KEYWORDS: Microsoft Foundation Class Library, Composites, Sensors, Structural health monitoring, Transducers, Satellites, Actuators, Damage detection, Packaging, Ferroelectric materials
Inflatable-rigidizeable composite space structures are an emerging technology that could revolutionize the design of large on-orbit satellites. These structural systems have the advantages of low mass, high packaging efficiency, low life cycle cost, low part counts, and high deployment reliability. As they are rigidized on-orbit, they do not depend on internal pressure to maintain their shape once deployed. However, as thin-walled structures, micrometeoroids and orbital debris (MMOD) are still a potential threat to their structural integrity. Such impacts will create punctures on the structure of varying sizes related to the size and kinetic energy of the debris/meteorite. For closed-cell geometries, such as booms or struts, MMOD objects can penetrate the outer wall twice, once on initial impact and once upon exiting the
structure. As impact damage and structural degradation will be cumulative over time, being able to monitor the structural integrity of these satellites would be of great interest. Impedance-based structural health monitoring schemes using distributed piezoelectric transducers are one possible approach. In this study, several Macro-Fiber Composite (MFC) piezoelectric devices were installed on a representative space-inflatable rigidizeable composite boom and used in ground tests as collocated sensor-actuators for detecting
and assessing simulated micrometeoroid/orbital debris strike damage. Electrical impedance signatures were compared before and after application of the simulated damage to determine the extent of the damage sustained. Both small and large footprint MFC piezocomposite sensor/actuators were shown to be effective in characterizing simulated MMOD punctures along the entire length of the boom.
Rail lines are subject to many types of damage that, in the worst cases, can cause train derailments. The damage can arise from either manufacturing defects or external factors, possibly even terrorist acts to disrupt the civil infrastructure. Current rail inspection techniques require train traffic to be interrupted while workers and equipment move along the track. Moreover, a technician with rail testing experience is required to analyze the results. This paper focuses on simple proof of concept experiments to determine if impedance based structural health monitoring may be used to detect anomalies in rail tracks, and in particular broken rails. The technique applies a very low voltage (one volt) high frequency wave to a structure, measures its response and determines the location and extent of a rail break. The monitoring device is envisioned to run off of ambient vibration and thermal gradients provided by passing trains and daily thermal cycles, store the energy and utilize the stored energy periodically to inspect the track (according to the track usage schedule). If damage occurs or starts to occur, a warning signal would be transmitted to substation then broadcast to the appropriate operator listing the location and extent of the damage.
An effective integrated structural health monitoring system must include a method of sensing and a process of damage identification that are optimized to work together. The result is a system that provides an automated and quantified assessment of damage present in a structure. Two candidates for such a symbiosis of sensing and damage identification are impedance-based measurement and statistical process control. The impedance-based structural health monitoring method uses a high frequency signal to excite a structure through a bonded piezoelectric patch and measures the impedance response of the excited structure across a frequency spectrum. In structural damage cases such as threads loosening or a crack developing, the structure in question will begin to show a change in impedance. Once measured, a damage sensitive feature from this impedance change can be statistically quantified into different damage cases by statistical process control. This paper addresses impedance measurements from experimental structures and a subsequent statistical method for quantitatively determining when the impedance signature of the structures has changed significantly enough to warrant the classification of “damaged”. Simple features and hypothesis testing algorithms are explored in an effort to create real-time solutions and reduce the complexity of damage identification for future use in low resource integrated structural health monitoring systems.
KEYWORDS: Sensors, Microsoft Foundation Class Library, Ferroelectric materials, Structural health monitoring, Space operations, Solids, Rockets, Actuators, Composites, Intelligence systems
Many of the structures responsible for the launch, ground turnaround and support operations of the space shuttle are still being used well past their design life. This has led to an increased interest in monitoring these structures in order to decrease the risk of breakdowns or structural failure. One monitoring method which has shown promising results for such applications is the impedance-based structural health monitoring technique. This paper presents results from proof-of-concept tests on the launch pad's orbiter access arm bolted connection, solid rocket booster support post, mobile launch platform heat shield and crawler transporter bearing. These tests showed that the impedance method can provide a permanent structural health monitoring solution to NASA's ground structures. In addition several positive and negative aspects of the impedance method were discovered or highlighted. Modifications for future tests are suggested.
The goal of this research is to reduce the likelihood of failure and the cost of maintenance of critical bolted joints. To reduce the self-loosening mode of failure, the concept of a self-sensing and self-healing bolted joint has been developed. This concept combines piezoelectric-based health-monitoring techniques with shape memory alloy (SMA) actuators to restore tension in a loose bolt. Many practical issues need to be addressed before the self-healing bolted joint can become a reality. One of the primary issues is the actuation of the SMA washer. The relatively large mass of the SMA washer and low resistance because of its short length make resistive heating particularly difficult. In addition, the large mass of the members connected by the joint can often act as a heat sink for what heat is generated. Therefore, a series of models was developed to assess the viability of resistive heating and provide an estimate for the power requirements for effective actuation. Modeling and experimental testing have shown that the use of external heater can be used to actuate SMA actuator with conventional power sources. By making the SMA washer substantially easier to actuate, this method provides a convenient alternative to the resistive heating, which requires very large currents needed for heating, and make the adaptive joints more accessible to real-field applications. This paper summarizes considerations needed to design SMA actuators, experimental setup and procedures, and several implementation issues and can be used as a guideline of future investigation.
This paper presents the current research on impedance-based structural health monitoring technique at the Center for Intelligent Material Systems and Structures. The basic principle behind this technique is to apply high frequency structural excitations (typically higher than 30 kHz) through the surface-bonded piezoelectric transducers, and measure the impedance of structures by monitoring the current and voltage applied to the piezoelectric transducers. Changes in impedance indicate changes in the structure, which in turn can indicate that damage has occurred. Three examples, including a bolted joint, gas pipeline and composite structure, are presented to illustrate the effectiveness of this health monitoring technique to the wide variety of practical field applications. Although many proof-of-concept experiments have been performed using the impedance methods, the impedance-measuring device (HP4194A) is still bulky and expensive. Therefore, we have developed an operational amplifier-based turnkey device that can measure and record the electric impedance of a PZT. The performance of this miniaturized and portable device has been compared to our previous results and its effectiveness has been demonstrated. This paper summarizes the experimental setup, procedures, considerations needed to implement the device in field applications.
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