Structural Health Monitoring of Composite Laminates Through Ultrasonic Guided Wave Beam Forming

The availability of military platforms is affected by design methods, damage detection, maintenance, and logistics. Structural health monitoring provides a means of detecting damage, which can be used to predict future structural performance of the platform and effectively schedule maintenance. Ultrasonic guided waves can be employed as an active structural health monitoring technique in a variety of ways. Recent progress in detecting damage in composite laminates is summarized. The paper culminates with a description of new phased array technology for beam forming to improve material coverage. 1.0 INTRODUCTION The availability of a military platform is dependent upon the operability of all its systems. More specifically, the readiness of structures is influenced by many factors; especially design, damage detection, maintenance, and logistics. The focus of this paper is on damage detection in airframe structures, with emphasis on composite laminates. This is a part of the dynamic multidisciplinary field of structural health monitoring (SHM), which applies to military and civilian platforms ranging from aircraft, rotorcraft, and spacecraft to ships, trucks, and trains to power plants, chemical plants, and manufacturing plants as well as to bridges, buildings, pipelines, and dams. SHM provides information about the current state and projected performance of a structural system that enables decisions to be made regarding logistics and maintenance as well as safety. SHM has the potential to improve fleet readiness, safety, and design, and at the same time reduce whole life cycle costs. It includes diagnostics (detecting a change of state in the structure – damage), prognostics (how the damage affects the ability of the structure to perform its intended function in the future), and decision making about maintenance and operations. Effective structural health monitoring relies on the synthesis of nondestructive evaluation, damage mechanics, sensor technology, data acquisition, signal processing, life prediction modeling, as well as other technologies. SHM systems can be designed to mitigate problems associated with a known damage mode in the legacy fleet, or included in the design of new structures. This paper focuses on the diagnostics element of SHM, that is, damage detection. There are numerous techniques currently under investigation for diagnostics including for example: embedded fiber optic sensors for strain measurement, microelectromechanical system (MEMS) accelerometers for vibration measurement, active ultrasonics, passive acoustic emission monitoring, and electromechanical impedance measurements. The paper briefly describes each of these techniques and then zooms in for a closer look at recent successes with guided wave ultrasonics for laminated composites. The use of ultrasonic guided waves for nondestructive evaluation of structures is rapidly expanding due to increased understanding of the underlying wave mechanics and improvements in sensors and signal processing. The ability of guided waves to travel Structural Health Monitoring of Composite Laminates Through Ultrasonic Guided Wave Beam Forming P1 2 RTO-MP-AVT-157 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED long distances, and therefore to monitor a large volume of material from one location, makes them very attractive for use in an SHM system. One feature of a structure necessary for monitoring with ultrasonic guided waves is that it be a wave guide, i.e., it has boundaries or interfaces that channel the ultrasonic energy such as plates, shells, pipe, and rail for example. This is not a very restrictive requirement. Carbon fiber reinforced polymer (CFRP) laminates are being used more and more in aircraft structures due to their high strength to weight and stiffness to weight ratios. The increased performance that they enable outweighs their relatively high cost. However, the damage modes in composite laminates are completely different and more complex than the well-known ones in conventional metal alloys. Furthermore, damage typically initiates much earlier in the life of the structure and often occurs inside the material where it is invisible. Thus, development of SHM diagnostics is critical. Detection and quantification of inter-ply delaminations, matrix cracking and degradation, and fiber fracture is necessary for prognostics. Finally, the paper presents progress in development and application of ultrasonic guided waves for damage detection in composite laminates. This discussion is organized around the sensing technology employed and includes: surface mounted piezoelectric sensors, air coupled transducers, annular array sensors, embedded piezoelectric fibers, and phased arrays for ultrasonic beam forming. 2.0 DAMAGE DETECTION TECHNOLOGY OVERVIEW Some of the more common and generally applicable technologies for damage detection within SHM are briefly summarized. This is not intended to be a comprehensive list. 2.1 Fiber Optic Sensors Fiber optic sensors can be embedded in, or surface mounted to, structural materials. Fiber Bragg gratings (FBG) enable these passive sensors to measure strain at a point much like a conventional resistance strain gage. FBG are processed by focusing ultraviolet light on a small section of a doped optical fiber at a prescribed interference angle. Exposure of the fiber core to ultraviolet light modulates the local index of refraction, which creates a narrow band optical wavelength filter. As broad band light travelling down the fiber encounters a grating, light having the wavelength of the grating is reflected. Deformation of the FBG due to loads changes the grating spacing and hence the wavelength of the reflected light, which can be detected. Because FBG can be designed to reflect light at various wavelengths they can be multiplexed on a single optical fiber. Optical fibers with many FBG are capable of measuring the strain field at hot spots in structural materials, where large local strains indicate the presence of damage. More details and application to vibration monitoring are described by Todd et al. [2001]. 2.2 MEMS Accelerometers Accelerometers are widely used to measure vibrations for condition based maintenance and SHM of machines and equipment [Adams, 2007]. The use of microelectromechanical systems (MEMS) as accelerometers enables them to be extremely small in size, embedded, wireless, and integrated with signal conditioning. MEMS accelerometers are very simple devices, consisting of a cantilever beam with a proof mass. 2.3 Ultrasonics Ultrasonics and radiography are broad classifications of methods widely used for nondestructive evaluation and testing. These methods are less applicable to SHM due to equipment requirements, that is, many of the Structural Health Monitoring of Composite Laminates Through Ultrasonic Guided Wave Beam Forming RTO-MP-AVT-157 P1 3 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED methods are best suited for a dedicated laboratory and it is impractical to permanently affix the necessary instrumentation to the structure. One major exception is ultrasonic guided waves that can be generated by lightweight piezoelectric transducers permanently affixed to the structure. Guided waves have strong potential for SHM because they enable monitoring of a large volume of material from a single location. The structure must have boundaries or interfaces that make it a wave guide and channel ultrasonic energy in certain directions. Section 3 describes ultrasonic guided wave methods for SHM in more detail. 2.4 Acoustic Emissions The initiation and propagation of damage results in a release of energy. In extreme cases, the strain energy converted to sound energy is in the audible range. Usually this is not the case, but the sound energy propagates through the structure nonetheless. Acoustic emission (AE) sensors passively listen to the structure by measuring the pressure due to the acceleration from the passage of sound waves. The sensor has a piezoelectric element that converts the mechanical signal to an electrical signal. The frequencies of AE are often in the 100 kHz to 1 MHz range. Features of an AE event such as counts, peak levels, and energies are correlated with the damage event. It is important to distinguish damage events from background noise and non-damage related events. AE monitoring can be applied to airframe, bridge, storage tank, and building structures. 2.5 Electromechanical Impedance The electromechanical (E/M) impedance method is a variant of the mechanical impedance method of Lange [1978] and others. The E/M impedance method couples the mechanical impedance of the subject structural material to the electrical impedance measured by a piezoelectric wafer active sensor (PWAS) as described by Giurgiutiu [2008]. The PWAS is used as a high frequency modal sensor. The peaks and valleys of the real part of the electrical impedance measured between the sensor electrodes reflects the mechanical response spectrum of the structure. Damage can be detected through spectral changes characterized by simple statistical equations or probabilistic neural networks. Giurgiutiu’s [2008] chapter on E/M impedance methods provides example applications for spot welds, bonded joints, composite overlays for the infrastructure, and aging aircraft panels. 3.0 ULTRASONIC GUIDED WAVE METHODS Some of the ultrasonic guided wave methods under development for SHM applications are summarized in this section. Again, this is not a comprehensive list, but rather it focuses on some of the more exciting possibilities and culminates with phased array beam forming. 3.1 Surface Mounted Piezoelectric Sensors Lead zirconate titanate (PZT) is the most commonly used material for surface mountable transducers because of it has relatively high piezoelectric coupling coefficients and it is reasonably affordable. These transducers are referred to as piezoelectric wafer active sensors (PWAS) by Giurgiutiu [2008] and are the subject of his monograph on structural health monitoring. Figure 1 shows a thin PWAS disk sensor. Typically these sensors have electrodes on the top and bottom surfaces and are polarized through the thickness. As an actuator, the through thickness electric potential is converted to a radial displacement that propagates into the base material as a stress wave. The sequence is reversed when the PWAS functions as a receiver. Below, PWAS applications for detection of fatigue damage and for tomography are discussed. Structural Health Monitoring of Composite Laminates Through Ultrasonic Guided Wave Beam Forming P1 4 RTO-MP-AVT-157 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Figure 1: Surface mountable PZT sensor. 3.1.1 Fatigue Damage Unlike metallic alloys, fatigue damage can initiate in CFRP materials relatively early in their life. Thus, detection of fatigue damage as early as possible is quite important. In a recent study [Lissenden et al., 2007], CFRP laminates were subjected to constant amplitude cyclic bending to induce fatigue damage. Cycling was interrupted periodically for ultrasonic monitoring by an array of six PWAS bonded to the top of the laminate. Nine different pitch-catch (through transmission) paths and a frequency range of 100-600 kHz were used. The signal difference coefficient (SDC), between the pristine laminate and the current state was calculated for each pitch-catch path. Figure 2 shows the evolution of the SDC as a function of the cyclic loading applied to the CFRP. In this case all of the pitch-catch paths between sensors in the array were able to detect damage for frequencies ranging from 200-400 kHz. This provides an earlier indication of damage than compliance changes.