Damage evolution in a bonded nonwoven glass fiber network under cyclic compression

This study is concerned with nonwoven materials are made of network of glass fibers and consolidated by local bonds, such as geometric entanglement, local thermal fusion, or chemical binders. The objective of the study is to gain understanding of the damage accumulation processes during cyclic compression. The micromechanisms of damage during tensile loading of nonwoven materials have widely been studied and it is found that fiber–fiber bond failure followed by frictional sliding between fiber bundles, leading to the localization of damage is a dominant failure mechanism. However, the fracture mechanism a fiberglass network under compression is less understood. The compressive load response is nonlinear due to fiber-to-fiber contact. Comparing microCT images of samples before and after loading, we infer that fiber fracture is the dominant failure mechanism. Based on these observations, we hypothesize that the damage accumulation under cyclic compressive loading emerges indeed from fiber fracture, but that due to the accumulations of such fractures, high-stress locations are successively eliminated from the microstructure. In order to establish the validity of the hypothesis, the study reports on cyclic loading experiments on a range of strain ranges, in combination with acoustic emission measurements, as well as on a computational modeling effort. Strain-controlled cyclic (low-cycle fatigue) compression tests were carried out on samples of extracted from nonwoven fiberglass mats to determine the evolution of the stress–strain response and the degradation of the material. A global damage variable, D, is introduced to account for the change in modulus with cycle number N. It was found that the degradation is well described by with the degradation exponential k. It was found that although the modulus exhibits noticeable interspecimen variation, k remained consistent for multiple specimens from the same material system; however, it is dependent on the applied strain. In order to further elucidate the fracture mechanism of the fiber network associated with the global strain, in-situ investigation by monitoring the fiber fracture in the structure with the strain history is hereby proposed using acoustic emission (AE) detection technique. In contrast to AE where the system is directly coupled to the sample, here the AE signal is acquired indirectly via the load platens. Based on microCT imaging, the association between acoustic events and fiber fracture can be made. The measurements therefore allow us to link the damage onset with global strain and the influence of AE activity level during multiple load cycles on the material deterioration. Finite element models based on microCT images are used in conjunction with the XFEM approach to further obtain insight into the damage accumulation process. This approach will finally enable us to provide an explanation for the damage evolution law

Active Dense Tensegrity Structure: A Novel Concept For Shape Morphing Structures

Shape morphing is of relevance for advanced engineering systems such as future aerospace vehicles. Tensegrity structures, a structure comprised of a set of discontinuous compressed struts held together with a continuous web of tensioned cables, can function as the foundation of a shape morphing system (Keith et al, 2007). However, the classic tensegrity structures may not be ideal in all configurations because the space within the structure is not utilized. As an alternative, the present work proposes a dense tensegrity system as an assembly of cables carrying tensions only and polyhedral elements under compression only. An active dense tensegrity can be achieved by equipping the dense tensegrity with active cables. Shape Memory Alloy (SMA) wires are capable of working as the actuation cables in the dense tensegrity. As a potential solution to shape morphing wings, active dense tensegrity structures are desired to possess significant differences in bending stiffness (K) when considering the bending direction (up or down). Active dense tensegrity structures were manufactured using tetrahedra shaped particles as the compression elements, and carbon fibers were used as the tensional cables such that an assembly in a dense planar array was obtained. Three types of polyhedral elements were considered: regular and homogeneous tetrahedra, truncated tetrahedra, and “Janus-type” tetrahedra made of part soft and hard solid. For assemblies of regular tetrahedra the tensegrity possesses the same stiffness in both deflection directions (K, upward/K, downward=1 due to symmetry), while for the tensegrities of the truncated tetrahedra (K, upward/K, downward=25.2) and of the “Janus-type” tetrahedra (K, upward/K, downward=4.7), significant bending stiffness asymmetries are realized. A basic theory for the mechanical response of the passive dense tensegrity systems is presented. SMA wires were integrated into the tensegrity in a span-wise manner as the actuation systems. Experiments were performed on the tensegrity with truncated tetrahedra. The results show that the actuation system using SMA wires is capable to induce controlled bending deflection into dense tensegrity structure. Various types of Truncated tetrahedral were generated with different percent of cut-off portion in order to make several active dense tensegrity structures. They are 33%-off, 25%-off, 15%-off, and 0%-off (regular). Passive deflection tests are performed on these samples and results were generated for analyzing stiffness. Active response from bending tests revealed that the maximum bending deflection are 24 mm, 14 mm, 8 mm, and 0 mm for above four different samples in respect. The active dense tensegrity has been demonstrated as a potential solution for shape morphing wing concepts. In the future, the active dense tensegrity will be modified into active shape morphing wings for Ornithopter applications. Also, the extra space created within the dense tensegrity structure has the potential capability for the wings to store fuels for the Ornithopters

Real-time determination of laser beam quality by modal decomposition

We present a real-time method to determine the beam propagation ratio M2 of laser beams. The all-optical measurement of modal amplitudes yields M2 parameters conform to the ISO standard method. The experimental technique is simple and fast, which allows to investigate laser beams under conditions inaccessible to other methods.Comment: 8 pages, 4 figures, published in Optics Expres

Random incidence transmission loss of a metamaterial barrier system

It has been shown previously that a panel comprising a cellular array can yield a normal incidence transmission loss in a specified low frequency range that is significantly larger than that of a homogeneous panel having the same mass per unit area. The cellular metamaterial considered consists of a periodic arrangement of unit plates held in a grid-like frame. However, when the incident sound field is diffuse, the relative advantage of the metamaterial barrier is reduced or eliminated. Here it will be shown through a sequence of experimental measurements that the relative advantage of the metamaterial barrier can be restored by creating a hybrid system consisting of a layer applied to the front surface of the material that causes sound to approach the barrier at normal incidence, and a layer on the rear surface of the material that compensates for the transmission loss minimum that normally follows the peak in a metamaterial barrier transmission loss. In the implementation considered here, the front layer consists of a lattice structure, and the rear layer consists of high performance glass fiber. The role of each of these components will be illustrated using measurements of transmission loss of a 1:2 m square panel system

Design und Implementierung eines optimierenden VHBC-Compilers für die Virtual Hardware Machine und Realisierung der Virtual Hard

Die vorliegende Arbeit beschreibt die Optimierung des VHBC-Compilers, die Erweiterung der Eingabedateiformate des Compilers um EDIF-Netzlisten, seine Anpassung an die veränderte Architektur der VHM und die Realisierung dieser Architektur mittels VHDL. Es wird der Aufbau und die Arbeitsweise des VHBC-Compilers erläutert und die neue Architektur der VHM ausführlich beschrieben. Dem geht ein Vergleich mit bestehenden Ansätzen rekonfigurierbarer Hardware und eine Analyse der Schwachpunkte der bestehenden VHM und des VHBC-Compilers voraus.This work describes the optimization of the VHBC-compiler, its extension to the input format EDIF, its adjustment to the changed architecture of the VHM and the realization of the VHM by means of VHDL

Mechanical Properties of Interlocking Assemblies on a Rhombille Tiling

The use of glue-less assembly methods has permitted the construction of rigid structures for centuries. Japanese interlocking wood joints and stereotomic structures by repetitious stacking of unit blocks are classical examples. The implementation of interlocking structures occurs when materials such as mortar and nails are unavailable or undesired. There has been a recent revival of interest in these construction methods as modern manufacturing tools enable new form and function. As humanity continues to innovate, materials possessing mechanical properties such as heightened flexibility without compromising strength or increased resistance to fracture will be needed. As one such example, this work examines interlocking assemblies emerging from a rhombille tiling. Rhombille tilings are formed by using three rhombuses to create a regular hexagon, then tessellating those hexagons. The resulting assembly is one of disphenoids and has either triangular or hexagonal symmetry. The elements are arranged such that the assembly forms a hexagonal plate with two thirds the density of a solid plate of equal thickness. Rotation free and restricted states are realized. The mechanical properties of this interlocked assembly are examined in finite element analysis and experiments performed on physical models realized by 3D printing. Initial results suggest a chiral response to loading paths in the hexagonally symmetric arrangement. Triangularly symmetric arrangements suggest load paths based on concentric or patterned hexagons. These load patterns are distinctly different from those in comparable solid plates. All assemblies have shown fracture resistance where damage is localized to few elements, leaving the remainder of the plate intact