• Gower, Mike
• Paepegem, Wim Van
• Williamson, C.
• Mera, R.
• Shaw, R.
• Makris, Andreas
• Smits, Arwen
• Van Hemelrijck, Danny
• Lamkanfi, Ebrahim
• Ramault, Carla
• Clarke, Andrew

Abstract

This paper will give an overview of the existing biaxial test methods for composite materials from the early beginning till the currently used methods. An evolution in both the biaxial testing capabilities and the applied inspection methods will be shown.<br/>The use of composite materials in aerospace, aviation and automotive industry has increased rapidly in recent years. In general, composite laminates are developing multiaxial stress states [1]. However, there is little existing capability to evaluate the full multiaxial (or even the biaxial) response of composite materials, even though large demand for such information exists [2].<br/>Different experimental techniques and specimens have been used to produce biaxial stress states. These techniques may be classified into two categories [3]: (i) tests using a single loading system and (ii) tests using two or more independent loading systems. In the first category the biaxial stress ratio depends on the specimen geometry or the loading fixture configuration, whereas in the second category it is specified by the applied load magnitude.<br/>Examples of the first category are bending tests on cantilever beams, anticlastic bending tests of rhomboidal shaped composite plates [4] and bulge tests [5]. Anticlastic bending allows the material to be analyzed in the second and fourth quadrants of the two-dimensional stress space. However, each stress ratio requires a specific shape of the plates. In the hydraulic bulge test, pressure is applied to the surface of a round or elliptical flat specimen. A stress gradient appears in the thickness of the specimen [6] and it has been proved that the developing stress fields are non-homogeneous due to the gripping of the specimens edges. Like the bending method, this technique requires a different specimens shape for each stress ratio.<br/>Examples of the second category are thin-wall tubes subjected to a combination of tension / compression and torsion or internal / external pressure, and cruciform specimens under in-plane biaxial loading. The technique with the thin-wall tube is the most popular one [7], because it allows tests with any constant load ratio to be performed. However, it presents some inconveniences [8]: (i) radial stress gradients may not be negligible depending on the thickness of the tube, (ii) real construction components in fibre reinforced composite materials are often flat or gently curved and differ a lot from tubular specimens, (iii) thin-wall tubes are not easy to fabricate and (iv) obtaining a perfect alignment and load introduction is not straightforward. The most realistic technique to create biaxial stress states consists of applying in-plane loads along two perpendicular arms of cruciform specimens. Biaxial creep testing machines have been developed using a deadweight lever system where loads are applied by means of ropes which pass over pulleys [9]. The use of hydraulic actuators represents a very versatile technique for the application of the loads. The main differences between the techniques are the use of one actuator per loading direction or two. For the techniques using one actuator per loading direction [10] the centre of the specimen will move during a test causing side bending of the specimen. Systems with two actuators per loading direction [11] with a close-loop servo control, allow the centre of the specimen standing still. The biaxial test devices used at the Free University of Brussels [12], at Qinetiq [13] and at the National Physical Laboratory are of this type. Various inspection techniques are nowadays applied on the specimens under biaxial stress states e.g. digital image correlation, ESPI, acoustic emission. Also post failure analysis inspection techniques are available e.g. X-ray photography, ultrasonic inspection, fractographic analysis. <br/>To conclude, we can state that the development of the biaxial testing techniques, led to a better understanding of the behaviour of composite materials under biaxial stress states, but that further research in this domain is needed.

Topics

• impedance spectroscopy
• surface
• composite
• positron annihilation lifetime spectroscopy
• Photoacoustic spectroscopy
• bending flexural test
• ultrasonic
• two-dimensional
• acoustic emission
• creep