Gianni Nicoletto

Fatigue crack growth in bonded DCB specimens

Fatigue crack growth tests were conducted on double cantilever beam bonded specimens with the aim to characterize an adhesive for structural applications. The tests were conducted in lab air at two different load ratios, R=Pmin/Pmax, and at two different loading frequencies, f. Crack propagation was monitored using the unloading compliance method. The da/dN vs. ΔG experimental showed the typical sigmoidal shape of bulk materials. Crack closure considerations were successfully used to explain the R-ratio effect. The role of loading frequency was also addressed.

On the visualization of heterogeneous plastic strains by Moiré interferometry

Moire interferometry is a valuable tool for investigations of the mechanics of materials. It is characterized by high-sensitivity and full-field capability. In this paper, the applicability of moire interferometry and microscopic magnification to the visualization of the heterogeneous nature of the plastic strains in a polycrystalline material is considered. Plastic deformation of a coarse-grained aluminum is considered in detail.

Sliding wear behavior of nitrided and nitrocarburized cast irons

In this work the friction and the wear resistance behavior of surface treated spheroidal and lamellar cast irons was studied. Thermochemical surface treatments included nitriding and nitrocarburizing. After characterization of the surface layers, dry sliding wear tests were performed on a prototype pin-on-disk apparatus in which cast iron disks were mated to hardened steel pins. The influence of the sliding velocity on the response of the various combinations of material and surface treatment was determined. Also, the influence of the contact geometry was investigated using hemispherical and flat pins.

Thermo-mechanical finite element analysis in press-packed IGBT design

Abstract The reliability of press-packed insulated gate bipolar transistors (IGBTs) depends on satisfactory contact conditions applied at assembly stage and maintained throughout the service life. The objective of this work is the simulation of stresses and strains in press-packed IGBTs due to assembly and thermal cycling. Single-chip as well as multi-chip devices were analyzed with 2D and 3D models including an elastic–plastic material description and the contact between components using the abaqus code. The assembly process was initially modeled and the factors affecting the contact pressure uniformity between contact disks and chip discussed. The thermal cycling associated with accelerated stress test was then introduced to examine contact pressure evolution as well as local stress/strain concentrations and stick/slip conditions. The device sensitivity to potential damage initiation due to thermo-mechanical fatigue and/or fretting is addressed.

Power cycling on press-pack IGBTs: measurements and thermomechanical simulation

Abstract Press-pack IGBTs are increasing their market-share, especially for traction applications. As packaging performance is a key factor for a successful product, there is a great interest in defining optimal solutions in terms of geometry, materials and mechanical loading. To support IGBT reliability assessment we developed a testing rig for accelerated testing of a single chip under controlled pressure conditions. In parallel, we created a thermomechanical simulation of the chip/testing rig assembly for the determination of internal stresses and strains due to actual operation. Test results and preliminary failure analysis following power cycling show the possibility of predicting the degradation according to different mechanisms induced by the combined effect of pressure and temperature fluctuation.

Thermo-mechanical simulation of a multichip press-packed IGBT

Abstract The reliability of press-packed integrated gate bipolar transistors (IGBT) depends on satisfactory contact conditions applied at assembly stage and mantained throughout the service life. The objective of this work is the simulation of the thermo-structural behavior of a multichip IGBT during initial assembly and subsequent uniform thermal cycling using the finite element method. A detailed axisymmetric FE model of the 3D-device is developed to assess multi-zone contact conditions. Elastic-plastic material behavior and Coulombian friction on contact surfaces are prescribed. The role of dimensional tolerances on contact conditions is discussed. The thermal cycling associated to accelerated testing is then introduced to determine the contact pressure evolution as well as local stick/slip conditions. The device sensitivity to potential damage initiation due to thermo-mechanical fatigue and/or fretting is addressed.

Casting Porosity and Long-Life Fatigue Strength of a Cast Al-Alloy

The sand casting process usually generates porosity that is highly detrimental especially to the fatigue behavior of Al-Si alloys. Since pores favor early fatigue crack initiation, the total fatigue life is crack-propagation-dominated and, for a given stress level, strongly depends on the initiating pore size. Here long life fatigue strengths were estimated using specimens extracted from different AlSi7Mg castings. Detailed metallographic characterization of specimen sections was carried out to determine pore size distributions and correlate it to test results. Murakami’s experimental procedure for applying the largest extreme value statistics to small defect characterization is applied and discussed.

Fatigue crack growth behavior of Inconel 718 produced by selective laser melting

Additive layer manufacturing has recently gained a lot of interest due to the feasibility of producing metallic components directly from a computer-aided design file of the part. Selective laser melting, one of the main additive layer manufacturing technologies, is currently capable of producing nearly ready-to-use parts made of metallic materials. Their microstructure, however, differs substantially from that produced by conventional manufacturing. That is why a detailed study and knowledge of the relation of specific microstructure, parameters of the selective laser melting process and mechanical properties is of utmost significance. This study reports on the investigation of the fatigue crack growth behavior in Inconel 718 superalloy produced by selective laser melting. The fatigue crack growth curve and the threshold values of the stress intensity factor for propagation of long cracks were experimentally determined on compact-tension specimens fabricated using a RENISHAW A250 system and the recommended processing parameters. The fatigue crack growth rates and the fatigue crack paths both in the threshold and in the Paris region were investigated. The crack propagation curves and the crack propagation threshold were compared with literature data describing the behavior of conventionally manufactured material. The mechanism of fatigue crack growth was discussed in terms of the specific microstructure produced by selective laser melting.

Strain Heterogeneity and Damage Localization in Nodular Cast Iron Microstructures

Fracture mechanisms of ferritic/pearlitic nodular cast iron are influenced by the heterogeneous matrix microstructure. This contribution is aimed at understanding the role of a heterogeneous matrix on the localization of damage due to mechanical loading. Therefore, bend specimens made of different nodular cast iron were plastically deformed and the degree of strain heterogeneity determined by comparison with a homogenized response. To address these observations elastic-plastic finite element models of local microstructures were developed. Computed strain maps are examined to understand the link between microstructural features and conditions for damage localization.

Metallographic Characterization and Fatigue Damage Initiation in Ti6Al4V Alloy Produced by Direct Metal Laser Sintering

Direct Metal Laser Sintering (DMLS) is a complex process where a part is build-up by localized melting of gas atomized powder layers by a concentrated laser beam followed rapid solidification. The microstructure of DMLS produced material is substantially different from that of conventionally manufactured materials, although the ultimate strength is similar. However, yield strength and elongation and especially fatigue behavior may vary considerably according to the process parameters and post fabrication heat treatment because they affect structural heterogeneity, porosity content, residual stresses, and surface conditions. Fatigue tests of DMLS Ti6Al4V alloy are interpreted in the light of a thorough metallographic and fractographic investigation. The fatigue crack initiation for three different cyclic stress directions with respect to build direction is determined by fractography.