Integrated Design and Analysis Tools for Reduced Weight, Affordable Fiber Steered Composites

With their high specific stiffness, corrosion resistance and other characteristics, especially their outstanding performance in product weight loss, fiber-reinforced resin matrix composites are widely used in the aviation, shipbuilding and automotive fields. The difficulties in minimizing defects are an important factor in the high cost of composite material component fabrication. Fiber steering is one of the typical means of producing composite parts with increased strength or stiffness. However, fiber waviness is an important defect induced by fiber steering during the fiber placement process. Meanwhile, the laying speeds of the inner and outer tows along the path width direction are different during the fiber steering process, resulting in different interlaminar bond strengths. Therefore, the fiber waviness and uneven interlaminar bonding strength during fiber steering not only affect the dimensions of a composite product, but also influence the mechanical properties of the part. This study aims to reduce fiber waviness and improve interlaminar bonding uniformity along the path width direction using a multi-piece compaction roller. By analyzing the mechanism of the generation of fiber waviness, the interlaminar bonding strength for each tow during fiber steering is investigated. Through analyzing and optimizing the compaction force, laying temperature and laying velocity during fiber steering experiments, the optimization approach is verified.

Design, manufacturing, and testing of a variable stiffness composite cylinder

Effect of structural parameters on design of variable-stiffness composite cylinders made by fiber steering

Non-destructive investigation of thermoplastic reinforced composites

For Variable Stiffness (VS) composites with steered curvilinear tow paths, the fiber orientation angle varies continuously throughout the laminate, and is not required to be straight, parallel and uniform within each ply as in conventional composite laminates. Hence, the thermal properties (conduction), as well as the structural stiffness and strength, vary as functions of location in the laminate, and the associated composite structure is often called a “variable stiffness” composite structure. The steered fibers lead not only to the alteration of mechanical load paths, but also to the alteration of thermal paths that may result in favorable temperature distributions within the laminate and improve the laminate performance. Evaluation of VS laminate performance under thermal loading is the focus of this chapter. Thermal performance evaluations require experimental and numerical analysis of VS laminates under different processing and loading conditions. One of the advantages of using composite materials in many applications is the tailoring capability of the laminate, not only during the design phase but also for manufacturing. Heat transfer through variable conduction and chemical reaction (degree of cure) occurring during manufacturing (curing) plays an important role in the final thermal and mechanical performance, and shape of composite structures. Three case studies are presented in this chapter to evaluate the thermal performance of VS laminates prior to and after manufacturing. The first case study is a numerical analysis that investigates the effect of variable conductivity within the VS laminate on the temperature and degree of cure distribution during its cure cycle. The second case study is a numerical analysis that investigates the transient thermal performance of a rectangular VS composite laminate. Variable thermal conductivity will affect the temperature profile and results are compared to a unidirectional composite. The effect of fiber steering on transient time and steady state solutions is compared to the effect of unidirectional fibers. The third case study is an experimental one that was conducted to evaluate the thermal performance of two variable stiffness panels fabricated using an Advanced Fiber Placement (AFP) Machine.

Fibre waviness is a frequently encountered problem in composite design and manufacturing as it can severely influence mechanical properties of components. In this work we propose a new method for the detection and quantification of out-of-plane fibre waviness in carbon fibre composites using Talbot-Lau grating interferometry. The sensitivity of X-ray dark-field imaging to the orientation of carbon fibres is exploited to visualize fibre waviness by radiographic imaging and reduce measurement times in comparison to full computed tomography scans. We show that fibre waviness can be qualitatively detected by single radiographic images providing a valid option, e.g. for in-line monitoring of similar specimens. Furthermore, quantitative evaluation of fibre waviness angles can be performed by repeated radiographic imaging over an angular range of roughly 10°. Therefore, the number of required projection images can be reduced significantly. With this method, we evaluated wave angles at an error of less than ±1.5° compared to results achieved by full computed tomography scans.

Mapping the natural variation in whole bone stiffness and strength across skeletal sites

The reliability of overhead electric and telecommunication lines depends principally on the quality of their support structures. These structures are generally made of wood, metal or concrete. The complexity of a natural substance such as wood requires a thorough analysis of the various factors that influence its overall quality. In the case of wood poles, such factors include initial forest growth pattern, the species of wood and its preservative treatment, ageing characteristics, and its various mechanical defects such as knots, cracks etc. The accumulation of knowledge on the effect of the various variables that contribute to the overall quality of a wood support structure permits an optimum use of such a resource. For example, less variability and higher strength of wood support structures permits optimum loading and spacing between structures, thus reducing the number needed in a specific length of an overhead line. If one assumes that in Western Europe 1 wood pole is employed for every 2 inhabitants, and that this proportion increases in less densely populated countries such as the US and Scandinavia, the economics of optimum use of wood as a resource soon become apparent. In less developed countries, the proportions and the economics vary depending on the natural resources such as wood that they employ. The goal of this research is to establish, thanks to non destructive evaluations, a general ageing probabilistic law of the wooden pole based on two distinguished laws: one on the pole in studying the influence of a grading of the bad elements based on a normal law: left-truncation of a normal distribution, point 1; and another one based on the in-field wooden pole in exploiting the different parameters such as: the age of the its chemical treatment, its species, its knots etc. in order to define the pole's damage law, point 2. Statistical distribution law of the wooden pole after grading by non destructive sorting (ultrasounds) of the high mechanical performances supports: This distribution law is a Gaussian law or evolves to a Log or Weibull's law with 3 parameters according to the inspected species. This grading allows a revalorization of the properties of the poles and of the design values while guaranteeing an index of reliability required by the design standards, or in improving directly this nominal reliability (economic gain and reliability gain). Statistical distribution law of an aged in-field population (20-50 years old) approached by a bi-modal law which depends on: The distribution law of the component (see point 1) and its minimal extreme law, which is asymmetrical, for an observation on 50 years. The statistical distribution at the time t of the residual mechanical performances of a group of supports making a local net, evaluated by non destructive methods. The non destructive evaluation is based on the measurements of physical variables (density, biological moisture content) and some descriptive variables from natural origins (diameter, knots, cracks...) and from accidental origins (diameter reduction, lightning cracks...). The statistical distribution at the time t is then obtained on the basis of a model of multivariate non destructive evaluation, generalized to the whole of species and treatments. This model is the other concrete goal to reach in this thesis. As a conclusion, the research demonstrates the influence and the interaction of the pole grading (distribution at t0) on the modelisation of the distribution at ti (multivariate non destructive model). The data used for the mentioned modelisations come from a significant international database with a large amount of inspected wood poles and with studied cases. This database is the synthesis of about 15 years of research and development leaded by IBOIS-EPFL and its international partners. The probabilistic approaches are then validated by a huge database allowing thus to be directly exploitable. On this basis, all the standards dealing with the poles and dealing with the controls and maintenances of a wooden pole networks, could be re-examined for a double gain: Concerning the economy: by increasing the capacity of the poles profiting of an objective quality assurance, and by increasing the life time of the in-field in knowing how to purge only the ones which are under the critical threshold of damage Concerning the reliability: by increasing the reliability of the network from the stage new pole, by eliminating the weakest components, and by maintaining this reliability during all the life time of the network thanks to a cyclic preventive maintenance (every 5 to 8 years) and the replacement of only the weakened poles.

The development of unmanned aircraft presents the challenge of designing structural components that are both lightweight and environmentally friendly, while maintaining high mechanical performance to withstand aerodynamic loads and protect internal systems. This study investigates the effect of resin type and fiber orientation on the fracture toughness and specific strength of composite materials reinforced with pineapple leaf fiber—an agricultural waste with renewable potential. An experimental approach was employed to quantitatively assess the influence of resin type and fiber orientation on the mechanical performance indicators. The result indicates that epoxy resin combined with 0° fiber orientation yields the highest fracture toughness at 31.66 MPa√m, while polyester resin with the same fiber orientation achieves the highest specific strength at 59,642.2 N.m/kg. These findings demonstrate that optimizing fiber orientation and resin selection significantly enhances the mechanical properties of natural fiber composites. Pineapple leaf fiber, when processed appropriately and paired with suitable resin systems, offers a promising, sustainable alternative to synthetic fibers in lightweight structural applications.

In vivo bone remodeling rates determination and compressive stiffness variations before, during 60 days bed rest and two years follow up: A micro-FE-analysis from HR-pQCT measurements of the berlin Bed Rest Study-2

A design algorithm to model fibre paths for manufacturing of structurally optimised composite laminates

The crack propagation resistance through a porous or microstructurally heterogeneous brittle solid with local variability in strength and stiffness has been simulated. Specifically, the simulation probes the behavior of porous brittle materials in the range of porosity less than those of cellular materials and greater than those of microstructures that are in the category of dilute porosity. The simulation plane consists of a triangular network of points interacting with each other through both linear central force springs and bond angle springs, incorporating an appropriate element of a noncentral force contribution. Explicit microstructural details were incorporated into the model and the simulation was first carried out under conditions of uniaxial tensile strain in order to investigate the mechanisms of subcritical damage evolution, leading to quasi-homogeneous fracture. In order to investigate material strength and stiffness variability on the scale of a representative volume element for coherent fracture events in a crack tip stress gradient, the explicit microstructural results were incorporated into a simulation with boundary conditions characteristic of the displacement field of an infinite Mode I crack. To impart some 3D realism to the primarily 2D simulations a special 2D super-element was devised, which incorporated variability information as might be sampled by a crack front in three dimensions. For a given porosity, in general, only small differences were found between nominally diverse microstructures in terms of their tensile toughness, maximum strength and elastic moduli. The strongest dependence of the overall fracture toughness was found to come from the average porosity. The variability in local element strength and stiffness on the scale of the porosity produced highly tortuous crack paths, roughly on the scale of the chosen representative volume element. The tortuosity of the crack was largest where local variability of strength and stiffness was uncorrelated. Examples of microcrack toughening and crack bridging were observed.

Mechanical properties of concrete reinforced with reused tyre steel fibres

Corewood and compression wood, both with high S2 microfibril angles, are the worst parts of the tree most in need of improvement in wood quality. This study focuses on the characteristics of cellulosic reinforcement in wood with high S2 microfibril angles lying between 35° and 60°, as well as the probable influence of these characteristics on wood longitudinal mechanical properties. Stiffness and tensile strength were measured on 221 thin radiata pine wood strips. The results indicate that at high angles, the average values of wood stiffness and tensile strength are low with relatively large variations. The variations of stiffness and strength are weakly correlated with the variation of S2 microfibril angles, regardless of wood type. Further study involved examining the characteristics of cellulose microfibrils within the S2 layer using dual axis electron tomography for samples with the same microfibril angle but differing mechanical performance. From the fact that the severe compression wood possessed fewer cellulose microfibrils and had abundant dislocations, and that at many kinking points these cellulose microfibrils were broken down into shorter dislocation segments, it was concluded that the characteristics of cellulose microfibril reinforcement are responsible for the inferior stiffness and tensile strength in the severe compression wood compared with normal corewood with the same high S2 microfibril angle.

An experiment was designed to determine the critical design parameters for a metal/composite joint in a composite electronic enclosure where both thermal and mechanical performance are important. A combination bolted and bonded single-lap joint that simulates the interface between the thermal wall and the card guide was studied under static loading with power supplied through a surface-mounted chip resistor. The following parameters were varied: fiber orientation, type of thermally conductive pitch fiber, type of adhesive, adhesive thickness, bolt spacing, bolt size, and input power. Load, strain, and temperature from five thermocouples were collected during a tensile test with a 0.02 in/ min. strain rate. A fractional factorial experimental design was used. A dual failure mechanism was consistently recorded: failure of the adhesive bond followed by bolt shear failure. Initial screening results indicated that a configuration can be achieved that significantly enhances both mechanical and thermal performance.