Automated tape or fiber placement (ATP/AFP) with in-situ consolidation has been identified as a promising manufacturing technique for thermoplastic (TP) composites, which are highly in-demand in the aerospace industry for future aircraft structural applications. This manufacturing technique is attractive since it has the potential to eliminate the energy and time consuming autoclave consolidation. However, low quality is one of the biggest challenges in the application of in-situ consolidation. It seems that pressure contribution to the bond quality has received less attention than thermal one. Therefore, this work is initiated to simulate the pressure distribution between the compaction roller and the mandrel.
The objective in this work is to predict the pressure distribution in the contact area between the rubber-covered roller and the mandrel in room temperature by finite element methods (FEM). Rubber material is characterized by mechanical testing and then modeled as hyperelastic material. 3rdd-order Ogden material model is found to well describe the strain- stress behavior of rubber material. The material constants are then implemented in FE models as an input. The pressure distribution is predicted by FE modes and the influence of compaction force, rubber thickness, pre-stretching force on the pressure distribution are discussed, followed by an optimization between compaction force and rubber thickness. Experiments are conducted to validate the FE models regarding the three influencing factors, that are: (a). compaction force, (b). with/without pre-stretching and (c). various rubber thickness. Rubber deformation can be captured by the digital image correlation (DIC) Pressure distribution is obtained from the Prescale pressure measurement film produced by Fujifilm®, which are the films that show different color densities under different pressures. Experimental results of strain and pressure distribution are compared with FEM results, followed by discussions and conclusions.
It can be concluded that pre-stretching force should be used to avoid rubber-roller separation and prevent rubber from moving out to the sides. Both of the compaction force and rubber thickness affects the maximum pressure and contact length linearly. A trade- off between compaction force and rubber thickness can be made based on the force limit of the equipment and pressure requirements. ... Read more

Future wind turbines require larger rotor blade radius, leading to an increase of rotor swept area. The larger rotor swept area results in more energy production of the wind turbine. Further increase is however limited as a result of the structural weight, which induces a large bending moment and could result in buckling of the rotor blade. An innovative design solution resulting in a lower weight blade design could be acquired by introducing a grid stiffened structure as a substitution of the conventional sandwich structure. Design and analysis showed an increase in specific stiffness of the grid stiffened structure for higher loading conditions. In an attempt to validate the obtained results, one sandwich and one orthogrid panel has been manufactured and tested. ... Read more

For several years T-bolts have been a popular choice for joints in the field of wind energy, specifically for connecting the blade roots to the hub of the wind turbine. Their use is mandated by the geometry of the joint and they perform very well under pure axial loading. However recent analyses have shown significant bending stresses in the T-bolts preventing their use to their full capacity. These bending stresses are unavoidable due to the presence of a slew bearing between the blade root and the hub. The bending stresses generated while loading the blade root, causes the blade root-bearing joint to gradually open, causing excessive loading of the T-bolt above a certain load.
It is hypothesized that modifying the blade root design to reduce the effects of local bending can open up the possibility of reducing its mass and cost. To test this hypothesis, the blade root is initially studied and the stress ratio is identified as an appropriate joint performance parameter. The performance of the joint is boosted by increasing the pretension of the bolt. After an initial phase of over designing the joint to reduce the constraint stresses, the joint optimization is carried out using the Sequential Quadratic Programming algorithm. The optimization culminates with the mass reducing by roughly 110Kg and the material cost reducing by approximately 13% per blade root. The number of bolts reduces from 88 to 52. Thus, a simpler design is achieved, that promises simpler and cheaper manufacturability, higher reliability and lesser sites for crack nucleation in the laminates. The current design strategy at Suzlon is to employ a greater number of T-bolts with thinner shanks. Curiosity in the field of cost optimization that initiated from within Suzlon has proved that there exists a different design strategy that holds great promise for delivering structurally equivalent if not better designs with improved cost, mass and reliability. ... Read more

A novel continuum damage mechanics model for 2D woven fabrics has been developed and implemented in a VUMAT subroutine for Abaqus/Explicit. The model takes into account the shear non-linearity, the toughening mechanisms associated to tensile failure, and the influence of shear stresses in the initiation and propagation of tensile damage.

The non-linear shear behavior is reproduced by means of a Ramberg-Osgood equation. Permanent deformation, and the degradation of the secant shear modulus associated to the accumulation of matrix damage have been coupled to the formulation of this unidimensional plasticity law. The physical reference required to define completely the shear constitutive response proposed can be obtained from the experimental tensile test of ±45 off-axis coupons. Failure initiation in tension is identified using Hashin's quadratic failure criterion, which accounts for the interaction of shear stresses in the promotion of tensile failure. A bilinear softening relation was selected to represent the combination of fibre breakage and toughening mechanisms characteristic of the intralaminar tensile fracture of these materials. Effort was placed on the development of a procedure for calibrating the softening relation associated to this failure mode. Specifically, this work addresses the applicability of linking the definition of a bilinear tensile damage law in homogenized continuum damage mechanics models for woven composites to the shape of a crack growth resistance curve measured with compact tension tests.

The constitutive model was validated including it in a set of finite element models of unnotched and open hole coupons with different multistacking sequences, under quasi-static tensile loading conditions, and comparing the results obtained by the simulation of the coupons against an experimental benchmark. A good correlation was achieved between the ultimate strength predicted with finite element analysis and the experimental data. ... Read more

Deployable structures using tape-springs are a promising option to minimize the volume occupied by satellites at launch and reduce launching costs. The attractive characteristics of tape-springs made of ultra-thin woven composites are their high specific stiffness and bi-stable behavior. However, the most important shortcoming is the complexity of their mechanical analysis. Textile composites are formed by the weaving of bundles of fibers creating a very complex microstructure, and tape-springs present a highly nonlinear mechanical behavior subjected to multi-axial loadings. Therefore, conventional mechanical models developed for unidirectional lamina no longer apply since they do not take into account stress gradient effects through the representative volume element (RVE). In this work, a multi-scale approach is validated to predict the failure initiation of tape-springs. Micromechanical models are used to predict analytically the stiffness properties of the fiber bundles, while FE mesoscale models describe the woven structure and provides stiffness and strength properties for computations on macroscale. A case study of hybrid laminates composed of woven and unidirectional layers is analyzed for its application on tape-springs. Manufacturing defects related to ply waviness of the UD layer have been detected and taken into account to estimate the reduction of the overall stiffness properties. Finally, a dedicated failure criteria based on the force and moment resultants have been used to predict failure initiation of the tape-springs under multi-axial loading conditions. A good correlation was found between the finite element analysis and experimental observations. ... Read more

Following in the footsteps of the automotive industry with the successful implementation of Formula E, the E-SPARC design is the world’s first all-electric racing aircraft. E-SPARC’s mission is to proof the feasibility of a sustainable and high performance alternative for the current state-of-the-art in aerobatic racing. Thereby, the aim is to present a design worthy of competing in the popular Red Bull Air Races. Given the combination of being the world’s fastest growing international motorsport with the commitment towards reducing the carbon footprint [1], Red Bull Air Races provide the optimal platformfor the E-SPARC design. The leading design question is therefore whether an all-electric racing aircraft can be designed with performance characteristics equal to or exceeding the performance characteristics of the current competition. This report describes the design decisions and outcomes taken during the preliminary design phase, continuing upon the pusher canard configuration that was selected during the conceptual design phase... ... Read more

Grid-Stiffened composite structures have found their way into a variety of apphcations for their structural efficiency, cost and inherent damage tolerance. Where previous studies and research have focused on global optimization of the far-field grid structure, the current work focuses on the development of an efficient though versatile load introduction zone at the edge of the structure. A global lightweight design requires structural efficiency throughout the structure. Therefore, having a versatile design for the load introduction zones allows for optimization of the far-field structure and can result in a design that efficiently transfers loads to the next structure. To this end a design is developed where the ribs are interwoven with a laminate which can then easily be connected to the next structure. A parametrized Finite Element Method model has been verified through correlation with two different designs which also proves versatility of the rib-to-laminate concept. Using this FEM model the load introduction zone can be optimized for a variety of grid structures and layouts. Since limitations and effects of the manufacturing process can make or break a design, the two analyzed designs are manufactured and tested to prove feasibility and performance. Their quality is assessed and used as input for improvement of the FEM model. The end result is a parametrized and verified concept for load introductions that can be used in the giobfil optinrization of a grid-stiftened composite structure. ... Read more