xy(max) xy(adim) 0p (6) xx(max) xx(adim) 0p (7) where xy(adim) is the maximum shear stress normalized, xy(max) is the maximum shear stress, xx(adim) is the maximum flexural stress normalized, xx(max) is the maximum normal stress, all at the interface. The stresses do not occur at the same point at the interface. A variance study ANOVA presented by Odorczyk [4] indicated film thickness, friction coefficient and its interaction are factors of high significance for shear stresses. For tensile flexural stresses, material of substrate, thickness and structure and its interactions are highly significant. The velocity is not significant for both stress cases. From the stresses calculated in Eqs.(6) and (7), were calculated the principal stresses, tensile flexural and shear for the same points. With the principal stresses, assuming the greatest stress state at the interface, were calculated values for stress intensity factors KI and KII for brittle fracture failure modes I and II. Considering that a crack can propagate in any direction, the KI and KII parameters for the principal stress directions were investigated. It was considered a crack length of about 0.4 film thickness. The stress intensity factors KI and KII were obtained by Eq. (8) t II I a K K (8) It was adopted a mixed fracture criterion [6] considering KI and KII according to Eqs (9) and (10) 1 n IC II m IC I K K K K (9) n IIC IC K K (10) assuming m = n = 2. Also, it were considered the values of fracture toughness KIc = 1 and KIIc = 2, since the present study investigates just the competition between modes of fracture I and II. It were obtained KI and KII values for combinations of thickness, µ, type of substrate (steel and aluminum), and TiN film structure FGM x MGF. A variance study ANOVA was performed to investigate the influence of the parameters mentioned on the mixed fracture criteria shown in Eq. (9), to the main directions normal and shear. The experiment structure is shown in Table (1). Table 1. Influence of parameters on KI/KIc and KII/KIIc. SUBSTRACT MATERIAL AL(-1) STEEL (+1) FILM STRUCTURE FGM (-1) HMG (+1) FGM (-1) HMG (+1) THICKNESS 0,5 1 2 0,5 1 2 0,5 1 2 0,5 1 2 FRICTION 0,05 0,25 0,50 0,05 0,25 0,50 0,05 0,25 0,50 0,05 0,25 0,50
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