Category High-Performance Construction Materials

Lamb-wave-based approach

Lamb waves are the most commonly used plate waves for health monitoring of plate-like structures. The propagation of a Lamb wave depends a great deal on the selected frequency and the material thickness. The Lamb-wave-based approach has been successfully applied to the health monitoring of composite structures, metallic structures and steel – reinforced concrete structures.

• Health monitoring of composite structures. Delamination is a major concern for in-service composites. Su et al. [49] applied four distributed piezoelectric transducers to generate and monitor the ultrasonic Lamb wave with a narrowband frequency in quasi-isotropic carbon fiber/epoxy (CF/EP) laminates. Toyama et al. [50] investigate the effects of transverse cracking and delamination on the S0 mode velocity in carbon fiber reinforced plastic (CFRP) cross-ply laminates. Experimental results showed that both the stiffness and the velocity decreased as the transverse crack density increased. Metallic structures are common in many areas of engineering, particularly in aerospace, ground and sea transportation.

• Health monitoring of metallic structures. Due to aging, fatigue and erosion of metals, there can be cracks, holes or other types of damage inside or on the surface of a metallic structure. Staszewski et al. [51,52] proposed a laser-based Lamb wave sensing approach for a rectangular metallic plate by using two piezoceramic disks as actuator and sensor. The study showed the potential of the method for simple and rapid detection of the location of damage in a structure. Tua et al. [53] proposed an approach to locate and determine the extent of linear cracks in homogeneous aluminum plates based on the flight time of Lamb wave by using piezoelectric actuators and sensors. Hiber-Huang transform is used to process the sensor signal to determine accurate flight time and also estimate the orientation of the crack. In situ health monitoring is important in the maintenance of metallic structures. Rajic et al. [54] proposed an in situ health monitoring system based on a piezoceramic wafer element to the detection of specimen fatigue under cyclic loading condition. Lamb waves propagating through the beam test specimen are sensed using small surface-mounted piezoelectric transducers, then analyzed for indications that relate to the onset of fatigue. The study and the experimental results show the great potential for developing similar automated in situ structural health monitoring systems for application to operational structures such as aircraft.

• Health monitoring of concrete structures. Steel-reinforced concrete (RC) is widely used in civil infrastructure. Steel reinforced concrete (RC) structures usually serve under harsh environments. Wang et al. [55] studied the Lamb-wave-based health monitoring of both fiber-reinforced composites and steel – reinforced concrete. The piezoelectric sensor network is installed in selected rebars in areas such as the deck, the columns of bridges, and the footing area of columns for the purpose of health monitoring. Experimental results showed that cracks or debonding damage in RC structures can be detected by the proposed built-in active sensing system.

[1] SAC is a joint venture of the Structural Engineers Association of California (SEAOC), the Applied Technology Council (ATC), and California Universities for Research in Earthquake Engineering (CUREe), formed specifically to address both immediate and long-term needs related to solving the problems of the Welded Steel Moment Frame (WMSF) connection. (October 1997).

[2] Astaneh-Asl, A. 2002. Seismic Behavior and Design of Composite Steel Plate Shear Walls. Steel TIPS Report, Structural Steel Educational Council, Moraga, CA.

Vibration-characteristic-based approach

The vibration-characteristic approach utilizes the piezoelectric actuator to generate certain wave to propagate in the structure and compares the structural vibration-characteristic parameters (modal shape, model frequency, damping, stiffness, etc.) or vibration-characteristic response curves (frequency response, time response, transfer function, etc.) with those of the healthy state to detect damage.

Piezoelectric-based active sensing system is commonly utilized in the health monitoring of civil structures. In the piezoelectric-based active sensing system, one piezoelectric transducer is used as actuator to send excitation waves. Other distributed piezoelectric transducers are used as sensors. The crack or damage inside the structure acts as a stress relief during the wave propagation path. The amplitude of wave and the transmission energy will decrease due to the existence of a crack. The decrease in value of the transmission energy will be correlated with the degree of the damage inside. Shown in Fig. 9.25 is such a system with two embedded piezoelectric transducers. The piezoelectric-based active sensing has the following advantages. 1. This method can be operated at a very low cost; 2. The piezoelectric-based active sensing has high sensitivity in local areas which is good for locating damage; 3 Health monitoring can be conducted on the in-situ concrete structure without using extra bulky equipment. Besides, a piezoelectric actuator can be excited in a broad frequency range which is ideal for health monitoring. The piezoelectric-based active sensing method is an effective and economical approach for health monitoring of large civil concrete structure.

The piezoelectric-based active sensing method has been successfully applied to the health monitoring of civil structures such as composite structures and concrete structures. The delamination of a composite is a critical issue because it can cause failure of the composite structure. One consequence of delamination in a composite structure is a change in its stiffness. This change in stiffness will degrade the modal frequencies of the composite structure. Okafor [41] conducted theoretical and experimental studies to investigate the effect of delamination on modal frequencies of composite beams with a piezoelectric actuator. Experiments results showed that the third and fourth modal frequencies degrade significantly with increasing delamination size. Saafi et al. [44]
proposed the active damage interrogation technique to conduct health monitoring of composite reinforced concrete structures. This system uses an array of piezoelectric transducers (PZT) attached or embedded within the structure for both actuation and sensing. Experimental results showed a distinct difference between the transfer function of the healthy reinforced concrete specimens and that of the damage reinforced concrete specimens. Song et al. [45,46] developed smart aggregates based on piezoelectric materials for health monitoring of two full-size concrete bent-caps (specimens W1 and W2). The experimental setup for the testing of the concrete bent-cap is shown in Fig. 9.26.

Hydraulic actuator A

Hydraulic actuator D

N orth

Fig. 9.26 Experimental setup for the testing of the concrete bent-cap.

For the health monitoring of the first concrete bent-cap specimen W1, the smart aggregates are positioned in a planar configuration at one end. For the health monitoring of the second concrete bent-cap specimen W2, the smart aggregates (a total of 10) are placed at the spatial position shown in Fig. 9.27. The smart aggregates were manufactured by embedding a small PZT patch (0.8 cm x 0.8 cm x 0.0267 cm) in a small (about 2.5 cm3) concrete block before curing. The smart aggregates were placed at the desired position in the concrete bent cap before casting, as shown in Fig. 9.28. Both specimens W1 and W2 were subjected to

Fig. 9.27 The location of 10 smart aggregates (labeled by PZT1-10) for the structural health monitoring of concrete bent-cap W2. The 10 smart aggregates are embedded in 4 different planes (labeled by Plane I-IV).

Fig. 9.28 The location of smart aggregates before casting.

destructive tests with loadings applied by the four hydraulic actuators. During the experiments, the crack growth on both specimens was monitored by microscopy (MS) and LVDTs with results shown in Figs. 9.29 and 9.30, respectively.

Fig. 9.30 Crack width measured by
microscopy (MS) and LVDT for
specimen W2. (NE: northeast location;
NW: northwest location)

Fig. 9.32 Damage index curve vs. load
(specimen W2) of PZT2

With wavelet packet analysis, a damage index (DI) was defined to present the damage status of a concrete structure with embedded smart aggregates [34]. When the DI value is zero, there is no damage. When the DI value is one, the structural is completely damaged. For specimens W1 and W2, damage index curves are shown in Figs. 9.31 and 9.32, respectively. From the damage index curve of specimen W1, as shown in Fig. 9.31, the damage index is close to the critical value when the load is around 40 kips (276 MPa), while at the same load value, from the LVDT and MS result of W1, as shown in Fig. 9.29, the crack width has just begun to increase on the surface. From the experimental result of the
second specimen W2, the damage index reaches the critical point around 74 kips (510 MPa), as shown in Fig. 9.32, while at the same load value the crack width on the surface has just begun to increase, as shown in Fig. 9.30. This means that severe cracks have happened before the crack width begins to increase on the surface. For both specimens, the critical points in the proposed method are earlier than the critical point deduced from LVDT and microscopy results for both concrete bent-caps. This implies that the PZT sensors are more sensitive than LVDTs or microscopy for health monitoring [47,48].

Impedance-based approach

The impedance-based qualitative health monitoring technique is a real­time structural damage detection method. Due to the electromechanical coupling property of piezoelectric materials, the measured electrical impedance is directly related to the mechanical impedance, and will also be affected by the presence of damage. Sun et al. [39] conducted an automated real-time health monitoring system on an assembled truss structure. Chaudhry et al. [33] conducted local area health monitoring of aircraft by measuring the impedance of piezoelectric transducers. Tseng et al. [41] presented the results of an experimental study for the detection and characterization of damages using PZT transducers on aluminum specimens. The impedance characteristics of the PZT transducer are extracted to detect damage. Besides metallic structures, concrete structures are also suitable for the impedance-based health monitoring method. Soh et al. [42] surface-bonded piezoceramic (PZT) patches to carry out health monitoring during the destructive load testing of a prototype reinforced concrete (RC) bridge.

Application of piezoelectric material to structural health monitoring

In recent years, piezoelectric materials have been successfully applied to the structural health monitoring of composite structures, metallic structures and concrete structures. Extensive theoretical and experimental research has been conducted. The piezoelectric-based health monitoring approach is a nondestructive evaluation (NDE) method that is suitable for health monitoring of inaccessible in-situ civil structures without using additional expensive and bulky equipment.

According to the principle of the testing method, piezoelectric-based health monitoring can be classified into three major categories: (a) impedance domain-based approach, (b) vibration-characteristic-based approach and (c) Lamb wave-based approach.

Shape restoration using superelastic SMAs

There is a specific type of application of superelastic SMA wires for structural control purpose that is different from the aforementioned examples. This application uses the shape restoration property of superelastic SMA wires. For example, Sakai et al. [37] researched self­restoration of a concrete beam using superelastic SMA wires. The experimental results show that the mortar beam with SMA wires recovers almost completely after incurring an extremely large crack.

In recent work [38] at University of Houston, a more efficient way to use superelastic SMA wires in the form of stranded cables to achieve a relatively large restoration force was developed. Shown in Figs. 9.23 and

9.24 is a concrete beam (24 in x 4 in x 6 in) reinforced with fourteen 1/8-inch diameter superelastic stranded cables using the method of post tensioning to achieve a 2% pre-strain. Each cable has seven strands and each strand has seven superelastic wires. Special clamps are used to hold the superelastic strands/cables without slippage. After loading at 11,000 lb, a large crack appeared (Fig. 9.23). Upon subsequent unloading, the crack closed (Fig. 9.24) under the elastic restoration force of the superelastic SMA cables. This research also demonstrates that the stranded cable provides a new and effective way to use SMAs for civil applications.

Fig. 9.23 Large crack in the beam before activation of the SMA.

Fig. 9.24 Crack width reduced in the beam after activation of the SMA.

Application of Active SMA Devices for Structural Vibration Control

9.8.1 SMAs for active structural frequency tuning

For a structure vibrating at its resonant frequency, the vibration can be reduced by actively tuning the resonant frequency of the structure. Upon heating, SMA actuators embedded or installed in structures will increase the stiffness of the host structure, so that the natural frequency of the structures can be actively tuned. By active frequency tuning, the vibration control for the structure can be achieved. This is the basic principle of SMAs for semi-active structural vibration control. For example, McGavin and Guerin [36] reported a proof-of-concept experiment in which the frequency of a steel structure is adjusted by using SMA wire actuators in real time. They achieved about 32% change of the natural frequency.

SMA column anchorage and SMA beam-column connection

Connectors or connections in various structures are prone to damage during an earthquake. SMAs have been used to protect a beam-column connection or a ground-column connection. An SMA-made anchorage

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Fig. 9.12 Through-thickness resistance (thick curve) during uniaxial compression (strain
shown by the thin curve) at a stress amplitude of -17.4 MPa for a 24-lamina quasi-
isotropic continuous carbon fiber epoxy-matrix composite [8].

Fig. 9.21 Schematic of SMA bar anchorage for a column [26].

Fig. 9.22 Schematic of SMA connector for steel structures [34].

(Fig. 9.21) or auxiliary (Fig. 9.22) is able to not only reduce the relative motion between the two connected parts by dissipating energy, but also tolerate a relatively large deformation without a connection failure.

Pulsating tension loading tests and numerical simulations were conducted on an exposed-type column base with anchorages (Fig. 9.21), each of which is made of a Nitinol rod and a steel rod [26]. The work shows that this type of anchorage is very effective in dissipating energy and reducing vibration of the column. In addition, it shows that the Nitinol segment of the anchorage can recover its original length after cyclic loadings.

The loading tests of the two full-scale SMA enhanced steel beam- column connections (the shaded parts in Fig. 9.22) demonstrate that the connections exhibit a stable and repeatable hysteresis for rotations up to 4% to absorb the vibration energy [34]. Also it is shown that the SMA enhancement is able to sustain up to 5% strain without permanent damage.

SMA dampers for bridges

Both superelastic and martensitic SMAs can be used as damper elements for bridges. Li et al. [30] numerically studied an application of a superelastic SMA damper for vibration reduction of a cable-stay bridge. This SMA-cable damper system is illustrated in Fig. 9.19. The numerical simulation shows that the proposed SMA damper can effectively suppress the cable’s vibration.

In [31], the testing and simulation analysis results of a full-scale superelastic SMA bar restrainer (Fig. 9.20) used for seismic retrofit of a multi-span simply supported bridge were reported. The results have shown that the SMA restrainer more effectively reduced the relative hinge displacements at the abutment and it provided a large elastic deformation range in comparison with conventional steel restrainer cables. In addition, the SMA restrainer effectively limits the motion of the bridge decks.

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Fig. 9.20 Schematic of the setup of SMA restrainer for
a simple-supported bridge [31].

Casciati et al. [1] studies an application of a large martensitic Nitinol bar as a seismic protection device in a bridge. The finite element analysis used shows the applicability of the martensitic Nitinol bar in energy dissipation in relation to both the static and dynamic response to strong earthquakes. The analysis conclusion is in agreement with the experimental results.

SMA braces for frame structures

The SMA wire braces are installed diagonally in frame structures. As the frame structures deform under excitation, the SMA braces dissipate energy through the stress-induced martensitic transformation (for the superelastic SMAs) or the martensitic reorientation (for the martensitic SMAs). Several different scaled-down prototypes of the SMA braces were designed, implemented and tested. The examples include the brace consisting of 210 loops of Nitinol wire installed on a six-story two-bay by two-bay steel frame, as shown in Fig. 9.16 [22], the re-centering

Fig. 9.18 Schematic the SMA braces for a frame structure [26].

braces tested on a real two-story building [13,14], and the hybrid braces made of rigid segments (mostly steel wire) and the Nitinol wires (illustrated in Figs. 9.17 and 9.18) [22,24,26]. The SMA braces have self-centering capability, high stiffness for small displacements and good energy dissipation. The effectiveness of the SMA braces for the vibration suppression is dependent on the pre-strain and the geometry of the SMA wire brace [26,28].

SMA energy dissipation devices

SMA energy dissipation devices exist in the form of braces for framed structures [13,14,22-29], dampers for cable-stayed bridges [30] and for simply supported bridges [31,32], anchorages for columns [26,33], beam-column connections [34], and retrofitting devices for historic buildings, as shown in Fig. 9.15 [21,35].