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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.

In order to optimize the stiffness/weight ratio and to reduce the manufacturing time, Additive Manufacturing (AM) components are built up by a solid skin and a partially filled core. The filled/tot...

This work explores the influence of Additive Manufacturing (AM) parameters on the Mode I fracture toughness of polymer bonded joints. The motivation is that realizing AM parts to be later assembled...

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.

Recent trends in car body manufacturing indicate that the use of a consistent mix of lightweight materials in conjunction with adhesive bonding, not only allows substantial weight reduction, but can also drive down costs and potentially enables more intelligent designs. Yet, presently, it is highly desirable that the introduction of structural adhesives is made with minimum impact on the automotive body framing systems. Adhesives commonly deployed in car body production (i.e. body-in-white) are applied without any prior surface preparation and final curing is carried out during the baking process that follows the electrophoretic coating. Several structural adhesives have been tailored to the conditions dictated by the automotive production process. The aim of this work is to assess the mechanical properties of adhesive joints bonded with automotive grade hot setting epoxy adhesives employed for the assembly of frame and panels. Single-lap and T-joints were fabricated and adhesive hardening was carried out following the typical curing cycle employed in the process chain of automobile production. The resulting mechanical properties were assessed before and after exposure to a standard accelerated thermal cycling under controlled humidity. The evaluation of the mechanical behavior was done through a combination of testing and analysis, which included the assessment of fracture surfaces using optical microscopy to resolve the locus of failure.

Hybrid joints consist of mechanical connections in which both a traditional mechanical joint (i.e. riveting, clinching, welding) and adhesive bonding coexist in order to obtain a better performance...

This paper tries to relate the laser treatment process parameters to the morphological properties of laser-treated aluminum alloy surfaces and the adhesive joint fracture load in butt joint configuration to describe their pivotal role as surface roughness design parameters. In these contexts, the role of the individual process parameter and energy density on the surface and joint characteristics were experimentally investigated. Surface roughness was modeled and validated to demonstrate that it is designable and controllable. Moreover, to globally understand the effect of the laser parameters on joint fracture load, experimental results were analyzed following a DoE framework. This study shows that laser-treatment parameters contribute significantly to surface roughness control. Model-based surface morphology and the consequent average surface roughness can be accurately predicted, i.e., groove characteristics of the surface can be accurately controlled by regulating the laser-treatment parameters. Optimal laser parameters allow designing a surface that maximizes the adhesive joint fracture load.

Surface texturing produced by laser ablation is an efficient and effective technology for treating substrates to improve adhesive bond strength. In the literature, its effect has been extensively studied for different adherends materials, adhesives and laser sources. Laser ablation produces both morphological and chemical modifications of the surfaces, promoting mechanical interlocking and chemical bonding between the adhesive and the substrates. In this work, the effect of pulsed Yb-fiber laser ablation over the quasi-static mode I fracture energy of Double Cantilever Beam (DCB) aluminum bonded joints has been assessed for different combinations of processing parameters, with the aim of optimizing the treatment for industrial purposes. The mechanical tests show that the treatment becomes effective when a laser energy density threshold is overcome. On the other hand, a further increase in the energy density leads to a slight reduction of the joints fracture energy. This is related to the viscosity of the adhesive and to the high roughness produced by high energy treatments, resulting in the presence of air bubbles in the adhesive layer. In order to understand this phenomenon, the treated surfaces are characterized from the morphological point of view using a 3D optical profiler and SEM analysis.

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.

This work emphasizes the critical need of surface preparation to improve the performance of adhesively bonded 3D printed Acrylonitrile Butadiene Styrene (ABS) adherends obtained using Fused Filament Fabrication (FFF). In particular, mechanical abrasion and two different plasma pre-treatments, i.e., Atmospheric Pressure Plasma (APP) and Low-Pressure Plasma (LPP), were compared through the qualitative analysis of surface wettability, using static and dynamic contact angle measurements, and the quantitative evaluation of surface roughness measured using optical profilometry. In addition, mechanical tests were carried out using the single lap joint configuration and the interaction between the treated ABS substrates and three different adhesive materials, epoxy, polyurethane and modified silane, were carefully ascertained. The results indicated that the plasma process at low pressure (LPP) enabled a substantial decrease of contact angle (θ 80o). The improved wetting was accompanied by a relevant increase in the joint strength. The actual mechanism of fracture shifted from adhesive failure, typical of solvent cleaned and abraded surfaces, to full cohesive failure within either the substrates of the adhesive layer. Besides, a strong influence of the adhesive selected for manufacturing was reported. Indeed, using a tough bi-component epoxy adhesive in conjunction with the LPP pre-treatment led to substrate failure, representing the largest possible enhancement of joint strength. The shear strength was over 300% of that obtained with reference solvent cleaned, and 80% larger than that recorded on abraded surfaces.

In order to prevent the generation of a potential weakness at the substrate/adhesive interface of adhesively bonded joints, many methods were developed during the decades to pre-treat the substrates surfaces before the deposition of the adhesive. Experimental tests were carried out to simulate the real environmental conditions in which the joints have to work characterizing the mechanical properties of the joints caused by the exposition to high temperature, moisture, presence of chemical agents. The industrial need towards long-scale problem lead to the development of specific accelerated ageing methods able to induce in few weeks the same damage mechanisms within the joints which arise in years during their working life. In this work, different surface laser pre-treatments were studied with a focus on the influence that pre-treatments produce over the variation of the tensile failure load of Single Lap Joints (SLJ), previously subjected to different accelerated ageing cycles. Simple degreasing and grit blasting were also considered as reference treatments. The materials chosen for the manufacturing of the substrates were an aluminium alloy (AA 6082-T6) and a stainless steel (AISI 304). Three different accelerated ageing techniques were tested and compared to each other: (i) a cycle involving the simultaneous presence of high temperature gradient and moisture (method A), (ii) the immersion into an alkaline foam-forming cleanser (method B), and (iii) the immersion into an acid foam-forming cleanser (method C). The results showed that, while the method A did not significantly modify the mechanical strength of the joints, the method B and C resulted detrimental for the mechanical performance of the joints, even if their sensitivity to the tested pre-treatments was different. In particular, for both aluminum and stainless steel joints, it was noticed that the laser pre-treatment was able to reduce the loss of strength produced by the ageing process in comparison with the two reference pre-treatments. However, this result was also dependent of the specific value of energy density used for the laser ablation during the joints pre-treatment.

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.

Nanosecond pulsed laser texturing was performed on pigmented polyethylene (PE) specimens with a 1064 nm fiber laser with the aim of improving adhesive-bonded joint strength. A Design-of-Experiments (DoE) was employed to optimize process parameters and determine the effects of average laser power, scanning velocity and hatch spacing on the resulting failure load of PE joints bonded with Teroson 9399. Failure load increased with laser energy dose up to the onset of macroscopic melting. Laser scanning strategies minimizing heat accumulation yielded best results as the energy dose could be increased as much as possible prior to onset of melting. The process exhibited highest sensitivity to heat accumulation in the laser scanning direction, favoring large pulse separation distances in the scanning direction and moderate pulse overlap in the lateral direction. With a focused spot diameter of 60 μm and a repetition rate of 20 kHz, a maximum failure load of 0.93 kN (1.49 MPa, standard deviation 0.01 kN) was achieved with a crossing-line laser scanning strategy, 25 μm hatch spacing, 3 W average power and 2500 mm/s scanning velocity. Under these conditions, purely cohesive failure took place yielding a 79% improvement in failure load over standard primed joints.

The capability of the Atmospheric Pressure Plasma Treatment (APPT) to increase the shear strength of adhesively bonded Single Lap Joints (SLJ) realized with polymeric adherends was investigated, by exploring the different response obtainable by changing the surface-to-nozzle distance and the treatment speed and by considering an industrial application as target. Beyond APPT, abrasion and chemical treatments were also performed and considered as reference. Three thermoplastic resins were used as adherends: polypropylene (PP), polyethylene (PE) and polyamide 66 (PA66). In addition to the mechanical characterization, even the variation of the surface free energy associated with the different combinations of parameters employed for the APPT was evaluated by means of optical contact angle (OCA) measurements. Moreover, for some representative combinations of substrates and APPT parameters, the changes of the polymer chemical structure were assessed by means of infra-red spectroscopy (IR). Finally, the sensitivity of the mechanical behavior to the time between the treatment and the deposition of the adhesive was assessed. The results showed that, in a plasma treated joint, a decrease of the surface-to-nozzle distance assured however an increase of the shear strength with respect to the chemical treated samples, provided that the treatment speed was correspondingly risen up. The trend of the wettability with the process configuration appeared to be consistent with the one exhibited by the joint shear strength, while any apparent influence of the delay time was detectable within 24 h, which allowed to assume that APPT represents a reliable pre-treatment technique for industrial applications.

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.

The effect provided by the state of the surface on the quality of the adhesion, as well as the sensitivity of the position of the locus of failure to the surface morphology, is known to be one of the most crucial issue to be addressed when evaluating the capability of the bonded joint to withstand any mechanical stress. Therefore, the need for the substrates to undergo a pre-treatment before being bonded is to be considered. In this work, different pre-treatments were selected to be applied over aluminum and stainless steel adherents’ surfaces with the goal to produce single lap joints to undergo cyclic loading until complete failure. In particular, the experimental campaign aimed to correlate the morphology generated by the different surface pre-treatment (laser ablation, grit blasting and simple degreasing) with the quality of the fatigue performance, measured as the number of cycles to failure. Result of this research shows that the surface morphology generated by the laser ablation was able to reduce or avoid interfacial failures, leading to an increase of the fatigue performances if compared with grit blasted and degreased joints.

A reliable screening and non invasive method based on the use of microextraction by packed sorbent coupled with desorption electrospray ionization-high resolution mass spectrometry was developed and validated for the detection of new psychoactive substances in oral fluid. The role of different sample substrates in enhancing signal intensity and stability was evaluated by testing the performances of two polylactide-based materials, i.e. non-functionalized and functionalized with carbon nanoparticles, and a silica-based material compared to commercially available polytetrafluorethylene supports. The best results were achieved by using the non-functionalized polylactide substrates to efficiently ionize compounds in positive ionization mode, whereas the silica coating proved to be the best choice for operating in negative ionization mode. LLOQs in the low μg/L, a good precision with CV% always lower than 16% and RR% in the 83(±4)-120(±2)% range, proved the suitability of the developed method for the determination of the analytes in oral fluid. Finally, the method was applied for screening oral fluid samples for the presence of psychoactive substances during private parties, revealing mephedrone in only one sample out of 40 submitted to analysis.

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.

The cohesive zone model (CZM) has found a wide acceptance as a tool for the simulation of debonding in adhesive 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 of two-dimensional (2D) defects in joints. The procedure has been implemented in the finite-element software (Abaqus®) using software-embedded subroutines. That model was then extended to 3D cracks with quasi-straight crack front, where G could be evaluated by contour-integral on slices along the crack front.Inspiring to the approach used by Xie and Biggers in the case of virtual crack extension technique, the aim of this work is to extend the fatigue crack growth evaluation to 3D cracks of general shape. The new procedure has been compared with the previously developed one in the case of planar, 3D cracks showing a good agreement. The evolution of a corner crack is also shown as a qualitative ...

Purpose – In this work, the cohesive zone model (CZM) developed by some of the authors to simulate the propagation of fatigue defects in two dimensions is extended in order to simulate the propagation of defects in 3D. The paper aims to discuss this issue. Design/methodology/approach – The procedure has been implemented in the finite element (FE) solver (Abaqus) by programming the appropriate software-embedded subroutines. Part of the procedure is devoted to the calculation of the rate of energy release per unit, G, necessary to know the growth of the defect. Findings – The model was tested on different joint geometries, with different load conditions (pure mode I, mode II pure, mixed mode I/II) and the results of the analysis were compared with analytical solutions or virtual crack closure technique (VCCT). Originality/value – The possibility to simulate the growth of a crack without any re-meshing requirements and the relatively easy possibility to manipulate the constitutive law of the cohesive elements makes the CZM attractive also for the fatigue crack growth simulation. However, differently from VCCT, three-dimensional fatigue de-bonding/delamination with CZM is not yet state-of-art in FE softwares.

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.

Structural adhesives are widely used in industrial applications dealing with the problem of assembly and bonding. Despite the several advantages brought by this technique, one of the main issue is represented by the need of the surface to be mechanically and/or chemically treated with the aim to make it suitable for the adhesive deposition. Previous works demonstrate how the surface treatment by laser ablation seems to enhance the joint strength with respect to the untreated material. In particular, the effect of pulsed Yb-laser ablated aluminium surfaces over the mode I energy release rate of Double Cantilever Beam (DCB) specimens was investigated and a significant growth of the fracture toughness compared with the untreated and the grit blasted joints was observed. In this work, an investigation concerning the durability of the mechanical properties of aluminium joints treated with several representative parameters configurations was conduced. These setting configurations were used in an experimental campaign with the aim of verifying their suitability varying the type of the test (fatigue tests) and the environmental conditioning of the specimens (quasi-static tests after ageing cycles). Concerning the fatigue behavior, the ranking of the laser parameters configuration according to the increase of toughness with respect the degreased and the grit blasted samples seemed quite consistent with the results obtained in the quasi-static test campaign. When an accelerated ageing cycle in control of temperature and relative humidity was applied, a general lowering of toughness affected every tested specimen. This effect was however more marked in the grit blasted sample than in the laser treated ones. Therefore, a relative improvement of the mechanical performance when using some laser ablation configurations instead of the grit blasting, under the same conditions, when the adhesively bonded joints were aimed to undergo some critical environmental exposure, was recorded.

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.

The state of the surface plays a crucial role in defining the mechanical properties of bonded joints, in particular, the chemical state of the adherend surface and its structural morphology have be...

Hybrid joints in this article are simultaneous use of two different joining techniques, one of them being adhesive bonding. They, in principle, bring together the advantages of adhesive bonding and traditional joining techniques such as spot welding, riveting or clinching concerning static and fatigue strength, stiffness and energy absorption. Therefore, the joint may be mechanically “tailored” on the application depending on design goals and service conditions. The aim of this work is to evaluate to what extent, or under which conditions it is beneficial to use hybrid weld-, rivet- or clinch–bonded joints in comparison with simple adhesive, spot-welded, riveted or clinched joints. The experimental analysis was conducted using the Design of Experiments (DoE) methodology and the influence of the material, geometrical factors and environment on static strength, stiffness and energy absorption was assessed through the analysis of variance (ANOVA). Hybrid and simple joints were then compared in terms of mechanical response under the various conditions tested.

A fracture mechanics-based model for fatigue failure prediction of adhesive joints has been applied in this work. The model is based on the integration of the kinetic law of evolution of defects originated at stress concentrations within the joint. Final failure can be either brittle (fracture toughness-driven) or ductile (tensile/shear strength-driven) depending on the adhesive. The model has been validated against experiments conducted on single-lap shear joints bonded with a structural adhesive. Three different kinds of adhesives, namely a modified methacrylate, a one-part epoxy and a two-part epoxy supplied by Henkel, have been considered and three different overlap lengths have been tested. Fracture toughness and fatigue crack growth properties of the adhesives have been determined with mode I tests. The number of cycles to failure has been successfully predicted in several cases. It is interesting to notice that in the case of joints loaded at the same average shear stress, the shorter the joint, the longer the duration. This fact is also captured by the model.

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.