied; 4) ability to run tests by controlling the amount of deformation; 5) low cost of instrumentation to perform several tests simultaneously; 6) adaptability of the frequency of loading. The purpose of this chapter is to present a material testing machine for fatigue bending stress level that meets all the requirements listed above and to show the results of damage tests of composite material specimens and of the delaminations propagation in bonded joints.