The Monitoring And Identification Of Damage

Published Date: 02 Nov 2017

[1] INTRODUCTION

The monitoring and identification of damage at an early stage is a very essential factor of in industries like mechanical, civil and aerospace engineering. The different damage identification methods at present are either visual or localized experimental methods which includes acoustic or ultrasonic methods, magnetic field methods, radiograph, eddy-current methods and thermal field methods

The experimental procedure requires the visibility of the damage and the easily accessible during inspection of the structure. Due to the limitations in the experimental procedures, damage detection is possible either on the surface or near the structural surface. The continued research development is required for additional global damage identification methods which can be used to complex structures for the identification of vibration characteristics changes. The monitoring, testing and continuous evaluation is required to ensure the safety of structures. The modal based damage identification methods contribute the increase in speed and computing memory. It also increases the advancements in finite element method and non-contact or remote monitored sensors. The subject of damage identification due to the dynamic property changes or system response has received a lot of attention in the recent years. The commonly measured modal parameters such as frequencies, mode shapes and modal damping are the functions of the physical properties (mass, damping and stiffness) of the structure. The identified changes of these modal properties, such as reduction in stiffness are mainly due to the presence of damage or loose connection in the structure. Therefore, the changes in modal properties are useful for the identification of damage as damage indicators. The changes in vibration characteristics provide an information regarding damage in a structure. Damage is a local response of the structure which can be captured by higher frequency modes. But, excitation of the structure at higher frequency response is more difficult. The global response of the structure can be captured at lower frequency mode. But these lower frequencies are less sensitive to local structural changes. It is necessary to produce more energy for the measurement of response at higher frequencies than at lower frequencies. In vibration based damage identification method, when the response of the system changes from linear to nonlinear the time histories are sufficient to detect the damage. The advanced methods of examining vibration data for the identification of structural damage have gained lot of interest over the last 20 to 30 years.

Damage can cause sudden failure in a structure during high load operation, which may lead to catastrophic consequences. An early damage detection method for structural failure is one of the most important parameter in maintaining the integrity and safety of structures. Structural health monitoring (SHM) denotes a reliable system with the ability to detect and interpret adverse change in a structure due to damage or defect. Many damage detection techniques such as X-ray imaging, ultrasonic scans, infrared thermograph, and eddy current can identify damages have been proposed for structural health monitoring system. Those techniques are somehow difficult to implement in-service aircraft testing and in-site space structures. Changes in the physical properties of the structures due to damage will alter the dynamic responses such as natural frequencies, damping and mode shapes and these physical parameter changes can be extracted to estimate damage information. Therefore, the dynamic response of structures can offer unique information on defects that may be contained within the structures.

Objectives

1. Fabrication of Glass/Epoxy composite beam by hand lay-up process.

2. Evaluation of frequency changes

The procedure used to evaluate the variation of frequency change should be identical when it is verified by two different procedures, namely, experimental and analytical. In the present work both the experimental as well as the numerical (FEM) procedures are used to estimate the frequency changes before they are compared.

3. Experimental procedure

The composite beam of the dimensions L:W:T with ratio 80 : 6 : 1 is considered for the experiment. RION SA-73 Sound and Vibration Dual Channel analyzer is used, to extract the natural frequencies, along with B&K-Type 4344 pickup and RION PH-51 impact hammer. Only a few of the lower order frequencies, including the fundamental frequency, were extracted from the experiment. To start with, the natural frequencies of the undamaged cantilever beam were measured. Then the damage was generated and propagated to the desired width by a thin saw cut (around 0.8 mm thick). Four specimens with the same geometry, material and damaged at locations 0.2L,0.4L,0.6L and 0.8L, with an increment of 0.2¸ from the fixed end, and with crack (considered as damage) depths of 0.1B to 0.5B, with an increment of 0.1B, at each location were tested. The "ZOOMING" capability of the analyzer was used to observe the change in frequency as crack depth increased. From experimentally measured frequency values the percentage of change in the first three bending frequencies with respect to the undamaged beam for each damage location and depth are calculated.

4. By using same geometry and material properties, Finite Element Method is carried out on the cantilever beam and measured frequency values. The percentage of change in the first three bending frequencies with respect to the undamaged beam for each damage location and depth are calculated. These results were compared with the experimental results.

5. Four damage detection algorithms are applied to the measured dynamic response data.

6. Compare the effectiveness of different damage detection algorithms.

7. Perform numerical finite element simulations of damaged beams and validate the

8. By using damage detection algorithms the damage indicators and damage severities were calculated.

Modeling the single and double edge crack (Considering crack as damage): The additional boundary conditions at the crack location can be established such that the beam can be replaced with two intact beams connected at the crack location.

Analysis of free vibration characteristics of the cracked composite beam: To facilitate the analysis on a cracked composite beam, clamped free (a cantilever beam) boundary conditions are assumed. Changes in natural frequencies and mode shapes with respect to the crack location, crack ratio, fiber angles and fiber volume fraction are obtained and plotted.

Crack detection based on changes in natural frequencies. While natural frequencies are relatively easier and more accurately measured than other modal parameters, solving an inverse problem for crack detection based only on changes in natural frequencies is not so easy, considering that fact that natural frequency has a global nature while damage in most cases is a local phenomenon. However, if the crack is the most possible failure mode and no other form of damage exists, detecting the crack by natural frequencies is possible.

Crack identification by using Damage detection algorithms

The following Damage detection algorithms were used to locate the damage in the composite beam buy using FE analysis. The modal parameters that change locally are mode shapes, i.e., bending mode shape.

(1) The Curvature Mode Shape Method :

(2) The Damage Index Method,

(3) The Gapped Smoothing Method.

(4) The Laplacian operator Method.

Three types of analysis was studied by using FEM software ANSYS

MODAL ANALYSIS

HARMONIC ANALYSIS

TRANSIENT ANALYSIS

Structural health monitoring has been receiving a growing amount of interest from researchers in various fields of engineering. Generally, the non-destructive inspection methods are used to investigate the critical changes in the structural parameters to prevent sudden or unexpected failure. There are many techniques and approaches involved in the non destructive evaluation (NDE) of structural systems and which are classified as local or global methods. The local method designed to provide information about a relatively small region of the structure under study by using local measurements such as acoustic techniques, thermal field methods or curvature approach. The global method used to determine the general state of the system by using global measurements such as measurement of natural frequencies.

In this study, the local methods are used to detect, locate and quantify damages which concentrate on a part of the structure. Each NDE technique has its own limitations; therefore, not all NDE techniques are physically capable of detecting all damages. The damage in a structure causes reduction in the structural stiffness which produces significant changes in the dynamic properties, such as natural frequencies, mode shapes and structural damping. Damage can be detected from dynamic analysis using natural frequencies and mode shapes. The measurement of natural frequencies is easier than that of changes in structural damping. The changes in natural frequencies are useful for the indication of the existence of damage, but it is not sufficient to locate the damage. To determine the location of the damage, mode shapes information is required. This work focuses on damage detection techniques that use displacements and curvature mode shapes which can be obtained by both experimental as well as numerical means. The difference between the undamaged and damaged state is performed to improve the damage detection.

Composite materials can be defined as the combination of two or more materials on a microscopic scale to form a useful material. The advantage of composites is that the overall properties are superior when compared to the individual constituents. During the formation of a composite material the properties that can be improved usually include strength, stiffness, corrosion resistance, surface finish, weight and fatigue life. Due to the high strength-to-weight and stiffness-to-weight ratios, fiber-reinforced composites have been extensively used for many applications, such as aerospace structures and high-speed turbine machinery. Therefore, mechanics of fiber-reinforced composites have been intensively studied and handbooks guiding the design and testing have also published. The present research is concerned with numerical investigation on the vibration characteristics of a cracked composite beam and plate. Specifically, the topic on a composite laminated cantilever beam that has an edge crack and vibrates in bending will be studied to identify, locate the damage along with damage severity. By using various fracture mechanics, a large amount of research has focused on the stress and deformation analyses of cracked composites and relatively less investigation has been devoted to studying the vibration characteristics of cracked composite structures. In recent decades supported with intensive theoretical and experimental analyses fracture mechanics of composite materials were well developed.

Structural health monitoring (SHM) has emerged as a well-recognized field of technology with a tremendous growth in the last few years. The main objectives of structural health monitoring involve the integration of smart materials, sensors, data processing, telemetry, modern computational power, decision-making algorithms into structures to detect damage, access the integrity and even predict the remaining life time based on projected loading and environmental conditions. Structural health monitoring may have an array of sensors attached on or imbedded in the structure in service which monitors continuously in real time physical parameters that can be further processed for damage detection and health status assessment. Structural health monitoring techniques especially benefit aerial and ground vehicles, aerospace structures, bridges, buildings, offshore systems. In the field of structural design and dynamics, large amount of articles appeared about research and applications with SHM. The initial step for a feasibility of SHM understands damage mechanisms of the materials as well as dynamics and failure mechanisms of structures under working conditions. This information provides guidelines for further implementation of sensor arrays, data processing and decision making. The primary objective of SHM is to detect a various damage cases at the earliest possible stage to prevent catastrophic failure. The damage on a structure usually changes the mass, stiffness and damping distribution of the structure either locally or globally. Due to the change of vibration characteristics of the structure, the evaluation of vibration responses may be used to detect the damage. The fundamentals in SHM also include development of more advanced sensors and implementation of smart materials used for measuring accurately small changes of physical parameters in real time. The important parameters for the implementation of SHM technique are data processing, feature extraction, statistical analysis and final decision making.

The structural damage is defined as the deviation in geometry or change in the material property of the structure. It may degrade the integrity or causes an unintentional response of the structure. During structure’s service life the damage may be formed at different stages. The occurrence of unpredictable damage may cause catastrophic failure and also possesses a potential threat to human lives. The possible sources leading to structural damage are the raw material imperfection, welding flaws during the fabrication and fatigue accumulation during the machine operation. Effective engineering solutions are highly desirable to identify, locate and quantify the damage. It is also required to predict the situation of the damage at the earliest stage. The structural maintenance schedules depend on periodic assessments usually conducted through visual inspections recommended by manufacturers or through checks based on the past experience. In some cases the machine has to be taken out of service or even dismantled to carry out the inspection or the accessibility is also a problem in situations where some critical locations are beyond reach due to confined spaces. Many researchers have focused on vibration- based damage detection methods in order to overcome the limitations. In the vibration-based damage detection methods, the modal parameters consist of both global and local information of the structural dynamic properties.

The main reason for the majority of the attention given to vibration-based damage detection methods is that, the modal parameters contain both global and local information of the structural dynamic properties The advantage of this approach is that the applicability does not depend on the structural material in use, which means that it can be used for non-metallic or composite structures..

The implementation of vibration-based damage detection consists of the following basic steps:

(1) Determine the optimal parameters used for damage indication.

(2) Conduct vibration testing and collect all necessary data by choosing appropriate excitation, response locations, transducer type, the number of channels, and data acquisition system.

(3) Analyze and interpret the test results.

(4) The corresponding health status of the structure is then assessed according the selected criterion.

By using different techniques the selected feature parameters and the way to interpret the measured data were investigated by many researchers. Some authors directly used the change of modal parameters such as natural frequencies or mode shapes as the damage indicators. While some authors using information derived from basic modal parameters to detect the structural damage.

The implementation of vibration-based damage detection method may encounter many significant challenges. Damage is a local phenomenon, which may not influence the global dynamic response of a structure. The sensitivity varies from one structure to another. For damage indication the frequency reduction has significant limitations. Damping is probably the most sensitive structural parameter and accurately obtaining is also difficult. Mode shapes can be used for damage identification. To obtain meaningful mode shapes high spatial resolution is required which needs large number of transducers. Most of the work reported in literature is based on analytical simulations and lack of experimental validation. The measurements are always contaminated with noises and the outcomes from the field tests may be quite different to those derived from analytical simulations.

Structures are required to work safely during its service life. But, the damages cause a breakdown period on the structure. Damages in a structure may be hazardous due to static or dynamic loadings. Therefore, the damage detection plays an important role for structural health monitoring applications. Steel used in construction and machinery industries are commonly beam type structures. The common structural defect is the presence of damage in a structure due to many reasons. The damage not only causes a local variation in the stiffness but it also affect the mechanical behavior of the entire structure. Damages may be due to fatigue under service conditions or initiated during manufacturing processes which are very small in size. Due to the fluctuating stress conditions, these small damages will propagate. If these propagating damages reach their critical size and remain undetected, then a sudden structural failure may occur. The objective is to carry out vibration analysis on a cantilever beam with and without damage. The parameters affecting dynamic response of the structure are physical dimensions, boundary conditions and the material properties. In addition to the above parameters, the presence of damage modifies the dynamic response of the structure. The factors greatly influence the dynamic response of the structure are the position of damage, the depth of crack, the orientation of damage and number of cracks.

The damage identification methods based on changes in dynamic characteristics provides global way to evaluate the structural condition. These methods are based on the modal parameters (i. e., natural frequencies, mode shapes, modal damping ratios, etc.) which are a function of the physical properties of the structure (stiffness, damping, mass and boundary conditions). Therefore, changes in the stiffness or flexibility of the structure will cause changes in the modal properties.

A new structural damage detection methods that can be applied to structures has led to the development of methodologies that examine changes in the vibration characteristics of the structure is required. In view of the above, the structural damage identification is important to examine some of the global damage detection methodologies.

The first phase of the work consists of single edge crack and the second phase of the work consists of double edge cracks. The crack was considered as damage. The simulation results were presented with different damage location with different crack depths. By using various damage algorithms from different literature papers, damage location and damage severities were calculated and presented.