Issue 35

W. Ozgowicz et alii, Frattura ed Integrità Strutturale, 35 (2016) 11-20; DOI: 10.3221/IGF-ESIS.35.02 13 used for recording the acoustic impulses. The measurement system AE was connected with a devise for fastening the samples by mechanic clamps, from which the AE signal if emitted by a quartz waveguide and an AE sensor. For measuring the AE a WD sensor produced by the Physical Acoustic Corporation and an amplifier of the authors’ own production were used. This system was provided with special filters permitting the elimination of frequencies corresponding to the range of free vibrations of the strength machine. The arrangement of the blocking system, as well as the recording of AE have been presented in Fig. 1. Figure 1 : Simplified block diagram measuring and recording system of the AE Metallographic tests were carried out on longitudinal microsections of statically stretched samples. The preparation of the microsections consisted in standard operations of immarging the samples in chemohardening resin, grinding and mechanical polishing on the machine Struers Labo Pol-21, and etching them in a 2% solution of hydrofluoric acid in water. The structure was observed on a scanning light microscope OLYMPUS GX71F with magnifications within the range from 200 to 1000 times. For fractographic tests fractures of torn-off samples after their decohesion in tensile tests were applied, using for this purpose the scanning microscope (SEM) ZEISS SUPRA 25 with the electron part GEMINI of voltage 20 kV at magnitudes of 60 to 2000 times. R ESULTS he results of mechanical tests of the PLC effect of the bronze CuSn6P in the tensile test at a temperature of 20÷400  C and a strain rate amounting to about 1.2·10 -3 sec -1 are to be seen on the curves σ-ε (Fig. 2 and 3) and have been gathered in Tab. 2. After continuous casting and homogenizing annealing of the investigated alloys their work- hardening curves display a qualitatively similar course of oscillation of stresses, mainly a mixed one (A+B+C), starting as soon as the stresses corresponding to the yield strength (R p0.2 ) has been reached and disappear in the zone of unstable flow at the moment of the rupture of the sample. The temperature of the tensile test has been found to influence considerably the range of occurrence of the PLC effect and in a less distinct way the shape and intensity of the recorded oscillation of the stresses. The PLC effect does not appear in samples stretched at room temperature of up to about 150 o C and above 300  C. In the investigated alloys the PLC effect is initiated similarly, in the case of values of critical deformations (  C ) in the range from 0.5 to 1% of deformation, coinciding with yield point of the material. The highest intensity of the oscillation of stresses, and thus also of the unstability of plastic deformation PLC has been found in samples tested at a temperature of amounting to 250  C (Tab. 2), basing on the quantitative description of the PLC effect, applied in stereology, making use of the calculation software MACLAB. For the sake of a more complete description of serration recorded on the curves  -  , the coefficient of expansion of the line ( R L ) was determined, as well as the average of the stress of oscillation in the given range of deformation ( A ) and the frequency of their occurrence ( f ) with this interval, applying the following two Eqs. (1) and (2): ' L L R L  (1) T