/ Composites: analysis yielded a value of -141 MPa(compressive)for res Sinter 1600 debonding Fig. 7 shows the regression of Vickers indentation data for the R1550C for Ih stress free material MA and hybrid laminate A/AZ 30°Ch Details on this method can be found elsewhere [34 30°Ch 800 4.2. Piezo-sped 3°Ch Piezo-spectroscopic technique is a valuable method to 50C/h measure the stress amount and distribution in laminated 50%C/h The stress distribution along cross sections of the multi 150200250 layered samples can be de etermine heat treatment time/h the characteristic Rl, R2 doublet produced by the chromo- Fig. 4. Thermal cicle for densification laminated composites. phonic fluorescence of Cr't impurities in Al2O3. The relation between a line shift in a fluorescence spectrum and the stress state has been described by Grabner [60]. In polycrystalline, If the value of Kie is measured independently by a reliable fine grained and untextured Al2 O, samples subjected to a method(for example, Chevron-Notched Beam(CNB)59]), normal stress a, a luminescence line undergoes a change the parameter x can be evaluated by means of the best fit of a in frequency Av given by the tensorial relationship regression of the experimental data of P and co, measured he ma material △p=-11: When a residual stress is present, Eg. (4)becomes P where, Lii(the trace of piezo-spectroscopic matrix) is =x+Yo (5) piezo-spectroscopic coefficient relating frequency to stress Our spectrometric apparatus (T 64000 Horiba/Jovin-Yvon) where utilized a 400 mW argon-ion laser beam operating at a wavelength of 488 nm as the excitation source. An optical CI crack length in the stressed material microscope lens was used both to focus the laser beam on the y 1.29 geometrical factor (for a halfpenny sample and to collect the scattered signal, and a micron-scale magnification was obtained. When focussed by means of the residual stress optical microscope using a X 20 optical lens, the laser spot on the sample was 5 um in size. Thermal and instrumental whereas the other symbols have the same meaning as above. fluctuations were compensated by monitoring the spectrum Using the value for KIc(measured using the CNB method) from a Hg/Ne discharge lamp. The recorded spectra were and x(obtained from Eq (4)), regression of the experimental analyzed with a commercial software(LabSpec 4.02, Horiba/ data of P and cI through Eq. (5)allows us to calculate res. Jobin-Ivon). The frequency shifts were obtained by subtractin For the system considered in this paper and for a thickness ratio the centre frequency of the peak, measured for the reference among layers of dissimilar composition of about l, this material in the unstressed state, from the centre frequency of Lamination procedu Monolithic alumina Laminated alumina 「MA after intering afmer n ated A/AZ Fig. 5. Production of laminated composites.If the value of KIc is measured independently by a reliable method (for example, Chevron–Notched Beam (CNB) [59]), the parameter c can be evaluated by means of the best fit of a regression of the experimental data of P and c0, measured on the MA material. When a residual stress is present, Eq. (4) becomes KIc Zc P c3=2 1 CYsres ffiffiffiffi c1 p (5) where c1 crack length in the stressed material Y 1.29 geometrical factor (for a halfpenny shape crack) sres residual stress whereas the other symbols have the same meaning as above. Using the value for KIc (measured using the CNB method) and c (obtained from Eq. (4)), regression of the experimental data of P and c1 through Eq. (5) allows us to calculate sres. For the system considered in this paper and for a thickness ratio among layers of dissimilar composition of about 1, this analysis yielded a value of K141 MPa (compressive) for sres. Fig. 7 shows the regression of Vickers indentation data for the stress free material MA and hybrid laminate A/AZ. Details on this method can be found elsewhere [34]. 4.2. Piezo-spectoscopic technique Piezo-spectroscopic technique is a valuable method to measure the stress amount and distribution in laminated structures [13,35]. The stress distribution along cross sections of the multi￾layered samples can be determined from the spectra related to the characteristic R1, R2 doublet produced by the chromo￾phoric fluorescence of Cr3C impurities in Al2O3. The relation between a line shift in a fluorescence spectrum and the stress state has been described by Grabner [60]. In polycrystalline, fine grained and untextured Al2O3 samples subjected to a normal stress s, a luminescence line undergoes a change in frequency Dn given by the tensorial relationship Dn Z 1 3 Piisjj (6) where, Pii (the trace of piezo-spectroscopic matrix) is the piezo-spectroscopic coefficient relating frequency to stress. Our spectromettric apparatus (T 64000 Horiba/Jovin-Yvon) utilized a 400 mW argon-ion laser beam operating at a wavelength of 488 nm as the excitation source. An optical microscope lens was used both to focus the laser beam on the sample and to collect the scattered signal, and a micron-scale magnification was obtained. When focussed by means of the optical microscope using a !20 optical lens, the laser spot on the sample was 5 mm in size. Thermal and instrumental fluctuations were compensated by monitoring the spectrum from a Hg/Ne discharge lamp. The recorded spectra were analyzed with a commercial software (LabSpec 4.02, Horiba/ Jobin–Ivon). The frequency shifts were obtained by subtracting the centre frequency of the peak, measured for the reference material in the unstressed state, from the centre frequency of Fig. 5. Production of laminated composites. Fig. 4. Thermal cicle for densification laminated composites. G. de Portu et al. / Composites: Part B 37 (2006) 556–567 561