102,103 research outputs found
Dynamic behaviour of polyolefin thermoplastic hot melt adhesive under impact loading conditions
No full textDynamic behavior of polyolefin thermoplastic hot melt adhesive under impact loading conditions
R. Ciardiello1, A. Tridello1, G. Belingardi1, L. Goglio1.
1 Politecnico di Torino, Department of Mechanical and Aerospace Engineering, Torino, 10129, IT.
The mechanical behaviour of adhesive joints under impact loadings is an active area of research due to significant industrial interests. Furthermore, the absence of a unique adopted standard for the study of bonded joints under impact loading increases the academic interests for this topic [1]. In this work, the static and the dynamic response of adhesive joints, bonded with a polyolefin hot-melt adhesive (HMA), were investigated by means of Single Lap Joint (SLJ) tests. The adhesive studied in this work is used in automotive application for bonding plastic internal and external plastic components [2], such as plastic bumpers that can be subjected to impacts during its life. The mechanical and thermal properties of this adhesive are presented in [3]. The main aim of this study is to test standard specimens, SLJ, under dynamic impacts with the use of a modified Charpy pendulum in order to compare the differences between static and dynamic behaviour. The substrate used in this activity are made of a polypropylene copolymer with 10% in weight of talc. Figure 1 shows the testing machine with the clamping system of the specimen. These special fixtures were designed by Goglio et al. [4] with the aim to apply a dynamic load on the tested SLJ. The specimen is fixed to the hammer at the front end, as shown in the right part of Figure 1; the back end is connected to a transverse tail, which hits the two stoppers fixed on the pendulum base, shown in the red circle of Figure 1. The fixtures hold the specimen during the fall of the hammer and transmit the load. A tail in aluminium alloy with T cross section was used, in order to guarantee a high stiffness during the impact, without adding excessive inertia to the system. The system is able to perform dynamic tests for SLJ specimens up to 3.75 m/s.
Figure 1: Charpy pendulum used for the experimental tests.
Mechanical tests show that there is a clear influence of the load rate on force-displacement diagram and on the maximum force for the tested adhesive. Figure 2 illustrates the differences between a representative curve of quasi-static test and dynamic tests with two different velocities.
Figure 2: Force vs linear displacement: comparison between quasi-static and dynamic tests.
Figure 3 shows the average values of the peak force and absorbed energies. This Figure illustrates that the velocity increase leads to an increase of the maximum force while the adsorbed energy significantly decreases by comparing quasi-static and dynamic tests.
Figure 3: Peak loads and absorbed energy of the quasi-static and dynamic tests.
Finally, the fracture surfaces of the SLJ specimens were assessed by means of visual inspection. This analysis showed that the joint separation in the quasi-static tests is mostly cohesive, whereas it becomes completely adhesive in dynamic tests.
[1] J.J.M. Machado, E.A.S. Marques and L.F.M. da Silva, J. Adhes., (2017). https://doi.org/10.1080/00218464.2017.1282349.
[2] G. Belingardi, V. Brunella, B. Martorana and R. Ciardiello, in Adhesives applications and properties, Cap.13, p.341, A. Rudawska Ed. (INTECH, Rijeca, 2016).
[3] E. Koricho, E. Verna, G. Belingardi, B. Martorana, and V. Brunella, Int. J. Adhes. Adhes. 68, 169–181 (2016).
[4] L. Goglio and M. Rossetto, in Proceedings of ESDA2006 8th Biennial ASME Conference on Engineering Systems Design and Analysis, 637-643 (2006)
Adhesives in Very High Cycle Fatigue testing
No full textThe versatility of adhesive bonding creates new opportunities also for the experimental activities. One example is Very High Cycle Fatigue (lives up to 1E+10 cycles) [1] testing, in which the axial vibration generated by a piezoelectric transducer in the range of kHz is applied to the specimen by means of a horn-shaped adapter, which increases the vibration amplitude. Continuity between horn and specimen is ensured by a butt joint, which must be both capable to transmit the motion without disturbing wave propagation and suitable of being assembled/disassembled easily each time the specimen is replaced. In the experimental campaigns carried out in Politecnico over the last years [2], this goal was successfully accomplished by means of adhesive bonding. Cyanoacrylate was chosen to minimize the thickness of the adhesive layer and ease the bonding/debonding procedures. As the strength of the metal specimen is more than one order of magnitude higher than that of the adhesive, to prevent failure in the joint the key issue was to ensure that the joint section represents a maximum in the displacement mode shape and, therefore, a node (i.e. nil value) in the stress mode shape. To do so, a detailed finite element analysis was carried out to tune the horn-specimen setup. Indeed, no failure in the joints occurred during the tests, although the alternating stresses in the specimens were in excess of 500 MPa. In a different perspective, a controlled deviation from the above mentioned condition can represent a technique to test the adhesive under fatigue: by offsetting the bond from the node of the stress mode shape, the adhesive undergoes a stress cycle. As the general stress level is high, the offset must be controlled precisely. Again, the problem was investigated by finite element modelling, which enabled to assess the desired condition and identify the material parameters (elastic constants, damping) governing the case. References [1] C. Bathias, P.C. Paris. Gigacycle fatigue in mechanical practice (CRC Dekker, New York, 2005). [2] D.S. Paolino, A. Tridello, G. Chiandussi, M. Rossetto. Fatigue Fract. Eng. Mater. Struct. 37:570-579 (2014)
Studio delle gelate tardive e precoci verificatesi nella pianura veneta nel periodo 1992-2001
No full textStudio della siccità in Veneto negli anni 1961-2004: SPI (Standardized Precipitation Index).
No full textAn innovative testing technique for assessing the VHCF response of adhesively bonded joints
No full textIn the last few years, the use of adhesive joints for structural applications hasrapidly increased and adhesives are more often subject to fatigue loads during their in‐service life. In presence of a rapidly varying load, such as a high‐frequency vibration, adhesively bonded joints may undergo fatigue lives in the Very High Cycle Fatigue (VHCF) region that are significantly larger than those investigated in usual high‐cycle fatigue tests. The present paper proposes an innovative testing technique for performing accelerated fully reversed tension‐compression VHCF tests on adhesive butt‐joints. The procedure for the design of the adherends is described and then experimentally validated.Ultrasonic VHCF tests are finally carried out on a cyanoacrylate butt‐joint upto 10^9 cycles: experimental results show that the proposed testing equipment permits an effective assessment of the VHCF response of the adhesive in a limited testing time
WeedTurf: software for improving summer annual weeds control in turf
No full textSummer annual grass species are problem weeds in turf. Understanding their emergence dynamics could be useful for timing herbicide treatments and maximizing their efficacy. With this aim, a prediction model was developed that can simulate
the emergence of four summer annual grass weeds in turf (Digitaria sanguinalis, Setaria glauca, Setaria viridis and Eleusine indica). By using weather data and soil characteristics, the model provides the percentage of emergence reached by a certain weed species at any given time. This information can provide enhanced recommendations for programming application times of pre- and post-emergence herbicides. This paper presents the prototype of the software that implements the model. The software is designed to be intuitive and easy to use even by inexpert users, and is a modular and flexible system that can modify and extend model application to other species according to user requests
Residual properties in damaged laminated composites through nondestructive testing: A review
No full textThe development of damage tolerance strategies in the design of composite structures constitutes a major challenge for the widespread application of composite materials. Damage tolerance approaches require a proper combination of material behavior description and nondestructive techniques. In contrast to metals, strength degradation approaches, i.e., the residual strength in pres-ence of cracks, are not straightforwardly enforceable in composites. The nonhomogeneous nature of such materials gives rise to several failure mechanisms and, therefore, the definition of an ulti-mate load carrying capacity is ambiguous. Nondestructive techniques are thus increasingly re-quired, where the damage severity is quantified not only in terms of damage extension, but also in terms of material response of the damaged region. Based on different approaches, many nonde-structive techniques have been proposed in the literature, which are able to provide a quantitative description of the material state. In the present paper, a review of such nondestructive techniques for laminated composites is presented. The main objective is to analyze the damage indexes related to each method and to point out their significance with respect to the residual mechanical perfor-mances, as a result of the working principle of each retained technique. A possible guide for future research on this subject is thus outlined
Influence of testing velocity on mechanical strength and failure modes of single-lap joints made with hot-melt adhesive and polyolefin substrates
No full textThe mechanical behaviour of adhesive joints under dynamic loadings is an active area of research due to their significant industrial applications. Furthermore, the absence of a unique adopted standard for the study of bonded joints under impact loading increases the scientific and practical interests in this topic. In this work, the static and the dynamic responses of adhesive joints, bonded with a polyolefin hot-melt adhesive (HMA), were experimentally investigated by means of Single Lap Joint (SLJ) specimens, tested using a tensile machine and an instrumented pendulum. The substrates used in this activity were made of a polypropylene copolymer with 10% in weight of talc. In the last decades, HMAs have been used in automotive application for bonding internal and external plastic components. Indeed, HMAs are capable to bond several kinds of materials, including plastics that are difficult to bond with other adhesives. The results of the mechanical tests show that there is a clear influence of the loading rate on the force-displacement diagram and on the maximum force for the tested adhesive. Failure modes were finally analysed and compared. A change in the failure mode was noticed: in quasi-static tests the SLJs failed cohesively within the adhesive, whereas in dynamic tests the SLJs failed adhesively, with low energy absorption
Topology and fibre orientation simultaneous optimisation: A design methodology for fibre-reinforced composite components
No full textAdditive manufacturing for fibre-reinforced composite structures is rapidly diffusing, since it enables the production of lightweight structural parts characterized by complex geometries and tailored fibre orientations. Therefore, the development of design methodologies capable to simultaneously optimize the shape of the fibre-reinforced composite part and the fibre orientation in the additive manufacturing process is, at present, of utmost interest among industries and research centres. In this paper, a novel simultaneous optimisation method capable to optimise the topology and the local fibre orientation is proposed. The method is computationally cheap, fast convergent and permits to avoid stress peaks, working efficiently on 2D and on 3D models. The analytical formulation of the problem and the optimisation algorithm are at first described. The optimisation criteria are based on the uniform strain energy density distribution and the fibre alignement along the principal stress direction. The proposed method is then verified with several benchmarks from the literature and with a 3D illustrative example, confirming that it can be effectively and efficiently employed for the optimisation of composite components to be produced through additive manufacturing
A new methodology for thermostructural topology optimization: Analytical definition and validation
No full textIn the last few years, the rapid diffusion of components produced through additive manufacturing processes has boosted the research on design methodologies based on topology optimization algorithms. Structural topology optimization is largely employed since it permits to minimize the component weight and maximize its stiffness and, accordingly, optimize its resistance under structural loads. On the other hand, thermal topology optimization has been less investigated, even if in many applications, such as turbine blades, engines, heat exchangers, thermal loads have a crucial impact. Currently, structural and thermal optimizations are mainly considered separately, despite the fact that they are both present and coupled in components in service condition. In the present paper, a novel methodology capable of defining the optimized structure under simultaneous thermomechanical constraints is proposed. The mathematical formulation behind the optimization algorithm is reported. The proposed methodology is finally validated on literature benchmarks and on a real component, confirming that it permits to define the topology, which presents the maximized thermal and mechanical performance
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