1,721,029 research outputs found
The effect of bond-line thickness on fatigue crack growth rate in adhesively bonded joints
No full textThe effect of adhesive thickness on fatigue crack growth in an epoxy film adhesive (FM94) was investigated, using a combination of experiments and numerical modelling. For the range of thicknesses investigated an increased thickness led to an increased crack growth rate. It was found that the energy required per unit of crack growth did not depend on the adhesive thickness. In contrast, the energy available for crack growth does depend on the adhesive thickness. The numerical analysis confirms that the energy required per unit crack growth is not sensitive to the adhesive thickness, but that the plastic energy dissipation increases with the thickness. The experimental results imply that this increase of plasticity has an anti-shielding effect, as the crack growth rate is increased.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Structural Integrity & Composite
Preliminary tests on additive-manufactured Al-Sc specimens for the setup of a numerical model for Laser Shock Peening
Full text linkAluminum-Scandium alloys offer a great potential in aerospace applications due their high corrosion resistance and improved strength properties. Furthermore, these alloys have been quali ed for laser additive manufacturing (AM), producing parts with static strengths rivalling their conventionally manufactured counterparts. However, laser processing also results in large residual stresses that can severely affect fatigue properties and result in geometric
distortion. A proven method for reducing the fatigue-related problems in metallic structures is to drive compressive residual stresses into the affected area by means of Laser Shock Peening
(LSP). This surface treatment is very effective in bulk structures, improving life performances of fatigue-sensitive aeronautical components, such as jet engines turbine blades or helicopter
gearboxes. On the other hand, quite a limited number of studies has been presented on the effect of LSP on fatigue crack growth in thin components and laser AM structures. This work presents first the results of preliminary tensile tests on additive manufactured Al-Sc specimens. The tensile strengths of as-built and heat-treated samples are compared. Then, a reliable and computationally time-effective numerical model of laser peening is reviewed, referring to
case studies investigated earlier. In view of applying LSP to additive manufactured Al-Sc components, the effects of different laser parameters and geometries are discussed. Finally, the possible drawbacks of the LSP treatment are addressed, in order to exploit its full potential in increasing the fatigue life of AM components
The microsatellite research program at Universita' di Bologna
No full textIn the Second Faculty of Engineering of the University of Bologna, the Aerospace group in Forlì has started a new microsatellite research program. The first step consists of the design and setup of an amateur radio ground station, recently installed and implemented in the University laboratories. At the same time, researchers, PhD and graduate students are directly involved in the satellite design. The microsatellite weighs about 20kg and consists of a cubical prism, 300mm side, with a modular structure made of six shop-machined Al trays, kept together by eight stainless steel bars. Four Al/Al honeycomb sandwich lateral panels, which are the support structure of glued solar panels, complete the satellite structure. This architecture has been selected in order to have a multi-purpose bus, to be used with minor changes for several missions, accommodating payloads with different volume and power requirements. This paper reports on the current status of the ground station implementation and of the microsatellite bus design and manufacturing
A general formulation of the potential energy release rate for a three-dimensional hyperelastic body containing a plane crack
No full textIn this paper a general expression of the potential energy release rate G for a three-dimensional fracture mechanics problem is supplied. Under the hypothesis of a quasi-static growth phenomenon, the distribution of the vector a, i.e. the velocity field of the fracture propagation, is assumed unknown along the crack front. This assumption leads to a general formula of G for a three-dimensional hyperelastic body containing a plane crack. Moreover, imposing a stationary condition of G with respect to a, the analytical problem of the crack front shape evaluation is formulated. A unique solution exists for the problem, which is described by a system of two non-linear equations. Practical applications of the theory can be obtained by the use of finite element analysis results, together with a numerical solution of the two equations in the unknown components of the fracture propagation velocity
Una formulazione analitica per il calcolo quasi-statico della geometria del fronte di una fessura piana
No full textIn questo lavoro viene fornita una espressione generale della velocità di rilascio della energia potenziale, indicata
con il simbolo G , relativa ad un problema di meccanica della frattura elastica tridimensionale: tale espressione
viene ricavata ipotizzando che il fenomeno della propagazione di una fessura piana sia quasi-statico e assumendo
incognita la distribuzione della velocità di avanzamento del fronte di frattura che viene descritta da un campo
vettoriale indicato con il simbolo a. Le assunzioni fatte portano ad una formula generale di G , valida per un
corpo fessurato tridimensionale iperelastico, che viene così a dipendere esplicitamente da a e dal suo gradiente
spaziale. Sulla base di questo primo risultato, imponendo una condizione di stazionarietà di G rispetto al vettore
a, viene formulato il problema analitico per il calcolo della evoluzione della forma del fronte di una frattura piana
durante la fase dipropagazione. Nel lavoro viene dimostrato che, nell’ambito della validità delle ipotesi fatte,
esiste una sola soluzione del problema, cioè, un unico campo vettoriale a che può essere calcolato risolvendo un
sistema di due equazioni non lineari. Le applicazioni pratiche della teoria esposta possono essere ottenute
mediante la messa a punto di opportune analisi agli elementi finiti e ricavando numericamente la soluzione del
sistema delle due equazioni che descrivono l’evoluzione del fronte di frattura
A General Formulation of the Potential Energy Release Rate for a Three Dimesional Hyperelastic Body containing a Plane Crack
Full text linkIn this paper a general expression of the potential energy release rate G for a three-dimensional fracture mechanics problem is supplied. Under the hypothesis of a quasi-static growth phenomenon, the distribution of the vector a, i.e. the velocity field of the fracture propagation, is assumed unknown along the crack front. This assumption leads to a general formula of G for a three-dimensional hyperelastic body containing a plane crack. Moreover, imposing a stationary condition of G with respect to a, the analytical problem of the crack front shape evaluation is formulated. A unique solution exists for the problem, which is described by a system of two non-linear equations. Practical applications of the theory can be obtained by the use of finite element analysis results, together with a numerical solution of the two equations in the unknown components of the fracture propagation velocity
Closed form solutions for stress intensity factors in an MSD panel with a review of different approaches to the problem
No full textCalcolo del fattore di intensita' degli sforzi in pannelli irrigiditi in presenza di danneggiamento multiplo
No full textInfluence of thermal cycling on the low velocity impact response of CFRP laminates
No full textLow velocity impact tests were carried out on unidirectional CFRP laminates previously aged with up to 80 thermal cycles between temperatures of -16 °C/+80 °C, in order to assess the effect of repeated temperature changes on the impact behavior of the material. The specimens subjected to the highest numbers of thermal cycles present a lower energy absorption during impact. In addition, the load drops that are usually visible in the contact force history of impacted laminates, and are attributed to the propagation of delamination, tend to disappear as the number of thermal cycles increases. On the other hand, both visual inspection and optical microscope analysis showed that no significant difference is induced by thermal cycling in the extent of delaminations. Thermal aging probably induced an embrittlement in the material, due to matrix microcracks that are generated at the lowest temperatures
A numerical approach to the disbonding mechanism of adhesive joints
No full textThe need for lightweight structures in aeronautics is leading to a strong interest in adhesively bonded joints. Incomplete knowledge of their fatigue behaviour is a major obstacle to their application. At present, the prediction of the disbonding growth is yet an open question. This work aims to develope a numerical model for the computation of the disbonding growth in an adhesive joint. The scope is calculating the energy release under quasi-static conditions in order to relate it to the fatigue disbond growth through the existing analytical models. A finite element model for the prediction of disbond growth under quasi-static loading has been implemented in Abaqus, by introducing a cohesive zone model which is able to capture the process zone around the crack tip and to enforce an energy-based failure criterion. The model, which had originally been developed for double cantilever beam specimens under mode I, was extended to mode II loading. Numerical simulations are validated by comparison with experimental results on double cantilever beam coupons in mode I and with literature results on end notched flexure coupons in mode II conditions. The results from tests and simulations are in accordance with each other. The presented model is a suitable option for the estimation of fracture mechanics parameters in cases in which complex geometry and loads prevent the application of analytical theories
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