Classification Of Damage And Damage Identification Methods

Published Date: 02 Nov 2017

The objective of this chapter is based on a previous detailed review (Doebling,et al.[1996]) of the modal-based damage detection literature. The field of damage detection is broad and includes both local and global methods. However, there are some critical issues to the success of using mechanical vibration characteristics for the identification of damage of a structure. The main factors are excitation and measurement considerations, which include the selection and the location of sensors. Similarly, it is also important to the type and location of the excitations.

CLASSIFICATION OF DAMAGE AND DAMAGE IDENTIFICATION METHODS

The damage on a structure and its effects can be divided as linear and nonlinear. In case of linear damage, the linear-elastic structure remains same before and after damage. In linear damage, the change in the geometry and/or the material properties of the structure causes the changes in modal properties. But, the structural response can be modeled by using linear equations of motion. Whereas, in nonlinear damage the structure behaves initially as linear-elastic and after the introduction of damage it behaves like a nonlinear manner. The fatigue crack is an example of nonlinear damage. It opens and closes during normal operating vibration environment. The problem of linear damage was addressing most of the technical literature.

The damage-identification methods can also defined as four levels:

First level : Determination that damage is present in the structure.

Second level : First level plus determination of the geometric location of the damage.

Third level : Second level plus quantification of the severity of the damage.

Fourth level : Third level plus prediction of the remaining service life of the structure.

DAMAGE DETECTION BASED ON CHANGES IN BASIC MODAL PROPERTIES

Frequency Changes

A large amount of literature review observed with reference to damage detection by using shifts in resonant frequencies. The changes in vibration frequencies were mainly due to the changes in structural properties. Therefore, modal methods can be used for damage identifications. But, the frequency shifts have practical limitations which can be resolved with the help of present and future research work. The damage due to low sensitivity frequency shifts requires precise measurements or damage at larger levels. It should be noted that frequency shifts have significant practical limitations for applications to the type of structures considered in this review, although ongoing and future work may help resolve these difficulties. The somewhat low sensitivity of frequency shifts to damage requires either very precise measurements or large levels of damage. The modal frequencies of the structure are a global property and these shifts cannot be used to identify the damage existence. Generally, the frequencies cannot provide spatial information related to the structural changes. The spatial information about structural damage can be obtained through multiple frequency shifts. The different combination of changes in the modal frequencies is due to the changes in the structure at different locations.

This chapter is based on a previous detailed review of the modal-based damage detection literature (Doebling,et al.[1996]). As mentioned previously, the field of damage identification is very broad and encompasses both local and global methods. Many different issues are critical to the success of using the mechanical vibration characteristics of a structure for damage identification and health monitoring. Among the important issues are excitation and measurement considerations, including the selection of the type and location of sensors, and the type and location of the excitations. The effects of damage on a structure can be classified as linear or nonlinear. A linear damage situation is defined as the case when the initially linear-elastic structure remains linear-elastic after damage. The changes in modal properties are a result of changes in the geometry and/or the material properties of the structure, but the structural response can still be modeled using linear equations of motion. Nonlinear damage is defined as the case when the initially linear- elastic structure behaves in a nonlinear manner after the damage has been introduced. One example of nonlinear damage is the formation of a fatigue crack that subsequently opens and closes under the normal operating vibration environment. The majority of the studies reported in the technical literature address only the problem of linear damage detection.

Another classification system for damage-identification methods, defines four levels of damage identification, as follows :

• Level 1: Determination that damage is present in the structure

• Level 2: Level 1 plus determination of the geometric location of the damage

• Level 3: Level 2 plus quantification of the severity of the damage

• Level 4: Level 3 plus prediction of the remaining service life of the structure

To date, modal-based damage identification methods that do not make use of some structural model primarily provide Level 1 and Level 2 damage identification. When modal-based methods are coupled with a structural model, Level 3 damage detection can be obtained in some cases. Level 4 prediction is generally associated with the fields of fracture mechanics, fatigue life analysis, or structural design assessment and, as such, is not addressed.

DAMAGE DETECTION BASED ON CHANGES IN BASIC MODAL PROPERTIES

Frequency Changes

The amount of literature related to damage detection using shifts in resonant frequencies is quite large. The observation that changes in structural properties cause changes in vibration frequencies was the impetus for using modal methods for damage identification and health monitoring. An effort has been made to include the early work on the subject, some papers representative of the different types of work done in this area, and papers that are considered by the authors to be significant contributions in this area.

It should be noted that frequency shifts have significant practical limitations for applications to the type of structures considered in this review, although ongoing and future work may help resolve these difficulties. The somewhat low sensitivity of frequency shifts to damage requires either very precise measurements or large levels of damage. Also, because modal frequencies are a global property of the structure, it is not clear that shifts in this parameter can be used to identify more than the mere existence of damage. In other words, the frequencies generally cannot provide spatial information about structural changes. An exception to this limitation occurs at higher modal frequencies, where the modes are associated with local responses. However, the practical limitations involved with the excitation and extraction of these local modes, caused in part by high modal density, can make them difficult to identify. Multiple frequency shifts can provide spatial information about structural damage because changes in the structure at different locations will cause different combinations of changes in the modal frequencies.

The Forward Problem

The forward problem, which usually falls into the category of Level 1 damage identification, consists of calculating frequency shifts from a known type of damage. Typically, the damage is modeled mathematically, then the measured frequencies are compared to the predicted frequencies to determine the damage.

Cawley and Adams [1979] give a formulation to detect damage in composite materials from frequency shifts. They start with the ratio between frequency shifts for modes i and j, . A grid of possible damage points is considered, and an error term is constructed that relates the measured frequency shifts to those predicted by a model based on a local stiffness reduction. A number of mode pairs are considered for each potential damage location, and the pair giving the lowest error indicates the location of the damage. The formulation does not account for possible multiple-damage locations. Special consideration is given to the anisotropic behavior of the composite materials.

Friswell, et al. [1994] present the results of an attempt to identify damage based on a known catalog of likely damage scenarios. The authors presume that an existing model of the structure is highly accurate. Using this model, they computed frequency shifts of the first n modes for both the undamaged structure and all the postulated damage scenarios. Then ratios of all the frequency shifts were calculated. For the candidate structure, the same ratios were computed, and a power-law relation was fit to these two sets of numbers. When the body of data is noise-free, and when the candidate structure lies in the class of assumed damages, the correct type of damage should produce a fit that is a line with unity slope. For all other types of damage the fit will be inexact. The likelihood of damage was keyed on the quality of the fit to each pattern of known damage. Two measures of fit were used: the first was related to the correlation coefficient; the second was a measure of how close the exponent and coefficient were to unity. Both measures were defined on a scale from 0 to 100. It was hypothesized that damage was present when both measures were near 100. (Tracy and Pardoen, [1989]) present other approaches to forward problem.

The Inverse Problem

The inverse problem, which is typically Level 2 or Level 3 damage identification, consists of calculating the damage parameters, e.g., crack length and/or location, from the frequency shifts.

Lifshitz and Rotem [1969] present what may be the first journal article to propose damage detection via vibration measurements. They look at the change in the dynamic moduli, which can be related to the frequency shift, as indicating damage in particle-filled elastomers. The dynamic moduli, which are the slopes of the extensional and rotational stress-strain curves under dynamic loading, are computed for the test articles from a curve-fit of the measured stress-strain relationships at various levels of filling.

Stubbs and Osegueda, [1990] developed a damage detection method using the sensitivity of modal frequency changes that is based on work by Cawley and Adams [1979]. In this method, an error function for the ith mode and pth structural member is computed assuming that only one member is damaged.The member that minimizes this error is determined to be the damaged member. This method is demonstrated to produce more accurate results than their previous method in the case where the number of members is much greater than the number of measured modes. The authors point out that this frequency-change sensitivity method relies on sensitivity matrices that are computed using a FEM. This requirement increases the computational burden of these methods and also increases the dependence on an accurate prior numerical model. To overcome this drawback, Stubbs, et al.[1990] developed a damage index method.

MODE SHAPE CHANGES

Fox [1992] shows that single-number measures of mode shape changes such as the MAC are relatively insensitive to damage in a beam with a saw cut. Again this highlights the problem that too much data compression can cause in damage identification. "Node line2MAC," a MAC based on measurement points close to a node point for a particular mode, was found to be a more sensitive indicator of changes in the mode shape caused by damage. Graphical comparisons of relative changes in mode shapes proved to be the best way of detecting the damage location when only resonant frequencies and mode shapes were examined. A simple method of correlating node points—in modes that show relatively little change in resonant frequencies—with the corresponding peak amplitude points—in modes that show large changes in resonant frequencies—was shown to locate the damage. The author also presents a method of scaling the relative changes in mode shape to better identify the location of the damage.

MODE SHAPE CURVATURE/STRAIN MODE SHAPE CHANGES

An alternative to using mode shapes to obtain spatial information about sources of vibration changes is using mode shape derivatives, such as curvature. It is first noted that for beams, plates and shells there is a direct relationship between curvature and bending strain. The practical issues of measuring strain directly or computing it from displacements or accelerations are discussed by some researchers.

Stubbs, et al. [1990] present a method based on the decrease in modal strain energy between two structural DOF, as defined by the curvature of the measured mode shapes.

Chance, et al. [1994] found that numerically calculating curvature from mode shapes resulted in unacceptable errors. They used measured strains instead to measure curvature directly, which dramatically improved results.

Chen and Swamidas [1994], (Dong, et al. [1994]), present other studies that identify damage and its location from changes in mode shape curvature or strain-based mode shapes.

CRITICAL ISSUES FOR FUTURE RESEARCH IN DAMAGE IDENTIFICATION AND HEALTH MONITORING

This section contains a summary of the critical issues, as perceived by the authors, in the field of modal-based structural damage identification and health monitoring. The purpose behind this section is to focus on the issues that must be addressed by future research to make the identification of damage using vibration measurements a viable, practical, and commonly implemented technology.

One issue of primary importance is the dependence on prior analytical models and/or prior test data for the detection and location of damage. Many algorithms presume access to a detailed FEM of the structure, while others presume that a data set from the undamaged structure is available. Often, the lack of availability of this type of data can make a method impractical for certain applications. While it is doubtful that all dependence on prior models and data can be eliminated, certainly steps can and should be taken to minimize the dependence on such information.

Almost all of the damage-identification methods reviewed in this report rely on linear structural models. Further development of methods that have an enhanced ability to account for the effects of nonlinear structural response has the potential to enhance this technology significantly.

The number and location of measurement sensors is another important issue that has not been addressed to any significant extent in the current literature. Many techniques that appear to work well in example cases actually perform poorly when subjected to the measurement constraints imposed by actual testing. Techniques that are to be seriously considered for implementation in the field should demonstrate that they can perform well under the limitations of a small number of measurement locations, and under the constraint that these locations be selected a priori without knowledge of the damage location.

An issue that is a point of controversy among many researchers is the general level of sensitivity that modal parameters have to small flaws in a structure. Much of the evidence is only demonstrated for specific structures or systems and not proven in a fundamental sense. This issue is important for the development of health- monitoring techniques because the user of such methods needs to have confidence that the damage will be recognized while the structure still has sufficient integrity to allow repair.

Many researchers noted with regards to long-term health monitoring of structures such as bridges and offshore platforms, the need to reduce the dependence upon measurable excitation forces. The ability to use vibrations induced by ambient environmental or operating loads for the assessment of structural integrity is an area that merits further investigation.

It is clear, though, that the literature in general needs to be more focused on the specific applications and industries that would benefit from this technology, such as health monitoring of bridges, offshore oil platforms, airframes, and other structures with long design life, life-safety implications and high capital expenditures.

Additionally, research should be focused more on testing of real structures in their operating environment, rather than laboratory tests of representative structures. Because of the magnitude of such projects, more cooperation will be required between academia, industry, and government organizations. If specific techniques can be developed to quantify and extend the life of structures, the investment made in this technology will clearly be worthwhile.