Experimental study of tool–part interaction during autoclave processing of thermoset polymer composite structures

Autoclave manufacturing of thermoset polymer matrix composite structures with high dimensional fidelity requires a good understanding of various parameters affecting process-induced warpage and application of this knowledge to minimize the warpage through appropriate process control. One important contributor is the interaction between a composite part and the tool on which the part is laid and cured. This experimental study quantified the tool–part interaction by measuring the static and dynamic frictional coefficients as a function of process time, using a friction test fixture specially designed to simulate the autoclave environment. Temperature ramp rate was varied to understand the effect of autoclave cure cycle on the friction coefficients. Measured friction coefficients were maximum at the start of the cure cycle and varied as a function of degree of cure (α) and ramp rate owing to change in the tool–part interface, cure shrinkage, resin/composite properties, residual stress, and mode of interface failure.     

Tool–part interaction in composites processing. Part I: experimental investigation and analytical model

The ability to process composite structures with a high degree of dimensional control remains a barrier to further implementation of composite materials in commercial applications. Of the numerous types of process induced deformations that occur, the warpage of flat laminates due to tool–part interaction remains a poorly understood phenomenon. This paper presents an experimental study of the effect of process conditions and part aspect ratio on tool–part interaction induced warpage. For a given lay-up and material, part aspect ratio was found to have a much greater influence than autoclave pressure on warpage, while the tool surface condition was not observed to have any significant effect. The results of the study are embodied in an empirical relation, which can be a useful guide to predict laminate warpage over a range of industrially relevant conditions. In addition, a simple analytical model is proposed which agrees well with the experimentally determined relationships. A complementary numerical model is presented in a companion paper.     

An inelastic Timoshenko beam element with axial–shear–flexural interaction

An enhanced beam element is proposed for the nonlinear dynamic analysis of skeletal structures. The formulation extends the displacement based elastic Timoshenko beam element. Shear-locking effects are eliminated using exact shape functions. A variant of the Bouc–Wen model is implemented to incorporate plasticity due to combined axial, shear and bending deformation components. Interaction is introduced through the implementation of yield functions, expressed in the stress resultant space. Three additional hysteretic degrees of freedom are introduced to account for the hysteretic part of the deformation components. Numerical results are presented that demonstrate the advantages of the proposed element in simulating cyclic phenomena, in which shear deformations are significant.     

An alternative method for manufacturing high-strength CP Ti–SiC composites by accumulative roll bonding process

For the first time, the accumulative roll bonding (ARB) process was used as an effective alternative method for manufacturing Ti–SiC composites and compared with the monolithic ARBed Ti. High-strength monolithic commercially pure titanium (CP Ti) and CP Ti–SiC composites with effective uniform reinforcement distribution were fabricated by this process. The tensile test, Vickers hardness measurements and SEM observations were done for the characterization of materials. A significant increase in yield and tensile strength and a drastic decrease in elongation were observed by applying 8 cycles of ARB process. An unexpectedly slight decrease of yield and tensile strength along with elongation was observed after the sixth ARB cycle for the monolithic sample. It was attributed to the weakening of the bond between the titanium layers in the final cycles. Strength of the composite samples was higher than that of the monolithic sample and did not decrease in the final ARB cycles. This was caused by the significantly improved distribution of SiC particles in the titanium matrix.     

Multiscale aggregating discontinuities method for micro–macro failure of composites

The application of a multiscale method, called the multiscale aggregating discontinuities (MAD) method, to the failure analysis of composites is described. Two distinct features of the MAD method are the use of perforated unit cells, and the extraction of coarse-grained failure information. In the perforated unit cell, all subdomains of the unit cell that are not strictly elliptic are excluded, which enables the decomposition of its stable and unstable material. By means of these concepts, it is possible to compute an equivalent discontinuity at the macroscale, including both the direction and the magnitude of the discontinuity. This equivalent discontinuity is then passed to the macroscale along with the computed stress from the unit cell. The macroscale discontinuity is injected into the macro model by the extended finite element method (XFEM) procedure. In this paper, the method is improved by adding hourglass modes to the unit cell deformations, which better model growing cracks. Several examples comparing the MAD method with direct numerical simulations are presented.     

Novel automated method for evaluating the morphological changes of cellulose fibres during extrusion-compounding of plastic–matrix composites

A novel method based on fluorescence optical microscopy has been developed for determining the fibre geometrical changes occurring during the melt processing of cellulose-reinforced composites, which are known to be closely related with composite properties. Determination of these changes is still a tedious and challenging task because existing methods are not well developed yet. The novel method proved its ability for explaining the screw configuration effects on the attrition bore by the fibres during extrusion-compounding of plastic–matrix composites. The percentage of fibres longer than the critical length parameter was revealed to highlight the mechanical degradation of fibres during compounding. The percentage of fines exhibited the clearest correlation with the differences in fibre content of composites. Relationships found between composite tensile properties and fibre characterization parameters revealed the ability of the novel method for explaining the effects of composition and processing on composite properties.     

Microcellular processing of polylactide–hyperbranched polyester–nanoclay composites

The effects of addition of hyperbranched polyesters (HBPs) and nanoclay on the material properties of both solid and microcellular polylactide (PLA) produced via a conventional and microcellular injection-molding process, respectively, were investigated. The effects of two different types of HBPs (i.e., Boltorn H2004^? and Boltorn H20^?) at the same loading level (i.e., 12%), and the same type of HBP at different loading levels (i.e., Boltorn H2004^? at 6 and 12%), as well as the simultaneous addition of 12% Boltorn H2004^? and 2% Cloisite^?30B nanoclay (i.e., HBP–nanoclay) on the thermal and mechanical properties (both static and dynamic), and the cell morphology of the microcellular components were noted. The addition of HBPs and/or HBP with nanoclay decreased the average cell size, and increased the cell density. The stress–strain plots of all the solid and microcellular PLA-H2004 blends showed considerable strain softening and cold drawing, indicating a ductile fracture mode. Among the two HBPs, samples with Boltorn H2004^? showed higher strain-at-break and specific toughness compared to Boltorn H20^?. Moreover, the sample with Boltorn H2004^? and nanoclay exhibited the highest strain-at-break (626% for solid and 406% for microcellular) and specific toughness (405% for solid and 334% for microcellular). Finally, the specific toughness, strain-at-break, and specific strength of microcellular samples were found to be lower than their solid counterparts.     

A coupled symmetric BE–FE method for acoustic fluid–structure interaction

A coupled symmetric BE–FE method for the calculation of linear acoustic fluid–structure interaction in time and frequency domain is presented. In the coupling formulation a newly developed hybrid boundary element method (HBEM) will be used to describe the behaviour of the compressible fluid. The HBEM is based on Hamilton's principle formulated with the velocity potential. The state variables are separated into boundary variables which are approximated by piecewise polynomial functions and domain variables which are approximated by a superposition of static fundamental solutions. The domain integrals are eliminated, respectively, replaced by boundary integrals and a boundary element formulation with a symmetric mass and stiffness matrix is obtained as result. The structure is discretized by FEM. The coupling conditions fulfil C¹-continuity on the interface. The coupled formulation can also be used for eigenfrequency analyses by transforming it from time domain into frequency domain.     

Hydroxyapatite–zirconia composites preparedby precipitation method

Chemical routes to prepare ceramic precursor powders are frequently envisaged, especially when the aspects related to the microstructure are important and need to be controlled. An understanding of which parameters of synthesis and thermal treatment can influence the mechanical properties of hydroxyapatite compounds is essential for the production of such materials. Hydroxyapatite–zirconia composites have been prepared, in this study, by a precipitation method. This led to the formation of homogeneous powders with a very defined particle-size distribution. Ceramic pellets prepared from these powders were easily compacted and sintered without cracking. As expected, the presence of the zirconia phase improved composite densities and appeared to have an important role in thermal stabilization of the hydroxyapatite phase.     

A numerical method for a void–crack interaction under cyclic loads

This paper presents a numerical method to model a general system containing cracks and voids in an infinite elastic plate under remote cyclic loads. By extending Bueckner’s principle suited for a crack to a general system containing cracks and voids, the original problem is divided into a homogeneous problem (the one without cracks and voids) subjected to remote loads and a void-crack problem in an unloaded body with applied tractions on the surfaces of cracks and voids. Thus, the results in terms of the stress intensity factors can be calculated by considering the latter problem, which is analyzed easily by using the hybrid displacement discontinuity method (a boundary element method). Further, a fatigue growth technique of a mixed-mode crack is combined with the numerical approach to simulating a void–crack interaction problem under cyclic loads. Test examples are included to illustrate that the numerical method is very simple and effective for analyzing a void–crack interaction problem.     

A coupled ES-FEM/BEM method for fluid–structure interaction problems

The edge-based smoothed finite element method (ES-FEM) developed recently shows some excellent features in solving solid mechanics problems using triangular mesh. In this paper, a coupled ES-FEM/BEM method is proposed to analyze acoustic fluid–structure interaction problems, where the ES-FEM is used to model the structure, while the acoustic fluid is represented by boundary element method (BEM). Three-node triangular elements are used to discretize the structural and acoustic fluid domains for the important adaptability to complicated geometries. The smoothed Galerkin weak form is adopted to formulate the discretized equations for the structure, and the gradient smoothing operation is applied over the edge-based smoothing domains. The global equations of acoustic fluid–structure interaction problems are then established by coupling the ES-FEM for the structure and the BEM for the fluid. The gradient smoothing technique applied in the structural domain can provide the important and right amount of softening effects to the “overly-stiff” FEM model and thus improve the accuracy of the solutions of coupled system. Numerical examples of acoustic fluid–structure interaction problems have been used to assess the present formulation, and the results show that the accuracy of present method is very good and even higher than those obtained using the coupled FEM/BEM with quadrilateral mesh.     

An analytical model for predicting the freeze–thaw durability of wood–fiber composites

The development and validation of an analytical model that predicts the onset of frost-induced damage in wood–plastic composites (WPCs) is presented in this work. The mathematical model is based on the mechanics of a hollow cylinder subjected to an internal pressure caused by the expansion of freezing moisture bound in the wood–fiber reinforcement. The model is substantiated using experimental data from several published studies. Using a stochastic approach, the model is implemented to analyze the effect of wood fiber specie, fiber volume fraction, and matrix material properties on the frost resistance of fully and partially saturated WPCs. Results show that WPCs with high fiber contents, high moisture contents, and low polymer tensile strengths are most susceptible to frost-induced damage. Data also suggest that the use of softwood fibers (e.g., pine, spruce) and polymers with low moduli and high tensile strengths enhances the frost-resistance of WPCs.     

An efficient Peaceman–Rachford splitting method for constrained TGV-shearlet-based MRI reconstruction

As a fundamental application of compressive sensing, magnetic resonance imaging (MRI) can be efficiently achievable by exploiting fewer k-space measurements. In this paper, we propose a constrained total generalized variation and shearlet transform-based model for MRI reconstruction, which is usually more undemanding and practical to identify appropriate tradeoffs than its unconstrained counterpart. The proposed model can be facilely and efficiently solved by the strictly contractive Peaceman–Rachford splitting method, which generally outperforms some state-of-the-art algorithms when solving separable convex programming. Numerical simulations demonstrate that the sharp edges and grainy details in magnetic resonance images can be well reconstructed from the under-sampling data.     

Coupled CFD–DEM method for undrained biaxial shear test of methane hydrate bearing sediments

Methane hydrate (MH), a potential source of future energy, is extensively deposited in marine sediments. It is essential to understand the mechanical properties of methane hydrate bearing sediments (MHBS) for applications relevant to mining and geotechnical engineering. This study aims to investigate the undrained shear strength of MHBS through coupled computational fluid dynamics and discrete element method (CFD–DEM) numerical approach. The Tait’s fluid state equation is implemented into the Navier–Stokes equation-based CFD, while the DEM is used to model granular particle system of MHBS. The CFD–DEM tool is first verified by two typical geomechanics problems where analytical solutions are available. The simulations show that the stress–strain behavior of MHBS depends on temperature, back pressure and MH saturation, as observed in reported experimental results. The presence of MH alters the hardening response of clean sand into softening response due to the bonding effects of MH. The friction angles and cohesions described by total stress and effective stress both increase as the back pressure and MH saturation increase or the temperature drops. There is significant localization in MH bond breakage events but no localization effect is observed in fluid flow and excess pore pressure distribution. This is because fluid is mostly controlled by the boundary conditions instead of specific fluid–particle interactions locally in the simulated quasi-static loading.     

A simple interaction law for viscous–inviscid interaction

The viscous–inviscid interaction (VII) philosophy for modelling aerodynamic boundary layers is discussed. ‘Traditionally’ the shear-layer equations are solved with pressure prescribed by the inviscid flow, but then the solution breaks down in a singularity related to flow separation. In the quasi-simultaneous coupling approach this singularity is overcome by making use of an interaction law. A novel mathematical analysis is presented of the essential properties of such interaction laws, which is based on classical theory for non-negative matrices. The performance of a highly simplified interaction law is demonstrated for separated airfoil flow beyond maximum lift.     

Coupling of membrane element with material point method for fluid–membrane interaction problems

This paper proposes a coupled particle–finite element method for fluid–membrane structure interaction problems. The material point method (MPM) is employed to model the fluid flow and the membrane element is used to model the membrane structure. The interaction between the fluid and the membrane structure is handled by a contact method, which is implemented on an Eulerian background grid. Several numerical examples, including membrane sphere interaction, water sphere impact and gas expansion problems, are studied to validate the proposed method. The numerical results show that the proposed method offers advantages of both MPM and finite element method, and it can be used to simulate fluid–membrane interaction problems.     

Encapsulation and microwave hybrid processing of Al 6061–Graphene–SiC composites

The demand for new aluminum alloy–based metal matrix composites with combinations of novel reinforcements, processed through innovative methods are very much needed for critical engineering applications. With this perspective, the current research work is aimed at the development of Al 6061 composites reinforced with two-dimensional Graphene nanoflake-encapsulated SiC. Ultrasonic liquid processing method is used to disperse the Graphene flake and the mixture is ball milled by adding SiC to achieve the encapsulation. Subsequently, the Al 6061 powder is added to the milled mixture and consolidated through uniaxial vacuum hot press followed by microwave hybrid sintering. Scanning electron microscope (SEM) analysis, X-ray diffraction analysis, hardness, density, and microstructure analysis were carried out on developed composites. Raman analysis was carried out to analyze the distortion on Graphene physical structure during various processing stages. Further, effects on novel combination of material with combined processing approach on flexural and tribological behavior have been analyzed.     

Immersed smoothed finite element method for fluid–structure interaction simulation of aortic valves

This paper presents a novel numerical method for simulating the fluid?Cstructure interaction (FSI) problems when blood flows over aortic valves. The method uses the immersed boundary/element method and the smoothed finite element method and hence it is termed as IS-FEM. The IS-FEM is a partitioned approach and does not need a body-fitted mesh for FSI simulations. It consists of three main modules: the fluid solver, the solid solver and the FSI force solver. In this work, the blood is modeled as incompressible viscous flow and solved using the characteristic-based-split scheme with FEM for spacial discretization. The leaflets of the aortic valve are modeled as Mooney-Rivlin hyperelastic materials and solved using smoothed finite element method (or S-FEM). The FSI force is calculated on the Lagrangian fictitious fluid mesh that is identical to the moving solid mesh. The octree search and neighbor-to-neighbor schemes are used to detect efficiently the FSI pairs of fluid and solid cells. As an example, a 3D idealized model of aortic valve is modeled, and the opening process of the valve is simulated using the proposed IS-FEM. Numerical results indicate that the IS-FEM can serve as an efficient tool in the study of aortic valve dynamics to reveal the details of stresses in the aortic valves, the flow velocities in the blood, and the shear forces on the interfaces. This tool can also be applied to animal models studying disease processes and may ultimately translate to a new adaptive methods working with magnetic resonance images, leading to improvements on diagnostic and prognostic paradigms, as well as surgical planning, in the care of patients.     

Reactive processing and properties of nickel aluminide–carbon nanotube composites

This paper for the first time investigates combustion synthesis of 3Ni-Al–carbon nanotube (CNT) powder compacts simultaneously under two modes of ignition (electrically activated reaction synthesis (EARS) and electrically ignited self-propagating high-temperature synthesis (EISHS)), using a novel setup. The resulting phases, microstructure, porosity, and hardness were investigated and discussed. An increase in CNT (0–3 vol.%) content led to an increase in product porosity and microstructural inhomogeneity. This was seen most dramatically in the EISHS mode compared to the EARS. The increased pore content in EISHS regions resulted in lower hardness values. However, in regions of the compact that experienced EARS, an increase in CNT content led to an increase in microhardness, in spite of the higher porosity levels. The latter result highlights the important benefit of adding CNTs to nickel aluminides.