Issue 31

J. Lopes et alii, Frattura ed Integrità Strutturale, 31 (2015) 67-79; DOI: 10.3221/IGF-ESIS.31.06 74 For hybrid beams however a modified version of Eq. (2) is used to account for the different stiffness of the constituents in the laminate [3]:       2 2 2 3 3 3 4 3 4 metal CFRP F s t E b s t s t s t t E              (3) where: s and t are the specimen thickness and metal layer thickness in millimetres respectively metal E and CFRP E are the elastic modulus of the metal and CFRP respectively in GPa. Eq. (1) and (2) were used for every individual width and thickness of the specimens as they differ slightly from nominal dimensions due to manufacturing tolerances. Tab. 3 presents the experimental results with the average maximum ILSS for all types of beams and a comparison between the reference beam and the hybrid beams. The average maximum ILSS is in the range of [125 MPa – 130 MPa] which corresponds to a maximum load of [3.4kN – 3.5 kN] Types of beams ILSS (MPa)   std ILSS (MPa)  Hybrid Reference ILSS ILSS (MPa) Hybrid Reference ILSS ILSS Reference beam 130.03 1.40 - - Vacuum Blasting (one day storage) 129.87 3.76 -0.16 0.999 Vacuum Blasting 128.20 2.36 -1.83 0.986 Grit Blasting 126.11 5.00 -3.92 0.970 Pickling 125.63 2.15 -4.4 0.966 Table 3 : ILSS Experimental results – Comparison between reference beam and the hybrid beams with different surface treatments. Fig. 7 shows the typical behaviour of reference and hybrid specimens: An almost linear elastic displacement, followed by a gradual yielding of the resin rich neutral fibre (in the case of the reference beam) or CFRP/metal interface (in the case of the hybrid beam), until the beam reaches its maximum load. Failure occurs shortly after. The crack propagates in one of two ways: By a sudden and continuous propagation or by several steps due the heterogeneity of the resin. Figure 7 : Plot of typical examples of Reference beam and Hybrid beam with vacuum blasting.