Evaluation of additively manufactured stacks for thermo-acoustic devices

The thermo-acoustic effect provides a means to convert acoustic energy to heat and vice versa without the need for moving parts. This is especially useful to construct mechanically-simple and robust energy harvesting devices, although there are limitations to the power-to-volume ratio achievable. The mechanical and thermal properties as well as geometry of the porous stack that forms a set of acoustic waveguides in thermo-acoustic devices are key to its performance. In this study, we evaluate various additively manufactured polymer stacks against more conventional ceramic stacks using a benchtop thermos-acoustic refrigerator rig that uses air at ambient pressure as its working fluid. Influence of stack parameters such as material, length, location, porosity and pore geometry are examined using experiments and correlated to simulations using DeltaEC, a software tool based on Rott's linear approximation. Structure-performance relationships are established by extracting scaling laws for power-to-volume ratio and frequency-thermal gradient dependencies. It is found that additively manufactured stacks can deliver performance comparable to ceramic stacks while being more affordable and customizable for thermo-acoustic transduction applications.

Fracture modelling of plain concrete using non-local fracture mechanics and a graph-based computational framework

In this article, we developed a thermodynamically consistent non-local microcracking model for quasi-brittle materials with application to concrete. The model is implemented using a novel graph-based finite element analysis (GraFEA) approach that allows for (i) the probabilistic modeling of the growth and coalescence of microcracks, (ii) the modeling of crack closure using a kinematics-based approach, and (iii) the modeling of rate effects on microcracking. The developed theoretical model and its computational framework is also implemented into the dynamics-based Abaqus/Explicit finite element program through a vectorized user-material subroutine interface. We further demonstrate the procedure for obtaining the parameters (including the non-local intrinsic material length scale, which governs the fracture process) and consequently validate the simulations with independent experimental results.

Rate and Temperature Effect in Nanoindentation Testing on Hardness in SLM IN718

The microstructure and mechanical hardness of Inconel 718 (INC718) hexagonal honeycomb cellular structure manufactured by Selective Laser Melting (SLM) was studied in this work. Non-heat-treated SLM-produced samples with cell wall thicknesses of 0.4, 0.6 and 0.8 mm were studied. The hardness was measured using MTS Nanoindenter. For room temperature, continuous hardness measurements over penetration depths up to 2 µm under three different strain rates of 0.02, 0.05 and 0.08 s−1 was performed. For the 100 and 200°C, single hardness measurements at eight different depths were performed. The grain size was found to change considerably as the cell wall thickness changed from 0.6 mm to 0.4 mm compared to the change from 0.8 mm to 0.6 mm. similar trend in mechanical hardness reduction and strain rate sensitivity changes were observed between the three samples. The microstructure and hardness showed anisotropy between the planes parallel and perpendicular to the build planes as well. Temperature and strain rate indentation size effect model developed by the second author was modified and used to evaluate the intrinsic material length scale used in gradient plasticity theory.

An assessment of phase field fracture: crack initiation and growth

The phase field paradigm, in combination with a suitable variational structure, has opened a path for using Griffith’s energy balance to predict the fracture of solids. These so-called phase field fracture methods have gained significant popularity over the past decade, and are now part of commercial finite element packages and engineering fitness- for-service assessments. Crack paths can be predicted, in arbitrary geometries and dimensions, based on a global energy minimization—without the need for ad hoc criteria. In this work, we review the fundamentals of phase field fracture methods and examine their capabilities in delivering predictions in agreement with the classical fracture mechanics theory pioneered by Griffith. The two most widely used phase field fracture models are implemented in the context of the finite element method, and several paradigmatic boundary value problems are addressed to gain insight into their predictive abilities across all cracking stages; both the initiation of growth and stable crack propagation are investigated. In addition, we examine the effectiveness of phase field models with an internal material length scale in capturing size effects and the transition flaw size concept. Our results show that phase field fracture methods satisfactorily approximate classical fracture mechanics predictions and can also reconcile stress and toughness criteria for fracture. The accuracy of the approximation is however dependent on modelling and constitutive choices; we provide a rationale for these differences and identify suitable approaches for delivering phase field fracture predictions that are in good agreement with well-established fracture mechanics paradigms. This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.

Effect of Infill Pattern, Infill Density, and Infill Angle on the Printing Time and Filament Length of 3D Printing

<p class="Abstract">To optimize the 3D printing process, the influence of its parameters on the performance of the printing process needs to be investigated. This research investigates the effect of infill pattern, infill density, and infill angle on the printing time and the filament material length. First, this research collected the printing time and the filament length data for each combination of infill pattern, infill density, and infill angle. The data collection was conducted by implementing Repetier-Host v.2.1.6 software as a data acquisition tool. Then, the General Linear Model was applied to analyze the effect of infill pattern, infill density, and infill angle on the printing time and filament length. Based on the analysis, higher infill density increases the printing time for each infill pattern and each infill angle. Also, higher infill density increases the filament length for each infill pattern and each infill angle. The implementation of the Gyroid type of infill pattern reduces the required printing time for each density. Meanwhile, the implementation of the 3D honeycomb type of infill pattern increases the filament length for each infill angle. The use of the 45° infill angle increases the filament length and printing time. To reduce the filament length and printing time, the 90° infill angle should be implemented.</p>

Designing of Dynamic Spectrum Shifting in Terms of Non-Local Space-Fractional Mechanics

In this paper, the applicability of the space-fractional non-local formulation (sFCM) to design 1D material bodies with a specific dynamic eigenvalue spectrum is discussed. Such a formulated problem is based on the proper spatial distribution of material length scale, which maps the information about the underlying microstructure (it is important that the material length scale is one of two additional material parameters of sFCM compared to the classical local continuum mechanics—the second one, the order of fractional continua—is treated herein as a scaling parameter only). Technically, the design process for finding adequate length scale distribution is not trivial and requires the use of an inverse optimization procedure. In the analysis, the objective function considers a subset of eigenvalues reduced to a single value based on Kreisselmeier–Steinhauser formula. It is crucial that the total number of eigenvalues considered must be smaller than the limit which comes from the ratio of the sFCM length scale to the length of the material body.

On size-dependent bending behaviors of shape memory alloy microbeams via nonlocal strain gradient theory

This paper focuses on the size-dependent behaviors of functionally graded shape memory alloy (FG-SMA) microbeams based on the Bernoulli-Euler beam theory. It is taken into consideration that material properties, such as austenitic elastic modulus, martensitic elastic modulus and critical transformation stresses vary continuously along the longitudinal direction. According to the simplified linear shape memory alloy (SMA) constitutive equations and nonlocal strain gradient theory, the mechanical model was established via the principle of virtual work. Employing the Galerkin method, the governing differential equations were numerically solved. The functionally graded effect, nonlocal effect and size effect of the mechanical behaviors of the FG-SMA microbeam were numerically simulated and discussed. Results indicate that the mechanical behaviors of FG-SMA microbeams are distinctly size-dependent only when the ratio of material length scale parameter to the microbeam height is small enough. Both the increments of material nonlocal parameter and ratio of material length-scale parameter to the microbeam height all make the FG-SMA microbeam become softer. However, the stiffness increases with the increment of FG parameter. The FG parameter plays an important role in controlling the transverse deformation of the FG-SMA microbeam. This work can provide a theoretical basis for the design and application of FG-SMA microstructures.

Stability Analysis of a Periodic Fluid-Conveying Heterogeneous Nanotube System

In this paper, the stability of a periodic heterogeneous nanotube conveying fluid is investigated. The governing equations of the nanotube system are derived based on the nonlocal Euler–Bernoulli beam theory. The dynamic stiffness method is employed to analyze the natural frequencies and critical flow velocities of the heteronanotube. The results and discussions are presented from three aspects which reveal the influences of period number, material length ratio and boundary conditions. In particular, we make comparisons between the heterogeneous nanotubes with periodic structure and the homogeneous ones with the same integral values of material properties along the longitudinal direction to isolate the influences of periodic distribution. According to the simulation results, we can conclude that with a proper selection of period number in terms of length ratio, the stability of the constructed nanotube can be improved.