Smart self-sensory carbon-based textile reinforced concrete structures for structural health monitoring

The goal of this study is to develop a structural health monitoring methodology for smart self-sensory carbon-based textile reinforced concrete elements. The self-sensory concept is based on measuring the electrical resistance change in the carbon roving reinforcement and by means of an engineering gage factor, correlating the relative electrical resistance change to an integral value of strain along the location of the roving. The concept of the nonlinear engineering gage factor that captures the unique micro-structural mechanism of the roving within the concrete matrix is demonstrated and validated. The estimated value of strain is compared to a theoretical value calculated by assuming a healthy state. The amount of discrepancy between the two strain values makes it possible to indicate and distinguish between the structural states. The study experimentally demonstrates the engineering gage factor concept and the structural health monitoring procedure by mechanically loading two textile reinforced concrete beams, one by a monotonic loading procedure and the other by a cyclic loading procedure. It is presented that the proposed structural health monitoring procedure succeeded in estimating the strain and in clearly distinguishing between the structural states.

High temperature strain sensors based on gallium phosphide whiskers

The paper presents a study of tensoresistive characteristics of p-type GaP whiskers with [111] crystallographic orientation coinciding with the direction of the maximal piezoresistive effect for this material. The authors present a newly-developed technology of creating the ohmic contacts to GaP crystals that allows using these crystals at high temperatures (400—600°C). Tensoresistive characteristics of p-type GaP whiskers were studied in the strain range of ±1,2•10–3 rel. un. These studies show that the gauge factor for these crystals at 20°C is rather large. Thus, for p-type GaP crystals with a resistivity of 0.025—0.03 Ω•cm, the gage factor is in the range of 90—95. The study of tensoresistive properties shows that in the temperature range of 20—300°C for p-type GaP crystals with the resistivity of 0,01—0,03 Ω•cm, the gage factor decreases as the temperature rises, but in the temperature range of 300—550°C for this crystals, very slight temperature dependence of the gage factor was observed. In this temperature range, the temperature coefficient of gage factor is no more than –0,03%/°Ñ. In the temperature range of 300—500°C, the value of gage factor is high (40—50). It could be noticed that in the entire investigated temperature range, the strain sensors based on p-type GaP whiskers have the linear resistance vs. strain dependence in the strain range of ±5,0•10–4 rel. un. The developed strain sensors based on p-type GaP whiskers have high mechanical strength at the static and dynamic strain (more than 108 cycles), which makes them operable in dynamic mode.

Piezoresistance Behaviour of CFRP Cross-Ply Laminates with Transverse Cracking

This paper describes the piezoresistance behavior in CFRP cross-ply laminates with transverse cracking loaded in tension. The resistance change due to transverse cracking and the gage factor (the rate of resistance change per mechanical strain) for each transverse crack density were experimentally measured during loading/unloading cycles. The resistance change-strain curves in the unloading/ reloading processes show the bilinear behavior where two gage factors are defined as the slopes of the bilinear curve. The residual resistance change after full unloading increases almost linearly with the mechanical strain while the gage factors do not always increase with the strain. The residual resistance change and gage factors are associated with transverse crack density on the basis of an equivalent resistance circuit.

Sensor Characteristics of Carbon Fiber Mat

Structural health monitoring (SHM) is becoming a popular topic. Carbon fiber reinforced concrete (CFRC) is an intrinsically smart material that can sense strain. The resistivity increases reversibly under tension and decreases under compression. A new skin-like sensor —cement-based smart layer had been put forward, which can serve as whole field strain sensor. The smart layer is satisfactorily consistent with concrete structure. The smart layer is a thin carbon fiber mat cementbased composite material layer with finite electrodes. It can cover the surface of concrete structure, and provide on-line reliable information about the deformation of whole concrete structure. The static characteristics of the new-type sensor had been researched. Its gage factor is 20-25 under tension and 25-30 under compression within the elastic deformation range. Furthermore the smart layer has satisfactory linearity and repeatability. In this paper, the sensor characteristics of the bare carbon fiber mat have been reached. The resistivity of carbon fiber mat has good agreement with strain under uniaxial tension. The gage factor can be up to 375, and the sensor limit can be up to 10000 microstrain. The strain and the fractional change in electrical resistance .R/R0 are totally reversible and reproducible under cyclic loading and amplitude-variable cyclic tensile loading.

Piezoresistance

Piezoresistivity, the change in electrical resistivity with mechanical stress, is commonly used to monitor static or slowly varying stresses and strains. The sensitivity of piezoresistive elements are often compared by means of a strain gage factor: G = Δ