This paper is intended to reconcile the stress-based and strain-based formulations for material failure criteria, where a longstanding and deep division is present. The two approaches do not naturally agree with each other, and they not genuinely complement each other, either. Most popular criteria are stress-based when originally proposed, including the maximum stress, Tresca, von Mises, Raghava-Caddell-Yeh and the Mohr criteria. Their formulations are unique and self-consistent, i.e., capable of reproducing the input data. Their strain-based counterparts, with the maximum strain criterion being considered as the strain-based counterpart of the maximum stress criterion, are neither unique nor necessarily self-consistent. It has been proven that the self-consistent ones reproduce their respective stress-based counterparts identically in effect with a disadvantage of requiring an additional material property to apply, without a single benefit. For the Mohr criterion as a special case, a strain-based counterpart is simply infeasible in general. All undesirable features of strain-based criteria are rooted in a single source: The failure strains can only be measured under a uniaxial stress state, which corresponds to a combined strain state in general, not a uniaxial strain state! Given the arguments presented, the reconciliation proves to be biased completely towards the stress-based side if mathematics, logic and common sense prevail over perception and prejudice.
Mechanical metamaterials are engineered materials with unconventional mechanical behavior that originates from artificially programmed microstructures along with intrinsic material properties. With tremendous advancement in computational and manufacturing capabilities to realize complex microstructures over the last decade, the field of mechanical metamaterials has been attracting wide attention due to immense possibilities of achieving unprecedented multi-physical properties which are not attainable in naturally-occurring materials. One of the rapidly emerging trends in this field is to couple the mechanics of material behavior and the unit cell architecture with different other multi-physical aspects such as electrical or magnetic fields, and stimuli like temperature, light or chemical reactions to expand the scope of actively programming on-demand mechanical responses. In this article, we aim to abridge outcomes of the relevant literature concerning mechanical and multi-physical property modulation of metamaterials focusing on the emerging trend of bi-level design, and subsequently highlight the broad-spectrum potential of mechanical metamaterials in their critical engineering applications. The evolving trends, challenges and future roadmaps have been critically analyzed here involving the notions of real-time reconfigurability and functionality programming, 4D printing, nano-scale metamaterials, artificial intelligence and machine learning, multi-physical origami/kirigami, living matter, soft and conformal metamaterials, manufacturing complex microstructures, service-life effects and scalability.
The objective of this study is to investigate the effect of endurance and takeoff distance on the result of an adapted pre-sizing of an unmanned aerial vehicle (uav). Pre-sizing methods aim to define an initial geometry of the uav that necessitates little modifications in the next phase of uav design: detailed dimensioning. Endurance and takeoff distance are two important parameters, among others, that the end user wishes to control. The non-linear formulas and equations used in pre-sizing do not give a direct relation between these two parameters and the initial geometry. In this work, a summary presentation of an adapted pre-sizing method is given. A parametric study is then conducted of the effect of endurance and takeoff distance in two cases, an uav with one engine and one with two engines. The results will help to quantify and control the effect of the required parameters on the geometry at early stages of an uav design project and assist designers when they must choose one or two motors to better meet the end user's expectations.
In this paper, the linear hydrodynamic stability analysis of a viscoelastic fluid in a plane channel flow driven by a constant pressure gradient is investigated numerically. A nonlinear eigenvalue problem for the perturbed state is obtained from a generalized Orr-Sommerfeld equation. The Chebyshev spectral collocation method with expansions in Lagrange's polynomials are used for the numerical calculation. The complex wave speed, the critical Reynolds and wavenumber numbers are calculated for different parameters. We mainly examine the combined effects of relaxation time (via the Deborah number) and delay time (via the elasticity number) on the onset of instability. Results show that the relaxation time has a destabilizing effect and the retardation time has a stabilizing effect, with a displacement towards the long-wave region. Moreover, an important feature occurs when the Reynolds number is equal to the ratio of both characteristic times, the Jeffrey's fluid model behaves like the Newtonian model.
Improvement of the efficiency of thermoelectric modules requires the improvement of their performance. Thus, an optimisation method will be studied in this paper, as well as the analysis of the management of the operating conditions. By reversing the flow of hot and cold heat transfer fluids, we can reduce the temperature rise at the cold junctions and improve the performance. Water is the heat transfer fluid we have chosen for this study. The hot heat transfer fluid will flow in a channel to heat the hot junctions, and the cold heat transfer fluid will flow in a channel to cool the cold junctions. In the first case, the flows of the two heat transfer fluids are in the same direction. Subsequently, a second study will be carried out by opposing the flow direction of the heat transfer fluids. The results obtained show that by opposing the flow direction of the cold heat transfer fluid, we obtain a greater generated power than that obtained in the case of a flow in the same direction, which will allow us to have a better efficiency.
