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Influence of the laser ablation surface pre-treatment over the ageing resistance of metallic adhesively bonded joints
Full text linkIn 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
A Knowledge-Based Approach for a CAD-Embedded Tool Preventing Hygienic Design Issues in Food Processing Machinery
No full textHygienic design concerns the need for making food processing machines and plants compliant with a series of specific requirements aimed at preventing bacteria contamination of food products. The sources available in literature for hygienic design guidelines are mainly represented by guidelines provided by specific organizations (e.g.: EHEDG in Europe and 3A Sanitary Standards in US), but a structured database and an engineering tool allowing for a quick reference by the designer is still missing. The aim of this paper is to illustrate the preliminary work aimed at developing a CAD-based tool able to analyze CAD models of machines and plants and to automatically recognize those features which break some hygienic design rules, allowing therefore the possibility to detect and fix them in an early stage of the design process. This work started from the collection of several guidelines, considering both the literature sources and also the hygienic design knowledge developed within companies. The acquired knowledge is then stored into a database and structured consistently with a specific syntax according to a selected ontology. A case study with two practical examples is exploited to validate the method. Results show the usefulness of the approach to avoid the need for several design reviews and the benefit that a tool implementing this knowledge-based system could bring to the designer
Experimental Study of the Influence of the Surface Preparation on the Fatigue Behavior of Polyamide Single Lap Joints
No full textThe 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
Influence of high frequency on the fatigue life of metallic single lap joints
Full text linkPolymer adhesives are known to exhibit time dependent mechanical behaviour, that becomes more and more evident approaching the glass transition temperature, Tg. When subjected to fatigue, bonded joint lifetime can be therefore affected by the loading frequency and amplitude. According to the literature, the influence of the frequency on the fatigue behavior manifests as a rise of the temperature in the adhesive due to the hysteretic heating, which is related in turn to Tg and to the viscoelastic properties of the adhesive. In this paper, cyclic loading was applied to single lap shear joints until failure at frequencies in the range of 70-90 Hz, that are little explored by the existing literature. The aim is twofold: on one side, to investigate the possibility to speed up fatigue tests in comparison to tests performed in the range of 7-9 Hz; on the other side, to explore the possibility to generate stress-life data in a range of number of cycles between 106 and 108, therefore two order of magnitude higher then usual, without exceeding with the time required for the tests. The joints were made with both aluminum and stainless-steel substrates bonded with a structural epoxy adhesive, to manufacture a Single Lap Joint (SLJ). Two loading ratios R (respectively 0.1 and 0.4, defined as the minimum over the maximum force of the fatigue cycle) and different load ranges were applied. Digital Image Correlation (DIC) technique was used to assess the presence of creep strains. A preliminary investigation consisting in a Dynamic Mechanical Analysis (DMA) of bulk adhesive for the range of frequency of interest, and in the measurement of the temperature of the adhesive layer during some selected fatigue tests was carried out. No significant changes of the viscoelastic properties of the adhesive were found for the frequency of interest, and, at the same time, temperature mea-surements revealed that the temperature increased by a few degrees, remaining in any case far from the glass transition temperature. The test results showed that only small effect due to the application of a high frequency cyclic loading on the fatigue life apparently occurs for tests carried out at the lowest load ratio, in particular when the applied loads were relatively low, and the number of cycles at failure relatively high. On the opposite, the results of tests carried out at the highest load ratio are affected by the loading frequency, and it has been related to the presence of significant creep deformation within the adhesive layer
Influence of laser ablation-induced surface topology on the mechanical behaviour of aluminium bonded joints
No full textEffect of laser patterned microscale interlocking features on aluminum adhesive-bonded single lap joints
No full textMany attempts have been made to enhance the strength or toughness of adhesive-bonded joints by fabricating macroscopic interlocking features on adherends or resorting to 3D-printing to produce microscale surface structures. Laser ablation has recently been proposed as a method to manufacture serrated surface profiles on aluminum single lap joints with the aim of improving strength; however, increases have so far been limited due to joint failure resulting from peeling phenomena at the end of the overlap, making enhancements in mechanical interlocking ineffective in the loading direction. The present work seeks to overcome these limitations, increasing the role of interlocking in strengthening laser patterned joints by employing deeper tooth-like features to increase geometric interlocking and thicker substrates to minimize the effect of bending-induced peeling on the initiation and development of failure. Several different patterns have been produced and tested, including both symmetrical (identical adherends) and asymmetrical (complementary adherends) with different feature orientations with respect to the loading direction. The results have revealed that, on single lap joints with thicker adherends, by using deeper asymmetrical features, the strength can be increased by 40 % compared to that obtained with flat adherends
Effect of the surface morphology over the fatigue performance of metallic single lap-shear joints
Full text linkTowards computer-aided hygienic design: Definition of a knowledge-based system for food processing equipment
Full text linkHygienic design requires the definition of rules allowing the correct development of food processing systems. The knowledge collection in this field would certainly help designers and engineers in developing hygienic-compliant systems. This paper aims to provide a knowledge-based (KB) system for gathering hygienic design guidelines for the design of food processing machinery and equipment. The KB system is based on a specific ontology that has been used to collect 78 hygienic design rules from different sources. The rules repository can be considered a backbone for the subsequent development of a CAD-based tool for an automatic search and detection of non-compliant design features. Starting with a CAD model, the KB system was used to check the compliance of a fish stick production machinery. Results highlight how the adoption of the KB system in the early design phase would anticipate hygienic design issues avoiding several design reviews
Influence of atmospheric pressure plasma process parameters on the mechanical behavior of thermoplastic joints
No full textThe 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
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