Structural integrity of hierarchical composites

Interface mechanical problems are of paramount importance in engineering and materials science. Traditionally, due to the complexity of modelling their mechanical behaviour, interfaces are often treated as defects and their features are not explored. In this study, a different approach is illustrated, where the interfaces play an active role in the design of innovative hierarchical composites and are fundamental for their structural integrity. Numerical examples regarding cutting tools made of hierarchical cellular polycrystalline materials are proposed, showing that tailoring of interface properties at the different scales is the way to achieve superior mechanical responses that cannot be obtained using standard material

Sulla stima di macro, micro e nano-durezza di materiali metallici mediante analisi elasto-plastiche agli elementi finiti

Il lavoro sintetizzato nel presente articolo si pone come obiettivo primario quello di investigare la possibilità di stimare, mediante un approccio elasto-plastico agli elementi finiti, la durezza dei materiali metallici convenzionali, e questo sia a livello macroscopico, che a livello microscopico, che, infine, a livello nanoscopico. Per verificare validità e accuratezza della metodologia FEM sviluppata, sono state condotte una serie di analisi sperimentali su tre materiali metallici aventi caratteristiche metallurgiche estremamente diverse: una lega d’alluminio (Al 7075-T6), un acciaio a basso tenore di carbonio (BS970-En3B) e, infine, un acciaio austenitico (AISI 316L). L’indentazione Vickers è stata simulata con analisi elasto–plastiche agli elementi finiti considerando carichi di prova nell’intervallo tra 490 N e 490 µN e calibrando le simulazioni numeriche mediante curve monotone tensione–deformazione ottenute da prove di trazione eseguite utilizzando provini sia di dimensione convenzionale che aventi larghezza della zona calibrata dell’ordine dei 100 µm.La sistematica comparazione tra risultati sperimentali e simulazioni numeriche ha posto in evidenza come l’aumentare del valore della durezza misurata al diminuire della dimensione dell’impronta possa essere imputata al ruolo giocato dalla reale morfologia del materiale, ruolo che diventa predominante sulla plasticità convenzionale quando le dimensioni della superficie indentata diventano comparabili con le dimensioni medie della grana cristallina delle leghe esaminate. Tali fenomeni, pertanto, non hanno consentito di estendere l’utilizzo della meccanica del continuo fino ad un livello nanoscopico per determinare correttamente i valori della durezza. Alla luce di questi risultati è stata, però, proposta una semplice metodologia di correzione delle stime eseguite mediante gli elementi finiti che si è dimostrata un valido strumento da utilizzarsi in situazioni di interesse pratico per stimare la durezza dei materiali metallici, indipendentemente dalla dimensione della superficie indentata

On the overall accuracy of the Modified Wöhler Curve Method in estimating high-cycle multiaxial fatigue strength

The aim of the present paper is to systematically investigate the accuracy of the so-called Modified Wöhler Curve Method (MWCM) in estimating high-cycle fatigue strength of plain and notched engineering materials damaged by in-service multiaxial load histories. In more detail, the MWCM, which is a bi-parametrical critical plane approach, postulates that initiation and Stage I propagation of fatigue cracks occur on those material planes experiencing the maximum shear stress amplitude (this being assumed to be always true independently from the degree of multiaxiality of the applied loading path). Further, the fatigue damage extent is hypothesised to depend also on the maximum stress perpendicular to the critical plane, the mean normal stress being corrected through the so-called mean stress sensitivity index (i.e., a material constant capable of quantifying the sensitivity of the assessed material to the presence of superimposed static stresses). In the present investigation, the overall accuracy of the MWCM in estimating high-cycle fatigue strength was checked through 704 endurance limits taken from the literature and generated, under multiaxial fatigue loading, by testing both plain and notched samples made of 71 different materials. Such a massive validation exercise allowed us to prove that the MWCM is highly accurate, resulting in 95% of the estimates falling within an error interval equal to ±15%

Near tip strain evolution under cyclic loading

The concept of ratchetting strain as a crack driving force in controlling crack growth has previouslybeen explored at Portsmouth using numerical approaches for nickel-based superalloys. In this paper, we reportthe first experimental observations of the near-tip strain evolution as captured by the Digital Image Correlation(DIC) technique on a compact tension specimen of stainless steel 316L. The evolution of the near-tip strainswith loading cycles was studied whilst the crack tip was maintained stationary. The strains were monitored overthe selected distances from the crack tip for a given number of cycles under an incremental loading regime. Theresults show that strain ratchetting does occur with load cycling, and is particularly evident close to the crack tipand under higher loads. A finite element model has been developed to simulate the experiments and thesimulation results are compared with the DIC measurements

Case studies in numerical simulation of crack trajectories in brittle materials

Statistical Fracture Mechanics, formalism of few natural ideas is applied to simulation of cracktrajectories in brittle material. The “diffusion approximation” of the crack diffusion model represents cracktrajectories as a one-dimensional Wiener process with advantage of well-developed mathematical formalismand simplicity of creating computer generated realizations (fractal dimension d = 1.5). However, the most ofreported d values are in the range 1.1-1.3. As a result, fractional integration of Wiener processes is applied forlowering d and to generate computer simulated trajectories. Case studies on numerical simulation ofexperimentally observed crack trajectories in sandstone (discs tested in indirect tensile strength test) andconcrete (compact tension specimens tested in the quasi-static splitting tensile test) are presented here

Interference fit effect on holed single plates loaded with tension-tension stresses

This paper deals with the influence of interference fit coupling on the fatigue strength of holed plates. The effect was investigated both experimentally and numerically. Axial fatigue tests have been carried out on holed specimens made of high performance steel (1075MPa of Ultimate strength and 990MPa of Yield strength) with or without a pin, made of the same material, press fitted into their central hole. Three different conditions have been investigated: free hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress-life (S–N) curves pointed out an increased fatigue life of the interference fit specimens compared with the free hole ones. The numericalinvestigation was performed in order to analyse the stress fields by applying an elastic plastic 2D simulation witha commercial Finite Element software. The stress history and distribution along the contact interference of the fitted samples indicates a significant reduction of the local stress range due to the externally applied loading (remote stress) since a residual and compressive stress field is generated by the pin insertion

Characterisation of crack tip fields under non-uniform fatigue loading

The paper analyses previously reported work, which uses digital image correlation to measure fatigue crack closure. As well as determining crack opening loads, the information on crack shape may be used to estimate the stress intensity factor, as well as other parameters in more complex models of crack tip fields. A number of specimens were subjected to single overload cycles, which produced a significant retardation in crack growth rate. The method previously applied to the analysis of constant amplitude loading is here used to analyse the single overload case. The stress intensity factor history is found to be very different in the two cases and the consequences of this observation for analysis of fatigue crack propagation are discussed

Structural durability assessment of welded offshore K-nodes by different local design concepts

The structural durability design of complex welded structures should not rely only on one single design method but should apply different methods for assuring the reliability of the assessment. In this context the application of the structural stress concept, notch stress concept and crack propagation concept are discussed through the example of K-nodes used in energetic offshore constructions like oil platforms or wind power plants, presenting the state of the art

A numerical study of squeeze-film damping in MEMS-based structures including rarefaction effects

In a variety of MEMS applications, the thin film of fluid responsible of squeeze-film dampingresults to be rarefied and, thus, not suitable to be modeled though the classical Navier-Stokes equation. Thesimplest way to consider fluid rarefaction is the introduction of a slight modification into its ordinaryformulation, by substituting the standard fluid viscosity with an effective viscosity term. In the present paper,some squeeze-film damping problems of both parallel and torsion plates at decreasing pressure are studied bynumerical solving a full 3D Navier-Stokes equation, where the effective viscosity is computed according toproper expressions already included in the literature. Furthermore, the same expressions for the effectiveviscosity are implemented within known analytical models, still derived from the Navier-Stokes equation. In allthe considered cases, the numerical results are shown to be very promising, providing comparable or evenbetter agreement with the experimental data than the corresponding analytical results, even at low air pressure.Thus, unlike what is usually agreed in the literature, the effective viscosity approach can be efficiently applied atlow pressure regimes, especially when this is combined with a finite element analysis (FE

Micro void coalescence of ductile fracture in mild steel during tensile straining

The ductile fracture occurs mainly in three stages i.e. void nucleation, void growth and the voidcoalescence. The present work focuses on the study the coalescence of existing micro void in a ductile material,mild steel. The specimen with holes in square array at various angle to load axis have been tested. The holeswere machined in the specimen and assuming those hole as the voids. The growth and coalescence behavioursduring tensile straining were observed both in macro and micro levels. Since the existing facility is not adequateto make hole size in micron, this work has been carried out by making hole upto 500 micron. The results arecompared with other specimen with bigger size hole and without any hole. Also the effects of micro voids(present in the material) on the progress of crack have been studied. It is found that with same amount of voids,present in different positions, the mechanical properties of the material are altered