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

G. Di Franco, G. Marannano, A. Pasta and G. Virzi Mariotti

Universita degli Studi di Palermo, Dip. di Meccanica, Viale Delle Scienze – 90128 Palermo


1. Introduction

Polymeric and composites materials are used increasingly as structural parts in industry and therefore many informations on mechanical properties (creep, relaxation, fatigue life) are necessary.

Composite materials behavior subjected to fatigue load is very complex due to non homogeneous and anisotropic properties, and it has been studied for a long time; however, composite materials design is still based on very long fatigue tests and high safety factors are used.

Composites industry uses various types of resin (usually epoxy or polyester resin) and reinforced fibers (usually fiberglass). Many industrial components and consumer goods are made in this way, such as parts for boats, car components, etc.

Composites with polymer matrix are used by the industries with much performed resins and stubborn and rigid reinforced fiber. Composite materials are used primarily in aerospace, military and automotive industries, however, are also utilized in sports such as golf, fishing, skiing (and snowboarding) and in the naval industry (Marannano & Virzi Mariotti 2008).

These materials have very high mechanical properties such as low weight, high strength and stiffness, good formability and high design flexibility. Many theoretical studies (Van Paepegem & Degrieck, (b) 2001; Van Paepegem & Degrieck, 2002 ;Marannano & Pasta 2006; Natarajan et al. 2005) are dedicated to the study of crack propagation, applying the concepts of fracture mechanics. Fatigue failure can be described as a sequence of two phases:

• crack formation;

• crack propagation.

The crack propagation has been studied carefully, ignoring the formation crack, and pre­cracked specimens are used for this purpose; the study requires the development of equipping, methodologies and specialist analysis. Fatigue studies usually require several days (sometimes weeks) of load cycles to obtain an appreciable damage. The tests show inhomogeneous results, so it is necessary to do many repetitions to get a more accurate

estimate of fatigue life. In this case it is very important to develop a specific approach to fatigue tests based on the use of materials testing machines (FTM) to avoid the utilization of expensive hydraulic machines. Some ideal characteristics of FTM machines are:

1. adaptability to different geometries and rigidities of the specimens;

2. facility to perform various conditions of load (alternate or pulsatory load);

3. possibility to develop fatigue studies by recording the obtained data from different tested materials, which can be applied any criterion to predict the fatigue life;

4. possibility to measure strains;

5. low cost of instrumentation to perform several tests simultaneously;

6. adaptability of load frequency.

This chapter presents a FTM materials testing machine and experimental fatigue tests, to study composite materials behavior subjected to fatigue bending plane; the machine is designed to perform specific fatigue tests with different boundary conditions (such as frequency, elastic modulus, etc.). The development of a fatigue testing machine in plane bending starts with the need to perform multi-axial fatigue tests on metallic materials (Frustey & Laserre, 1989) and to make fatigue-corrosion tests (Berchem & Hocking, 2006), on aluminum alloys (Beck et at 2002; Van Paepegem & Degrieck, (a) 2001) and on a-brass (Sugeta et al. 2006). It is very difficult to find specifics to FTM machines for composite and polymeric materials in the literature (Trotignon, 1995; Caligiana et al. 2003; Marannano et al. 2007), while applications on bonded joints are not frequent. Initially the properties of composite materials under static conditions are evaluated, then fatigue curves are obtained for different loading conditions. Wohler curve are obtained using N10 criterion, corresponding to the loss of 10% of the initial stiffness. Finally, the model of residual resistance was adopted as a tool to describe the fatigue behaviour.