Shiv P. Joshi

Damage detection in CFRP by electrical conductivity mapping

Carbon-fiber-reinforced polymer (CFRP) composites derive their excellent mechanical strength, stiffness and electrical coductivity from carbon fibers. The mechanical deformation and electrical resistance are coupled in these fibers that make them inherently sensors. Thus CFRPs can be considered as a self-monitoring material without any need for additional sensing elements. However, for this to become reality the conductivity map of the entire structure needs to be constructed and the relationships between the conductivity and various use- and damage-related variables need to be established. Experimental results demonstrate that internal damage, such as fiber fracture and delamination, decreases the conductivity of composite laminates. In general, the information about the damage size and position can be obtained by utilizing electrical impedance tomography (EIT), but the traditional EIT is not capable of extracting this information when the medium possesses highly anisotropic electrical conductivity. Above a certain level of anisotropy, it is advantageous to modify the traditional EIT. This paper presents a method of extracting the damage size and position for highly orthotropic (unidirectional) CFRPs. The results are obtained without the need for complex calculations, thus enabling damage detection in real time. Experimental observations indicate that a practical EIT has a potential of being a cost-effective health and usage monitoring technique (HUMT) for CFRPs.

Non-linear constitutive relations for piezoceramic materials

Piezoelectric materials produce electric charges when mechanically deformed and an electric potential causes a mechanical deformation. This property makes them suitable for sensor and transducer applications. Understanding of the electroelastic constitutive behavior is critical for predicting the response of a structure with embedded piezoelectric material. A concise formulation of relevant non-linear constitutive relations is presented in this paper.

Comparison of Morphing Wing Strategies Based Upon Aircraft Performance Impacts

Conventional aircraft are designed for a specific mission and/or set of performance requirements. These requirements often cause the performance for certain mission segments to be compromised because of fixed wing geometry. While some devices, such as flaps, have been used to augment the wing geometry, new materials are being developed that could allow significant wing morphing capability. This paper demonstrates the impact of a morphing wing on aircraft performance and provides a method to compare various morphing strategies.

An experimental investigation of through-thickness electrical resistivity of CFRP laminates

In conjunction with longitudinal and inplane transverse resistivity, the through-thickness electrical resistivity can be utilized in developing simple and cost-effective means of health and usage monitoring of CFRP laminated composites. Most of the research has been concentrated on understanding the electrical conductivity only in the fiber direction. The objective of this work is to investigate the through-thickness electrical resistivity of CFRPs. The paper first describes the external factors that should be eliminated to obtain a true specific resistivity in through-thickness direction. After establishing a procedure to measure the resistivity, the effect of degree of cure, in-plane prepreg tape interfaces within a ply, material system, and ply orientation in a laminate is presented. The following observations are made. The partial curing increases the resistivity. In-plane interfaces within a ply create wide scatter in resistivity values. Significant variation from one carbon-fiber/epoxy system to another can be used for identifying the material system of a CFRP component. The specific resistivity in through-thickness direction increase with increasing angle between the fibers of adjacent plies.

Structural Health Monitoring with Piezoelectric Wafer Active Sensors for Space Applications

Ultrasonic guided waves inspection using Lamb waves is suitable for damage detection in metallic structures. This paper will present experimental results obtained using guided Lamb waves to detect flaws in aluminum specimens with design features applicable to space applications. Two aluminum panels were fabricated from a variable-thickness aluminum top plate, with two bolted I-beams edge stiffeners and four bonded angle stiffeners. Artificial damages were introduced in the two panels: cracks, corrosions, and disbonds. The proposed investigation methods used bonded piezoelectric wafer active sensors to excite and receive Lamb waves. Three wave propagation methods were used: pitch-catch, pulse-echo, and the embedded ultrasonic structural radar. In addition, we also used a standing-wave damage detection technique, the electromechanical impedance method. The paper will present in detail the salient results from using these methods for damage detection and structural health monitoring. Where appropriate, comparison between different methods in detecting the same damage will be performed. The results have demonstrated the ability of piezoelectric wafer active sensors working in conjunction with guided Lamb waves to detect various types of damages present in complex geometry structures typical of space applications.

Mechanical properties of shape memory polymers for morphing aircraft applications

This investigation addresses basic characterization of a shape memory polymer (SMP) as a suitable structural material for morphing aircraft applications. Tests were performed for monotonic loading in high shear at constant temperature, well below, or just above the glass transition temperature. The SMP properties were time-and temperature-dependent. Recovery by the SMP to its original shape needed to be unfettered. Based on the testing SMPs appear to be an attractive and promising component in the solution for a skin material of a morphing aircraft. Their multiple state abilities allow them to easily change shape and, once cooled, resist large loads.

Elastic wave generation by piezoceramic patches

Elastic flexural and longitudinal waves can be generated in structure by using piezoelectric transducers. These waves may be used to obtain fundamental properties of the medium or to locate flaws. Bulk waves can be created in a thin piezoelectric plate by applying an electrical signal to interdigital electrodes deposited on each side of the plate. These transducers can be used to generate ultrasonic bulk waves with a wide range of frequencies and amplitudes controlled by a number of electrodes and a delayed voltage independently applied to each electrode. Numerical results demonstrating the finite element simulation of piezoelectric actuators and sensors in generating and detecting elastic bulk waves are presented. Experimental observations of flexural waves generated hy surface mounted piezoceramic plates are compared with the finite element results. Bulk wave generation and detection using interdigital electrodes are simulated and their potential application as probes in smart structures is discussed.

Static structural response of plates with piezoceramic layers

A finite element formulation is developed to analyze laminated plates with arbitrarily placed piezoceramic patches. However, only isotropic layered plates are analyzed as illustrative examples because of the primary emphasis on the effect of piezoceramic patch shapes. The technique is applied to obtain static response and stress fields due to the application of electric field to the piezoceramic patches. Different shapes of piezoceramic patches are inserted in a square aluminium plate. The actuation stress fields due to the piezoceramic patches are discussed in detail. It is observed that the shape of the patches has a direct effect on the stress field within the patch and in the surrounding material. The choice of the shape of the patches depends on the desired stress field, however, sharp corners should be avoided. The circular patch is subjected to the in-plane hydrostatic stress field. The stresses are homogeneous in the elliptical patch. The stresses are higher near the edges of the square and rectangular patches.

Behavior of laminated composites under monotonically increasing random load

A model using stochastic function theory is proposed to predict the behavior of laminated composites under random loads. Random in-plane loading is defined as a Gaussian process. A numerical procedure for predicting damage evolution and its effect on deformation history of laminated composites is developed. The probabilities of failure of a mesovolume at ply level are used in reducing ply stiffness. The probability of mesovoliime failure is calculated based on the theory of excursions of random process beyond the limits. Three modes of failure, i.e., fiber breakage, matrix failure in transverse direction, as well as matrix or interface shear cracking, are taken into account. The results of a numerical study of the effects of loading speed and dispersion on damage evolution and final failure of a Kevlar/epoxy [O/ ± 30/90]

Closed-Form Expressions for Higher Order Electroelastic Tetrahedral Elements

laminated composite are presented as an illustration. Cases of tension, shear, and complex in-plane loading of the laminate are analyzed for a vast range of loading speeds. The results indicate that the laminate strength and failure strain are higher at higher loading speeds. The analytical results are qualitatively compared with the available experimental observations.