Publication Date

2017-12-19

Availability

Embargoed

Embargo Period

2019-06-12

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Mechanical Engineering (Engineering)

Date of Defense

2017-10-06

First Committee Member

Landon R. Grace

Second Committee Member

Michael R. Swain

Third Committee Member

Emrah Celik

Fourth Committee Member

Francisco J. De Caso y Basalo

Abstract

Bismaleimide (BMI) is a polymeric thermoset material that is suitable for high temperature structural and electrical applications, particularly when used as a matrix in fiber-reinforced composites for radar-protecting structures (radomes) on commercial and military aircraft. In this role, BMI’s high service temperature and high strength-to-weight ratio are of significant importance in supersonic aircraft. A main disadvantage of this material is the propensity to absorb environmental and aircraft related fluids in many applications, even in hot, dry environments. Moisture absorption has been known to be a deleterious factor in the service life of aircraft structures, especially in the case of dielectric property degradation. In this thesis, the effects of hygrothermal aging conditions and elevated temperatures on quartz-fiber-reinforced bismaleimide specimens were investigated via a split-post dielectric resonant technique at X-band (10 GHz), dynamic mechanical analysis (DMA), and flexural strength assessments. Furthermore, BMI/quartz moisture absorption kinetics were modeled by the three-dimensional hindered diffusion model (3D HDM) and its one-dimensional version of the Langmuir-type absorption model. These methods utilize least square regression to recover composite diffusivities, equilibrium moisture content, and molecular binding and unbinding probabilities with an error of less than 1%. In split-post dielectric experiments, a 1% increase in contaminant content by weight resulted in a 7.8%, 4.5%, and 2.5% increase in relative permittivity of the material due to water, deicing fluid, and propylene glycol, respectively. A more significant impact was seen in the material’s loss tangent, where a 1% increase in contaminant content by weight is responsible for a 378.5%, 592.99%, and 441.49% increase in loss tangent due to the aforementioned fluids, respectively. Since water is the most ubiquitous, a simple desorption method was performed and it was found that relative permittivity and loss tangent increase were reversed, relative to the initial moisture content. Exposure to temperatures at approximately 60% of Tg (316°C) coupled with relatively low moisture content (1.21%) and cyclical (1 Hz) flexural strain on the order of 10-5 were sufficient to induce fiber-matrix debond. This debond was confirmed by scanning electron microscopy (SEM) and was associated with a discrete decrease in flexural strength of up to 26%. The sensitivity of the DMA to the fiber-matrix interface integrity provides a beneficial tool for assessing the effects of thermal and moisture conditions on the onset of fiber-matrix debond, which can be detrimental to radomes in operation.

Keywords

Bismaleimide\Quartz Composite; Moisture Diffusion; Viscoelastic; Dielectric; Aircraft Radome

Available for download on Wednesday, June 12, 2019

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