VARIABILITY IN THE FAILURE OF COMPOSITE TUBES SUBJECTED TO COMBINED AXIAL AND TORSIONAL LOADINGS DUE TO MANUFACTURING DEFECTS AND NONDETERMINISTIC MATERIAL PROPERTIES

Tubes made with polymer-matrix fiber-reinforced composite materials are widely used in automobile, mechanical and aerospace engineering applications. Composite tubes are increasingly manufactured using the modern Automated Fiber Placement (AFP) technique. The ply manufacturing parameters and the tube manufacturing parameters have considerable influence on the quality of the manufactured composite tubes. Manufacturing defects and variations in the material properties are inevitable in composite tubes due to the inherent unavoidable variations in these parameters. The commonly identified manufacturing defects include voids, fiber waviness, variation in volume fraction, and fiber misalignment. These have considerable influence on the mechanical behavior and failure of the composite tube. In the present work, the effects of the fiber misalignment and the variations in the material properties on the failure behavior of uniform-diameter composite tubes subjected to combined axial and torsional loadings are determined considering the First-Ply Failure (FPF) characteristics. The first-ply failure envelopes of the composite tube are developed based on the Classical Laminate Theory and Finite Element Modeling and Analysis. Existing works in the literature are used to validate the three-dimensional finite element model of the uniform-diameter composite tube developed using the commercial software ANSYS®. The variations in the first-ply failure loading limits of the uniform-diameter composite tube made of a Carbon Fiber Reinforced Polymer (CFRP) composite material are investigated using the Monte Carlo Simulation (MCS) method, considering the random variability in the material properties and the fiber misalignment. The random variables corresponding to the material properties and the fiber misalignment are generated. For the composite tube with a sample set of simulated random variables the corresponding first-ply failure envelope is determined. The ensemble of such failure envelopes is developed based on an adequate number of simulations from which the probabilistic distributions of the first-ply failure loadings are determined. Design aspects are brought out.

Effect of Extrusion Parameters on Short Fiber Alignment in Fused Filament Fabrication

Additive manufacturing by material extrusion such as the widespread fused filament fabrication is able to improve 3D printed part performance by using short fiber reinforced composite materials. Fiber alignment is critical for the exploitation of their reinforcing effect. This work investigates the influence extrusion parameters have on the fiber alignment by conducting set of experiments on the process parameters determining whether the flow under the nozzle is convergent or divergent. A strong impact of flow conditions during extrusion line shaping on the fiber alignment is observed and two extremes are tested which show a large difference in strength, stiffness and strain at break in tensile testing along the extrusion lines. From highest to lowest fiber alignment, strength is reduced by 41% and stiffness by 54%. Fiber misalignment also leads to inhomogeneous strain fields in the layers when tested perpendicular to the extrusion lines. It is demonstrated that material flow after the nozzle has a high impact on the material properties of short fiber reinforced 3D printed parts and needs to be considered in process design.

Adherence Kinetics of a PDMS Gripper with Inherent Surface Tackiness

Damage and fiber misalignment of woven fabrics during discontinuous polymer processing remain challenging. To overcome these obstacles, a promising switchable elastomeric adherence gripper is introduced here. The inherent surface tackiness is utilized for picking and placing large sheets. Due to the elastomer’s viscoelastic material behavior, the surface properties depend on loading speed and temperature. Different peeling speeds result in different adherence strength of an interface between the gripper and the substrate. This feature was studied in a carefully designed experimental test set-up including dynamic thermomechanical, as well as dynamic mechanical compression analyses, and adherence tests. Special emphases were given to the analyses of the applicability as well as the limitation of the viscoelastic gripper and the empirically modeling of the gripper’s pulling speed-dependent adherence characteristic. Two formulations of poly(dimethylsiloxane) (PDMS) with different hardnesses were prepared and analyzed in terms of their applicability as gripper. The main insights of the analyses are that the frequency dependency of the loss factor tanδ is of particular importance for the application along with the inherent surface tackiness and the low sensitivity of the storage modulus to pulling speed variations. The PDMS-soft material formulation exhibits the ideal material behavior for an adhesive gripper. Its tanδ varies within the application relevant loading speeds between 0.1 and 0.55; while the PDMS-hard formulation reveals a narrower tanδ range between 0.09 and 0.19. Furthermore, an empirical model of the pulling speed-dependent strain energy release rate G(v) was derived based on the experimental data of the viscoelastic characterizations and the probe tack tests. The proposed model can be utilized to predict the maximum mass (weight-force) of an object that can be lifted by the gripper

Bio Inspired Self-Curing Composite: A Leap into Augmented Enactment

Relentless progress has been made on composite materials, their manufacturing processes and their structural design in past few decades. Nevertheless, the approval of composite materials in all engineering disciplines is constrained due to its susceptibility to various kinds of defects during manufacturing stage viz porosity, foreign body inclusion, incorrect fiber volume, bonding defect, fiber misalignment, ply misalignment, incorrect curing cycle, wavy fiber, ply cracking, delamination, fiber microstructural defects etc. Hence there was a requirement of techniques to somehow overcome these defects during the service life of composites being used in various structures and equipment. This promising field of research has made great progress over the past several years, but many procedural encounters are still to be overcome, and there exists a great need for focused research to address several areas of concern. On the other hand, nature has materials that have curing potential and repair strategies ensuring their survival. Sustained development in the field will produce new curing chemistries that possess greater stability, faster kinetics. Tailor-made placement of curing agents is dynamic research subject at the cutting edge of self-curing. New bio-imitative curing agents are closely connected to vascular networks. The purpose of this technical paper is to sort the methodology in line with ongoing research efforts in composites. A perspective on current and future self-curing approaches using this biomimetic technique is offered.

100 Gb/s Silicon Photonic WDM Transmitter with Misalignment-Tolerant Surface-Normal Optical Interfaces

A 4 × 25 Gb/s ultrawide misalignment tolerance wavelength-division-multiplex (WDM) transmitter based on novel bidirectional vertical grating coupler has been demonstrated on complementary metal-oxide-semiconductor (CMOS)-compatible silicon-on-insulator (SOI) platform. Simulations indicate the bidirectional grating coupler (BGC) is widely misalignment tolerant, with an excess coupling loss of only 0.55 dB within ±3 μm fiber misalignment range. Measurement shows the excess coupling loss of the BGC is only 0.7 dB within a ±2 μm fiber misalignment range. The bidirectional grating structure not only functions as an optical coupler, but also acts as a beam splitter. By using the bidirectional grating coupler, the silicon optical modulator shows low insertion loss and large misalignment tolerance. The eye diagrams of the modulator at 25 Gb/s don’t show any obvious deterioration within the waveguide-direction fiber misalignment ranger of ±2 μm, and still open clearly when the misalignment offset is as large as ±4 μm.

FE Analysis of the Influence of Fiber Orientation to Shearing and Wrinkling of Fiber Reinforced Thermoplastic Parts

This work presents the influence of deviations of fiber orientations of warp and weft yarns to the shear stress vs. shear angle behavior and the formation of wrinkles of fabric reinforced thermoplastics. FE results of bias-extension tests and the forming of a double dome part will be investigated with angular misalignment of the yarns and/or deviations of the blank to its loading direction. The prepregs or organic sheets may have imperfections like fiber misalignment or the prepregs of multilayered sheets are twisted against each other. Furthermore, there may be deviations to the idealized orientation caused by the operator or cutting machine for example.