Basic Biomechanics for Craniofacial Surgeons: The Responses of Alloplastic Materials and Living Tissues to Mechanical Forces

Many terms such as twist, compress, bend, and stretch, describe how materials behave when subjected to mechanical stresses. Subjective adjectives to describe the property of materials such as hard or brittle are imprecise and impedes proper understanding of important principles needed in planning and performing surgical treatments. The viability of tissue and time dependent variables effect healing and compound the issue. Some parameters are time dependent (strain rate), while others are nearly independent of time (Young’s modulus). The craniofacial skeleton and enveloping soft tissues are viscoelastic composite materials which undergo time-dependent changes upon loading. The ability to remodel and respond to environmental changes makes them “smart,” reenforcing where needed and removing where not required based on a set of predetermined upper and lower thresholds. This mini review has 7 sections on engineering principles that underpin craniofacial surgery: (1) The general concept of mechanics: load, force, stress, strain, compression, tension, shear, stress-strain curves and values derived from them such as Young’s modulus, fatigue damage, and load- shearing. (2) Material properties of bone and suture and structural engineering of the craniofacial skeleton in normal and pathological conditions. (3) Fixation using wires, screws, and plates: anatomy and function of screws and plates, locking plates, lag screws, internal and external fixators. (4) Biomechanics of distraction osteogenesis and the effects of radiation. (5) Finite element analysis and other computational biomechanical tools. (6) Virtual surgical planning, cutting guides, and intra-operative navigation. (7) Tissue engineering: design goals, criteria, and constraints. An appreciation and understanding of these biomechanical principles will help craniofacial surgeons to facilitate intrinsic optimization and better treat complex morphological problems, helping one achieve the most favorable and durable results. The biological responses to mechanical stress are extremely important as well, but due to space constraints, they will be the subject of a separate dedicated review.

Dissipative properties of composite structures. 1. Statement of problem

Object and purpose of research. The object under study is a sandwich plate with two rigid anisotropic layers and a filler of soft isotropic viscoelastic polymer. Each rigid layer is an anisotropic structure formed by a finite number of orthotropic viscoelastic composite plies of arbitrary orientation. The purpose is to develop a mathematical model of sandwich plate. Materials and methods. The mathematical model of sandwich plate decaying oscillations is based on Hamilton variational principle, Bolotin’s theory of multilayer structures, improved theory of the first order plates (Reissner-Mindlin theory), complex modulus model and principle of elastic-viscoelastic correspondence in the linear theory of viscoelasticity. In description of physical relations for rigid layers the effects of oscillation frequencies and ambient temperature are considered as negligible, while for the soft viscoelastic polymer layer the temperaturefrequency relation of elastic-dissipative characteristics are taken into account based on experimentally obtained generalized curves. Main results. Minimization of the Hamilton functional makes it possible to reduce the problem of decaying oscillations of anisotropic sandwich plate to the algebraic problem of complex eigenvalues. As a specific case of the general problem, the equations of decaying longitudinal and transversal oscillations are obtained for the globally orthotropic sandwich rod by neglecting deformations of middle surfaces of rigid layers in one of the sandwich plate rigid layer axes directions. Conclusions. The paper will be followed by description of a numerical method used to solve the problem of decaying oscillations of anisotropic sandwich plate, estimations of its convergence and reliability are given, as well as the results of numerical experiments are presented.

Topology optimization for 3D microstructures of viscoelastic composite materials

Materials with high stiffness and good vibration damping properties are of great industrial interest. In this paper, a topology optimization algorithm based on the BESO method is applied to design viscoelastic composite material by adjusting its 3D microstructures. The viscoelastic composite material is assumed to be composed of a non-viscoelastic material with high stiffness and a viscoelastic material with good vibration damping. The 3D microstructures of the composite are uniformly represented by corresponding periodic unit cells (PUCs). The effective properties of the 3D PUC are extracted by the homogenization theory. The optimized properties of the composites and the optimal microscopic layout of the two materials phases under the conditions of maximum stiffness and maximum damping are given by several numerical examples.

Contact characteristics of steel-rubber rollers based on modified contact theory considering viscoelasticity

This paper mainly studied the contact characteristics of steel-rubber rollers. First, according to the dynamic mechanical experimental data of rubber, the hyperelastic-viscoelastic composite constitutive model was constructed to accurately describe the properties of rubber. Then, for the contact problem of steel-rubber rollers, the elastic modulus of rubber presents an instantaneous change due to the influence of rubber's viscoelasticity, so the Hertz contact theory was modified through the instantaneous elastic modulus. Next, a dimensionless parameter [Formula: see text] was constructed to judge the viscoelastic influence degree of rubber. Finally, based on modified contact theory and the dimensionless parameter [Formula: see text], the influence of compressive force and rolling speed on the contact characteristics of steel-rubber rollers was analyzed. The research in this paper is helpful to the analysis of dynamic contact problem of rollers structure and the development of contact theory.

VISCOELASTIC RESPONSES OF MDF KERF STRUCTURES

Medium-density fiberboard (MDF), comprised of chopped wood fibers and epoxy resin is commonly used in building construction. Given the current trend in freeform architecture, there is a need to generate complex geometric structures out of common construction materials. In this study, kerfing (relief cutting) method is used to create flexible and moldable shapes out of relatively rigid wood panels. The kerf panels can be easily formed into various complex shapes for outdoor facades and/or indoor paneling. The natural wood fibers and epoxy resin in MDF combine to produce a viscoelastic composite material that exhibits time-dependent changes in mechanical behavior, i.e., material stiffness/compliance. The work described herein comes from a study designed to develop a better understanding of the viscoelastic response of kerf unit cells and panels. Experimental creep tests were performed on dog-bone specimens under uniaxial loading conditions1in order to determine the viscoelastic response of the MDF. The experimental results were used to develop a model to be used in simulations. The simulations were carried out on a kerf unit cell, e.g., a square interlocked kerf, and kerf structures of complex geometries in order to investigate the time-dependent changes in the deformations of the kerf structures. From this study, a systematic design of kerf panels with complex shapes will be developed in order to minimize the time-dependent changes of kerf structures.

DYNAMIC RESPONSES OF ARCHITECTURAL KERF STRUCTURES

Kerfing is a subtractive manufacturing approach to create flexible freeform surfaces from stiff planar materials. The kerf structures are used in both indoor and outdoor architectures for wall paneling, outdoor façade and pavilion. In addition to their physical appeal, these structures have potential applications in tuning the dynamics responses in buildings, e.g., indoor acoustic, vibration suppression, etc. To exploit these novel applications of kerf structures, this paper presents a study on the dynamic responses of kerf structures made up of Medium Density Fiberboard (MDF). MDF is a viscoelastic composite material comprising of wood fiber networks and epoxy. The influence of the material behavior, i.e. viscoelasticity of MDF is considered in determining the dynamic response of the kerf panels. Two kerf panels with similar kerfing pattern but different cut density and arrangement are studied for their modal responses. A 3D beam element is used to model the mechanical responses of the kerf panels. With the understanding of the dynamic response of these kerf panels, their applications in altering the indoor acoustics and the wind responses of the buildings can be better comprehended.

REPAIR OF POWER HYDRAULIC CYLINDERS USING NEW POLYMER COMPOSITE MATERIALS

The development and implementation of a new repair technology based on the use of polymer parts made of high-performance antifriction composites in tribo-couplings of power hydraulic cylinders will reduce the cost and repair time, significantly increase the reliability of the entire hydraulic system. (Research purpose) The research purpose is in increasing the efficiency of repairing of power hydraulic cylinders of agricultural machinery by restoring tribo-conjugations with new antifriction composites based on modified caprolon. (Materials and methods) The article presents the study of the physical and mechanical characteristics of samples of polymer composites according to well- known and original methods. Authors used computer programs and original methods of equipment suppliers in the study of compounding processes, rheological and tribotechnical tests. Theoretical research using the provisions of theoretical mechanics, elasticity theory and thermophysics has been conducted. A numerical study of the introduction of a spherical indenter into a sample of a viscoelastic composite material was carried out using the ANSYS software package, the MAPDL module. (Results and discussion) The article analyzes the obtained data on the elastic-strength, rheological and tribotechnical characteristics of the studied compositions of polyamide composites. The three-component composite containing 2 percent shungite, 0.5 percent graphite, 6 surfactants has the best complex of these properties. It has been proposed to restore the operability of the piston assembly and the front cover during the repair of the power hydraulic cylinder to change their design with the installation of guide support rings made of polyamide composite into the cover and piston of the hydraulic cylinders. This will lead to a significant reduction in the wear rate of the tribo-coupling parts and reduce the complexity of repairing hydraulic cylinders. (Conclusions) Restoration of tribo-couplings of power hydraulic cylinders with the installation of a guide ring made of a new polymer composite ensures a decrease in the maximum contact stresses, the rate of their increase with an increase in the gap and, as a result, a decrease in the wear rate of the contacted parts.

A DIGITAL TWIN FOR AUTOMATED LAYUP OF PREPREG COMPOSITE SHEETS

The composite sheet layup process involves stacking several layers of a viscoelastic prepreg sheet and curing the laminate to manufacture the component. Demands for automating functional tasks in the composite manufacturing processes have dramatically increased in the past decade. A simulation system representing a digital twin of the composite sheet can aid in the development of such an autonomous system for prepreg sheet layup. While Finite Element Analysis (FEA) is a popular approach for simulating flexible materials, material properties need to be encoded to produce high-fidelity mechanical simulations. We present a methodology to predict material parameters of a thin-shell FEA model based on real-world observations of the deformations of the object. We utilize the model to develop a digital twin of a composite sheet. The method is tested on viscoelastic composite prepreg sheets and fabric materials such as cotton cloth, felt and canvas. We discuss the implementation and development of a high-speed FEA simulator based on the VegaFEM library. By using our method to identify sheet material parameters, the sheet simulation system is able to predict sheet behavior within 5 cm of average error and have proven its capability for 10 fps real-time sheet simulation.