Michael A. Stubblefield

Various shape memory effects of stimuli-responsive shape memory polymers

One-step dual-shape memory polymers (SMPs) recover their original (permanent) shape upon small variation of environmental conditions such as temperature, electric field, light, magnetic field, and solvent/chemicals. For advanced applications such as aerospace and medical devices, complicated, multiple-step, spatially controllable, and two-way shape memory effects (SMEs) are required. In the past decade, researchers have devoted great effort to improve the versatility of the SME of SMPs to meet the needs of advanced applications. This paper is intended to review the up-to-date research endeavors on advanced SMEs. The problems facing the various SMPs are discussed. The challenges and opportunities for future research are discussed. (Some figures may appear in colour only in the online journal)

Investigation into FRP repaired RC columns

Due to the aging of the infrastructures in the US, repair and rehabilitation of damaged steel reinforced concrete structures using fiber reinforced plastics (FRP) are increasingly becoming a topic of interest in the infrastructure community. In this study, a finite element analysis using ANSYS® was utilized to conduct a parametric analysis. Experiments were also conducted to justify the finite element analysis results. A reasonable agreement was found between the finite element analysis and the test results. The effect of the thickness, stiffness, and fiber orientation of the FRP layers as well as the interfacial bonding between the FRP wraps and the concrete on the strength and stiffness of the repaired columns was evaluated using the finite element modeling.

Investigation of prepreg bonded composite single lap joint

Adhesive bonded single lap joint has been used extensively in laminated composite structures. Using neat resin adhesives, however, the joint strength is comparatively low and the fabrication time is long. In order to increase the joint strength and reduce the fabrication time, two types of fiber pre-impregnated (prepreg) composites were used to bond composite single lap joints. Test specimens were prepared per ASTM D 3165-95 standard. Ninety days of accelerated conditioning using seawater and ultraviolet radiation were conducted to investigate the long-term performance of prepreg bonded single-lap joint in an offshore environment. The shear strength of various specimens was obtained using tension tests. Two types of neat resin bonded specimens were also used for comparisons. Finite element analysis was implemented to justify test results. Parameters affecting the load carrying capacity of prepreg bonded composite single lap joints were investigated based on finite element analysis results.

Louisiana: a model for advancing regional e-Research through cyberinfrastructure

Louisiana researchers and universities are leading a concentrated, collaborative effort to advance statewide e-Research through a new cyberinfrastructure: computing systems, data storage systems, advanced instruments and data repositories, visualization environments and people, all linked together by software programs and high-performance networks. This effort has led to a set of interlinked projects that have started making a significant difference in the state, and has created an environment that encourages increased collaboration, leading to new e-Research. This paper describes the overall effort, the new projects and environment and the results to date.

Crack Initiation Process of DCB Specimens Based on First-order Shear Deformation Theory

The current work develops an analytical model which can consider the crack initiation process of double cantilever beam (DCB) specimens. The current model is based on the first-order shear deformation beam theory, and thus includes the effect of shear deformation in the beams on the crack initiation process. The relationship between the remote peel load P and loadline deflection u is explicitly established based on a parametric equation of crack tip separation δ for the crack initiation process. The nonlinear response in the ascending branch of the loading process is captured by the present analytical model. With properly defined cohesive laws (such as exponential type), it might not be necessary to define a clear final separation δf for the crack propagation. The comprehensive comparisons with test and numerical results validate the accuracy of the present model for predicting the crack initiation and propagation of DCB specimens. This model can be used for predicting the debonding process of adhesively bonded composite joints.

Shear Strength Characteristics of an Ultrasonic Welded Lap Shear Joint

Experimental attempts were carried out on ultrasonically joining glass fiber composite materials using fiber reinforced adhesive. Two sets of specimens with different energy guides were investigated. All the samples failed by shear at the interface of the lap joint. Finite element analysis was conducted to justify the test results and the effect of adherend surface treatment.Copyright

Engine Performance and Emission Products of Pure Diesel and Multi-Feedstock Blended Biodiesel

Due to the ever-reducing conventional petroleum resources, considerable research on renewable energy sources such as biodiesel as a possible “greener” substitute fuel for internal combustion engines is needed. This study aims to compare the engine performance and emission results of various blends of pure diesel and a multi-feedstock (MFS) biodiesel when used in a naturally aspirated air-cooled, single-cylinder direct injection diesel engine. The engine was coupled to a dynamometer for torque measurement and output data transmitted to a PC for post-processing and displayed using customized programs in the computer. Engine combustion products — Nitrogen Oxide emissions (NOx), Hydrocarbons (HCs), Carbon monoxide (CO) and Carbon dioxide (CO2) — were measured and are presented alongside performance properties including brake-specific fuel consumption (BSFC), engine efficiency, torque and power. The experimental results show that, relative to diesel, biodiesel had approximately 3–24% decrease in torque, 4–11% decrease in power, 11–32% increase in BSFC and 8–29% general reduction in engine efficiency. However, biodiesel reduced the emissions of CO (1.5–6%), CO2 (13–34%) and unburned HCs (3–25%), while NOx emissions were increased significantly (12–48%). These results indicate that smaller percentages of biodiesel (20% or less) could be blended with pure diesel and used in a diesel engine, without any engine modifications, as an alternative and environmentally friendly fuel and without significantly compromising engine performance.Copyright

Rheological Analysis of Curing Process of Epoxy Prepreg Used as Composite Pipe Joints

The rheological properties of curing process of epoxy prepreg were measured by Bohlin Rheometer. The variations of storage modulus, loss modulus and viscosity are monitored vs. the cure time and temperature. Viscosity profiles were described by different models. Except the first order viscosity models, new viscosity models based on Boltzmann function were proposed. In the new models, a parameter called critical time was introduced. Critical time is a function of temperature and also meets an Arrhenius law. The activation energy calculated by critical time closes to that obtained by initial viscosity. The kinetic rate constants in the old and new models are comparable at each temperature, and the kinetic activation energies calculated from rate constants in the old and new models are very close. The fitting results show that the proposed new viscosity models are better than the old models for both isothermal and dynamic cure processes.Copyright

Transient Two-Dimensional Numerical Modeling of Asymmetric Curing Process

Modeling of a heat-activated coupling process of fiberglass reinforced epoxy composite pipe with a copper nickel 90/10 (Cu 90% Ni 10%) alloy pipe was investigated in this study. A nonlinear-coupled two-dimensional heat diffusion model was used to capture the essentials of in-situ thermal transport during the curing process through the thickness of the prepreg wrapping layers. The resulting nonlinear boundary value problem was solved using an Alternating Direction Implicit (ADI) finite difference model (FDM). Transient temperature distributions and degree of cure were predicted for the prepreg layers with and without a heating source at the side of the alloy pipe. A reasonable agreement was found between the predicted temperatures and the experimental results. Measures to improve the curing quality of prepreg layers were discussed based on the modeling results.© 2002 ASME