Prediction of Probabilistic Shock Initiation Thresholds of Energetic Materials through Evolution of Thermal-mechanical Dissipation and Reactive Heating

The ignition threshold of an energetic material (EM) quantifies the macroscopic conditions for the onset of self-sustaining chemical reactions. The threshold is an important theoretical and practical measure of material attributes that relate to safety and reliability. Historically, the thresholds are measured experimentally. Here, we present a new Lagrangian computational framework for establishing the probabilistic ignition thresholds of heterogeneous EM out of the evolutions of coupled mechanical-thermal-chemical processes using mesoscale simulations. The simulations explicitly account for microstructural heterogeneities, constituent properties, and interfacial processes and capture processes responsible for the development of material damage and the formation of hotspots in which chemical reactions initiate. The specific mechanisms tracked include viscoelasticity, viscoplasticity, fracture, post-fracture contact, frictional heating, heat conduction, reactive chemical heating, gaseous product generation, and convective heat transfer. To determine the ignition threshold, the minimum macroscopic loading required to achieve self-sustaining chemical reactions with rate of reactive heat generation exceeding the rate of heat loss due to conduction and other dissipative mechanisms is determined. Probabilistic quantification of the processes and the thresholds are obtained via the use of statistically equivalent microstructure samples sets (SEMSS). The predictions are in agreement with available experimental data.

Processing light with an optically tunable mechanical memory

Mechanical systems are one of the promising platforms for classical and quantum information processing and are already widely-used in electronics and photonics. Cavity optomechanics offers many new possibilities for information processing using mechanical degrees of freedom; one of them is storing optical signals in long-lived mechanical vibrations by means of optomechanically induced transparency. However, the memory storage time is limited by intrinsic mechanical dissipation. More over, in-situ control and manipulation of the stored signals processing has not been demonstrated. Here, we address both of these limitations using a multi-mode cavity optomechanical memory. An additional optical field coupled to the memory modifies its dynamics through time-varying parametric feedback. We demonstrate that this can extend the memory decay time by an order of magnitude, decrease its effective mechanical dissipation rate by two orders of magnitude, and deterministically shift the phase of a stored field by over 2π. This further expands the information processing toolkit provided by cavity optomechanics.

IRREVERSIBILITY OF FRICTION AND WEAR PROCESSES

Ways of energy dissipation by friction are analysed from a thermodynamic perspective. The non-equilibrium and irreversibility of processes in tribological systems are found to be sufficient conditions for energy dissipation. M. Planck’s currently prevailing opinion that mechanical work can be converted into heat without limitations, e.g., by means of heat, is demonstrated not to apply to the friction of solids subject to wear. Ranges of work conversion into friction heat are determined. The generation of tribological wear particles is dependent on work of mechanical dissipation and its components – surface and volume work. A friction pair or its fragments, where energy is directly dissipated, are treated as open thermodynamic systems. The processes in place are described with the first law of thermodynamics equation. The effect of friction heat and the work of mechanical dissipation on variations of internal energy, enthalpy, and energy transferred to the environment as heat are defined. These dependences should be addressed when planning and interpreting tribological tests.

Ultrathin complex oxide nanomechanical resonators

Complex oxide thin films and heterostructures exhibit a variety of electronic phases, often controlled by the mechanical coupling between film and substrate. Recently it has become possible to isolate epitaxially grown single-crystalline layers of these materials, enabling the study of their properties in the absence of interface effects. In this work, we use this technique to create nanomechanical resonators made out of SrTiO3 and SrRuO3. Using laser interferometry, we successfully actuate and measure the motion of the nanodrum resonators. By measuring the temperature-dependent mechanical response of the SrTiO3 resonators, we observe signatures of a structural phase transition, which affects both the strain and mechanical dissipation in the resonators. Here, we demonstrate the feasibility of integrating ultrathin complex oxide membranes for realizing nanoelectromechanical systems on arbitrary substrates and present a novel method of detecting structural phase transitions in these exotic materials.

Numerical modelling of pore-fluid-enhanced thermal spallation in granitic rock

This paper considers numerically the effect of pore-fluid on thermal spallation of granitic rock. For this end, a numerical model based on the embedded discontinuity finite element approach to rock fracture and an explicit scheme to solve the underlying thermo-mechanical problem is developed. In the present implementation, a displacement discontinuity (crack) is embedded perpendicular to the first principal direction in a linear triangle element upon violation of the Rankine criterion. In the thermo-mechanical problem, the heating due to mechanical dissipation is neglected as insignificant in comparison to the external heat flux. This leads to an uncoupled thermo-mechanical problem where the only input from the thermal part to the mechanical part is thermal strains. This problem is solved with explicit time marching using the mass scaling to speed up the solution. Finally, the fluid trapped into the micro-pores is modelled as a material that can bear only volumetric compressive stresses. A thermal spallation problem of a rock sample under axisymmetry is simulated as a numerical example.

Multiphysics Coupling In Aging Process of Filled-Rubber

Filled rubbers are used popularly in damping parts which can be found in automobile sector or in building sector. However, the mechanical properties of material depend sensitively on temperature, chemical composition and environment conditions. In fact, the mechanical dissipation due to damping process leads to the increase of temperature considerably. Aging process can be activated sequentially by the heat which result in the change of damping properties during usage time. This paper presents a new behavior model that considers the simultaneous effects of temperature, mechanical loads on the behavior of materials along with the aging of materials. With the assumption of internal variables related to aging phenomenon and visco-plastic behavior, the model is built in the thermodynamical framework. A fully coupled finite element formulation is proposed to solve simultaneously thermo, chemical and mechanical phenomenon appeared in this material. An example illustrates the number of applicability of the model to predict the behavior of materials under the effect of cyclic loads in extremely working conditions

Numerical modelling of heat generation during shear band formation in rock

This article gives a computational continuum mechanics answer to a question of how much heat is generated, in terms of temperature rise, during controlled shear band formation in a rock like material. This problem is treated as adiabatic heating due to mechanical dissipation at the material point level. Assuming that only the compressive strength of the rock is temperature dependent, the coupled system of the constitutive equations and the adiabatic heat equation can be solved as a second order polynomial equation for the viscoplastic multiplier at an integration point. A Mohr-Coulomb viscoplastic model with linear softening is employed for rock material description. Numerical simulations of a 2D strip under uniaxial compression at strain rates up to 10 1/s show that the temperature rise in a rock like material with a compressive strength of 100 MPa is less than two degrees.