Samuel Ibekwe

A review of stimuli-responsive polymers for smart textile applications

Stimuli-responsive polymers (SRPs) are smart materials which can show noticeable changes in their properties with environmental stimulus variations. Novel functionalities can be delivered to textiles by integrating smart SRPs into them. SRPs inclusive of thermal-responsive polymers, moisture-responsive polymers, thermal-responsive hydrogels, pH-responsive hydrogels, and light-responsive polymers have been applied in textiles to improve or achieve textile smart functionalities. The functionalities include aesthetic appeal, comfort, textile soft display, smart controlled drug release, fantasy design with color changing, wound monitoring, smart wetting properties and protection against extreme variations in environmental conditions. In this review, the applications of SRPs in the textile and clothing sector are elucidated; the associated constraints in fabrication processes for textiles and their potential applications in the near future are discussed.

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)

Stiffness Degradation of FRP Strengthened RC Beams Subjected to Hygrothermal and Aging Attacks

The past decade has witnessed an ever increasing interest in strengthening, repairing, retrofitting, andu pgrading of deterioratedco ncrete structures using fiber reinforcedplastics (FRP). Enhancedload carrying capacity by FRP strengthening has been observedby a large number of researchers through experiments at ambient environments. In a harsh environment, however, FRP will degrade. This may result in structural degradation of FRP strengthened concrete members. The possible structural degradation has become a major obstacle for the wide-spread acceptance of this new strengthening technique. In the present study, boiling water and ultraviolet (UV) radiation were used to study the structural degradation of concrete beams strengthened with GFRP and CFRP fabrics. A total of eighteen 170 7.62 15.2 cm steel reinforcedconcrete (RC) beams were prepared as control specimens, conditioned specimens, and unconditioned specimens. Finite element modeling was used to analyze stiffness degradation of FRP layers. The tested and calculated results showed that environmental attacks had a considerable effect on the structural degradation of FRP strengthened concrete beams. 57–76% of the strengthening efficiency and43–48% of the stiffness of FRP were lost after conditioning.

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.

Healing of shape memory polyurethane fiber-reinforced syntactic foam subjected to tensile stress:

In this study, healing of conventional non-shape memory syntactic foam embedded with shape memory polyurethane fibers was investigated per the biomimetic close-then-heal strategy. The syntactic foam was made of epoxy matrix dispersed with 30% by volume of glass microballoons, 5% by volume of shape memory polyurethane fibers, and three levels of thermoplastic healing agent (5%, 10%, and 15% by volume). Notched beam specimens were prepared and fractured by tension to create macroscopic cracks. Three levels of tensile stresses (26.5, 24.5, and 22.5 kPa) were applied to the fractured beam specimens during healing, in order to evaluate the healing capability of the composite under in-service conditions. It is found that the tensile stress level and healing agent content have a significant effect on the healing efficiency. Subjected to 22.5 kPa of tensile stress, the healing efficiency determined by peak tensile load is as high as 90%.

Impulse excitation study of elasticity of different precipitated microstructures in IN738LC at high temperatures

The elastic modulus of the cast superalloy IN738LC in various heat-treated conditions was determined with multiple specimens for each microstructure using the impulse excitation technique and the resonant frequencies while heating and cooling. Whereas the second and higher order harmonics were also excited in the high temperature range 700–1000 °C in 50 mm long specimens during controlled heating, analogous specimens 35 mm in length, impacted in similar fashion, did not excite the higher harmonics. Also, the 50 mm long specimens became excited and stayed in the second harmonic over broader temperature ranges during uncontrolled cooling inside the closed furnace. All precipitated conditions had nearly similar elastic data, varying from about 200 to 115 GPa, with small deviations, within 5%, found among multiple specimens of similar microstructures tested. Specimens with fine nano-size precipitates had a distinctly smaller rate of decrease in elastic modulus with increasing temperature, in contrast to a somewhat larger and nearly similar rate of decrease in specimens with coarse or medium-sized precipitates. This behavior is indicative of a larger average cohesive strength between the atoms and/or between the matrix and the precipitate particles in the former microstructure. The duplex size precipitate microstructure seemed to have both small and large drops in different specimens.

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

Healable and Repeatable Adhesively Bonded Joint

The adhesively bonded structure has to be replaced after the crack initiation and propagation. In a previous study, a biomimic two-step self-healing scheme (close-then-heal) by mimicking human skin has been proposed for self-healing structural-length scale damage. The adhesively bonded joint are prepared and to invest its feasibility and repeatability by fabricating a composite adhesive bonded joint with thermoplastic particles dispersed in a most commonly used epoxy based adhesive material. The fractured specimens were healed per the close-then-heal mechanism and tested again to fracture. This fracture-healing test lasted for 3 cycles.© 2011 ASME

Mechanical Properties of New Hybrid Materials: Metallic Foam Filled With Syntactic Foam

Syntactic polymer foam has received intensive attention and extensive application due to its remarkable low cost, lightweight, mechanical properties as well as its thermal, acoustic properties for multifunctional purpose. Electrically conductive polymers have the advantages of light weight, resistance to corrosion, good processability, and tunable conductivity. In a recent separated study, we proposed a novel conductive polymer which was based on the metallic foam filled with syntactic polymer foam. In this study, instead of focusing its unique multi-physical properties, we focus on characterizing the mechanical properties of this new conductive syntactic foam. Before the exploration of this new hybrid foam, an understanding of the mechanical properties is quite necessary. To this end, hybrid foams were prepared by varying the volume fractions of microballoons in the syntactic foam and types of microballoon materials: glass and polymer microballoons. The metallic foam adopted in this work was based on aluminum with an average relative density of 7% (the porosity is about 93%). Both compressive and bending tests were conducted. The current test results may provide the valuable baseline and also facilitate the further understanding of this hybrid foams as a core material in the advanced sandwiched pipe/pressure vessel structures featured by lightweight, impact tolerant, self-monitoring, thermal and acoustic insulation, and electromagnetic shielding.Copyright