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The peak stress method (PSM) allows a rapid application of the notch stress intensity factor (NSIF) approach to the fatigue life assessment of welded structures, by employing the linear elastic peak stresses evaluated by FE analyses with coarse meshes. Because of the widespread adoption of 3D modeling of large and complex structures in the industry, the PSM has recently been boosted by including four-node and ten-node tetrahedral elements of Ansys FE software, which allows to discretize complex geometries. In this paper, a Round Robin among eleven Italian Universities has been performed to calibrate the PSM with seven different commercial FE software packages. Several 3D mode I, II and III problems have been considered to investigate the influence of (i) FE code, (ii) element type, (iii) mesh pattern, and (iv) procedure to extrapolate stresses at FE nodes. The majority of the adopted FE software packages present similar values of the PSM parameters, the main source of discrepancy being the stress extrapolation method at FE nodes.

The use of engineering polymers for mechanical applications has seen increasing uptake due to properties such as low density, flexibility, ease of manufacturing and cost effectiveness. Despite these advantages, joining and assembly methods for these types of materials is still an open issue. Traditional assembly processes such as screw fastening and riveting are increasingly being replaced by new processes such as adhesive bonding. Engineering polymers, however, are difficult to bond using adhesives due to their low surface energy and low wettability. For this reason, surface chemical activation techniques with primers are often used. The utilization of various chemicals associated with such pre-treatments has a significant environmental impact. Within this context, the present paper aims to compare the environmental performance of four adhesive bonding pre-treatments: (i) mechanical (i.e., abrasion), (ii) chemical (i.e., primer), (iii) plasma and (iv) laser activation. The work was performed in three phases: (i) setup of the surface activation processes, (ii) mechanical characterization of bonded joints (static tests) and (iii) LCA analysis to evaluate and compare the different pre-treatments. The outcome of this study provides important insight into the development of laser and plasma technologies as sustainable surface activation methods for polymers through the creation of models correlating process parameters to the type of surface and joint strength.

Food packaging industry requires machines able to perform different tasks and carry out several functions. Machine modularization allows to feed customer’s needs creating a set of equipment with different features and technology. Module derivation is particularly important at the conceptual phase where main decisions are taken and where the degree of freedom are higher, avoiding subsequent costly modification. This study aims at investigating the adoption of engineering design process for the development of a tuna canning machine, deriving main modules for a definition of a product platform. The possibility to have a modular framework in this type of products allows to satisfy constraints coming from different markets and applications (i.e., product quality, adaptability, upgradability, assemblability, compliance with standards where the machine is installed, etc.). Modules were derived based on state-of-art approaches used for product development (i.e., functional analysis, module derivation and morphological matrix) and two examples (i.e., Cutter and Compactor & Shaper modules) were detailed to explain the developed design solutions. Results highlight how different design options can be adopted to overcome several issues (i.e., assemblability, upgradability) and fulfill requirements of different markets (i.e., product quality and aesthetic).

The purpose of this work is to compare the co-bonding vs. cold-bonding route on the adhesive joint performance of a CFRP (Carbon Fiber Reinforced Polymer) laminate–aluminum connection. In particular, the overlap shear, tensile strength and Mode I and Mode II fracture toughness will be evaluated. The adhesives for co-bonding and cold-bonding are, respectively, a thermosetting modified epoxy, unsupported structural film and a two-component epoxy adhesive, chosen as representative of applications in the high-performance/race car field. The emerging trend is that, in tensile e Mode I fracture tests, the failure path is predominantly in the composite. Mode II fracture tests instead resulted in a cohesive fracture, meaning that, under pure shear loading, the weakest link may not be the composite. The lap-shear tests are placed midway (cohesive failure for co-bonding and composite delamination for cold-bonding, respectively), probably due to the different peel stress values related to the different adhesive Young’s modulus. The exploitation of the full capacity of the adhesive joint, hence the possibility of highlighting better, different performances of co-bonding vs. cold-bonding, would require consistent improvement of the out-of-plane strength of the CFRP laminate and/or to someway redistribute the peel stress on the bondline.

The low quality of adhesion performance on polymeric surfaces has forced the development of specific pretreatments able to toughen the interface between substrate and adhesive. Among these methods, atmospheric pressure plasma treatment (APPT) appears particularly suitable for its environmental compatibility and its effectiveness in altering the chemical state of the surface. In this work, an experimental study on adhesively bonded joints realized using polyamide as substrates and polyurethane as the structural adhesive was carried out with the intent to characterize their fatigue behavior, which represents a key issue of such joints during their working life. The single lap joint (SLJ) geometry was chosen and several surface pretreatments were compared with each other: degreasing, abrasion (alone and followed by APPT) and finally APPT. The results show that the abrasion combined with APPT presents the most promising behavior, which appears consistent with the higher percentage of life spent for crack propagation found by means of DIC on this class of joints with respect to the others. APPT alone confers a good fatigue resistance with respect to the simple abrasion, especially at a low number of cycles to failure.

Adhesive bonding is the elective joining system between Carbon-Fiber Reinforced Polymer (CFRP) parts because, with respect to fastening, it allows a large connection area, no additional parts (hence weight saving) and no need to drill holes into the composite, that is always detrimental for the strength due to the possibility of developing damage. However, the choice of bonding CFRP should be evaluated as alternative to direct curing in terms of strength and durability, compared to cost and manufacturing time and complexity. In this work, a comparison between co-cured and co-bonded CFRP is done with respect to tensile strength and mode I and mode II fracture toughness, in order to understand whether co-bonding guarantees the same performance of a co-cured composite part.Adhesive bonding is the elective joining system between Carbon-Fiber Reinforced Polymer (CFRP) parts because, with respect to fastening, it allows a large connection area, no additional parts (hence weight saving) and no need to drill holes into the composite, that is always detrimental for the strength due to the possibility of developing damage. However, the choice of bonding CFRP should be evaluated as alternative to direct curing in terms of strength and durability, compared to cost and manufacturing time and complexity. In this work, a comparison between co-cured and co-bonded CFRP is done with respect to tensile strength and mode I and mode II fracture toughness, in order to understand whether co-bonding guarantees the same performance of a co-cured composite part.

The purpose of this work is to evaluate the influence of co-lamination vs. co-bonding on the failure behavior, and namely the fracture toughness, of carbon fibre reinforced (CFR) composite laminate joints in order to assess comparatively their performance. Since the strength of the laminate and ply texture are parameters affecting the strength of the joint, the comparison is extended to two different types of CFR pre-preg fibers, a satin T1100 with 2573 Nanoalloy® epoxy resin supplied by Toray and a twill T700 with ER450 toughened epoxy resin supplied by CIT, Toray group, representative of two different fields of application, racing and automotive, respectively.

In this study the fracture behavior of different cast iron microstructures subjected to tensile loading under different triaxialities is simulated by a finite element, 3-D Reference Volume Element approach. Three ferritic/pearlitic heterogeneous matrixes are considered which are representative of the class material grades for strength and ductility. Isotropic ductile and shear damage models are considered for the matrix constituents as concurrent damage mechanisms at the microscale, while graphite nodules are considered as voids acting as stress concentrators. Numerical results confirm experimental findings about local strain distribution and damage accumulation, and reproduce the engineering macroscopic behavior. The stress triaxiality is found to play a strong effect on the failure strain, extending the potentialities of this RVE modeling approach.

Adhesive bonding is the elective joining system between Carbon-Fiber Reinforced Polymer (CFRP) parts because, with respect to fastening, it allows a large connection area, no additional parts (hence weight saving) and no need to drill holes into the composite, that is always detrimental for the strength due to the possibility of developing damage. However, the choice of bonding CFRP should be evaluated as alternative to direct curing in terms of strength and durability, compared to cost and manufacturing time and complexity. In this work, a comparison between directly cured and co-cured, bonded CFRP is done with respect to mode I and mode II fracture toughness, in order to understand whether bonding guarantees the same performance of a co-cured composite part.

Material testing is a fundamental activity for the characterization of materials mechanical properties and for the certification of product quality. With concepts such as Smart Factories and Industry 4.0 coming to the fore, testing and measurement is moving away from laboratories and closer to the production floor: MaCh 3D is a miniaturized tensile testing machine developed for products and materials certification directly on the production site. The heart of the technology is the specimen with non-conventional geometry for tensile tests developed so as to be easily installed on the machine. The objective of this work is to illustrate the process of determining the geometry of specimen and fixtures by numerical analysis and their experimental validation, comparing the results with those obtained from specimens according to ASTM D638 standard, ASTM International, (1999).

Adhesive bonding is currently employed by automotive manufacturers to complement (or replace) welding in joining dissimilar materials. In order to reduce the impact on the existing manufacturing infrastructures, structural adhesives are deployed in the body shop but hardening is accomplished in the paint cure oven. Various adhesive formulations have been specifically developed for the implementation in the automotive manufacturing chain. However, it is very important to assess the mechanical behaviour of the joints which results from the peculiar curing strategy. In the present work, automotive grade single component epoxy and two component epoxy modified acrylic adhesives were evaluated. T-joints were fabricated using a cold rolled galvanized steel (FeP04) employed in the production of car body parts. The fracture toughness of the joints was determined using the test protocol proposed by the European Structural Integrity Society (ESIS). Optical microscopy was employed to ascertain the mechanisms of failure. The results indicated that both adhesives were able to provide a fairly good mechanical response with minimum preparation of the mating substrates. Moreover, the obtained values of fracture toughness were shown to be essentially independent of the adhesive layer thickness.

This work aims to explore innovative joining processes for additively manufactured components, and, in particular, to assess the feasibility of hybrid weld-bonded joints by comparing their performance with the baseline bonded and welded joint configurations. The novelty of the proposed solution lies in the fact that welding is achieved using a 3D printed material with conductive filaments, a solution derived from the use of embedded 3D printed circuits (direct printing) in the AM components. Direct printing can be used to obtain an accurate local control of the thermal cycle and to overcome geometrical limitations inherent to the process, as for example the need of access for the welding tools. The feasibility of the hybrid weld-bonded joint was assessed and, while for adhesive bonding the use of dedicated surface treatments was found to be necessary to improve the joint performance, the welding process was determined to be the most promising joining process, especially when directly integrated into a multi material additive manufacturing (MMAM) process.

Adhesive bonding can disclose the possibility to additively manufacture (AM) smaller, highly optimized objects to be bonded in post-production into/onto larger components. However, the infill shape and percentage can influence the fracture toughness of a bonded joint as it emerges from the literature Since the choice of the infill yields also a significant impact on manufacturing time, it is important to assess clearly its influence on bonded joint strength in order to balance strength and manufacturing issues. Therefore, in this work the Mode I fracture toughness of PLA bonded joints obtained by FFF will be assessed as a function of infill percentage, from solid (100% infill) to 20% infill. Test will be done using a Double Cantilever Beam (DCB) joint bonded with a PolyUrethane (PU), room temperature-curing adhesive.

This work deals with the experimental validation of the proposed specimens geometries which design process through Finite Element Method was presented during AIAS 2019 and published afterwards [1]. In particular, non-conventional miniaturized specimen geometry compliant to ASTM E8 and ISO 6892, specifically developed to work with MaCh3D [2], an innovative miniaturized tensile testing machine, were considered. The size-reduction of specimens is advantageous both in terms of material and equipment: smaller specimens require lower forces to be broken, hence a compact tensile testing machine can perform the mechanical characterization activity. In this experimental validation, mechanical properties determined using the three proposed MaCh3D miniaturized specimens are compared to full-size ISO 6892 samples. Specimens were cut from AISI 304 stainless steel plates, with different thickness according to specimens requirements. Good accordance has been found between the standard and non-standard geometries.

The cohesive zone model (CZM) has found wide acceptance as a tool for the simulation of delamination in composites and debonding in bonded joints and various implementations of the cohesive zone model dedicated to fatigue problems have been proposed in the past decade. In previous works, the authors have developed a model based on cohesive zone to simulate the propagation of fatigue defects where damage acts on cohesive stiffness, with an initial (undamaged) stiffness representative of that of the entire thickness of an adhesive layer. In the case of a stiffness that is order of magnitude higher than the previous one (for instance, in the simulation of the ply-to-ply interface in composites), the model prediction becomes inaccurate. In this work, a new formulation of the model that overcomes this limitation is developed. Finite element simulations have been conducted on a mode I, constant bending (constant G)-loaded double cantilever beam (DCB) joint to assess the response of the new model with respect to the original one for varying initial stiffness K0 and cohesive strength &sigma, 0. The results showed that the modified model is robust with respect to changes of two orders of magnitude in initial stiffness and of a factor of two in &sigma, 0.

The work illustrates how building parameters of the Additive Manufacturing process Fused Filament Fabrication can affect not only the mechanical properties but also the surface wettability and morphology. Wettability and morphology are relevant factors in bonded joints performance. Advantages of polymeric additive manufacturing are to allow a re-design of components with locally controlled properties and integrated functions. Major limitations are related to the lack of material testing standardization and constraints due to the build volume and to the object orientation for printability: the latter problem scan be addressed with the use of bonded joints that allow to create bigger assemblies from smaller parts optimally designed to take advantage of material orthotropy and without the structural drawbacks. In this study, two materials are considered, acrylonitrile butadiene styrene and polylactideacid. Wettability, surface morphology and mechanical strength have been determined at different combinations of nozzle temperature, print speed and layer thickness.

Recent literature works focused on the analysis of laser irradiation on the strength of adhesive joints under quasi-static loading conditions. It has been demonstrated that laser surface preparation allows to remove impurity and weak boundary layers from the mating substrates and, depending on the energy density, it is also able to modify surface morphology promoting mechanical interlocking. In previous works, the authors assessed the effect of Yb-fiber laser ablation over the quasi-static strength and toughness, of aluminum and stainless steel adhesively bonded joints. The experimental results demonstrated the ability of laser irradiation to improve the mechanical properties of the joints. The aim of this work is to extend the scope of previous investigations to fatigue loading. Double Cantilever Beam (DCB) samples with laser treated aluminum substrates have been bonded with a two component epoxy adhesive. For comparison standard degreasing and grit blasting have been also deployed for samples preparation. The results have been compared in terms of cycles to failure and the fracture surfaces have been analyzed by means of Scanning Electron Microscopy (SEM) in order to investigate the mechanism of failure.

In previous works, the authors showed that the interleaving of an electrospun nylon nanofibrous mat at the interface between adjacent plies of a composite laminate increases the delamination strength. In particular, the nanomat acts a net-like reinforcing web, enabling a ply-to-ply bridging effect. This reinforcing property of the nanomats can be potentially used in other applications which need to improve the fracture resistance of interfaces, such as adhesive bonding. The present work analyses the feasibility of an electrospun polymeric nanomat as adhesive carrier and reinforcing web in industrial bonding. Thus the adhesive is used to pre-impregnate a nylon nanofibrous mat that is then placed at the interface between two metal pieces and then cured. The aim of the work is first to assess the effectiveness of this procedure, by comparison of the mode-I fracture toughness measured with DCB (Double Cantilever Beam) tests with and without the reinforcement in the adhesive layer. For this purpose, a 2024-T3 aluminum alloy will be bonded using a general purpose, one-part epoxy resin with low viscosity.

Adhesively bonded metallic and/or composite material structures are present as principal structural elements in several fields, such as aeronautics, automotive, nautical, and wind energy. A numerical method able to reproduce three-dimensional fatigue debonding and, in the case of composites, delamination evolution in bonded structures is therefore necessary to improve their performances. In this paper, a comparison of the performances of a cohesive zone model developed by some of the authors and the Virtual Crack Closure Technique (VCCT) embedded in the software Abaqus (v. 6.11) is made on mode I, mode II, and mixed-mode I/II loaded cracks in bonded assemblies.

The effect of Nylon 6,6 electrospun nanofibers interleaved in composite laminate material is simulated numerically to investigate the differences in the fracture strength between a nanomodified laminate interface and the corresponding non-modified material. DCB and ENF mechanical test results from a previous work of some of the authors are used as reference for the identification of cohesive zone model by numerical simulations with the finite element software Abaqus using implicit time integration. A bilinear damage law came out to be necessary to match the experimental behavior of the nanomodified interface, while the virgin material can be represented through a simple linear damage law. The necessity of using a bilinear damage law has been related to the crack bridging and to the obstacle to crack growth caused by nanofibers.

This study demonstrates the feasibility of printing 3D composite objects based on acrylic photocurable formulations, containing in situ generated silver nanoparticles (AgNPs). In fact, the laser radiation of a commercial stereolithography printer was used to both selectively cure, layer by layer, the acrylic resin and to reduce a silver salt to AgNPs (having dimensions ranging between 10 and 25 nm). The most suitable formulation was developed using silver acetate to obtain 1% by weight of AgNPs in the final 3D structures. The presence of the filler causes an increase in the physical and mechanical properties of the samples that become significantly stiffer and stronger than the pristine matrix. Antibacterial properties and electrical conductivity measurements performed on the printed samples gave promising results for the use of the developed formulation for the building of 3D polymeric structures with improved multifunctional properties.

In this work a cohesive zone model is developed to simulate fatigue crack propagation along bonded interfaces. The model is implemented by means of the USDFLD subroutine in the commercial software Abaqus. In particular, the crack growth increment is translated into a damage increment per cycle in the cohesive elements ahead of the crack tip. The computation of the strain energy release rate, not available in output for cohesive elements in Abaqus, was specifically implemented.

Le giunzioni incollate guadagnano sempre più mercato, nel campo delle costruzioni in genere, dove è necessario un alleggerimento delle strutture. Nel caso di geometrie di giunto semplici il dimensionamento avviene attraverso relazioni analitiche che restituiscono il valore massimo delle tensioni, il quale deve essere inferiore al limite di utilizzo dell’adesivo stesso. Quando le geometrie sono complesse l’approccio analitico diventa impraticabile, di conseguenza si provvede a verificare la correttezza della soluzione mediante analisi agli elementi finiti (EF). L’introduzione del modello di zona coesiva nell'analisi EF permette di simulare il danneggiamento ed il cedimento del giunto in condizioni quasi-statiche e impulsive. In questo articolo si vuole implementare il modello per la simulazione della propagazione a fatica di difetti, utilizzando il software agli elementi finiti ABAQUS assieme a subroutine esterne interagenti con il modello EF stesso. Un punto focale dell'implementazione sarà il calcolo automatico del tasso di rilascio di energia G in modo indipendente dalla geometria del difetto stesso. I parametri del modello ricavati da prove di tenacità a frattura e propagazione di difetti a fatica in modo I, saranno utilizzati come riferimento per la convalida dell'implementazione.

This work simulates numerically Double Cantilever Beam and End Notched Flexure experiments on Carbon Fibre Epoxy Resin specimens that have been performed by some of the authors in a previous work. Specimens have been nanomodified by interleaving plies with a layer of electrospun nanofibres in the delaminated interface. Eight different configurations of nanofibres have been used as interleave, for a total of 9 configurations (8 nanomodified plus the virgin one) to be simulated for both kind of tests to identify the cohesive zone parameters corresponding to the effect of nanofibre diameter, nanolayer thickness and nanofibre orientation on the delamination behaviour of the composite. Results showed that a bilinear damage law is necessary for almost all nanomodified configurations, and presented a clear relationship between nanomat layer parameters and the cohesive energy of the interface.

The present research is part of the HY-LAP (HYbrid LAP-joints) project carried out at the Dept. of Mechanical Engineering of Politecnico di Milano in collaboration with the MUSP laboratory of Piacenza and the University of Parma. In particular, the present study deals with the fatigue behavior of hybrid lap-joints (obtained by USMW plus adhesive bonding) made of light alloys thin sheets. Fatigue tests were carried out at different stress ratios (0.1, 0.3 and 0.7) finding that a different failure mechanism takes place with higher applied mean stresses. The S-N results then allowed to appreciate the better performance of the hybrid joints with respect to the application of USMW alone, while the direct comparison of the fatigue behaviors of AA6022T4 and AZ31B alloys allowed to draw some interesting conclusions.

Il lavoro prende spunto dai risultati di un’analisi sperimentale del comportamento a fatica di giunzioni incollate di materiali compositi laminati di elevato spessore formati da strati di unidirezionale e di tessuto di fibra di carbonio. I giunti sono stati realizzati in modo tale da saggiare l’influenza della lunghezza di sovrapposizione (da 25,4 mm a 110,8 mm), della forma del giunto (con e senza rastremazione), e della composizione degli aderendi (sostituzione di uno degli aderendi in composito con uno in acciaio). Mediante analisi 2D elastiche con il metodo degli elementi finiti sono state ricavate le distribuzioni degli sforzi all’interno dello strato di adesivo, al fine di individuare un parametro utile alla descrizione del comportamento a fatica in termini di sforzi locali - numero di cicli a rottura. Il ruolo della fase di propagazione viene discusso alla luce di osservazioni dell’avanzamento della frattura, condotta su alcuni dei giunti testati.

The cohesive zone model (CZM) has found a wide acceptance as a tool for the simulation of delamination in composites and debonding in bonded joints. Recently, fatigue-devoted implementations of CZM have been proposed. In earlier works, the authors have developed a model of the cohesive zone able to correctly simulate the propagation of fatigue defects in two- dimensional geometry. The procedure has been implemented in the finite element solver (Abaqus) by programming the appropriate software-embedded subroutines. Part of the procedure is devoted to the calculation of the strain energy release rate, G, necessary to know the growth of the defect. The model has been then extended to 3D cracks with quasi-straight crack front, where G could be evaluated by the contour-integral on parallel slices along the crack front. The aim of this work is to extend the cohesive zone model calculation of G using contour integral and the crack front evolution to 3D cracks with non-straight crack front.