Sherif M. Daghash

Shape Memory Alloy Cables for Civil Infrastructure Systems

Shape memory alloys (SMAs) have attracted a great deal of attention as a smart material that can be used in various civil engineering applications due to their favorable mechanical properties such as ability to undergo large deformations, high corrosion and fatigue resistance, good energy dissipating capacity, and excellent re-centering ability. In contrast to the use of SMAs in the biomedical, mechanical and aerospace applications, which requires mostly small diameter of material, the larger size bars are usually needed in a civil engineering application. It is well known that properties of large-section SMA bars are generally poorer than those of wires due to difficulties in material processing. Furthermore, large diameter SMA bars are more expensive than thin SMA wires.Shape memory alloy cables have been recently developed as an alternative and new structural element. They leverage the superior mechanical characteristics of small diameter SMAs into large-size structural tension elements and possess several advantages over SMA bars. This study explores the performance of NiTi SMA cables and their potential use in civil engineering. The SMA cable, which has a diameter of 8 mm, is composed of 7 strands and each strand has 7 wires with a diameter of 0.885 mm. The uniaxial tensile tests are conducted at various loading rates and strain amplitudes to characterize the superelastic properties of the SMA cable and study the rate-dependent mechanical response of the SMA cable under dynamic loads. An optical digital image correlation measurement system and an infrared thermal imaging camera are employed to obtain the full-field strain and temperature fields. Potential applications of SMA cables in civil infrastructure applications are discussed and illustrated.Copyright

Bond-slip behavior of superelastic shape memory alloys for near-surface-mounted strengthening applications

The use of superelastic shape memory alloy (SMA) bars in the near-surface-mounted (NSM) strengthening application can offer advantages such as improved bond behavior, enhanced deformation capacity, and post-event functionality. This study investigates bond characteristics and load transfer mechanisms between NSM SMA reinforcement and concrete. A modified pull-out test specimen that consists of a C-shaped concrete block, where the NSM reinforcement are placed at the center of gravity of the block, was used for experimental investigations. The effects of various parameters such as epoxy type, bonded length, bar diameter, and mechanical anchorage on the bond behavior were studied. The slip of the SMA reinforcement relative to concrete was measured using an optical measurement system and the bond–slip curves were developed. Results indicate that the sandblasted SMA bars exhibit satisfactory bond behavior when used with the correct filling material in NSM strengthening applications, while the mechanical anchorage of SMA bars can significantly increase the bond resistance.

Fatigue Behavior Characterization of Superelastic Shape Memory Alloy Fiber-Reinforced Polymer Composites

Fiber-reinforced polymer (FRP) composites have been frequently used for strengthening concrete structures. However, conventional FRPs exhibit brittle behavior with relatively low ultimate tensile strains and limited energy dissipation capacity, and possess limited fatigue life. Superelastic shape memory alloys (SMAs) are a class of metallic alloys that can recover strains between 6% and 8% upon load removal. SMAs possess excellent corrosion resistance, enhanced energy dissipation abilities, and high fatigue properties. Small-diameter superelastic SMA strands are new form of SMA elements. SMA strands can replace conventional fibers to produce resilient composites with enhanced ductility and deformability. These composites can be used for concrete and steel infrastructure strengthening and energy absorption applications.

Mechanical Characterization of Polymer Nanocomposites Reinforced with Graphene Nanoplatelets

Fiber reinforced polymer (FRP) composites have been extensively used for strengthening concrete structures. To manufacture FRPs or bond them to concrete structures, usually thermoset polymers are used. The mechanical properties and integrity of these adhesives significantly affect the performance of FRP-strengthened structures. Graphene nanoplatelets (GNPs) are carbon-based functional fillers that possess large surface area and high aspect ratio. They are easy to be processed in the host matrix and have excellent material properties at a relatively low cost. This study investigates the tensile behavior of GNP-reinforced nanocomposites. Two different epoxy matrices, one ductile and another brittle, are considered. First, the effect of ultrasonication duration in dispersion of GNPs is studied. Then, specimens with different GNP concentration levels are prepared to assess the effect of GNP content on the developed nanocomposites. Monotonic uniaxial tensile tests are conducted to study the effect of GNP addition to tensile strength and tensile modulus of two different epoxy resins. Morphology of GNPs and the fracture surface of the developed nanocomposites are also observed using SEM to assess the dispersion of GNPs. Results shows that both tensile strength and tensile modulus of ductile epoxy increase with increasing GNP content up to 1 wt. %, while for brittle epoxy a significant increase in tensile modulus is observed with 2 wt. % GNP concentration together with a slight decrease in tensile strength.

Experimental investigation of bond in concrete members reinforced with shape memory alloy bars

Conventional seismic design of reinforced concrete structures relies on yielding of steel reinforcement to dissipate energy while undergoing residual deformations. Therefore, reinforced concrete structures subjected to strong earthquakes experience large permanent displacements and are prone to severe damage or collapse. Shape memory alloys (SMAs) have gained increasing acceptance in recent years for use in structural engineering due to its attractive properties such as high corrosion resistance, excellent re-centering ability, good energy dissipation capacity, and durability. SMAs can undergo large deformations in the range of 6-8% strain and return their original undeformed position upon unloading. Due to their appealing characteristics, SMAs have been considered as an alternative to traditional steel reinforcement in concrete structures to control permanent deformations. However, the behavior of SMAs in combination with concrete has yet to be explored. In particular, the bond strength is important to ensure the composite action between concrete and SMA reinforcements. This study investigates the bond behavior between SMA bars and concrete through pull-out tests. To explore the size effect on bond strength, the tests are performed using various diameters of SMA bars. For the same diameter, the tests are also conducted with different embedment length to assess the effect of embedment length on bond properties of SMA bars. To monitor the slippage of the SMA reinforcement, an optical Digital Image Correlation method is used and the bond-slip curves are obtained.

A New Class of Carbon Nanotube: Polymer Concrete with Improved Fatigue Strength

Polymer concrete (PC) has been used successfully as bridge deck overlays and in machine foundations. In such applications, PC is subjected to cyclic loading and thus is prone to fatigue failure. In this paper, we introduce a new method for improving the fatigue strength of PC using carbon nanotubes. To overcome the challenge of fatigue testing of concrete, a four point flexural displacement control test borrowed from Asphalt standards in AASHTO was used. In this test the displacement was ramped up from zero to 1.2 mm then the PC specimen (25 × 25 × 200 mm) was cycled between zero and 1.2 mm using a sinusoidal signal with a frequency of 0.5 Hz. Guided by AASHTO specifications, failure is defined as 50 % reduction in stiffness. Four PC mixes were tested. These mixes incorporated neat epoxy, and epoxy including 0.5, 1.0 and 1.5 % multi-walled carbon nanotubes (MWCNTs) by weight of epoxy resin. Damage in PC due to fatigue was evaluated with time. The experiments showed the ability of MWCNTs to improve fatigue strength by 61 and 100 % for PC incorporating 0.5 and 1.0 % MWCNTs respectively. PC incorporating 1.5 % MWCNTs reached 50,000 cycles without experiencing fatigue failure showing improvement above 520 %. Microstructural analysis of PC was conducted using scanning electron microscope (SEM) to explain the ability of MWCNTs to significantly improve fatigue strength of PC.