Issue 30

A. Carofalo et alii, Frattura ed Integrità Strutturale, 30 (2014) 349-359; DOI: 10.3221/IGF-ESIS.30.42 351 order to avoid the introduction of microdefects in the weld cord both in the as-welded condition and after PWHT heat- treating. TIG welded specimens are obtained taking care to guarantee the presence of a transversal welding cord in the specimen gage length. Weld cord has been removed through flattening and the welded plate has been subjected to an optimized aging heat-treating. Finally, specimens have been obtained by cutting and milling. However, thermal cycle of welding, aging heat-treating and final milling introduced specimen distortions, which was not compatible with the standard required for the execution of the mechanical tests, in particular low-cycle fatigue tests. Therefore, specimens have been plastically deformed at room temperature to respect the straightness tolerance. Flat specimens are not the usual choice to carry out high temperature mechanical test, due to the highest difficulty for a correct load application and a reliable strain measurement. However, a particular care has been used in order to achieve correct results from the mechanical test. Since a three-zone split furnace was used for heating, load application to specimen was obtained through serrated face grip designed and realized in Udimet720. This grip allows the application of a preload to the specimen in order to guarantee the backlash recover especially during low-cycle fatigue test. Even if the dimension of the specimen were opportunely reduced to limit the maximum load, grip was subjected to severe creep and plastic deformation that requires its substitution to complete the experimental program. Static test has been carried out following the indication of ASTM E8-04 and E21-09 standards, respectively for room temperature (RT) and high temperature 503°C, 704°C and 760°C. Sub-size specimen has been used for test at 503°C, in order to limit the maximum load during test. All the tests are carried out in displacement control imposing a speed of 0.1 and 0.5 mm and repeated on 3 and 5 specimens, respectively for room and high temperature. Test temperature is measured and controlled in three point using K thermocouples in contact in three different point of the specimen gage length. Low-cycle fatigue tests are carried out following the indication of ASTM E606-04 standard, using a triangular waveform in strain control and a frequency of 0.5 Hz. Specimen geometry has been changed with respect to the standard indication increasing the fillet radius to a value of 15 mm, in order to avoid failure due to excessive stress concentration (Fig. 1a). It was verified that this geometry was able to hold compressive strength close to yield strength without showing buckling and transversal deflection of the specimen. All the test are characterized by a strain ratio R ε = ε min /ε max = 0. Applied strain is in the range Δε = 0.4-0.9 %, therefore plastic deformation is low or absent and the expected fatigue life is relatively high. Run-out level was fixed at 10 5 cycles, after which test is continued in load control using the stabilized cycle stress range. Experimental test plan considers tests at room temperature (RT), 538°C and 760°C, both for base and TIG welded material. In this work only the results of RT and 538°C conditions are showed and discussed. Static and low-cycle fatigue tests were carried out on a servohydraulic testing machine 100 kN MTS810, while extensometers used for strain control test have a 10 mm gage length. Creep test are carried out on a dead-weight creep machine NORTEST TC50 equipped with adapters for flat specimen, which geometry is reported in Fig. 1b. Two different stress levels and a temperature of 704°C have been considered. Each test condition has been repeated on three specimens to guarantee the repeatability of the result. E XPERIMENTAL R ESULTS AND D ISCUSSION Static test tatic behaviours of laminated Waspaloy obtained in this work are resumed in Tab. 1, which reports the mean values of the most representative static parameters. All the values have been normalized considering a reference value equal to maximum strength. Yielding stress and tensile strength of the base material are coherent with the data reported in literature for Waspaloy [10, 18]. At temperature above 700°C, hardening disappeared justifying the decay of the tensile strength at high temperature. Finally, Young modulus undergoes a reduction of about 50% at 760°C, which is higher than that reported in literature for Waspaloy. A wide extension of the plastic zone is observed at temperature above 700°C (Fig. 2a), which is in contrast with the marked reduction observed in [10]. Assuming base material as a reference, the modification of the static behaviour induced by TIG is clearly highlighted. If the variability of yielding stress and Young modulus is excluded, which are data sensitive to error and numerical manipulation during elaboration, tensile strength is practically not affected by the presence of weld cord. As expected, a relevant change consists in a reduction of the plastic zone in all the condition. In particular, at the highest temperature, the presence of the weld cord restrained the extension of the plastic zone that was found for the base material (Fig. 2b). S