Effects of laser shock peening on the mechanisms of fatigue short crack initiation and propagation of AA7075-T651

A laser shock peening (LSP) treatment was performed on AA7075-T651 for maximum fatigue improvement. Surface and microstructural characterisation techniques (micro-hardness, SEM-EBSD, contact-profilometry) showed LSP surface modification was limited, and LSP generated deep compressive residual stresses above -300MPa. Fatigue testing showed a two-order magnitude increase in overall life, due to the mechanism of crack initiation changing from surface second-phase particles to subsurface crack initiation dependent on the local stress field. Modelling highlights the sensitive balance between surface roughness (including LSP-induced pits) and residual stress on the micro-mechanism of crack initiation, and how this can be used to maximise fatigue life extension

DogOnt - Ontology Modeling for Intelligent Domotic Environments

Abstract. Home automation has recently gained a new momentum thanks to the ever-increasing commercial availability of domotic components. In this context, researchers are working to provide interoperation mechanisms and to add intelligence on top of them. For supporting intelligent behaviors, house modeling is an essential requirement to understand current and future house states and to possibly drive more complex actions. In this paper we propose a new house modeling ontology designed to fit real world domotic system capabilities and to support interoperation between currently available and future solutions. Taking advantage of technologies developed in the context of the Semantic Web, the DogOnt ontology supports device/network independent description of houses, including both “controllable ” and architectural elements. States and functionalities are automatically associated to the modeled elements through proper inheritance mechanisms and by means of properly defined SWRL auto-completion rules which ease the modeling process, while automatic device recognition is achieved through classification reasoning.

Fatigue crack growth in pin-loaded cold-worked holes

An experimental program has been carried out for the evaluation of the influence of the split sleeve expansion process on the fatigue crack growth in 2024-T351 aluminium alloy specimens. In previous ICAF Symposia, some information has been already given about the first part of the experimental activity (Helsinki, 2015) and about the development of a numerical analysis method (Nagoya, 2017). Both such preliminary papers referred to open hole specimens, tested under Constant Amplitude loading, while recently a pin loaded hole configuration has been evaluated. The non-inspectability of the configuration required some particular experimental effort, in order to collect test results in terms of crack growth as a function of number of cycles. To this end, a marker load technique was adopted: a block of high R ratio cycles (R=0.9) was inserted in the R=0.1 sequence, with the aim of obtaining information about the crack front evolution at different number of cycles. Due to the three-dimensionality of the residual stress field, which is of lower intensity at the face of mandrel entrance, a 1 mm radius quarter-circular notch was inserted by means of EDM on the mandrel entry side face. Moreover, it was necessary to include also mechanically milled notches, of similar dimensions. The results show a rather regular front evolution, and have provided important material for the development of accurate numerical methods, based on the evaluation of the residual stress field and on the subsequent modification of the stress intensity factor distribution along the corner crack front. The numerical analysis methodology is a specialization of the technique, already presented in the Nagoya Symposium, to the problem of single or double corner crack in a pin loaded hole. In particular, the strong three-dimensionality of the stress field poses a challenge to the block-by-block propagation analysis

Fatigue and Damage Tolerance Assessment of 3-D Reinforced CFRP Bonded Joints

An experimental campaign has been carried out to investigate the behavior of co-bonded CFRP joints with a novel design incorporating a through the thickness local reinforcement. Different specimens were manufactured to investigate static and fatigue behavior, as well as delamination size after impact and damage tolerance characteristics. Mechanical performance of specimens with a reinforcement based on the insertion of spiked thin metal sheets between the co-bonded laminates were compared with purely co-bonded joints. This novel design demonstrated by tests a more predictable failure with respect to the standard design based solely on adhesive structural bonding. The tests allowed obtaining a significant number of experimental results that can be used to define preliminary design guidelines

Friction stir welds with enhanced fatigue strength and life post laser peening

This study aims to quantify the influence of laser peening on the structural integrity of a FSWelded aluminium fuselage joint. Fatigue tests with different stress amplitudes were performed for samples in as-machined and friction stir welded conditions to characterise the loss of fatigue strength and the effect of stress amplitude on the degradation of fatigue life. Optimisation of laser parameters was carried out to achieve an appropriate through-thickness stress profile. The optimised parameters were used to peen tensile coupon specimens and analyse the fatigue life recovery. The residual stress fields were quantified using the central incremental hole drilling method on peened and unpeened samples. It was found that both the microstructure and superposition of residual stresses from the welding process and peening treatment play a crucial role in the fatigue behaviour of the FSWelded joint. Using the parameters and layout described in this article, laser peening was able to increase the fatigue life of as-welded samples by a factor of 2.4 for the lower end of the load spectrum tested. This represents a recovery of 65% when compared to the pristine aluminium reference samples

Influence of tartaric-sulfuric acid anodic film on four-point bending fatigue behavior of AA 7050-T7451 samples

Tartaric-sulfuric acid (TSA) anodizing is an electrochemical process used for generating an anodic layer on the surface of aluminum alloys for aerospace applications. It must provide corrosion resistance and ensure no detrimental effect on the required fatigue properties for structural applications. This work aimed at analyzing the properties of the anodic layer and evaluating its influence on the fatigue behavior of the AA 7050-T7451. The film properties were analyzed using optical and scanning electron microscopies. The surface residual stress (SRS) field in the as-machined and anodized conditions was evaluated by the X-ray diffraction technique. The results showed that TSA induces an appreciable reduction in the fatigue life, especially at lower values of stress amplitude, due to the presence of defects at the interface between the substrate and the anodic film. No correlation was observed between the presence of the anodic layer and the development of a tensile SRS field

Interoperation Modeling for Intelligent Domotic Environments

This paper introduces an ontology-based model for domotic device inter-operation. Starting from a previously published ontology (DogOnt) a refactoring and extension is described allowing to explicitly represent device capabilities, states and commands, and supporting abstract modeling of device inter-operation