Mode I fracture toughness of fibre-reinforced concrete by means of a modified version of the two-parameter model

Abstract The present paper proposes a method to calculate Mode I plane-strain fracture toughness of concrete, by taking into account the possible crack deflection (kinked crack), even in the case of a far-field Mode I loading. As a matter of fact, during fracture extension, cracks may deflect as a result of microstructural inhomogeneities inside the material. Concrete is an inhomogeneous mixture due to aggregates embedded in the cementitious matrix, but additional inhomogeneities may be represented by fibres. Firstly, a two-parameter fracture model based on Mode I analytical expressions of the linear elastic fracture mechanics is employed. Then, in order to take into account the possible crack deflection as a result of the above inhomogeneities, a modified version of such a model is here discussed. Three-point bending tests on both plain concrete specimens and concrete specimens reinforced with micro-synthetic polypropylene fibrillated fibres are experimentally performed, and the modified model is applied

Improved Zn-based coatings for ipersandelin steel products

The protection of iron-based alloy products against corrosion is fundamental to preserve their mechanical properties in aggressive environments. Hot-dip galvanizing process represents one of the most used techniques to make protective coatings for such products. In order to improve both mechanical and chemical properties of coating, metallic elements may be added to the traditional zinc bath. In the present paper, two types of improved zinc-based coating are proposed: (i) A coating obtained employing a tin addition (3% in weight); (ii) A coating obtained employing aluminium (5% in weight), tin (1% in weight) and copper (0.5% in weight) additions. Firstly, the performance of such two types of coatings is experimentally investigated through bending tests on ipersandelin steel plate specimens, treated through different bath dipping times. The intermetallic phase thicknesses of coatings are measured for each dipping time, in order to evaluate the kinetic formation. Then, a Finite Element (FE) model is proposed in order to simulate the bending behaviour of the above specimens, both employing the measured phase thickness and implementing the loading and boundary conditions of the experimental tests. A numerical non-linear static analysis is performed. A quite satisfactory agreement between experimental and numerical results is observed, especially under plastic behaviour regime

Mechanical behaviour and phase transition mechanisms of a shape memory alloy by means of a novel analytical model

The aim of the present paper is to examine both the fatigue behaviour and the phase transition mechanisms of an equiatomic pseudo-elastic NiTi Shape Memory Alloy through cyclic tests (up to 100 loading cycles). More precisely, miniaturised dog-bone specimens are tested by using a customised testing machine and the contents of both austenite and martensite phase are experimentally measured by means of X-Ray diffraction (XRD) analyses. On the basis of such experimental results in terms of martensite content, an analytical model is here formulated to correlate the stress-strain relationship to the phase transition mechanisms. Finally, a validation of the present model by means of experimental data pertaining the stress-strain relationship is performed

contribution of date palm fibres reinforcement to mortar fracture toughness

Abstract The present paper aims to analyse the fracture behaviour of date-palm fibre-reinforced mortar. The vegetable fibres have been valorized over the last few years as a suitable reinforcement of cement-based matrixes used in non-structural applications. As a matter of fact, the desirable characteristics of such fibres (for instance, low environmental impact, biodegradability, low cost and density, and equitable stiffness and toughness) have led many designers to employ vegetable fibres in building composite materials. To the best knowledge of the authors, there are no studies in the literature related to the fracture behaviour of a cement-based mortar reinforced by a date-palm fibre mesh. In the present paper, the fracture behavior of a cement mortar reinforced by short fibres extracted from date-palm mesh is analysed. An experimental campaign is carried out on single edge-notched specimens, by examining five different values of fibre content. The experimental tests are performed under three-point bending loading and crack mouth opening displacement control. The Modified Two-Parameter Model proposed by the authors is herein adopted in order to compute the fracture toughness. Such a method is able to take into account the possible crack kinking occurring during the stable crack propagation, which is typical of quasi-brittle materials

Joined application of a multiaxial critical plane criterion and a strain energy density criterion in low-cycle fatigue

In the present paper, the multiaxial fatigue life assessment of notched structural components is performed by employing a strain-based multiaxial fatigue criterion. Such a criterion, depending on the critical plane concept, is extended by implementing the control volume concept reated to the Strain Energy Density (SED) approach: a material point located at a certain distance from the notch tip is assumed to be the verification point where to perform the above assessment. Such a distance, measured along the notch bisector, is a function of both the biaxiality ratio (defined as the ratio between the applied shear stress amplitude and the normal stress amplitude) and the control volume radii under Mode I and Mode III. Once the position of the verification point is determined, the fatigue lifetime is assessed through an equivalent strain amplitude, acting on the critical plane, together with a unique material reference curve (i.e. the Manson-Coffin curve). Some uniaxial and multiaxial fatigue data related to V-notched round bars made of titanium grade 5 alloy (Ti-6Al-4V) are examined to validate the present criterion

fatigue strength of welded joints under multiaxial non proportional loading

Abstract Fatigue behaviour of a fillet-welded tubular T-joint in the so-called H structural component of an agricultural sprayer is examined by using experimental strain measurements found in the literature and linear elastic finite element analyses. Two stress-based critical plane criteria are applied at a verification point located near the intersection between the end of brace and chord of the above-mentioned T-joint, where crack initiation and growth is experimentally observed

Two-parameter fracture model for cortical bone

The analysis of the bone fracture behaviour is fundamental for prevention, diagnosis and treatment of traumas. In the present paper, an experimental analysis of the fracture behaviour of a bovine femoral cortical bone is carried out, where specimens are extracted from a diaphysis. Fracture toughness is computed by employing a two-parameter fracture model originally proposed for concrete. In order to take into account the possibility of crack deflection (kinked crack) due to osteons orientation, a modified version of such a model is here discussed. The fracture toughness results are then compared with those reported in the literature, related to a femur of an 18-month-old bovine

Life estimation by varying the critical plane orientation in the modified Carpinteri-Spagnoli criterion

The modified Carpinteri-Spagnoli (C-S) criterion is a multiaxial high-cycle fatigue criterion based onthe critical plane approach. According to such a criterion, the orientation of the critical plane is linked to boththe averaged directions of the principal stress axes and the fatigue properties of the material. The latterdependence is taken into account through a rotational angle, ?. Then, the multiaxial fatigue strength estimationis performed by computing an equivalent stress amplitude on the critical plane. In the present paper, somemodifications of the original ? expression are implemented in the modified C-S criterion. More precisely, suchmodified expressions of ? depend on the ratio between the fatigue limit under fully reversed shear stress andthat under fully reversed normal stress (in accordance with the original expression), and can be employed formetals ranging from mild to very hard fatigue behaviour. Some experimental data available in the literature arecompared with the theoretical results in order to verify if the modified ?expressions are able to improve thefatigue strength estimation capability of the modified C-S criterion

Life estimation by varying the critical plane orientation in the modified Carpinteri-Spagnoli criterion

The modified Carpinteri-Spagnoli (C-S) criterion is a multiaxial high-cycle fatigue criterion based on the critical plane approach. According to such a criterion, the orientation of the critical plane is linked to both the averaged directions of the principal stress axes and the fatigue properties of the material. The latter dependence is taken into account through a rotational angle, ?. Then, the multiaxial fatigue strength estimation is performed by computing an equivalent stress amplitude on the critical plane. In the present paper, some modifications of the original ? expression are implemented in the modified C-S criterion. More precisely, such modified expressions of ? depend on the ratio between the fatigue limit under fully reversed shear stress and that under fully reversed normal stress (in accordance with the original expression), and can be employed for metals ranging from mild to very hard fatigue behaviour. Some experimental data available in the literature are compared with the theoretical results in order to verify if the modified ?expressions are able to improve the fatigue strength estimation capability of the modified C-S criterion

Analysis of behaviour of metallic structural components under fretting fatigue

La presente Tesi di Dottorato ha come oggetto l’estensione del criterio di fatica multiassiale, proposto da Carpinteri e collaboratori, all'analisi del comportamento a fatica di componenti strutturali metallici soggetti a contatto (fretting fatigue), in regime di alto numero di cicli. In particolare, tale estensione consiste nell’applicazione congiunta del criterio di Carpinteri et al. con approcci non-locali. L’obiettivo finale è quello di sviluppare nuovi criteri per la valutazione della vita a fatica e del piano di nucleazione delle fessure in componenti strutturali metallici in presenza di fatica da contatto. Nel Capitolo 1 viene introdotto il fenomeno della fatica nei componenti strutturali metallici soggetti a contatto, e delle relative problematiche ad esso associate. Inoltre, viene presentato il panorama storico al fine di definire l’evoluzione degli approcci impiegati nell’analisi del fenomeno. Il Capitolo 2 è dedicato all’analisi dello stato tensionale indotto all’interno di due corpi posti a contatto e soggetti a carichi tangenziali, di natura statica o ciclica, con particolare riferimento a contatti cilindrici e a contatti sferici. Vengono quindi fornite le espressioni analitiche, in forma approssimata e in forma chiusa, necessarie per la determinazione delle componenti del tensore degli sforzi in un punto generico del materiale. Tali componenti rappresentano i dati di ingresso per l’analisi a fatica. Nel Capitolo 3 viene presentata la procedura tipicamente impiegata per l’analisi del comportamento a fatica di componenti strutturali metallici soggetti a fatica da contatto. Tale procedura, in particolare, consiste nell’applicazione congiunta di criteri di fatica multiassiale con approcci non-locali, con il fine di tenere in considerazione la presenza degli elevati gradienti tensionali, tipici della fatica da contatto. Per tale motivo viene inizialmente illustrato il criterio di fatica multiassiale proposto da Carpinteri e collaboratori, originariamente proposto per la valutazione della vita a fatica di componenti strutturali metallici soggetti a carichi ciclici multiassiali (fatica pura). Successivamente, viene presentato lo stato dell’arte dei più comuni approcci non-locali basati sul concetto di distanza critica, al fine di individuare un punto rappresentativo del materiale in cui eseguire l’analisi a fatica del componente. Il Capitolo 4 è dedicato alla valutazione della vita a fatica di componenti strutturali metallici soggetti a fatica da contatto. Più precisamente, il criterio di Carpinteri e collaboratori viene esteso considerando quattro diverse tipologie di approccio non-locale, basate sul concetto di distanza critica: (i) approccio non-locale 1, in cui viene assunta una distanza critica costante; (ii) approccio non-locale 2, in cui viene proposta una nuova formulazione della distanza critica, funzione del gradiente tensionale; (iii) approccio non-locale 3, in cui viene proposta una nuova formulazione del Line Method (appartenente alla Teoria della Distanza Critica); (iv) approccio non-locale 4, in cui viene proposta una nuova formulazione dell’Area Method (appartenente alla Teoria della Distanza Critica). Al fine di verificare la validità dell’estensione considerata, vengono impiegati dati sperimentali reperiti in letteratura, relativi a contatti cilindrici e sferici. Infine, il Capitolo 5 è dedicato alla determinazione del piano di nucleazione della fessura in componenti strutturali metallici soggetti a fatica da contatto. Tale determinazione viene condotta estendendo il criterio di Carpinteri e collaboratori, attraverso tre distinte procedure: (i) procedura 1, nella quale il piano di nucleazione della fessura viene valutato come media della giacitura dei piani critici determinati lungo un dato percorso curvo; (ii) procedura 2, nella quale il piano di nucleazione della fessura viene assunto coincidente con il piano critico al centro di una regione di materiale, detta volume strutturale; (iii) procedura 3, nella quale il piano di nucleazione della fessura viene determinato impiegando il Critical Direction Method congiuntamente al criterio di Carpinteri et al. L’accuratezza di tali metodi viene valutata impiegando dati sperimentali, reperiti in letteratura, relativi a prove di fatica condotte in presenza di contatti cilindrici.The present Ph.D. Thesis deals with the extension of the multiaxial fatigue criterion proposed by Carpinteri and co-workers, in order to analyse the fatigue behaviour of metallic structural components subjected to fretting fatigue in high-cycle fatigue regime. In particular, such an extension consists in the joint application of the Carpinteri et al. criterion together with non-local approaches. The final objective is to develop new criteria for both fatigue life and crack nucleation plane evaluation in metallic structural components under fretting fatigue. Fatigue phenomenon in metallic structural components subjected to contact together with related problems are introduced in Chapter 1. Furthermore, the historical background is presented in order to define the evolution of approaches employed in the analysis of such a phenomenon. Chapter 2 deals with the stress state analysis induced inside two bodies pressed in contact and subjected to tangential loadings (static or cyclic), with particular reference to both cylindrical and spherical contacts. The analytical expressions are given in both approximate and closed form, in order to determine stress tensor components in a generic point of the material. Such components represent the input data for fatigue analysis. The procedure typically employed for fatigue behaviour analysis of metallic structural components subjected to fretting is presented in Chapter 3. In particular, such a procedure consists in the joint application of multiaxial fatigue criteria together with non-local approaches, in order to take into account the influence of high stress gradients which are typical of fretting fatigue. In such a context, the multiaxial fatigue criterion by Carpinteri and co-workers, originally proposed for fatigue life evaluation of metallic structural components subjected to multiaxial cyclic loadings (pure fatigue), is outlined. Then, the state-of-the-art of the most common non-local approaches based on the critical distance concept is presented, aimed to determine a representative material point where to perform the fatigue analysis of the structural component being examined. Chapter 4 deals with fatigue life evaluation of fretted metallic structural components. More precisely, the criterion by Carpinteri and co-workers is extended by considering four different non-local approaches, based on the critical distance concept: (i) non-local approach No.1, where a constant critical distance is assumed; (ii) non-local approach No.2, where a new expression of the critical distance is proposed, as a function of the stress gradient; (iii) non-local approach No.3, where a new formulation of the Line Method (belonging to the Critical Distance Theory) is proposed; (iv) non-local approach No.4, in which a new formulation of the Area Method (belonging to the Critical Distance Theory) is proposed. In order to verify the validity of the extension here examined, some experimental results from the literature, related to both cylindrical and spherical contacts, are employed. Finally, Chapter 5 deals with crack nucleation plane determination in metallic structural components under fretting fatigue. Such a determination is performed by extending the criterion by Carpinteri and co-workers through three different procedures: (i) procedure No.1, where the crack nucleation plane is evaluated as the average orientation of critical planes determined along a given curved path; (ii) procedure No.2, where the crack nucleation plane is assumed to be coincident with the critical plane at the centre of a material region, named structural volume; (iii) procedure No.3, where the crack nucleation plane is determined by employing the Critical Direction Method together with the criterion by Carpinteri et al. The accuracy of such methods is evaluated by means of experimental data, available in the literature, related to fatigue tests carried out under cylindrical contacts