Specific Shear Modulus Research Articles

The cylindrical mechanical metamaterial with high-level thermal-mechanical stabilities and high dynamic stiffness

Mechanical metamaterials with high-level thermal-mechanical stabilities are attracting increasing interest in aerospace. Although numerous configuration designs have been proposed, few of them can satisfy the demands of space explorations, mainly considering the inevitable geometric errors during the assembling process of the metamaterial using a number of parts. To address this issue, the design strategy of cylindrical mechanical metamaterials based on the building block assembly method is proposed here to achieve high-level thermal-mechanical stabilities and high dynamic stiffness. The mechanical and thermal properties of the metamaterial have been systematically studied in terms of theoretical predictions, Finite Element Analysis (i.e., FEA), and experimental measurements. The theoretical model proposed here has accurately predicted the thermal-mechanical behaviors of the metamaterial and demonstrated the capabilities of the metamaterial in achieving combined attributes of the good loading capability (i.e., 2.30 × 106 kN mm/kg for the specific compression modulus, 1.02 × 106 kN mm/kg for the specific shear modulus), the experimentally high thermal-mechanical stabilities (i.e., 0.037 ppm/°C), the high-level dynamic stiffness (i.e., 1013.84 Hz). Notably, the selective electron beam melting and the unique assemble method realize the feasibility of structurally integrated preparation of the metamaterial and achieve the reusability capability even once fabricated.

In-plane shear stiffness of diversified hexagonal honeycombs: A simple energy-based approach

The conventional hexagonal honeycomb is widely used in various practical applications. The re-entrant and semi re-entrant hexagonal forms derived from the conventional one have gained large attention in recent years due to their unusual Poisson's ratio and superior mechanical performance. However, the in-plane shear behavior of the semi re-entrant hexagonal honeycomb has not been reported yet. Especially, a comparative study on the shear stiffness of the three hexagonal honeycombs is lacking. A simple energy-based approach was used in this paper to establish theoretical models for predicting the in-plane shear modulus of the hexagonal honeycombs, which were later verified by the numerical simulations. Good agreement between the theoretical and numerical results is observed. The effect of the geometrical parameters on the shear modulus and the specific shear modulus were further discussed. It is found that the conventional hexagonal honeycomb displays higher shear modulus and specific shear modulus than both the re-entrant and semi re-entrant ones, while the re-entrant hexagonal honeycomb demonstrates the lowest values of shear modulus and specific shear modulus.

Simultaneous non-destructive identification of multiple elastic and damping properties of spruce tonewood to improve grading

Abstract The tonewoods used in musical instruments are finely selected and are subject to current and upcoming availability issues. Wood grading assesses the quality of the tonewood and depends on both subjective and objective criteria. Traditionally, grading criteria based on mechanical properties consider mainly the longitudinal direction. This paper investigates the elastic and damping properties of spruce wood used in the making of acoustical guitars and violins. In this context, the mechanical grading properties will be studied in multiple directions. A non-destructive vibrational characterisation method is used to simultaneously identify at least six mechanical properties of wood and leads to the determination of the elastic and damping parameters of wood along the different material directions. The main objective is to quantify the variability of the mechanical properties in different material directions as a function of the grading attributed by the wood seller. The results show that material anisotropy decreases as the grade decreases. This is attributed to strong increase of the specific elastic modulus in the radial direction and the specific shear modulus in longitudinal and radial plane. Moreover, grading seems to be assigned as a function of specific elasticity and loss factor in the longitudinal direction, which is tied in with the preferences of instrument makers. Following the assumption that high specific modulus is desirable, the classification based on instrument makers choices can be improved for new grading criteria that consider directions than longitudinal. Moreover, the substitution of wood species and the proposal of new materials exhibiting equivalent elastic and damping properties can be proposed.

Molecular dynamics simulations of thermally activated edge dislocation unpinning from voids inα-Fe

In this study, thermal unpinning of edge dislocations from voids in $\ensuremath{\alpha}$-Fe is investigated by means of molecular dynamics simulations. The activation energy as a function of shear stress and temperature is systematically determined. Simulations with a constant applied stress are compared with dynamic simulations with a constant strain rate. We found that a constant applied stress results in a temperature-dependent activation energy. The temperature dependence is attributed to the elastic softening of iron. If the stress is normalized with the softening of the specific shear modulus, the activation energy is shown to be temperature-independent. From the dynamic simulations, the activation energy as a function of critical shear stress was determined using previously developed methods. The results from the dynamic simulations are in good agreement with the constant stress simulations, after the normalization. This indicates that the computationally more efficient dynamic method can be used to obtain the activation energy as a function of stress and temperature. The obtained relation between stress, temperature, and activation energy can be used to introduce a stochastic unpinning event in larger-scale simulation methods, such as discrete dislocation dynamics.

Patient Specific Wall Stress Analysis and Mechanical Characterization of Abdominal Aortic Aneurysms Using 4D Ultrasound

The aim of this study was to perform wall stress analysis (WSA) using 4D ultrasound (US) in 40 patients with an abdominal aortic aneurysm (AAA). The geometries and wall stress results were compared with computed tomography (CT) in seven patients. Additionally, the WSA models were calibrated using 4D motion estimation, resulting in patient specific material parameters that were compared among patients. 4D-US images were acquired for 40 patients (AAA diameter 27-52mm). Patient specific AAA geometries and wall motion were extracted from the 4D-US. WSA was performed and corresponding patient specific material properties were derived. For seven patients, CT data were available and analyzed for geometry and wall stress comparison. The 4D-US based 99th percentile wall stress ranged from 198 to 390kPa. Regression analysis showed no significant relation between wall stress and diameter of the AAA. The similarity indices between US and CT were very good and ranged between 0.90 and 0.96, and the 25th, 50th, 75th, and 95th percentile wall stresses of the US and CT data were in agreement. The characterized patient specific shear modulus had a median of 1.1MPa (interquartile range, 0.7-1.4MPa). Based on the maximum AAA diameter, the AAAs were divided in a small, medium, and large diameter groups. The largest AAAs revealed an increased wall stiffness compared with the smallest AAAs. 4D ultrasound is applicable for wall stress analysis of AAAs, and offers the opportunity to perform wall stress analysis over time, also for AAAs who do not qualify for a CT or magnetic resonance imaging. Moreover, the patient specific material properties can be determined, which could possibly improve risk assessment.

Estimation of Modal Damping Ratio from Specific Shear Modulus for Monolithic Materials and Hybrid Cored Multilayer Composites

Damping properties are of significant importance in determining the dynamic response of structures, and accurate prediction of them at the design stage, especially in the case of light-weight structures is very desirable. Unfortunately, damping parameters cannot be deduced deterministically from other structural properties and recourse is generally made to data from experiments conducted on completed structures of similar characteristics. Such data is scarce but valuable, both for direct use in design and for furthering research into the phenomenon and modelling of damping. Till date, no work has been reported on the estimation of the damping ratio of the material purely from its elastic and physical constants. In this work, an effort has been made to obtain modal damping ratio for the fundamental mode of the structure from both analytical and experimental technique. The damping properties of PURE materials such as Aluminium, Polyurethane foam, Cork material, and of laminated composites (GFRP materials), are determined by classical modal analysis and also from their respective shear modulus and density constants. These theoretical developments in line with classical modal analysis, give impetus towards understanding damping mechanisms in Hybrid composite systems.

Hexachiral truss-core with twisted hemp yarns: Out-of-plane shear properties

This work describes a truss-core structure made of hemp/epoxy biocomposite based on a topology with auxetic (negative Poisson’s ratio) characteristics. Epoxy-based composites with 25wt.% of hemp yarns have been manufactured and characterized with tensile and flexural tests. Truss-core hexagonal chiral panels with the biocomposite core have been produced using a Resin Transfer Molding technique. The panels have been subjected to standard transverse shear tests, and their properties predicted with a Finite Element nonlinear modeling. The results show that the hexachiral biocomposite truss core exhibits specific shear modulus and strength significantly higher compared to the ones observed in previous demonstrators made of polymeric core.

Vibrational properties of Sitka spruce heat-treated in nitrogen gas

Sitka spruce (Picea sitchensis Carr.) wood was heated for 0.5–16.Oh at temperatures of 120°–200°C in nitrogen gas or air. The values for Young's modulus, shear modulus, and loss tangent were measured by free-free flexural vibration tests. X-ray diffractometry was carried out to estimate the crystallinity index and crystallite width. The results obtained are as follows: (1) Density decreased at higher temperatures and longer heating times. The specific Young's modulus, specific shear modulus, crystallinity index, and crystallite width increased during the initial stage and were constant after this stage at 120°C and 160°C, whereas they increased during the initial stage and decreased later when the temperature was high. Loss tangent in the longitudinal direction increased under all conditions, whereas that in the radial direction increased at 120°C and decreased at 160°C and 200°C. (2) From the relation between Young's modulus and moisture content, it can be safely said that Young's modulus is increased by the crystallization and the decrement in equilibrium moisture content, and that crystallization (rather than degradation) is predominant at the initial stage of the heat treatment, whereas the latter is predominant as the heating time increases. (3) It is implied that the specific Young's modulus, specific shear modulus, crystallinity index, and crystallite width decreased more in air than in nitrogen gas because of oxidation in air.