Analysis of impact induced damage in composites for wind turbine blades

Abstract

Glass fabric-reinforced polymer (GFRP) composites used in wind turbine blades are usually exposed to large-deflection bending impacts caused by wind storms, heavy rainfall, water splashes and hailstones in the offshore; and sand and dust impingement in the desert environments. Such loadings can cause deterioration of structural integrity and load-bearing capacity of the blade structure due to induced damage in the form of matrix cracking, delamination and fibre fracture. These types of damage mechanisms become more detrimental and pose a threat to the fatigue life of the turbine blades. In this work, first the load-bearing and energy absorbing capability of woven GFRP laminates is investigated under impact loading. Experimental tests are conducted to characterise the behaviour of GFRP composites under large-deflection dynamic bending in Izod type impact tests using Resil impactor. Impact tests are performed at various energy levels to determine the ultimate fracture toughness of the laminates. In these tests, the material demonstrated interply delamination damage due to weaker matrix at low energy levels. At higher impact energies, apart from delamination, the material also exhibited permanent deflection instead of catastrophic fabric fracture. The latter was due to the visco-elasto-plastic nature of the glass fibres apart from the thermoplastic matrix. The deformation behaviour and delamination damage ensued by dynamic loading is also studied by developing three-dimensional finite element (FE) model in Abaqus/Explicit commercial package. In FE model, multiple layers of bilinear cohesive-zone elements are defined at the damage locations. Stress-based criteria and fracture-mechanics techniques are used to assess damage initiation and its progression, respectively. Numerical results gave good correlation when compared to the dynamic response observed in experiments. The methodology developed here can be employed in damage tolerant design of wind turbine composite blades subjected to similar impact loading conditions.