Nikolaos D. Alexopoulos

Evaluation of the effects of variations in chemical composition on the quality of Al-Si-Mg, Al-Cu, and Al-Zn-Mg cast aluminum alloys

The effect of slight variations in chemical composition on the quality of cast aluminum alloys from three different major alloy systems was evaluated. For the evaluation of the alloy quality, an index QD adjusted to damage tolerance requirements that are currently involved for the design of advanced lightweight structures is used. The quality index QD accounts for tensile strength and ductility as well as for material failure through yielding or fracture. For this investigation, experimental results obtained for variations in chemical composition of the alloy systems Al-Si-Mg, Al-Cu, and Al-Zn-Mg were exploited. In total, castings from 37 different batches from 10 aluminum alloys, varying in chemical composition, were evaluated. Quality characterization and alloy quality ranking were made by evaluating results of 512 tensile tests using the index QD as well as, for comparison, the quality index Q, which is currently used by the industry. The results obtained involving the index QD seem to be more realistic, from the viewpoint of damage tolerance design requirements.

Definition of quality in cast aluminum alloys and its characterization with appropriate indices

In this paper, the issue of “quality” of cast aluminum alloys from various viewpoints is interpreted. Many methods to characterize the quality of materials are available; the methods used currently for the quality evaluation of cast aluminum alloys include nondestructive testing, characterization of the microstructure, and mechanical testing. With regard to mechanical testing, a number of quality indices have been devised to evaluate and characterize the quality of cast aluminum alloys. As these quality indices use different mechanical properties for the quality evaluation, they are expected to lead to different results. In this work, the application of proposed quality indices and their suitability is discussed for a number of situations, including minor variations in chemical composition, different solidification rate, solid solution and artificial aging heat treatments.

Mechanical Performance Evaluation of Cast Magnesium Alloys for Automotive and Aeronautical Applications

The potential of cast magnesium alloys for being used as structural materials in lightweight applications is assessed. The ability of the alloys for mechanical performance is evaluated and compared against the ability of widely used structural aircraft cast aluminum alloys. The specific quality index QDS, devised for evaluating both cast and wrought aluminum alloys, will be exploited to evaluate the ability of a number of cast magnesium alloys for mechanical performance. The exploited quality index QDS involves the material’s yield strength Rp to account for strength, the strain energy density W to account for both tensile ductility and toughness, and the material’s density . The effects of differences in chemical composition and heat treatment conditions on the mechanical performance of cast magnesium alloys have been assessed. The use of the quality index QDS has been proved to appreciably facilitate the evaluation of the mechanical performance of cast magnesium alloys and also the comparison between alloys of different base materials. The results quantify the gap to be closed such as to involve cast magnesium alloys in aircraft structural applications. DOI: 10.1115/1.2744407

Fracture mechanical behaviour of laser beam-welded AA2198 butt joints and integral structures

Purpose – Composite materials and metallic structures already compete for the next generation of single-aisle aircraft. Despite the good mechanical properties of composite materials metallic structures offer challenging properties and high cost effectiveness via the automation in manufacturing, especially when metallic structures will be welded. In this domain, metallic aircraft structures will require weight savings of approximately 20 per cent to increase the efficiency and reduce the CO2 emission by the same amount. Laser beam welding of high-strength Al-Li alloy AA2198 represents a promising method of providing a breakthrough response to the challenges of lightweight design in aircraft applications. The key factor for the application of laser-welded AA2198 structures is the availability of reliable data for the assessment of their damage tolerance behaviour. The paper aims to discuss these issues. Design/methodology/approach – In the presented research, the mechanical properties concerning the quasi-static tensile and fracture toughness (R-curve) of laser beam-welded AA2198 butt joints are investigated. In the next step, a systematic analysis to clarify the deformation and fracture behaviour of the laser beam-welded AA2198 four-stringer panels is conducted. Findings – AA2198 offers better resistance against fracture than the well-known AA2024 alloy. It is possible to weld AA2198 with good results, and the welds also exhibit a higher fracture resistance than AA2024 base material (BM). Welded AA2198 four-stringer panels exhibit a residual strength behaviour superior to that of the flat BM panel. Originality/value – The present study is undertaken on the third-generation airframe-quality Al-Li alloy AA2198 with the main emphasis to investigate the mechanical fracture behaviour of AA2198 BMs, laser beam-welded joints and laser beam-welded integral structures. Studies investigating the damage tolerance of welded integral structures of Al-Li alloys are scarce.

Fatigue Behavior of Inconel 718 TIG Welds

Mechanical behavior of reference and TIG-welded Inconel 718 specimens was examined in the present work. Tensile, constant amplitude fatigue, and fracture toughness tests were performed in ambient temperature for both, reference and welded specimens. Microstructure revealed the presence of coarse and fine-grained heat-affected zones. It has been shown that without any post-weld heat treatment, welded specimens maintained their tensile strength properties while their ductility decreased by more than 40%. It was found that the welded specimens had lower fatigue life and this decrease was a function of the applied fatigue maximum stress. A 30% fatigue life decrease was noticed in the high cycle fatigue regime for the welded specimens while this decrease exceeded 50% in the low cycle fatigue regime. Cyclic stress-strain curves showed that Inconel 718 experiences a short period of hardening followed by softening for all fatigue lives. Cyclic fatigue response of welded specimens’ exhibited cyclically stable behavior. Finally, a marginal decrease was noticed in the Mode I fracture toughness of the welded specimens.

Strain monitoring of cement-based materials with embedded polyvinyl alcohol - carbon nanotube (PVA-CNT) fibers

This article investigates the possibility of exploiting innovative polyvinyl alcohol fibers reinforced with carbon nanotubes (PVA-CNT fiber) as a strain sensor in cement mortars used in the restoration of Cultural Heritage Monuments. Two types of PVA-CNT fibers were embedded in the matrix at a short distance from the bottom of the beam and their readings were correlated with traditional sensors, e.g. strain gauges and Fiber Optic Bragg Gratings. The Electrical Resistance Change (ERC) of the embedded PVACNT fiber was in-situ monitored during four-point bending mechanical tests. For the case of coated PVA-CNT fiber, a linear correlation of the applied strain at the bottom surface of the specimen along with ERC values of the fiber was noticed for the low strain regime. For the case of incremental increasing loading – unloading loops, the coated and annealed PVA-CNT fiber gave the best results either as embedded or as ‘surface attached’ sensor that exhibited linear correlation of ERC with applied strain for the low applied strain regime as well as hysteresis loops during unloading. The article discusses their high potential to be exploited as strain/damage sensor in applications of civil engineering as well as in restoration of Monuments of Cultural Heritage.

Carbon Nanotube Structures with Sensing and Actuating Capabilities

We describe carbon nanotube (CNT) structures which are used as mechanical sensors, electromechanical actuators and shape memory materials. These structures include CNT mats and fibres of aligned CNTs. Mechanical sensors are based on the piezo-resistivity of the investigated CNT structures. They can be used as embedded sensors for sensing and damage monitoring of composites. CNT can also be used for novel actuator technologies. Indeed CNTs deform in response to charge injection and electrostatic phenomena. They can be stimulated under the form of electrodes in a given electrolyte. CNT structures can generate a large stress because of their stiffness. In other classes of actuating materials, carbon nanotubes can be used as fillers of shape memory polymers (SMPs). SMPs have applications in packaging, biomedical devices, heat shrink tubing, deployable structures, etc. CNTs are ideal materials to improve the stiffness of shape memory polymers, which is critical for achieving large stress recovery. Their electrical conductivity is of particular interest in the engineering of SMPs which can be heated via Joule’s heating and directly stimulated by an electrical current. We review in this chapter the properties of these new functional materials and highlight their potential for future applications.

Prediction of aircraft aluminum alloys tensile mechanical properties degradation using support vector machines

In this paper we utilize Support Vector Machines to predict the degradation of the mechanical properties, due to surface corrosion, of the Al 2024-T3 aluminum alloy used in the aircraft industry Pre-corroded surfaces from Al 2024-T3 tensile specimens for various exposure times to EXCO solution were scanned and analyzed using image processing techniques The generated pitting morphology and individual characteristics were measured and quantified for the different exposure times of the alloy The pre-corroded specimens were then tensile tested and the residual mechanical properties were evaluated Several pitting characteristics were directly correlated to the degree of degradation of the tensile mechanical properties The support vector machine models were trained by taking as inputs all the pitting characteristics of each corroded surface to predict the residual mechanical properties of the 2024-T3 alloy The results indicate that the proposed approach constitutes a robust methodology for accurately predicting the degradation of the mechanical properties of the material.

Experimental analysis of constant-amplitude fatigue properties in 6156 (Al-Mg-Si) sheet aluminum alloy

Fatigue mechanical behavior of wrought aluminum alloy (Al-Mg-Si) 6156 at T4 temper is experimentally investigated. Constant-amplitude fatigue tests, at fixed stress ratio R = 0.1, were carried out, and the respective stress–life diagram was constructed and compared against the competitive 6xxx aluminum alloys, for example, 6082 and 6061. Fatigue endurance limit of AA6156 was found to be approximately 155 ± 5 MPa, that is, almost 30% below yield stress Rp of the material. AA6156 presents almost 50% higher fatigue life in the high-cycle fatigue area and approximately 20% higher fatigue endurance limit, when compared with other 6xxx series aluminum alloys. Significant work hardening was induced due to fatigue and was experimentally validated by the measurements of residual stiffness of fatigue loops as well as of absorbed energy per fatigue loop. Work-hardening exponent was essentially decreased by almost 25% from the first fatigue cycles and up to 10% of fatigue life. Fracture surfaces of specimens loaded at applied stresses close to fatigue endurance limit exhibited signs of coarse voids due to the formed precipitates at the matrix. The fracture mechanism was a mixture of transgranunal and intergranular fracture for the fatigue specimens tested at higher applied fatigue loadings.

Strain Sensing of Glass Fiber Reinforced Coupons by Using Carbon Nanotube Doped Resin

Addition of different percentages of multi-wall carbon nanotubes (MWCNTs) on the mechanical behaviour of epoxy resin as well as glass fiber composite with symmetrical stacking sequence is assessed. GFRPs were manufactured with the vacuum assisted resin infusion (VARI) process. Performed tensile tests showed that the addition of the CNTs increased the modulus of elasticity with a simultaneously dramatic ductility decrease. The real merit of the CNT addition is the enhancement the composites multi-functionability; piezo-resistivity was recorded to further exploit the self-sensing ability of the innovative composites.Copyright