This work presents a comprehensive study on the impact damage tolerance of Sheet Moulding Compounds (SMCs). The performance of glass, carbon and hybrid glass/carbon SMCs are compared by means of tensile, compression, low-velocity impact and compression after impact experiments. Damage analysis of the impacted laminates was performed by ultrasonic and X-ray methodologies. The glass SMC exhibited the highest damage tolerance in low-velocity impact with the smallest damaged area, crack density and loss in compression after impact (CAI) strength. On the other hand, the carbon SMC demonstrated superior in-plane stiffness and strength, but exhibited a large damaged area and crack density under impact. The hybrid SMC displayed an optimal compromise, exhibiting intermediate tensile in-plane performance and excellent damage tolerance at lower impact energy levels, but suffered from extensive delamination at the highest impact energy. Overall, the findings highlight the suitability of hybrid SMCs for structural applications with potential impact risks.

This study investigates the vibrational performance of a new generation of hybrid composites based on a short fibre core for structural dynamic applications. The dynamic characterisation was conducted through dynamic mechanical analysis (DMA) and the free vibration of a cantilever beam at different lengths. The short fibre composite exhibited a superior specific damping capacity, producing a maximum damping of 0.42. The values extracted from DMA indicated no visco-plastic contribution from the PEEK resin system, implying that the internal microarchitecture drives the material damping, with friction at the fibre/matrix interface being the primary dissipation mechanism. The hybrid laminate presented an improvement in damping performance of 39% due to the addition of the short fibre core compared to a baseline quasi-isotropic laminate of similar flexural stiffness. These findings show hybridisation’s advantages in designing structural components with improved damping performance and reduced cost for a variety of dynamic applications in industries such as automotive.

This paper explores the use of compression moulding to produce structural hybrid composites based on short-fibre composites, combining a short-fibre core and uni-directional (UD) skins. A parametric study on processing parameters found low consolidation pressures provided a higher repeatability of the manufacturing process. The best mechanical response of the hybrid laminate was obtained through a 2-step consolidation process due to an increased fibre volume fraction. Despite a moderate 21.5% increase in cost compared to the short fibre material, the hybrid panels showed large increases in mechanical properties with an outstanding increase in flexural modulus of 330.4%. A semi-analytical constitutive model of the short fibre composite was developed to determine the variability in the mechanical response due to the stochastic microstructure. The results predicted a low scatter in strength, compatible with the requirements of structural applications. This study opens the path to the development of sustainable thermoplastic composites from recycled short-fibre compounds.

Short fibre and hybrid carbon fibre PEEK composite materials were tested in tension and compression under quasi-static and high strain rate conditions to observe the strain rate dependence. Multiple temperatures including room temperature, +85 and -50 degrees C were used to investigate the temperature dependence of the materials. The hybrid laminate comprised a consolidated short fibre core reinforced with outer UD plies in the 0 degrees orientation to provide maximum reinforcement whilst minimising the quantity of expensive UD composite used. Under compression, the beneficial effect of the hybridisation strategy was observed for all high-strain rate testing conditions, where the hybrid laminate outperformed the response of the individual constituents in terms of strength and strain rate dependency. The outer unidirectional (UD) layers contributed to confining the short fibre core, providing superior structural integrity. Under tension, the response was dominated by the UD layers with a 288% increase in strength at room temperature over the short fibre material. However, in the high temperature quasi-static case, the strength was dramatically reduced, by 64%, due to the debonding of the UD reinforcement. This study shows the suitability of hybrid composites for impulsive applications and provides material parameters for the future design of composite structures subjected to impact events.

In this work we present a new experimental technique alternative to Acoustic Emissions, which makes use of sound signal sensing, to complement the assessment of the mechanical behaviour of sand grains. A post-processing protocol is also proposed. By using this technique in combination with microscopy during in-situ compression tests, it was possible to capture complementary features in the complex mechanical behaviour of granular materials. In addition to that, dominant acoustic frequencies in the sound emitted by sand grains during failure were captured. The method has been applied to study the effects of chemical composition, morphology and grain sizes on the failure mechanics of three types of single sand grains. The advantages of this approach over other traditional techniques to capture failure events during cracking of granular materials have also been clearly demonstrated.

In this work, we explore the application of the novel technique Sound Measurements (SM) on E-glass fibre bundles. This technique relies on the ability to capture the sound emitted when fibres break. The method was used to monitor the failure of fibres and to generate a set of effective values of strength for the glass fibres within a bundle. The numerical experiments showed that not only did this methodology provides a more accurate representation of the features observed in the mechanical test, but also a better coefficient of determination R2 in relation to the Weibull analysis.

Voronoi tessellation techniques are widely accepted methods for the generation of representative models of polycrystalline microstructures of metallurgic and ceramic materials. Contrary to most of the Voronoi-based tessellation methods developed, the Laguerre Voronoi technique provides control over the size and shape of the cells, therefore allowing to simulate accurately the grain structure of a wide range of materials. This paper presents a method for the generation of numerical models of 3D polycrystalline microstructures, based on the Laguerre-Voronoi tessellation technique. An innovative approach to define the additional parameters required by the Laguerre-Voronoi formulation for the generation of realistic 3D microstructures is presented, providing the algorithm with information on the given microstructure from a set of 2D micrographs easily obtainable experimentally. The method implemented efficiently avoids degenerated cells (affecting the quality of the final structure) and finds the most representative set of input values by comparing 2D sections of the numerical model against 2D imaging of real polished surfaces. In this paper, the capability of the method developed is verified by reproducing the microstructure of polycrystalline alumina with various ranges of grain sizes, deriving from different sintering procedures.

The morphology of many naturally occurring and man-made materials at different length scales can be modelled using the packing of correspondingly shaped and sized particles. The mechanical behaviour of this vast category of materials - which includes granular media, particle reinforced materials and foams - depends strongly upon the shape and size distribution of the particles. This paper presents a method for the generation and packing of arbitrarily shaped polyhedral particles. The algorithm for the generation of the particles is based on the Voronoi tessellation technique, whilst the packing is performed using a geometrical approach, which guarantees the non-overlapping of the bodies without relying upon any, otherwise typically computationally expensive, contact detection and interaction algorithm. The introduction of three geometrical parameters allows to control the shape, size and spacial density of the polyhedral particles, which are used to build numerical models representative of densely packed granular assemblies, granular reinforced materials and closed-cell foams. Copyright (C) 2017 John Wiley & Sons, Ltd.

The effect of grain shape, size distribution, intergranular friction, confinement, and initial compaction state on the high strain rate compressive mechanical response of sand is quantified using Long Split Hopkinson Pressure Bar (LSHPB) experiments, generating up to 1.1 ms long load pulses. This allowed the dynamic characterisation of different types of sand until full compaction (lowest initial void ratio) at different strain rates. The effect of the grain morphology and size on the dynamic compressive mechanical response of sand is assessed by conducting experiments on three types of sand: Ottawa Sand with quasi-spherical grains, Euroquartz Siligran with subangular grains and Q-Rok with polyhedral grain shape are considered in this study. The adoption of rigid (Ti64) and deformable (Latex) sand containers allowed for quasi-uniaxial strain and quasi-uniaxial stress conditions to be achieved respectively. Additionally, the effect of intergranular friction was studied, for the first time in literature, by employing polymer coated Euroquartz sand. Appropriate procedures for the preparation of samples at different representative initial consolidation states are utilized to achieve realistic range of naturally occurring formations of granular assembly from loose to dense state. The results identify material and confining sample state parameters which have significant effect on the mechanical response of sand at high strain rates and their interdependency for future integration into rate dependent constitutive models.

The ability to model impact and penetration behaviour of granular materials - such as sand - largely depends on the understanding of the stress-strain and volume change characteristics at high strain rates. Split Hopkinson Pressure Bar has been used extensively in the past to evaluate strain rate dependent constitutive response of a wide range of advanced and natural materials. However, the deformation behaviour of geo-materials - including granular media - needs careful interpretation in order to provide data for calibration and validation of numerical models. In this paper, a new procedure for the determination of the smallest Representative Volume Element (RVE) of granular media is proposed. The procedure relies upon the simulations of consolidated granular assembly using Discrete Element Method (DEM). Results indicate the role of void ratio as an important state variable, which influences the dynamic mechanical performance of granular materials considerably. The existence of a lower limit for the dimensions of specimens used in the high-strain rate experiments on granular materials as a function of their consolidation state is demonstrated. The adoption of the RVE as the smallest specimen size allows the time for achieving dynamic equilibrium that is essential for reliable characterisation of rate dependent behaviour of granular or particulate materials. Using the concept of RVE, experimental results are generated, demonstrating the success of our proposed methodology. The identified RVE size will favourably impact the next generation of predictive modelling tools for impact and penetration problems in granular materials. (C) 2016 Elsevier Ltd. All rights reserved.

Quasi static and dynamic experiments were conducted to characterise the mechanical response of Etnean volcanic sand. Stress and strain histories were measured in near-uniaxial strain and near-uniaxial stress conditions at strain rates ranging between 5·10 −4 and 1.5·10 3 s −1 using bespoke experimental setups. The effects of the lateral confinement and initial consolidation state were assessed. Etnean volcanic sand exhibited a noticeable strain rate dependent behaviour when characterised in its loose consolidation state but not when densely packed before loading. The effect of volcanic particles impingement on Ti-6Al-4 V alloy was assessed by conducting dynamic experiments at different incident angles using targets of different geometry. The texture of thus eroded surfaces was analyzed by means of non-contact 3D-profilometry. The surface analysis provided insights on the erosion mechanisms and quantitative data on the roughness increment caused by the collision and rubbing with volcanic sand.

The process of comminution, occurring during compaction and impact on granular media, affects dramatically the mechanical response of the materials. The adoption of rigid particles-common in DEM simulations-fails to reproduce accurately the grain fracture. The often-adopted solutions of replacing a broken particle with a cluster of either overlapping or non-overlapping spheres have the main drawback of inducing, respectively, repulsive spurious forces that badly affect the inter-particles contact or an undesirable loss of mass. This paper presents a novel method to overcome the issues of mass loss without imposing overlapping of the particles. When a failure condition is reached, the domain is decomposed using a Laguerre-Voronoi tessellation approach. A dense agglomerate of tangent, non-overlapping particles, generated with an efficient geometrical packing algorithm, fills the polyhedral cell containing the failed particle. Since the polyhedral cell is, by construction, bigger than the failed particle, the approach developed allows for a drastic-if not complete-reduction of the mass loss within a time efficient, concurrent simulation of the comminution process.

A new procedure for the determination of the smallest Representative Volume Element (RVE) of granular media is proposed in the present investigation. The procedure is based on the simulation of consolidated granular assemblies using the Discrete Element Method (DEM). The existence of a lower limit for the dimensions of specimens used in the high-strain rate experiments on granular materials as a function of their consolidation state is demonstrated. The repeatability of the experimental results presented demonstrates the validity of the proposed method for the determination of the RVE. The results obtained show clearly the influence of chemical/physical composition, grain shape, initial consolidation state and type of confinement on the measured mechanical response.

SummaryThe morphology of many natural and man‐made materials at different length scales can be simulated using particle‐packing methods. This paper presents two novel 3D geometrical collective deposition algorithms for packed assemblies with prescribed distribution of radii: the ‘planar deposition’ and the ‘3D‐clew’ method. The ‘planar deposition’ method mimics an orderly granular flow through a funnel by stacking up spirally ordinated planar assemblies of spheres capable of achieving the theoretical maximum for monodisperse aggregates. The ‘3D‐clew’ method, instead, mimics the winding of a clew of yarn, thus yielding densely packed 3D polydispersed assemblies in terms of void ratio of the aggregate. The morphologies of such geometrically generated assemblies, achieved at several orders of magnitude reduced computational cost, are comparable with those consolidated uni‐directionally by means of discrete element method. In addition, significantly faster simulations of mechanical consolidation of granular media have been performed when relying upon the proposed geometrically generated assemblies as starting configurations. Copyright © 2015 John Wiley & Sons, Ltd.