Additive manufacturing and joints: Design and methods

The industrialization of the Additive Manufacturing (AM) processes is enabling the use of AM components as final product in several applications. These processes are particularly relevant for manufacturing components with optimized custom-tailored geometries. However, to fully exploit the potentiality of AM, the development of knowledge aimed to produce dedicated design methods is needed. Indeed, even if AM enables the manufacturing of new kinds of structures, e.g. 3D lattice structures, it introduces process-specific design input and limitations that needs design methods different to from the ones for subtractive manufacturing. Design for AM (DfAM) is a design methodology that aims to take advantage of new buildable geometries but taking into account also AM processed materials anisotropy and 3D printing constraints. Recent literature focused on the assembly of AM components and on the AM components joining to a main structure. The conclusion was that adhesive bonding is a promising joining process, especially considering its improved stress distribution compared to fastening, but at the time of writing a method that combines DfAM and adhesive bonding knowledge is not available. The work presented in this thesis focused on developing knowledge on design for AM and bonded joints. First step was evaluating testing methods for AM and producing data on materials properties. Secondly, the early works on tailoring approaches for AM joints, published recently in scientific literature, were analyzed. Then AM dedicated designs, modifications and testing methods were proposed both for the adherends (in the thickness and on the surfaces) and the joints. Specifically, an innovative joint design concept was introduced, i.e. using the 3D printing parameters as bonded joint design factors. Eventually, feasibility of performing joints using multi-material AM with conductive polymer to embed heating elements was assessed. The 3D printed through the thickness circuits is a cutting-edge approach to enable new solutions for joints structural monitoring and self-healing

An improved model to describe the repeated loading-unloading in compression of cellular materials

Cellular materials, often referred as foams or structural foams when used for energy absorption, are largely used to protect people and goods in the case of shocks and impacts. The detailed knowledge of their behavior is fundamental to design components for this aim. Peroni et al. (2008)-(2009) proposed a model able to describe the mechanical compression behavior of some polymeric material. Such model, based on the work by Rusch (1970), described the stress-strain curve as a sum of two contributions, the first for the elastic part and the second for the densification. More recently Avalle and Belingardi (2018) presented a more general model where the stress is calculated as a sum of three terms, one for the elasto-plastic phase, the second for the plateau, and a third for the densification. The model could include effects like density and strain-rate. However, those models allow to describe only the monotonic compression behavior: in several situations repeated impacts can happen with unloading followed by further reloading. Unfortunately unloading cannot be described by a linear relation between stress and strain (as is usually considered for metals). Unloading follows a non-linear law with a variable relation between stress and strain in the successive cycles: this requires a particularly complex model. In this work, a new model able to effectively reproduce the compression behavior of some polymeric cellular materials is presented. The model is validated and tuned on the basis of experimental tests with specimen subject to complex cycles of repeated loading and unloading. The model describes both the loading from different levels of residual compression and unloading from any value of compression level. The application to several materials justifies the generality of the method

Fatigue strength of plastics components made in additive manufacturing: first experimental results

Evolution of additive manufacturing (AM) techniques is making these innovative technologies more and more available and known to a larger audience. This allowed components built with AM techniques, especially metallic ones, to be effective in substituting similar components made with traditional technologies; with all the advantages of AM that make these components even more interesting in terms of performance. With plastics this process is relented also due to the chronic lack of established knowledge of the plastic materials, both in terms of strength, design criteria, both in long term behavior but also in static short-term properties. This work tries to give some useful information about the fatigue behavior of one class of material widely used with the mostly widespread AM technique for plastics, that is filament deposition modeling (FDM). The material considered is acrylonitrilebutadiene-styrene (ABS), used in countless components (electronic devices, household appliances, medical tools, and others) due to its excellent mechanical performances and relatively good workability. The property mainly analyzed in this work is fatigue behavior. Fatigue tests were performed in plane bending on specimen very similar to the type proposed and used by Nicoletto (2018) in different manufacturing and loading conditions. The obtained results offer an interesting insight into the properties of small components in ABS made by FDM and the effects of some influencing parameters: different stress-ratios were considered, as well as technological variations such as deposition direction. Experiments reveal that the scatter of fatigue data, even with the manufacturing uncertainties and defects typical of AM, can be controlled and within reasonable limits

The use of low pressure plasma surface modification for bonded joints to assembly a robotic gripper designed to be additive manufactured

The paper explores how different surface preparations modify the mechanical performance of bonded joints on components made in acrylonitrile butadiene styrene (ABS) processed by fused filament fabrication (FFF) additive manufacturing. Two alternative treatments are considered: surface abrasion compliant to the standard ASTM D2093-03 (17) and using low pressure plasma, an innovative solution. The assessment is performed on standard lap shear test specimens and structural epoxy adhesive. The bonding layer with abraded surfaces shows adhesive failure while after the low-pressure plasma treatment shows adherends failure. As case of study the bonding solution to perform the assembly is considered a jaw finger of a robotic gripper for the picking of garments from a table. The redesign of the finger availing of the performance of bonding with the new plasma treatment is proposed and discussed. Experimental testing assessed the feasibility of this innovative technical solution

Choice of the stress integration scheme for accurate large-deformation finite element analysis

The use of computational structural models that include geometrical non-linearity in many application cases may require high reliability in prediction of displacements. Nevertheless, large differences up to 60% on maximum total displacement have been found among results of static large-deformation analyses performed by means of the major commercial software packages in a simple benchmark study with linear material properties. In order to investigate the causes of such disagreement, the present work compares different finite element formulations including well-established stress update schemes. The various formulations are tested, and results are compared in three test cases. Rodriguez stress update algorithms have shown the best performance among methods reported in literature. Finally, the cause of the large differences found in the predictions of commercial codes is identified. It is linked to the energetic inconsistency of some stress update methods in the simulation of extension/compression loading conditions. Such inaccuracy is reproduced analytically by formulating and integrating the corresponding inconsistent constitutive equations. The identified problem is very important for designers, as it affects almost all the static simulations, which are the most common type of largedeformation analyses and usually involve extension/compression loading

Modeling the strength of laminated parts made by fused filament fabrication additive manufacturing

Fused filament fabrication (FFF, also known as fused deposition modeling) is the most popular 3D printing additive manufacturing technology: cheap 3D printers are largely widespread and most polymeric materials can be used. It is probably the most versatile additive manufacturing technology. The applications range from prototyping to the production of custom parts with structural capabilities: fiber reinforced plastics can also be used. However, the knowledge of the FFF materials and the design criteria for fused filament fabricated parts are scarcely known and this results in a very common skepticism in adopting this technology for technical structural components. As for the other processing technologies for plastics, included injection molding, the mechanical properties of the components strongly depend on the manufacturing parameters. As shown by some authors, since production by FFF produces a layered structure, it is possible to model the mechanical behavior by means of the classical lamination theory largely verified and known for composites. In those papers, it was shown that the equivalent elastic properties could be predicted as a function of the lamination stacking and angles. In this paper, based on experimental results obtained on symmetric balanced angle-ply laminated samples made of polyethylene terephthalate with added glycol (PETG) and polyamide (PA) subject to tensile loads, the Tsai-Hill criterion was applied to predict the strength. After identification of the strength parameters, a good correlation of the experimental with the model parameters, for all lamination angles, was obtained

Development of a gripper for garment handling designed for additive manufacturing

The paper presents how a robotic gripper specific for grasping and handling of textiles and soft flexible layers can be miniaturized and improved by polymeric additive manufacturing-oriented re-design. Advantages of polymeric additive manufacturing are to allow a re-design of components with integrated functions, to be cost-effective equipment for small batches production and the availability of suitable materials for many applications. The drawback is that for design validation extended testing is still necessary because of lacks in standardization and that the mechanical properties are building parameters dependent. The outcomes are a lower complexity of the design overall and lower number of components. These are pursued taking advantage of the anisotropy of the additive manufacturing processed polymer and assigning appropriate shapes and linkages in the mechanisms. Set of common materials (polylactide, polyethylene terephthalate, acrylonitrile butadiene styrene) and technical (acrylonitrile styrene acrylate, polycarbonate/polybutylene terephthalate blend) are tested to obtain data for the modelling

Development of a wearable device for the early diagnosis of neurodegenerative diseases

The progressive evolution of information and sensing technologies is giving pulse to the development of wearable mobile devices in search of life quality improvement. A relevant field of application is healthcare, with the development of wireless unobtrusive wearable solutions for the continuous remote health monitoring of patients. These wearable devices are particularly important for neurodegenerative diseases due to the possibility of early stage diagnoses through continuous monitoring to collect earlier significant data. Discovering specific symptoms and early defining medical treatments can delay, if not stop, the pathology progress whereas, once major symptoms like restricted or impaired mobility has appeared, the patients already underwent relevant and irreversible brain damage. The aim of this work is to show the development of the Neuroglass, a wearable smart glasses device for early stage diagnosis and monitoring of Parkinsonian-type neurodegenerative diseases. The designed frame is compliant to the standards and in order to embed the sensors to collect data from head and eyes movements since one of the early symptoms of Parkinson's disease has proven to be eye tremors. Preliminary laboratory tests, e.g. head accelerations measurements for different body movements, were carried out in order to choose properly the characteristics and positioning of the sensors; afterwards the device\u2019s frame was designed by means of a 3D parametric CAD and built by additive manufacturing. The design was validated by first experimental test on monitoring eye movements and blinks

ADDITIVE MANUFACTURING PROCESS PARAMETER INFLUENCE ON MECHANICAL STRENGTH OF ADHESIVE JOINTS, PRELIMINARY ACTIVITIES

open7The work illustrates how building parameters of the Additive Manufacturing (AM) process fused filament fabrication can affect not only the mechanical properties [1] but also the surface wettability and morphology. Wettability and morphology are relevant factors in bonded joints performance [2]. Advantages of polymeric AM are to allow a re-design of components with locally controlled properties [3] 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 can be addressed with the use of bonded joints that allow to create bigger assemblies from smaller parts optimally designed to take advantage of the anisotropy of the material and without the structural drawbacks due to other joining method, such as stress concentration in bolted joints. As for the Mechanical properties, they are obtained with uniaxial tensile tests using MaCh3D [4], an innovative cost effective solution for materials testing. The as built surface properties are investigated quantitatively and qualitatively using a plate specimen of 15 × 15 × 1.2 mm. Roughness parameters are measured by surface scanning with a CCI Taylor-Hobson 3D optical profilometer while contact angle values between specimens and a drop of Milli-Q water are measured in order to evaluate wettability. Different materials, such as ABS and PLA, are characterized at different combinations of nozzle temperature, print speed and layer thickness. The analysis of the collected data provide information on how building parameters can modify two fundamental aspects in adhesive joints such as surface roughness and wettability in order to maximize joint performance.openM. Frascio, L. Bergonzi, F. Moroni, A. Pirondi, M. Avalle, M. Monti, M. VettorFrascio, M.; Bergonzi, L.; Moroni, F.; Pirondi, A.; Avalle, M.; Monti, M.; Vettor, M

Review of Tailoring Methods for Joints with Additively Manufactured Adherends and Adhesives

This review aims to assess the current modelling and experimental achievements in the design for additive manufacturing of bonded joints, providing a summary of the current state of the art. To limit its scope, the document is focused only on polymeric additive manufacturing processes. As a result, this review paper contains a structured collection of the tailoring methods adopted for additively manufactured adherends and adhesives with the aim of maximizing bonded joint performance. The intent is, setting the state of the art, to produce an overview useful to identify the new opportunities provided by recent progresses in the design for additive manufacturing, additive manufacturing processes and materials’ developments