Fatigue crack monitoring with nonlinear acoustics

 

Mentor: Anthony Puckett

 

Abstract

This project will investigate the ability of Lamb waves to detect the presence of fatigue cracks in plates. This topic has been considered by other researchers previously; however, this project will explore the ability of Lamb waves to excite and exploit the nonlinear response of fatigue cracks in order to detect the fatigue cracks.

 

Project Outline

The process of implementing a damage detection strategy for aerospace, civil and mechanical engineering infrastructure is referred to as structural health monitoring (SHM). Here damage is defined as changes to the material and/or geometric properties of these systems, including changes to the boundary conditions and system connectivity, which adversely affect the system’s current or future performance. This project will consider the structural health monitoring of commercial aircraft and specifically the detection of fatigue cracks in the skins of the wings and fuselage.

 

Airplanes are subjected to many forms of cyclical loading (thermal, turbulence, etc.) and if the growth of fatigue cracking goes unchecked the results can be catastrophic. Fatigue cracks often appear around rivet holes, and if these cracks grow large enough to bridge two rivet holes then the structure is weakened and sometimes considerably. This project will investigate the ability of fixed piezoelectric patches (often referred to as PZTs) to determine the extent of fatigue cracking using Lamb waves.

 

Lamb waves are elastic waves that are guided by the top and bottom surfaces of a plate and can propagate long distances. In this research, a single PZT will excite Lamb waves in the plate and a second PZT will record the waves that reach it. The attributes of the propagating Lamb waves change as they propagate through a material. A change in geometry or impedance (i.e. density or stiffness) along the path of the wave will alter the form of the wave. An analysis of the changes in the received waveform can be used to identify and locate damage.

 

The experimental test specimen for this research will be idealized by using a flat plate with a single hole. This plate will be cyclically loaded to generate fatigue cracks. For reference, two plates (one with a hole and one without) not subjected to cyclic loading will also be analyzed. The plates will be analyzed using several techniques. Initially, a modal analysis will be conducted looking at both the flexural modes and the longitudinal modes to see if the presence of the cracks can be detected. Once the modal analysis is complete PZTs will be bonded permanently to the plates to explore Lamb wave techniques for detecting the fatigue cracks. For these experiments a data acquisition system with a power amplifier will be required to excite the PZTs. Matlab and Labview will be used for data acquisition, signal processing and modeling.

 

The opening and closing of fatigue cracks produces a nonlinear response. Unfortunately, usually a large amount of energy is required to open the crack. Initial experiments will vary the amplitude and frequency of the excited Lamb waves to identify the best frequencies and modes for detection. These experiments should also indicate if the PZTs are capable of exciting the nonlinear response of the fatigue cracks. If the PZTs are capable of exciting the nonlinear effects of the fatigue cracks then ability of time reversal acoustics to identify fatigue cracks will be explored. If the PZTs can not produce enough energy to excite the nonlinear response of the crack then a different technique will be considered. A mechanical shaker or an impact hammer will be used to impart additional energy into the system. At this point the PZTs may be able to pick out some nonlinear response. If not, then one PZT will excite a continuous sine wave in the plate and the other PZT will look for harmonics indicative of a nonlinear response.

 

The goal of this project is to advance the state of knowledge of the nonlinear response of fatigue cracks and identity appropriate SHM techniques for locating the cracks.

 

Schedule

Weeks

Tasks

1

Orientation

2

Background research on the topics of Lamb waves and the papers listed below.

3

Vibration testing/modal analysis of the plates with accelerometers.

4

Installation of PZT and MFC patches to Al panels. Hardware use orientation. Analyze modal analysis data. Write introduction and maybe literature review for conference paper.

5

Initial Lamb wave measurements. Explore frequencies (excite just S0 mode, just A0 mode, and combination of modes) and amplitudes. Model Lamb wave propagation to understand the different parts of the signals. Write modal analysis section.

6

Analyze Lamb wave measurements. Consider existing SHM techniques for identifying damage. Look for nonlinear effects in the data. Write experimental section.

7

Experimental investigation of nonlinear effects. Hammer strike or large amplitude ambient vibration with a continuous sine from the PZT to excite nonlinear effects. Write up initial Lamb wave measurements.

8

Final experiments as needed. Write up nonlinear section.

9

Finish writing and presentation.

 

Background Literature:

Atchley, A. (2005). “Not your ordinary sound experience: a nonlinear-acoustic primer,” Acoustics Today. 1(1), p19-24.

Fink, M. (1997). “Time reversed acoustics,” Physics Today, 50(3), p34-40.

Giurgiutiu, V. (2005). “Tuned Lamb wave excitation and detection with piezoelectric wafer active sensors for structural health monitoring,” Journal of Intelligent Material Systems and Structures. 16, p291-305.

Grondel, S, et al. (2002). “Fatigue crack monitoring of riveted aluminium strap joints by Lamb wave analysis and acoustic emission measurement techniques,” NDT&E Intl. 35, 137-146.

Ikegami, R. and Haugse, E.D. (2001). “Structural health management for aging aircraft,” Proceedings of SPIE. 4332, p60-67.