z200 口AAZ I mm 00150200250300350 Fig. 7. Residual stress in laminated composite(thickness ratio among the layers about 1)determined by indentation technique Ref [34] as grain size, presence of other phases, porosity, etc. Hence,a preliminary calibration procedure is required to determine the Luni value pertinent to each material. For this purpose, bending bars were obtained from laminated structures prepared with layers of the same composition (A and AZ, respectively), mounted on a 4-points bending jig under the laser beam focused using the optical microscope and loaded below the I mm fracture stress. After loading, the spectra were recorded every 40 um on going from the side in compression towards the side in tension of the specimen. The load value was converted into a stress value, o, using the standard 4-point-bending elastic equation, and then the peak shift, Av, was plotted as a function of the applied stress. The average Luni value was obtained from the slope of the a vs. Av plot. The stress distributions in the laminated structures were measured at a microscopic level by determining stress line profiles by means of automatic 10 um-spaced measurements carried out on the specimen cross sections with the aid of a computerised x-Y table moved with a lateral resolution of 0. 1 um along both axes Fig. 8, the cross sections of three different layered structures with the relative stress profiles are reported. From this figure several considerations concerning the stress distribution can be drawn. According to the theoretical mm prediction the layers with lower CTA are in compression while the layers with higher CTA are in tension. It is evident that the thickness ratio among the different layer effects the amount and the distributions of residual stress. The stresses dramatically increase in the proximity of the interface of the Fig. 6. Sections of different laminated structures. Note the different ratio among the layers of dissimilar materials dissimilar layers. At the surface the stress is lower than that in he inner layers evidencing a surface effect. This effect is alse present in the z-axis of the cross section and leads to a the peak recorded under stress. A standard value of frequency measurement of residual stress lower than the actual one. This for zero external stress was obtained acquiring an array of 100 is the reason why the measured stress are generally lower than spectra measured on the surface of sample AA(produced by that theoretically predicted. stacking only A layers)and averaging all the values of the peak centre 5. Mechanical properties An important characteristic of the piezo-spectroscopic technique is that the average uniaxial piezo-spectroscopic The observed residual stresses, deliberately introduced in coefficient IIunis which characterizes the linear dependence the structures by a proper design lead to a peculiar and superior between peak shift and stress, strongly depends on several mechanical and tribological properties of the systems. These parameters, which are related to the material and process such properties can be associated to the surface(contact damage andthe peak recorded under stress. A standard value of frequency for zero external stress was obtained acquiring an array of 100 spectra measured on the surface of sample AA (produced by stacking only A layers) and averaging all the values of the peak centre. An important characteristic of the piezo-spectroscopic technique is that the average uniaxial piezo-spectroscopic coefficient Puni, which characterizes the linear dependence between peak shift and stress, strongly depends on several parameters, which are related to the material and process such as grain size, presence of other phases, porosity, etc. Hence, a preliminary calibration procedure is required to determine the Puni value pertinent to each material. For this purpose, bending bars were obtained from laminated structures prepared with layers of the same composition (A and AZ, respectively), mounted on a 4-points bending jig under the laser beam, focused using the optical microscope and loaded below the fracture stress. After loading, the spectra were recorded every 40 mm on going from the side in compression towards the side in tension of the specimen. The load value was converted into a stress value, s, using the standard 4-point-bending elastic equation, and then the peak shift, Dn, was plotted as a function of the applied stress. The average Puni value was obtained from the slope of the s vs. Dn plot. The stress distributions in the laminated structures were measured at a microscopic level by determining stress line profiles by means of automatic 10 mm-spaced measurements carried out on the specimen cross sections with the aid of a computerised X–Y table moved with a lateral resolution of 0.1 mm along both axes. In Fig. 8, the cross sections of three different layered structures with the relative stress profiles are reported. From this figure several considerations concerning the stress distribution can be drawn. According to the theoretical prediction the layers with lower CTA are in compression while the layers with higher CTA are in tension. It is evident that the thickness ratio among the different layer effects the amount and the distributions of residual stress. The stresses dramatically increase in the proximity of the interface of the dissimilar layers. At the surface the stress is lower than that in the inner layers evidencing a surface effect. This effect is also present in the z-axis of the cross section and leads to a measurement of residual stress lower than the actual one. This is the reason why the measured stress are generally lower than that theoretically predicted. 5. Mechanical properties The observed residual stresses, deliberately introduced in the structures by a proper design lead to a peculiar and superior mechanical and tribological properties of the systems. These properties can be associated to the surface (contact damage and Fig. 6. Sections of different laminated structures. Note the different ratio among the layers of dissimilar materials. Fig. 7. Residual stress in laminated composite (thickness ratio among the layers about 1) determined by indentation technique Ref. [34]. 562 G. de Portu et al. / Composites: Part B 37 (2006) 556–567