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Predictive models play an important role for increasing the reliability of composite structures over time, but a great deal of experimental data is requested. In this paper, results from creep experiments on uniaxial E-glass fiber reinforced polymer (FRP) single ply laminates, performed at different stress levels, are presented. The tests duration was of 42 months. Analytical modeling of the viscous behavior of the tested GFRP composite, under linear and nonlinear viscoelastic hypotheses, is reported. A discussion on the comparison of creep strain response by Burgers model, with parameters obtained from fitting of data for different test duration, is also proposed. Finally, predictive finite element method (FEM) simulations were carried out for discussing the deferred behavior induced by creep, for composite layers used for repair purposes in hydrogen transportation pipes. Numerical results highlighted a non-negligible difference in creep strain values, pointing out that a model based on experimental tests with shorter duration leads to a conservative composite design.

An exact general solution of the following inverse problem is obtained: given a couple of beams adhesively bonded along a surface parallel to their axes, subject to traction / flexure load, find the unknown interface shear force. This solution is used to identify the cohesive law that characterizes the behaviour of adhesive layers subject to shear load. In order to generate data to be used to evaluate the identification methodology, three decohesion virtual tests are performed by means of detailed FE simulations. The series of tests is related to standard test configurations, and shows the correctness of the inverse solution and the efficiency of the proposed methodology, as it is more precise than the data reduction methodologies provided by literature. If ad hoc test configurations are designed, this identification methodology can be applied to adhesion between different materials and to adhesive layers with variable behaviour.

Composite systems are widely used in many engineering applications for new structures and strengthening of existing ones. Within the structural rehabilitation of civil constructions, the plating technique of beams with Fiber Reinforced Polymer (FRP) represents a quick and optimal intervention with respect to traditional ones. The failure of these composite systems usually occurs due to the FRP debonding, which corresponds to a mode II fracture of concrete specimens. In this paper, a new experimental setup for investigating the mode II fracture behavior of FRP-concrete composite structures is presented. The test equipment consists of both conventional equipment and a non-contact optical technique, Digital Image Correlation (DIC), and the test system was realized at the Design Machine Laboratory of the University of Salerno. A preliminary test was performed and the corresponding results are shown and discussed.

Adhesively bonded structures are widely used in many engineering fields for new structures and strengthening of existing ones. The failure of these joints are in some cases caused by long-term deformations. In this paper, an in house made experimental setup for investigating the creep behavior of epoxy resin specimens is presented. The test equipment consists of both conventional apparatus and a non-contact optical technique, Digital Image Correlation (DIC). Several tests were performed at different temperatures and the corresponding results were used for creep master curve construction by means of time-temperature superposition principle (TTSP).

A method is presented for studying bonded assemblies under pure mode III loading. It is based upon a novel fixture that can be mounted on a common universal testing machine and is able of realizing pure anti-plane shear conditions. Twisting of the bonded assembly generates a smoothly propagating circular crack front that allows for accurate measurements during the test. Data reduction is carried out based on the global load-displacement response and material properties are identified. Results are presented that demonstrate the capabilities of the proposed approach to provide mode III fracture parameters of adhesively bonded joints. © 2015 Elsevier Ltd.

By the means of a general analytic solution, suitable to identify the cohesive law that characterizes the behavior of adhesive layers subject to shear load and recently published by this author, a new test configuration is presented. This test configuration, named Twice Notched Flexure Test (TNF), is thought as alternative to the traditional End Notched Flexure (ENF) test or similar ones. It is expected to have several interesting improvement features. In order evaluating the TNF identification methodology, a de-cohesion virtual test is performed by means of detailed FE simulation. The simulation shows that, within the limits of the present model, the cohesive law can be deduced with very high precision.

Repair of structural elements with bonded fiber reinforced polymers (FRPs) is widely used in many engineering applications. Within the strengthening of civil structures, failure usually occurs due to FRP debonding by in-plane shear mode fracture. In this work, mode II fracture behavior of concrete specimens, reinforced with pultruded FRP, was investigated by the authors. Shear tests were performed by using both conventional equipment and a non-contact optical technique, Digital Image Correlation (DIC). Starting from the experimental data, the evaluation of the J-integral and of specimens' fracture toughness was carried out. Subsequently, a cohesive law was associated to the J-integral and thus identified by comparison with experimental data, by means of the theoretical approach proposed by Rice. The proposed cohesive zone (CZ) model can be adopted in a Finite Element (FE) code for simulating the debonding failure in composite structures.

In this work, a novel multi-hinged test fixture, which can apply a pure shear load to thin panels, is presented. The new device has been used to perform several experimental tests on the shear behaviour of aluminium alloy panels. Four different test configurations have been considered: stiffened and not stiffened panels, with and without a central rectangular cut-out. The specimens, made of Al 6082 T6, are tested under shear load boundary conditions up to buckling and post-buckling occurrence. Furthermore, finite element analyses have been performed in order to simulate the four panel configurations tests. The new test fixture has shown the capability to buckle a panel attaining very large out-of-plane displacements, if compared to the outcomes of a traditional picture frame test fixture, under the same external load. Numerical and experimental results are finally compared, showing a satisfactory agreement.

In this work, the performance of a new methodology, based on the Dual Boundary Element Method (DBEM) and applied to reinforced cracked aeronautic panels, is assessed. Such procedure is mainly based on two-dimensional stress analyses, whereas the three-dimensional modelling, always implemented in conjunction with the sub-modelling approach, is limited to those situations in which the so-called secondary bending effects cannot be neglected. The connection between the different layers (patches and main panel) is realised by rivets: a peculiar original arrangement of the rivet configuration in the two-dimensional DBEM model allows to take into account the real in-plane panel stiffness and the transversal rivet stiffness, even with a two dimensional approach. Different in plane loading configurations are considered, depending on the presence of a biaxial or uniaxial remote load. The nonlinear hole/rivet contact, is simulated by gap elements when needed. The most stressed skin holes are highlighted, and the effect of through the thickness cracks, initiated from the aforementioned holes, is analysed in terms of stress redistribution, SIF evaluation and crack propagation. The two-dimensional approximation for such kind of problems is generally not detrimental to the accuracy level, due the low thickness of involved panels, and is particularly efficient for studying varying reinforcement configurations, where reduced run times and a lean pre-processing phase are prerequisites.The accuracy of the proposed approach is assessed by comparison with Finite Element Method (FEM) results and experimental tests available in literature.This approach aims at providing a general purpose prediction tool useful to improve the understanding of the fatigue resistance of aeronautic panels.KEYWORDSDBEM, full scale aeronautic panel, 2D/3D crack growth, MSD, doubler-skin assembly, damage tolerance

In this work, a numerical-experimental characterization of pure mode III fracture toughness of bonded pultruded fibreglass composite joints is presented. Three types of specimen were considered, which differ for the fibres orientation of the bonded surfaces. The analysed angular orientations are: parallel (0 degrees-0 degrees), inclined (0 degrees-45 degrees), and perpendicular (0 degrees-90 degrees). The experimental investigation was realized by using the recently proposed torque shells test (TST). Numerical analyses, based on Finite Element Method (FEM), were performed in order to model a cohesive law of the adhesive interface. A minimization algorithm was also used for identification of cohesive zone model (CZM) parameters by comparing numerical and experimental outcomes. The results show how the TST provides with an almost pure mode III fracture loading condition when the adherends are non-isotropic materials. The comparison of experimental and numerical torque-angle curves, for every test case, shows a satisfactory agreement. As a result of the analyses, the influence of fibres direction on the mode III fracture behaviour of pultruded GFRP joints can be considered negligible for practical applications.

Fibre reinforced polymer (FRP) laminates are being currently used within industrial, automotive, naval, aerospace and civil fields. The reliability of composites over time can be evaluated by characterising their deferred behavior. Experimental investigations on this topic are limited due to the complexity in accurate testing. A creep test program was carried out in order to model the viscous behavior of the composites and their phases. Long term behavior of several unidirectional glass fibre reinforced polymer laminates, manufactured through the wet lay-up technique, and their constituent phases (matrix and fibre) is presented in this paper. Starting from the experimental data acquired for six months, a predictive micromechanical model was proposed for the laminates and the fibres, whilst a phenomenological non linear model, formulated by the authors, was used for the resin matrix. The tests were performed at the Structural Engineering Testing Hall of the University of Salerno.

In this work, powder compaction process was investigated by using a numerical material model, which involves Mohr-Coulomb theory and an elliptical surface plasticity model. An effective algorithm was developed and implemented in the ANSYS finite element (FEM) code by using the subroutine USERMAT. Some simulations were performed to validate the proposed metal powder material model. The interaction between metal powder and die walls was considered by means of contact elements. In addition to the analysis of metal powder behaviour during compaction, the actions transmitted to die were also investigated, by considering different friction coefficients. This information is particularly useful for a correct die design.

Predictive models, which enable the prediction of nanocomposite properties from their morphologies and account for polymer orientation, could greatly assist the exploitation of this new class of materials in more diversified and demanding market fields. This article focuses on the prediction of effective elastic properties (Young's moduli) of polymer nanocomposite films (copolyamide-6/nanoclay) using 3D analytical (based on the Mori-Tanaka theory) and 3D finite element (FE) models. The analytical model accounts for the orientation of polymer chains induced by drawing. 3D FE model exploits the representative volume element concept and accounts for the nanocomposite morphology as determined from transmission electron microscopy experiments. Model predictions were compared with experimental results obtained for nanocomposite films produced by means a pilot-scale film blowing equipment and collected at different draw ratios. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

The rather complex 3D fatigue crack growth behaviour of two anti-symmetric “bird wing” cracks, initiated from the two crack front corner points of a notched shaft undergoing torsion, is investigated by the Dual Boundary Element Method (DBEM) and by the Finite Element Method (FEM). Different criteria for the crack path assessment (Minimum Strain Energy Density, Maximum Principal Stress and Approximate Energy Release Rate) and for the Stress Intensity Factor (SIF) evaluation (COD and J -integral) are adopted. The SIF’s and the crack path, calculated by such different approaches, turn out to be well consistent with each other. Moreover the simulated crack path qualitatively agrees with experimental findings available from literature.

In this work an original creep prediction model is presented with a unified comprehensive formulation with primary, secondary and tertiary stages for variable stresses and temperature fields. The creep model is implemented in the ANSYS FEM code, using the Fortran subroutine usercreep, specifically suited for the cases examined, in order to assess the material behaviour more accurately than that provided by the mono-dimensional approach. Furthermore, the residual stresses that arise from thermo-mechanical loading and unloading cycles, under the temperature-dependent bilinear elastic–plastic hypothesis, are considered. In order to validate the model proposed, an investigation into the final stages of turbine blade is presented and discussed. The nonlinear properties of the material are taken from an industrial context and the literature. In addition to the temperature dependence of the properties of the material, geometric nonlinearities are also considered. The material adopted for model calibration is a polycrystalline Ni-based superalloy. Load conditions used in the simulations are obtained using a computational fluid dynamics (CFD) analysis under working conditions provided by industrial research and the constraints modelled are representative of the real joining condition of the blade. Finally, the simulation results are compared with those obtained by the Norton–Bailey prediction model.

The initial data needed to design metal powder compaction die are: compact shape and density, powder mix composition, compaction and radial pressure, part number and tool materials. The design targets are: diameters of insert and ring, sometimes number of rings and interference or interferences. The constraints include: no tensile stresses on the insert, no risk of relative motion at part ejection, no unwanted alteration of material microstructures and maximum stresses always below the allowable limits. Usually the design is based on engineering experience, company knowhow, and approximated analytical calculations and cost considerations.This study is focused on the use of numerical methods to determine the design parameters of dies for powder compaction. Both room temperature and warm compaction have been investigated. Numerical algorithms, implemented into FEM calculation codes, enable one to optimise the common diameter of insert and ring, corresponding to the lowest stresses on both items, or...

In this paper, the effectiveness of homogenization techniques for media with micro-structure subject to large deformations has been studied by comparing their micro- and macro-failure mechanisms. The material has been studied by considering its representative volume element (RVE) which entails all the geometric and constitutive information of the micro-structure. First, the formulation of the elastostatic problem governing the non-linear (large deformation and non-linear elastic) behavior of the structure of the RVE is presented. The RVE is subject to loading paths that produce uniform macroscopic strains . In this way it has been possible to use an homogenization procedure in order to simulate the overall behavior of the material, i.e. its constitutive tensor, at each point of the equilibrium path. Then, a macro failure surface has been defined as the locus of the points, in the macroscopic stretches space, corresponding to the loss of positivity of the macroscopic fourth order constitutive tensor in terms of the Biot stress [Encyclopedia of Physics, vol. 3(3), Springer-Verlag, Berlin]. Further, a micro-failure surface is defined as the locus of the points, in the overall stretches space, corresponding to the first critical point detected along the equilibrium path which can be characterized by an eigenmode compatible with the boundary conditions. Finally, a representative volume element, schematized by plane rods with strongly non-linear elastic constitutive behavior, is considered and the corresponding micro- and macro-failure surfaces are obtained in order to validate the proposed methodology.

This paper concerns the numerical characterization of the fatigue strength of a flat stiffened panel, designed as a fiber metal laminate (FML) and made of Aluminum alloy and Fiber Glass FRP. The panel is full scale and was tested (in a previous work) under fatigue biaxial loads, applied by means of a multi-axial fatigue machine: an initial through the thickness notch was created in the panel and the aforementioned biaxial fatigue load applied, causing a crack initiation and propagation in the Aluminum layers. Moreover, (still in a previous work), the fatigue test was simulated by the Dual Boundary Element Method (DBEM) in a bidimensional approach. Now, in order to validate the assumptions made in the aforementioned DBEM approach and concerning the delamination area size and the fiber integrity during crack propagation, three-dimensional BEM and FEM submodelling analyses are realized. Due to the lack of experimental data on the delamination area size (normally increasing as the crack propagates), such area is calculated by iterative three-dimensional BEM or FEM analyses, considering the inter-laminar stresses and a delamination criterion. Such three-dimensional analyses, but in particular the FEM proposed model, can also provide insights into the fiber rupture problem. These DBEM-BEM or DBEM-FEM approaches aims at providing a general purpose evaluation tool for a better understanding of the fatigue resistance of FML panels, providing a deeper insight into the role of fiber stiffness and of delamination extension on the stress intensity factors.

New generation jet engines are subject to severe reduced fuel consumption requirements. This usually leads to thin components in which damage issues such as thermo-mechanical fatigue, creep and crack propagation can be quite important. The combination of mechanical and thermal stresses usually leads to mixed-mode loading. Consequently, a suitable crack propagation tool must be able to predict mixed-mode crack propagation of arbitrarily curved cracks in three-dimensional space. To tackle this problem a procedure has been developed based on a combined FEM (finite element method) - DBEM (dual boundary element method) approach. Starting from a three-dimensional FEM mesh for the uncracked structure a subdomain is identified, in which crack initiation and propagation are simulated by DBEM. Such a subdomain is extracted from the FEM domain and imported, together with its boundary conditions (calculated by a previous thermal-stress FEM analysis), in a DBEM environment, where a linear elastic multiple crack growth analysis is performed. Once the crack propagation direction is determined a new crack increment can be calculated and, for the new crack front, the procedure can be repeated until failure. The proposed procedure also allows the consideration of the spectrum effects and creep effects: both conditions determine residual stresses that the crack will encounters during its propagation. The procedure has been tested on a gas turbine vane, getting sound results, and can be made fully automatic, thanks to in house made routines needed to facilitate the data exchange between the two adopted codes.

The main objective of the present work is the study of the effect of residual stresses, induced by a cold working split sleeve process, on the fatigue life of a holed specimen. The crack propagation is simulated by a two-parameters crack growth model, based on the usage of two threshold material parameters (ΔKthand Kmax,th) and on the allowance for residual stresses, introduced on the crack faces by material plastic deformations. The coupled usage of Finite Element Method (FEM) and Dual Boundary Element Method (DBEM) is proposed to simulate the crack propagation, in order to take advantage of the main capabilities of the two methods. The procedure is validated by comparison with experimental results (crack growth rates and crack path) available from literature, in order to assess its capability to predict the crack growth retardation phenomena.

The analysis of the overall mechanical behaviour of heterogeneous materials subject to large deformations is approached in this paper. First the problem of the homogenisation of composite materials made by regular cell assembly and characterised by a representative volume element (RVE) is stated by assuming non linear elastic constituents. The RVE is subjected to an enforced displacement path on its boundary and incremental homogenised moduli, defined in terms of Biot stress, are deduced. Then, micro- and macro-bifurcation criteria on the incremental solution are used to point out the possible unstable configurations of the RVE. Correspondingly, the micro- and macro-failure surfaces of the material are defined inside to the macroscopic strain space in order to detect the validity range of the overall moduli obtained by solving the homogenisation problem. Finally, two different microstructures are analysed and their incremental homogenised elastic moduli are estimated.

An inverse procedure is developed for computing the material parameters of the class of intrinsic cohesive-zone models for the analysis of structural adhesive joints presented in [Valoroso N, Champaney L. A damage-mechanics-based approach for modeling decohesion in adhesively bonded assemblies, Engng Fract Mech 2006;73:2774–801.]. In particular, using the same experimental data recorded as of ISO 25217 test protocol and Double Cantilever Beam bonded specimens, a deterministic identification that overcomes the usual limitations and difficulties of ISO 25217 is proposed to estimate the mode-I cohesive parameters. The driving rationale for our approach to identification is to set up an inverse procedure based on one single experimental test that uses the measured load–deflection curve and crack length as data set. A finite element model updating scheme is used that renders extensions to more general situations such as different geometries, large displacements and nonlinearities in the bulk material, possible with minor modifications. Experiments are conducted for symmetric DCB specimens under pure mode-I bending and an optimization problem is solved to find a suitable agreement between experimental data and finite element computations. Identification results are presented and the key role played by the crack progression in the identification is discussed to assess the effectiveness of the proposed approach.

In some powder metallurgy part forming, when high compacting loads would require a too large single shrink fitted die, or the inner die diameter is very large but the compaction area is modest, twice shrink fitted dies, which are relatively expensive and complex to build, should be used. In both cases, at any given compacting load, the design should minimise dimensions and stresses. The design targets are minimum die size, optimum intermediate diameter and optimum interference, with maximum stresses on each die item lower than those allowable for the selected materials. In dual mode, for fixed overall dimension, intermediate diameters and shrink fitting interference should enable to maximise the compacting load without exceeding the allowable maximum stresses. In the first part of this work, for cylindrical shapes (circular profile dies) and axisymmetric plane stress problems, dimensions, interferences and stress–strain states have been analytically calculated to optimise doubly shrink fitted dies in the abovementioned sense. In the second part, the same evaluations for axisymmetric generic problems have been carried out by finite element method (FEM) approach integrated in an optimisation procedure. The minimum overall dimensions of the die are presented as a function of the ratio between applied load and maximum allowable stress. Corresponding interferences and optimum diameters have been calculated. Some approximate equations for a quick and handy optimisation are presented. The differences between analytical and numerical results obviously depend on the consideration of the actually stressed length, or compact height, and total die height. The comparison indicates that FEM analyses are needed since analytical calculations generally give low approximations, especially for some die geometries.

Cellular solids such as honeycombs and foams are widely used as structural materials for their low density and high specific stiffness. Under compression or shear loading buckling may occur in cells, and complex deformation modes can result in sudden variations in the strength characteristics of the cellular material. In this paper, the behaviour of regular hexagonal honeycombs under general in-plane loading (compression, traction and shear) is examined by experimental tests and numerical evaluation. In addition, the influence of geometrical imperfections on the failure process is considered. A set of uniform deformation fields is imposed on several honeycomb models with different imperfections in order to determine the overall strength properties, the principal buckling modes and the post-buckling behaviour. Experimental tests are properly designed in order to obtain a complete nonlinear in-plane mechanical characterisation of an Al 3033 honeycomb. Moreover, all the numerical analyses are performed by means of an in-house developed FE code.

This paper presents an application of the Dual Boundary Element Method (DBEM) to the simulation of a fatigue crack propagation affecting the main cylinder of an extrusion press for aluminum sections. The crack initiates at the inner surface of the cylinder and propagates through the thickness causing a leakage of the pressurized oil and consequent production stop. The fatigue load is induced by the pressure variation inside the cylinder as needed to push each section through the extrusion hole. The aim of the simulation is to assess the most probable initial crack dimensions that, after the recorded in service fatigue cycles, lead to the final crack scenario. This was requested in order to assess if there was a rogue detectable flaw introduced by the manufacturing process. For validation purposes, the DBEM numerical results in terms of Stress Intensity Factors (SIFs) in the initial cracked configuration are compared with corresponding Finite Element Method (FEM) results. DBEM SIFs are calculated by both J-integral and Crack Opening Displacement (COD) approaches, whereas for FEM SIFs only COD is used.

This paper describes the application of a two-parameters crack growth model, based on the usage of two threshold material parameters (@DK"t"h and K"m"a"x","t"h) and on the allowance for residual stresses, introduced at the crack tip by a fatigue load spectrum or by material plastic deformations. The coupled usage of finite element method (FEM) and dual boundary element method (DBEM) is proposed in order to take advantage of the main capabilities of the two methods. The procedure is validated by comparison with available experimental results, in order to assess its capability to predict the retardation phenomena, introduced by a variable load spectrum or by a plastic deformation introduced with a tool on the panel (indentation). In particular two different tests are made: the first test involve a CT specimen undergoing a load spectrum and the second one involve a dented panel undergoing a constant amplitude fatigue load. In both cases a satisfactory numerical-experimental correlation will be proved. The main advantages of the aforementioned procedure are: the simplicity of the crack growth law calibration (few constant amplitude tests are sufficient without the need for any non-physical calibration parameters), and the possibility to simulate residual stress effects on crack propagation with a simplified approach, based on linear elastic fracture mechanics.

The deformation under radial pressure of rectangular dies for metal powder compaction has been investigated by FEM. The explored variables have been: aspect ratio of die profile, ratio between diagonal of the profile and die height, insert and ring thickness, radius at die corners, interference, different insert materials, i. e. conventional HSS, HSS from powders, cemented carbide (10% Co). The analyses has ascertained the unwanted appearance of tensile normal stress on brittle materials, also “at rest”, and even some dramatic changes of stress patterns as the die height increases with respect to the rectangular profile dimensions. Different materials behave differently, mainly due to difference of thermal expansion coefficients. Profile changes occur when the dies are heated up to the temperature required for warm compaction. The deformation patterns depend on compaction temperature and on thermal expansion coefficients.

If a certain physical phenomenon is so complex that simplifying hypotheses are inadequate to represent it, the determination of design parameters through analytical procedures is unsuitable. In these cases, a numerical algorithm may be the alternative solution, to simulate the phenomenon in a manner that is more faithful to the actual physical situation. The design parameters of compaction dies have been determined by FEM. To evaluate the effects of higher temperatures, as in warm compaction, the available literature data have been used to include the variations of characteristic parameters of the involved materials. A wide range of die dimensions has been covered, focusing the interest on progressively shorter dies or thin inserts. A die can be coded as short when its axial extension is small in comparison to the radial size, which can vary significantly. Different insert materials have been considered, from HSS to PM steels and Co-WC. The deviations from classical Lame's formulae depend on the g...