Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisor: Jingguang G. Chen, Dept. of Chemical Engineering. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 1992. / Vita. Includes bibliographical references (leaves [267]-272).

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 222-230). Also available via the Internet.

When a porous honeycomb core is glued to plane facings to make a sandwich construction, glue fillets (concave menisci) are formed around the core cell edges. It is known that glue fillets play an important role in strengthening the bond of the construction, but only few studies on the real function of the fillet have been reported. This thesis investigates the relationships between fillet size and bonding strength in sandwich construction followed by a stress analysis of the fillets. Sandwich panels with various fillet sizes were produced by means of a glue applicator of original design using a modified phenol-resorcinol resin glue, kraft paper honeycomb cores and Douglas fir plywood facings. Tensile strength tests normal to the sandwich specimens of 1 by 1 inch, and flexure tests on the sandwich beams of 3.75 by 12 inches were performed. Fillet rupture sizes and actual fillet dimensions were measured. A highly significant correlation was found between fillet size and bonding strength. Larger fillets provided greater bonding strength. When a sandwich was subjected to tensile load, a vertical shear failure took place at the center of the fillet concave meniscus regardless of fillet size. By assuming the uniformity of fillet shape, the following equation: [symbol omitted]= my + d , was found to express the relationship between the vertical shear stress [symbol omitted] at the fracture point B and the fillet height y at B, where m and d were constants. Too large fillets had tendency to form voids or bubbles within them resulting in lowering strength values. The appearance of fracture in the glueline in flexure test specimens was similar to that in the tensile test. Most of the sandwich specimens with smaller fillets failed in the glueline, while those with larger fillets mostly failed in core shear. This observation also indicated the superiority of larger fillets in bonding of honeycomb-to-plywood. The cause of glueline failure in the flexure test was deemed to result from a complex system of shear, compression and tensile stresses. However, a mathematical expression describing that system of stresses was not found. / Forestry, Faculty of / Graduate

Tensile properties of polyethylene fibres are shown to be very good in comparison to properties of other advanced composite reinforcing fibres. Nevertheless, the use of polyethylene fibres in polymeric matrix composites suffers due to a poor fibre to resin adhesion. However, its ballistic properties are excellent because of the poor adhesion and also fibre ductility. Applications involving structural use of polyethylene fibres are limited by, among others, the low compressive and shear strengths. These are affected strongly by the degree of adhesion. Improvements in bonding are expected to result in greater commercial appeal for the fibres as the property limitations are reduced. Ultra Violet radiation has been shown previously, in laboratory scale batch studies, to induce graft co-polymerization of monomers to polyethylene films. Improvements in wettability and adhesion result when the grafted polymer is compatible to the bonding medium. In this study the technique was adapted to bench scale, continuous fibre treatment, whereby the monomer was surface grafted to the polyethylene substrate. Acrylic acid monomer was used for this due to its relative safety, small molecular size, and high reactivity. Reaction initiation was provided by use of a benzophenone photosensitizer due to the stability of polyethylene to UV radiation. The reaction was performed by pre-coating the fibres with reactants, then exposure to UV radiation. Results of the continuous process for fibre treatment indicate that the monomer concentration and temperature of the preliminary soakings are key variables. Adhesion improvement was measured by single fibre pullout tests and interlaminar shear strength (ILSS) tests. Of these, the ILSS appeared to be more sensitive for judging small improvements. Tensile tests were used to judge property deterioration due to treatment, and flexural property tests gave a preliminary indication of material behavior. The ILSS showed marked improvement from 1.5 ksi for untreated material to over 5.2 ksi for the better treatments. A competing treatment, plasma, shows ILSS values around 3 ksi. The flexural test indicated that failure of UV-grafted polyethylene was in tension, whereas failure of plasma and untreated material was in compression. The study has proven successful in improving the adhesion of polyethylene fibres to an epoxy resin matrix. Commercial viability is currently being developed through decreased process residence times and irradiation exposures. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, August 2019 / An extension to classical lamination theory (CLT) is presented to analyse the natural fre- quencies and critical buckling loads of simply supported functionally graded plates. The variation of the through-thickness properties of the plate is governed by a power law which is subsequently represented by a polynomial series of su cient order and varies according to the law of mixtures or the Mori-Tanaka Homogenization method. The sti ness matrices are found, from which the position of the neutral plane is established which allows for the governing equations for the natural frequency and critical buckling load to be derived using the Rayleigh-Ritz method. The natural frequency and critical buckling loads are determ- ined for various volume indices, aspect and span ratios and the accuracy thereof is validated against 2D, 3D and quasi-3D solutions found in literature. A comparison with CLT found that the present study produces natural frequencies and critical bucking loads which are more accurate and which converge faster than CLT. / NG (2020)

The effects of room temperature tensile loading and five thermal loadings upon the development of transverse microcracks (TVM) in Celion 6000/PMR-15 graphite-epoxy laminates were investigated. Microcracks were observed using a replicating technique, microscopy and x-ray. The mechanical or thermal load at which microcracking initiates and the ply residual stresses were predicted using laminate analysis with stress and temperature-dependent materials. The TVM density as a function of tensile load was predicted using the multiple cracking theory proposed by Garrett, Parvizi and Bailey. Reasonably good correlation between theory and experiment was obtained for both the load at initial failure and the TVM density as a function of tensile load. It has been shown that TVM density and the load to initiate microcracking are functions of the type of loading and the laminate configuration. Generally, cross-ply laminates exhibit higher TVM density after thermal loading than do quasi-isotropic laminates, but the converse is true for tensile loading. Cross-ply laminates attain saturation TVM density prior to failure, but the TVM density of quasi-isotropic laminates continues to increase until failure. Edge effects have a significant influence on the development of TVM, and TVM present at the free edge of a laminate extend throughout the entire width of the laminate. <i>[Vita removed Feb. 15, 2011. GMc]</i> / Master of Science

Metal plates are being replaced by lighter but equally strong laminated composite plates in order to improve efficiency and increase performance of aerospace vehicles. But because of the complex construction of laminated plates they are very difficult to analyze. Conventional thin plate theories prove to be inadequate in predicting laminated composite plate behavior. Therefore, a finite element model which incorporates a first- order shear-deformation theory and nonlinear von Karman strains is described. The model is shown to accurately predict deflections in laminated composite plates due to nonuniform transient heat fluxes and transverse mechanical loads. / Master of Science

The present work deals with modes and mechanisms of failure in compression of angleply laminates. Experimental results were obtained from 42 angle-ply IM7/8551-7a specimens with a lay-up of [ (±9) / (=F9)] 6s where 9, the off-axis angle, ranged from 0° to 90°. The results showed four failure modes, these modes being a function of off-axis angle. Failure modes include fiber compression, inplane transverse tension, inplane shear, and inplane transverse compression. Excessive interlaminar shear strain was also considered as an important mode of failure. At low off-axis angles, experimentally observed values were considerably lower than published strengths. It was determined that laminate imperfections in the form of layer waviness could be a major factor in reducing compression strength. Previously developed linear buckling and geometrically nonlinear theories were used, with modifications and enhancements, to examine the influence of layer waviness on compression response. The wavy layer is described by a wave amplitude and a wave length. Linear elastic stress-strain response is assumed. The geometrically nonlinear theory, in conjunction with the maximum stress failure criterion, was used to predict compression failure loads and failure modes for the angle-ply laminates. A range of wave lengths and amplitudes were used. It was found that for 0° S 9 S 15° failure was most likely due to fiber compression. For 15° < 9 oS 35° failure was most likely due to inplane transverse tension. For 35° < e ~ 70° failure was most likely due to inplane shear. For e > 70° failure was most likely due to inplane transverse compression. The fiber compression and transverse tension failure modes depended more heavily on wave length than on wave amplitude. Thus using a single parameter, such as a ratio of wave amplitude to wave length, to describe waviness in a laminate would be inaccurate. Throughout, results for AS4/3502, studied previously, are included for comparison. At low off-axis angles, the AS4/3502 material system was found to be less sensitive to layer waviness than IM7 /8551-7 a. Analytical predictions were also obtained for laminates with waviness in only some of the layers. For this type of waviness, laminate compressive strength could also be considered a function of which layers in the laminate were wavy, and where those wavy layers were. Overall, the geometrically nonlinear model correlates well with experimental results. / Master of Science

Interlaminar stresses are estimated from plate theories by equilibrium. The elasticity equations of equilibrium are integrated with respect to the thickness coordinate z using the linear distribution in z of the in-plane stresses. This procedure, for example, requires fourth order derivatives of the out-of-plane displacement w with respect to the in-plane coordinates x and y to compute the interlaminar normal stress. Since compatible elements for the plate bending problem at most require the displacement and its first derivatives to be continuous across element boundaries, low degree interpolation polynomials are used. Thus, fourth order derivatives of the finite element polynomials are either meaningless, or at least inaccurate. In order to compute high order derivatives, an approximate polynomial solution of high degree to the governing partial differential equation for w(x,y) is determined using the finite element solution as a first approximation. A rectangular subdomain that may consist of several elements is selected from the finite element model. The displacement w(,y) over the subdomain is expanded in a Chebyshev series. Then collocation is used to determine the unknown Chebyshev coefficients such that the Chebyshev series matches displacement w and its normal derivative from the finite element solution at discrete points on the boundary of the subdomain, and the partial differential equation is enforced at discrete points within the subdomain. Interlaminar shear and normal stresses are computed from the third and fourth derivatives, respectively, of the Chebyshev series at the collocation points. / Master of Science