C.A. Rodopoulos

Study of the surface nanocrystallization induced by the esonix ultrasonic impact treatment on the near-surface of 2024-T351 aluminum alloy

A nanocrystalline surface layer of Al alloy induced by the Esonix Ultrasonic Impact Treatment process was studied by advanced electron microscopes. The results revealed that the process can effectively refine the surface coarse grains into equiaxed nanocrystalline grains (grain size: 8.0–10.0 nm) and elongated microbands (15–20 nm wide). The surface affected zone was measured to be approximately 10 µm. The microstructural features include nanograins, microbands, high-density dislocation, and twining structure.

The Effect of Surface Engineering Treatments on the Fatigue Behavior of 2024-T351 Aluminum Alloy

The work examines the effect of controlled shot peening (CSP), laser shock peening (LSP) and ultrasonic impact treatment (UIT) on the fatigue behavior of 2024-T351 aluminum alloy. The testing methodology has been designed to extract information regarding specific products of the treatments and their individual affect on fatigue damage. The work concludes that all three surface treatments improve the fatigue resistance of the material with the LSP covering the areas of safe-life and damage tolerance, the control shot peening can only benefit the area of short crack growth while the UIT proved to benefit both the short and long crack growth.

Characterization of 2024-T351 friction stir welding joints

Characterization of macrostructure, microstructure, hardness, precipitate distribution, residual stress, and cyclic deformation behavior of 2024-T351 friction stir welded joints has been conducted. Inhomogeneous microparameters governing the nonuniform residual stresses and cyclic strength are discussed. The cyclic strength of the weld microregimes is controlled by grain size and distribution of precipitates achieved during the weld process. The comprehensive information of micro-and macromechanics is used to assist in understanding the mechanism that governed the fatigue crack initiation, propagation, and life of the welded joints.

The Effect of Ultrasonic Impact Treatment on the Fatigue Resistance of Friction Stir Welded Panels

In this work, the results of an experimental study for assessing the effects of Ultrasonic Impact Treatment on the fatigue resistance of Friction Stir Welded aluminum alloy panels are presented. Although the significant compressive residual stress introduced on the material by ultrasonic impact treatment (UIT) was expected to cause retardation in the crack growth rate, this was only noted at low initial ΔΚ values. At high ΔΚ values, the effect of UIT practically diminishes. The phenomenon was attributed to the relaxation/redistribution of the residual stresses with fatigue damage. This provides an alarming situation where damage tolerance design relies on models where only the initial residual stress profile is taken into account without knowledge of the potential re-distribution of the residual stresses caused by the fatigue damage accumulation. The findings of this work also indicate that any FCG tests performed can only be considered as case-specific and conclusions can only be drawn for the case studied.

The use of Ultrasonic Impact Treatment to Extend the Fatigue Life of Integral Aerospace Structures

The work presents extensive experimental data proclaiming the potential enhancement on the fatigue life of integral structures using the Esonix Ultrasonic Impact Treatment.

Repair of Corroded Aerospace Aluminium Panels Using Ultrasonic Impact Treatment

The practise of identifying corrosion damage, air-blasting to remove loose material, grinding to remove corrosion, shot peening to increase the fatigue properties and finally back assembly onto the aircraft for many years has governed aircraft operators under the term “find and fix”. This empirical practise inevitably causes thickness and hence load bearing reduction while can locally overstress components. To avoid that a rule of thumb is implemented making sure that no more than 10% of the initial thickness of the material has been lost. Otherwise, the component should be replaced. It is not difficult to understand that such practise primarily lies within the expertise of the person performing the repair as well as the geometric complexity of the damage. If the corrosion damage is situated at a location which is difficult to grind or, and to peen then most certainly the quality of the repair will vary. The problem can have significant implication under cyclic loading considering that the residual stresses induced by shot peening will undergo continuous redistribution as part of their relaxation process, while irregular thicknesses can transform the design philosophy used for that component from safe life to fatigue damage tolerance. Herein, such repairs can start fatigue cracks following local stress raisers, due to stiffness loss especially when close to stiffeners or by redistributing shear strains when close to joints. The Ultrasonic Impact Treatments process is employing continuous ultrasonic vibrations at the ultrasonic transducer output end strengthened with hard materials (carbide-containing alloys, artificial diamonds etc.) and being in direct and generally continuous contact with the treated surface. During impact the near surface of the material experiences high strain rates as well as heating. The first is responsible for plastically deforming the material.

Crack Coalescence Modelling of FSW Joints

In the present work, Friction Stir Welds (FSW) of 2024-T351 aluminium alloys is characterised in terms of macrostructure, microstructure, hardness, precipitate distribution, and weld residual stress. Cyclic properties and fatigue endurance of the FSW joints are also investigated and discussed. Critical areas for natural fatigue crack initiation in FSW are pinpointed. The fatigue mechanism in FSW is identified to follow a multiple crack coalescence nature (Figure 1). The number of cracks participate in coalescence and the resulting crack growth rate is governed by the dynamic distance between the crack tips from crack initiation to coalescence. The above represents a complex condition for modelling.