F.W. Panella

Modification of creep and low cycle fatigue behaviour induced by welding

In this work, the mechanical properties of Waspaloy superalloy have been evaluated in case of welded repaired material and compared to base material. Test program considered flat specimens on base and TIG welded material subjected to static, low-cycle fatigue and creep test at different temperatures. Results of uniaxial tensile tests showed that the presence of welded material in the gage length specimen does not have a relevant influence on yield strength and UTS. However, elongation at failure of TIG material was reduced with respect to the base material. Moreover, low-cycle fatigue properties have been determined carrying out tests at different temperature (room temperature RT and 538°C) in both base and TIG welded material. Welded material showed an increase of the data scatter and lower fatigue strength, which was anyway not excessive in comparison with base material. During test, all the hysteresis cycles were recorded in order to evaluate the trend of elastic modulus and hysteresis area against the number of cycles. A clear correlation between hysteresis and fatigue life was found. Finally, creep test carried out on a limited number of specimens allowed establishing some changes about the creep rate and time to failure of base and welded material. TIG welded specimen showed a lower time to reach a fixed strain or failure when a low stress level is applied. In all cases, creep behaviour of welded material is characterized by the absence of the tertiary creep.

Characterization of Steel Welded Joints with Hybrid Projection and Capacitor Discharge Welding (CDW) Processes

This work studies the improved hybrid Capacitor Discharge Welding process (CDW), based on projection welding principles applied to stainless steel AISI 304. The innovative idea is to modify the igniting points geometry on the section to be welded and optimize the weld characteristics in order to guide the local fusion processes more uniformly on the whole area and enhance the weld properties. Eight different profile geometries for the contact surfaces have been realized in order to evaluate the geometry influence on joints quality and according to process parameters influence. The mechanical behavior of the welds has been verified with static characterization at room temperature and fatigue tests for welded samples with the better observed microstructure.

Damage Evolution of Composite Laminates with Digital Image Correlation

DIC (Digital Image Correlation) based methodology gives full field measure of the displacement using a well defined algorithm for matching the images of loaded and load free component, so that displacement in a plane can be evaluated for a certain number of grid reference points on the analysed surface. In this work, the authors present an application of DIC technique to analyse fatigue damage phenomena in two notched GFRC laminates under tensile load. Damage analysis based on optical DIC technique has been performed to detect the damaged areas on the specimen surface. The damage evolution and failure mechanism has been followed monitoring two parameters: the local hysteresis area of stress-strain cycles, the local stiffness variation.

Fatigue Properties of Al-Li Plates Joined by Friction Stir Welding

Al-Li alloys are characterized by a strong anisotropy in mechanical properties and microstructure with respect to the rolling direction. Plates of 2198 Al-Li alloy were friction stir welded by employing maximum rotation speed: 1000 rev/min and welding speed of 80 mm/min, both in parallel and orthogonal directions with respect to the rolling one. The joints mechanical properties were evaluated by means of tensile tests at room temperature. In addition, fatigue tests performed with a resonant electro-mechanical testing machine under constant amplitude control up to 250 Hz loading, were conducted in axial control mode with R(σmin/σmax)=0.33, for all the welding and rotating speed conditions. The fatigue crack propagation experiments were performed by employing single edge notched specimens.With the aim to characterize the weld performances, both the microstructure evolution at jointed cross sections, related to the welding variables, and the fractured surfaces were respectively analyzed by means of optical and scanning electron microscopy.

Experimental Assessment of Fatigue Reliability for High-Pressure Plunger Pumps

Current research at the University of Salento’s Department of Innovation Engineering aims to the development of the next generation of high-pressure pumps for Diesel common rail systems. In order to improve both performance and reliability, a better understanding of the mechanisms, underlying the fatigue behavior of high pressure reciprocating plunger pump, is necessary. Previous studies based on static and dynamic FEM simulations provided results to be confirmed by experimental tests. In this paper, we present an experimental approach to better assess the fatigue behavior of this critical component, toward an increase in both robustness, and speed and rail pressure. Based on the analysis of the worst contact conditions under service, the important parameters to be considered for the experimental set-up are isolated and selected; in particular the friction influence and the mutual position of the parts is crucial to establish the real stress state in the pump plunger.Copyright

Mechanical Characterization of Single Crystal Bars with Capacitor Discharge Welding and Laser Cladding

The efficiency of gas turbine systems used for energy production and for aeronautical engines construction can be improved through the elevated thermo-mechanical capabilities of advanced materials employed for blades production. It is well known in fact, the importance to increase the turbine inlet temperature; this temperature is limited by the highest working temperature of steels and super-alloys of which blades and rotor are made. Therefore it is necessary to study the employment of new materials more resistant to high temperatures.