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This paper presents a multi-functional strand capable of introducing prestressing force in prestressed concrete (PSC) girders and sensing their static and dynamic behavior as well. This innovative strand is developed by replacing the core steel wire of the strand used in PSC structures with a carbon fiber-reinforced polymer (CFRP) wire with a built-in optical Fiber Bragg Grating (FBG) sensor. A full-scale girder specimen was fabricated by applying this multi-function strand to check the possibility of tracking the change of prestressing force at each construction stage. Moreover, dynamic data could be secured during dynamic loading tests without installing accelerometers and made it possible to obtain the natural frequencies of the structure. The results verified the capability to effectively manage the prestressing force in the PSC bridge structure by applying the PC strand with a built-in optical sensor known for its outstanding practicability and durability.

This study aims to develop a prestressed concrete steel (PC) strand with an embedded optical Fiber Bragg Grating (FBG) sensor, which has been developed by the Korea Institute of Civil Engineering and Building Technology since 2013. This new strand is manufactured by replacing the steel core of the normal PC strand with a carbon-fiber-reinforced polymer (CFRP) rod with excellent tensile strength and durability. Because this new strand is manufactured using the pultrusion method, which is a composite material manufacturing process, with an optical fiber sensor embedded in the inner center of the CFRP Rod, it ensures full composite action as well as proper function of the sensor. In this study, a creep test for maintaining a constant load and a relaxation test for maintaining a constant displacement were performed on the proposed sensor-type PC strand. Each of the two tests was conducted for more than 1000 h, and the long-term performance verification of the sensor-type PC strand was only completed by comparing the performance with that of a normal PC strand. The test specimens were fabricated by applying an optical fiber sensor-embedded PC strand, which had undergone long-term performance verification tests, to a reinforced concrete beam. Depending on whether grout was injected in the duct, the specimens were classified into composite and non-composite specimens. A hydraulic jack was used to prestress the fabricated beam specimens, and the long-term change in the prestress force was observed for more than 1600 days using the embedded optical fiber sensor. The experimental results were compared with the analytical results to determine the long-term prestress loss obtained through finite-element analysis based on various international standards.

Ultrasonic vibration friction stir welding (UVFSW) has shown advantages in reducing welding defects and improving welding quality in aerospace, automobile, and power electronics. In this study, we design a novel 20-kHz integrated ultrasonic tool holder in FSW. The finite element model of FSW transducer is established, where the elastic modulus is measured by non-destructive acoustic. In the three transducer prototypes with alloy steel, the effect of prestress on resonant frequency is investigated and the ultrasonic vibration is measured. It proves that the resonant frequencies are well consistent between simulation model and the experiment by the elastic modulus testing and the prestress optimization. The ultrasonic amplitude of the pin is up to 24 μm. The experiment also indicates that the vibration is different with the steel material properties. Our findings can have a guidance to the design for a general ultrasonic actuator. The integrated FSW ultrasonic tool has potential to apply in a confined space in general machining equipment.

Form-finding of cable nets is the main topic of this paper. This initial stage of design path is grounded on the enhanced version of the Force Density Method. Apart from the basic form-finding it includes optimal shaping and adding self-weight of a cable structure. Minimal sum of cable lengths in the structure is treated here as a favourable initial configuration for reaching geometry and force distribution under prestress and self-weight. Regarding tensile forces obtained this way, cable sections can be proposed as the first approximation in further design process not included in this analysis. The basics of classic version of the Force Density Method are introduced in the paper. The nonlinear version of this method is used to solve an optimization problem of minimum weight cable net. The essentials of the procedures for achieving optimal shape and adding self-weight are also included and constitute the Extended Force Density Method proposed by the author. Defining proper input data for the self-weight analysis is crucial to find a new shape possibly close to the optimal one and is also discussed. A few examples of optimal or partially optimal cable nets are presented. It is shown that adding self-weight and elastic material properties can preserve the optimal shape with high accuracy. This allows to switch from the purely geometric problem of form-finding to the initial form of a structure with assumed sections and material. All calculations are performed with the use of the self-developed program UC-Form which is also briefly presented.

For the structural design of cable domes, the determination of prestress force distribution, the section of the structural components, and initial configuration are prerequisites for the subsequent detailed design of cable and strut sizes. To solve this problem, this paper elucidates the basic theory of the Minor Perturbation Method, introduces this theory into the field of force finding design for cable dome structures, and develops a new design method whose core is the comparison between the combined stress of each component conforming to mechanical characteristics of cable-strut structure and control stress, and meeting the convergence condition by adjusting the prestress level and cross-section of components. A corresponding design flow chart is established and programmed with finite element analysis software. Through the case studies of two different kinds of cable dome, it is proven that the proposed method and software program can simply, quickly, and effectively design the cable domes with an economic cross-section.

In view of the serious tensile residual stress distributed on the machined surface in cutting nickel-based superalloys GH4169, the prestressed cutting method has been adopted to actively control the surface residual stress. Numerical and experimental studies have been performed to investigate the effect of prestress on the cutting of GH4169. Based on the principle of prestressed cutting, a three-dimensional finite element model (FEM) of prestressed cutting has been established. Simulation results indicate that compared with cutting without prestress, remarkable increases of compressive residual stress and deeper distribution are observed. In addition, insignificant increments of cutting force and uniform serrated chip morphology are noted with higher prestress. Subsequently, a novel prestressed loading device for shaft parts has been prepared, and then an experimental setup has been developed. Single-factor experiments of prestressed cutting nickel-based superalloys have been carried out. It can be revealed that a significant rise of compressive residual stress is obtained as the prestress increases. Experimental and simulation results agree well in view of cutting force, chip morphology, and residual stress, which verify the effectiveness of the established FEM. A slight decrease in surface roughness and better machined surface integrity are found. Simultaneously, no additional work hardening appears on the machined surface.