Wahyu Lestari

Experimental Damage Identification of Carbon/ Epoxy Composite Beams Using Curvature Mode Shapes

Many composite materials and structures are susceptible to defects, which can significantly reduce the strength of structures and may grow to failure. To avoid the catastrophic failure of structures, development of a reliable method of structural health monitoring is one of the most important keys in maintaining the integrity and safety of structures. Dynamic response-based damage detection offers a simple procedure as an alternative to the conventional nondestructive evaluation techniques. However, this technique depends on the quality of measured data for its identification accuracy. In this article, experimental aspects of dynamic response-based damage detection technique on carbon/ epoxy composites are addressed. Smart piezoelectric materials are used as sensors or actuators to acquire the curvature modes of structures. These materials are surface-bonded to the beams. An impulse hammer is used as an actuating source as well. Four types of damage detection algorithms are evaluated for several possible damage configurations with two different excitation sources. The quality of damage identification with the four different detection algorithms is discussed. These experimental damage identification techniques using curvature modes and piezoelectric materials can be effectively used in damage detection and health monitoring of composite structures.

Curvature Mode Shape-based Damage Assessment of Carbon/Epoxy Composite Beams

In this article, a combined analytical and experimental damage assessment method using curvature mode shapes is developed. The curvature mode is selected due to its sensitivity to the presence of the damage and the localized nature of the changes. An analytical relationship between the damaged and the healthy beams is formulated, for which the effect of damage in the form of stiffness loss is accounted. This relationship is later used to estimate the extent of damage from the experimentally identified changes in structural dynamic characteristics. Surface-bonded piezoelectric sensors are used to directly acquire the curvature modes of composite structures, which simplify the identification procedure. The specimens are made of carbon/epoxy laminated composite beams. Several different types of damages are introduced in the beams (i.e., delamination, impact, and saw-cut damages) to simulate possible damage scenarios. Several limitations and remarks of the proposed experimental and damage identification approaches are discussed. The study shows that the present technique using curvature mode shapes and piezoelectric materials can be used effectively to locate the damage in the laminated composite structures.

Composite Structural Health Monitoring Through Use of Embedded PZT Sensors

The need to understand and monitor the integrity of structural components made of composite materials is becoming critical, due to an increase of the use of composites in aerospace, civil, wind energy, and transportation engineering. Off-the-shelf piezoelectric transducers embedded inside the composites or bonded onto the structure surface are a possible solution for on-line structural health monitoring and non-destructive evaluation: they can be used to generate Lamb waves, which are able to detect damage. This article focuses on the behavior of two sets of woven fiberglass/epoxy specimens, one with embedded, one with surface-mounted piezoelectric wafer transducers (lead zirconate titanate). The specimens are tested under axial tensile fatigue at high stress ratio, and the transducers are interrogated in pitch-catch mode at different stages of the specimens’ life, while they are subjected to the mean test load (the testing machine is paused). A novel signal processing technique based on wavelet thresholding/denoising and Gabor wavelet transform is discussed. This technique identifies changes in boundary conditions, loading/unloading prior to damage and during damage. It appears to correlate the contour area changes with the so-called characteristic damage state observed in the literature in composite laminates under tensile fatigue.

Health monitoring of structures - Multiple delamination dynamics in composite beams

In this paper, we discuss the dynamics of beams with multiple delaminations. The objective of the study is to use the dynamic response of the delaminated beams in health monitoring of composite structures. As a first step, we present experimental results on the vibrations of a cantilever beam with multiple delaminations. These experiments are conducted by exciting the beam with piezoelectric actuators. The response is detected by using a high-speed video camera to study the existence of delamination opening modes in a beam with multiple delaminations. Then, an analysis of natural frequencies and mode shapes, for a delaminated beam, is presented by using analytical techniques. The measured dynamic responses of the beam with multiple delaminations are also presented. The experimentally detected nonlinear response is discussed. Introduction Delaminations or interlayer cracks and impact damages are typical defects observed in composite structures. The presence of these defects changes the dynamic characteristics of the structure . Some of the specific changes in the observed dynamic characteristics include changes in natural frequencies, changes in the mode shapes and changes in damping ratios. These changes can be used to identify the existence, location and magnitude of a delamination or an impact damage, before they can grow to their critical sizes. Studies, to date, are concerned with the dynamics of beams and plates with a single delamination. In this paper, we specifically discuss the structural dynamics of a beam with multiple delaminations. This information can be used to develop health-monitoring techniques for composite beams with multiple delaminations. In an earlier paper, Luo and Hanagud studied the effects of defects, like saw-cuts, impact damage and delamination, on the structural dynamic characteristics of composite beams. They have shown that perturbations in natural frequencies and modes of a beam, due to defects, can be used for health monitoring of structures. In particular, it has been shown that an integral equation that considers perturbations in curvature modes and natural frequencies can be very effectively used to detect local defects. To distinguish delaminations from other types of monitored (identified) defects Luo and Hanagud have discussed nonlinear effects like superharmonic response in the dynamic response of a harmonically excited delaminated beam. They considered only a single delamination. In this paper we study beams with multiple delaminations. More specifically, we discuss dynamic response and the changes in natural frequencies, mode shapes due to multiple delaminations in a composite beam. One of the earliest models for vibration analysis of composite beams with delaminations was proposed by Ramkumar et al. This model simply used four Timoshenko beams connected at the delamination edges to model a composite beam with one throughwidth delamination. The predicted frequencies, based on this model, were consistently lower than the results of experimental measurements. Wang et al. improved the analytical solution by including coupling between flexural and axial vibrations of the delaminated sublaminates. Using an isotropic beam with splits and the classical beam model, they found that the calculated natural frequencies were closer to experimental results. With similar considerations, Nagesh Gummadi and Hanagud formulated a finite element solution for arbitrary composite beams. In the finite element models, they considered a classical composite beam model as well as the beam model with high order shear deformations. Mujumdar and Suryanarayan have proposed a model which imposed a constraint between the delaminated sublaminates to force them to have the same flexural displacement. This model was unable to * Ph.D. Student, School of Aerospace Engineering † Professor, School of Aerospace Engineering, AIAA Member Copyright

Dynamics-based Damage Detection of Composite Laminated Beams using Contact and Noncontact Measurement Systems

A reliable and effective damage detection technique is one of the significant tools to maintain the safety and integrity of structures. A dynamic response offers viable information for the identification of damage in the structures. However, the performance of such dynamics-based damage detection depends on the quality of measured data and the effectiveness of data processing algorithms. In this article, the experimentally measured data of two sensor systems, i.e., a surface-bonded piezoelectric sensor system and a noncontact scanning laser vibrometer (SLV) system, are studied, and their effectiveness in damage identification of composite laminated beams is compared. Three dynamics-based damage detection algorithms are evaluated using the data acquired from these two measurement systems. The curvature mode shape is selected as a parameter to locate damage due to its sensitivity. The piezoelectric sensors directly acquire the curvature mode shapes of the structures, while the SLV measures the displacement mode shapes. The difference in the measurement characteristics of these systems and their influence in the damage identification performance are addressed. The beam specimens are made of E-glass/epoxy composites, and several different types of damages are introduced in the beams (i.e., delaminations, and impact and saw-cut damages). This study provides a thorough assessment of the two sensor systems in damage detection of composite laminated beams and verifies the validity of dynamics-based damage detection methodology in locating the local defects in composite structures.

DETECTION OF AN EDGE NOTCH DEFECT BY USING A SINGLE MODE BASED METHODS

Reliable damage and effective damage detection has a significant role for utilization of the composite materials in structural applications. This paper is addressed to the problem of developing a damage detection technique for many damage occurring in a given structures is based on experimentally identified data and mathematical relationships. The mathematical expressions relate the natural frequency changes and the modes of the undamaged structure and the damaged structure containing damages at different locations. The curvature mode is selected in this method because of its sensitivity to the presence of the damage and the localized information that the curvature can provide. Damages are considered as point damages and the effects of the damage on both the stiffness loss and mass loss at each notch location are considered. Experiments are designed and conducted to validate the theoretical developments.

FATIGUE DAMAGE IDENTIFICATION IN COMPOSITE STRUCTURES THROUGH ULTRASONICS AND WAVELET TRANSFORM SIGNAL PROCESSING

This is an investigation on the damage behavior of fiberglass/epoxy specimens with embedded piezoelectrics under axial tensile fatigue. The specimen’s local and global damage states are complicated by the specimen’s own stretching under loading, which varies as a function of damage. A signal processing technique based on wavelet transforms is presented: denoised signals are processed with Gabor wavelet transforms, and the area of one of the contours is tracked throughout the fatigue life.

Sensing uniaxial tensile damage in fiber-reinforced polymer composites using electrical resistance tomography

Author(s): Lestari, W; Pinto, B; La Saponara, V; Yasui, J; Loh, KJ | Abstract:

Analysis of ultrasonic waveforms from smart sandwich composite structures under creep bending at elevated temperature

Sandwich structures with polyurethane foam core and glass fiber–reinforced polymer facesheets with three orientations were investigated experimentally and numerically under three-point bending tests at 80 °C, at a relatively low load level associated with a linear viscoelastic response. Off-the-shelf piezoelectric transducers were inserted inside one of the facesheets and were interrogated in pitch-catch at low ultrasonic frequencies during testing. The objective of this article is to investigate ability and sensitivity of the embedded transducers to detect creep deformation. The denoised received waveforms were analyzed in the time domain, where guided wave speeds were found to exhibit a drop due to temperature changes (most significant in the sandwich samples with off-axis orientation), followed by an increase, eventually reaching an asymptotic value. The waveforms were also processed in the joint frequency–time domain, with a novel signal processing technique built upon Gabor wavelet transforms and their contour lines. It is shown that this wavelet contour technique indirectly captures the trend of physically measured displacements and can differentiate among the three different fiber orientations in the facesheets, and among room temperature and 80 °C. This technique has the potential to effectively track creep time-dependent response and life performance in smart sandwich composites.

Structural health monitoring of glass/epoxy composite plates with MEMS PMN-PT sensors

Sensors constructed with single-crystal PMN-PT, i.e. Pb(Mg1/3Nb2/3)O3-PbTiO3 or PMN, are developed in this paper for structural health monitoring of composite plates. To determine the potential of PMN-PT for this application, glass/epoxy composite specimens were created containing an embedded delamination-starter. Two different piezoelectric materials were bonded to the surface of each specimen: PMN-PT, the test material, was placed on one side of the specimen, while a traditional material, PZT-4, was placed on the other. A comparison of the ability of both materials to transmit and receive an ultrasonic pulse was conducted, with the received signal detected by both a second surface-bonded transducer constructed of the same material, as well as a laser Doppler vibrometer (LDV) analyzing the same location. The optimal frequency range of both sets of transducers is discussed and a comparison is presented of the experimental results to theory. The specimens will be fatigued until failure with further data collected every 3,000 cycles to characterize the ability of each material to detect the growing delamination in the composite structure. This additional information will be made available during the conference.