Contact Behaviour of Composite Laminate under Quasi-Static Indentation Load

A viable solution to the ever-demanding weight-saving target in aerospace industry is the replacement of conventional engineering alloys with composite materials in primary structures. A major concern to the effective use of composite laminates is the substantial reduction in the compressive strength when the contact force has exceeded the delamination threshold load (DTL). This paper focuses on the study of the contact behavior of composite laminates under quasi-static indentation (QSI) forces. The effect of damage initiation and growth on contact behavior has been investigated via detailed assessment of the relation between the indentation force and the dent depth. Different phases corresponding to undamaged, local damage, global damage, and final failure of the laminate have been identified. A modification to the classical Hertz contact law has been proposed to account for the matrix material resistance to plastic deformation. Introduction Composite material has been widely used to replace the conventional metallic material for aircraft primary structures due to its higher specific stiffness and strength. The further application is however restricted by the susceptibility of current carbon-fibre/epoxy system to low velocity impact (LVI) damage, which may cause enormous strength reduction [1-4]. It is well recognized that the damage mechanisms of carbon fibre reinforced plastic material are more complicated than those of engineering alloys. This can be attributed to the inherent brittleness of both the carbon fibre and the matrix materials. Composite laminates can absorb impact energy mainly through elastic deformation and damage mechanisms, not via local plastic deformation as most conventional ductile alloys do [5]. Delamination and matrix cracking are the dominant failure mechanisms which interact with each other and contribute up to 60% degradation in compressive strength of composite laminates [6-7]. This paper presents a study of the contact response of composite laminates through a series of instrumented quasi-static indentation tests. The indentation force and dent depth are measured carefully to characterize the damage process of the composite laminate. A main focus of the study is the effect of the dent depth on the damage initiation and propagation. Experimental Procedure It has been reported that LVI tests can be approximated by the QSI tests [6, 8-9]. QSI tests can significantly reduce the number of samples in getting the contact force/dent depth data. In the present study, a series of instrumented indentation tests had been carried out using ASTM D626498 test standard [10] to characterise the damage resistance of the composite laminate. The unidirectional material is the carbon/epoxy prepreg HexPly UD/M21/35%/268/T700GC/300 supplied by the Centre of Composites, Airbus UK. Layup configurations of [45/0/90]s, [45/03/90]s, and [45/02/902/45]s corresponding to laminate thicknesses of 2mm, 3mm, and 4mm were cured in an autoclave with curing temperature of 180C and holding time of 120 minutes. The indentation force was applied to a hemispherical indenter of a diameter of 20mm under the displacement control of 2mm/min. Two loading approaches were used during the test. In a continual loading approach, indentation force was applied continuously until the final failure of the specimen. In a step-by-step loading approach, the specimen was loaded up to the predetermined level and then unloaded to measure the dent depth. The process was repeated for different load levels until the final failure of the same specimen. Results and Discussions The concept of delamination threshold load (DTL) is well documented [5-7]. The DTL is linked to the first sudden load drop in the impact force history associated with the delamination initiation. Fig. 1(a) shows the filtered impact load historie under different impact energy levels. The DTL value for the 4mm laminate is about 4.7kN. Fig. 1(b) shows the contact force versue displacement curves obtained under both the continual loading approach and the step-by-step loading approach. The solid line in Fig. 1(b) represents the test results from the continual loading approach. The dotted line represents the overall loading curve by piecing together all the individual load curves under different loading levels in the step-by-step loading approach. The existence of the knee point is clear in both curves in Fig. 1(b) and the DTL so obtained is around 4.2kN, which is close to the value determined in LVI tests. The 10% difference is considered acceptable due to the complexity in determining the DTL. The result demonstrates that the contact behaviour of composite laminates under LVI can be investigated through QSI tests.