Modelling size effects for static strength of brittle materials

The paper proposes a new model for the assessment of size effects affecting the fracture strength of brittle materials. The proposed model permits to accurately estimate the relation between the specimen strength, the initial defect size and to take into account the strength variation with respect to the tested volume. The proposed methodology is analytically defined and thereafter validated with the literature data obtained through tests on different types of brittle materials, and on specimens with increasing volume. A simple procedure for parameter estimation is also defined in the paper. The literature validation proves the effectiveness of the proposed methodology, with the resulting fitting models in well agreement with the experimental dataset and characterized by high values of coefficients of correlation, similar or larger than those obtained in the literature with different approaches

Modelling size effects for static strength of brittle materials

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Experimental and Numerical Investigation of a Lattice Structure for Energy Absorption: Application to the Design of an Automotive Crash Absorber

In this work, an experimental and numerical analysis of a lattice structure for energy absorption was carried out. The goal was to identify the most influencing parameters of the unit cell on the crushing performances of the structure, thus guiding the design of energy absorbers. Two full factorial plans of compression tests on cubic specimens of carbon nylon produced by fused deposition modeling (FDM) were performed. The factors were the beam diameter and the number of unit cells. In the first factorial plan, the specimen volume is constant and the dimensions of the unit cell are varied, while the second factorial plan assumes a constant size of the unit cell and the volume changes in accordance with their number. The results showed that the specific energy absorption increases with the diameter of the beam and decreases with the size of the unit cell. Based on these results, a crash absorber for the segment C vehicle was designed and compared with the standard component of the vehicle made of steel. In addition to a mass reduction of 25%, the improved crushing performances of the lattice structure are shown by the very smooth force-displacement curve with limited peaks and valleys

An innovative nondestructive technique for the local assessment of residual elastic properties in laminated composites

In this work, an innovative experimental methodology is presented for the assessment of damage severity in composites. The technique aims at determining the local variation of the elastic properties in the damaged region of a composite component. Based on the Impulse Excitation Technique (IET), the vibrational response of the inspected region is isolated by clamping its extremities through vacuum, thus allowing the assessment of local variations. Complementarily, a new analytical approach is derived for the assessment of the residual elastic properties of the damaged area from the measurement of the first resonant frequency. Validation of the proposed methodology is performed on two glass-fibre woven fabric composites, damaged by impact. The material properties of the damaged zone determined through the proposed technique are compared to the results of tensile tests performed on specimens cut from the impacted plates. In particular, the specimens are equipped with optic fibre in order to punctually measure the elastic parameters. Results show that the residual elastic properties assessed with the proposed technique are in very good agreement with those measured through the optic fibre, thus proving the effectiveness of the methodology

Comparison between Fractal and Statistical Approaches to Model Size Effects in VHCF

Size effects concern the anomalous scaling of relevant mechanical properties of materials and structures over a sufficiently wide dimensional range. In the last few years, thanks to technological advances, such effects have been experimentally detected also in the very high cycle fatigue (VHCF) tests. Research groups at Politecnico di Torino are very active in this field, observing size effects on fatigue strength, fatigue life and fatigue limit up to the VHCF regime for different metal alloys. In addition, different theoretical models have been put forward to explain these effects. In the present paper, two of them are introduced, respectively based on fractal geometry and statistical concepts. Furthermore, a comparison between the models and experimental results is provided. Both models are able to predict the decrement in the fatigue life and in the conventional fatigue limit

A new methodology for thermostructural topology optimization: Analytical definition and validation

In 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

An experimental-numerical methodology for the nondestructive assessment of the dynamic elastic properties of adhesives

In the last years, lightweight design has become a priority in many industrial sectors, like as the aerospace and the automotive industry, mainly due to the strict regulations in terms of gas emission and pollution. Together with lightweight materials, the use of adhesives to join different parts permits to significantly reduce the weight of mechanical assemblies. For a proper design of the joints, the mechanical properties of adhesives should be correctly experimentally assessed. However, the experimental assessment of the adhesive mechanical properties can be complex, since they can be hardly estimated from traditional experimental tests on lap joint or butt-joint specimens. The development of an experimental procedure for the assessment of the adhesive properties is therefore of interest. In the present paper, a methodology for the assessment of the dynamic elastic properties of adhesives, i.e., Young's modulus and the loss factor, is proposed. The procedure is based on the Impulse Excitation Technique and Finite Element Analyses (FEA). An automated routine has been written to assess the elastic properties by minimizing the difference between the frequency response obtained experimentally and through FEA. The proposed methodology has been experimentally validated to estimate the mechanical properties of an epoxy adhesive for automotive applications

Blunt notch effect on the fatigue response up to 10^9 cycles of selective laser melting Ti6Al4V specimens

In this paper, the influence of a blunt notch on the VHCF response of SLM Ti6Al4V specimens is investigated. Ultrasonic fully reversed tension-compression tests up to 10(9) cycles were carried out on unnotched specimens and specimens with a blunt notch. Unnotched specimens show a slightly larger fatigue response, with limited differences. All fatigue failures originated from defects, which are bigger in unnotched specimens, mainly due to the different risk volume of the tested specimens and the related size effect. Interactions between notch, stress gradient, and defect size distribution must be considered to properly assess the influence of notch on the VHCF response

Epoxy and Bio-Based Epoxy Carbon Fiber Twill Composites: Comparison of the Quasi-Static Properties

In recent years, interest in sustainability has significantly increased in many industrial sectors. Sustainability can be achieved with both lightweight design and eco-friendly manufacturing processes. For example, concerns on the use of thermoset composite materials, with a lightweight design and a high specific strength, have arisen, since thermoset resins are not fully recyclable and are mainly petrol based. A possible solution to this issue is the replacement of the thermoset matrix with a recyclable or renewable matrix, such as bio-based resin. However, the mechanical properties of composites made with bio-based resin should be carefully experimentally assessed to guarantee a safe design and the structural integrity of the components. In this work, the quasi-static mechanical properties of composite specimens (eight layers of carbon fiber fabric) made with commercially available epoxy and a bio-based epoxy resins (31% bio content) are compared. Tensile tests on the investigated resins and tensile, compression, shear and flexural tests have been carried out on composite laminates manufactured with the two investigated resins. A finite element model has been calibrated in the LS-Dyna environment using the experimentally assessed mechanical properties. The experimental results have proven that the two composites showed similar quasi-static properties, proving that bio-based composite materials can be reliably employed as a substitute for epoxy resins without affecting the structural integrity of the component but lowering their carbon footprint