2011 International Conference on Business, Engineering and Industrial Applications (ICBEIA)

Dynamic Characterization of Epoxy Plate Using

OMA with Free-Free End Conditions

Haizuan Abd Rahman, Ahmad Azlan Mat Isa Fakulti Kejuruteraan Mekanikal, Universiti Teknologi MARA

40450 Shah Alam, Selangor, Malaysia. E-mail address : haiz82@yahoo.com

ABSTRACT

Lightweight composite material is more preferable than metal and metal alloys in engineering applications however uncertainty in its failure prediction hinder widespread usage. Vibration response as damage detection is widely used however its usage on composite material is very restricted due to its nonlinear and non homogenous property. This paper attempts to utilize this method specifically operational modal analysis (OMA) on undamaged epoxy plate to extract the dynamic parameters and extend the procedure to damage detection on composite plate. To realize this, two experimental setups were implemented in this study which is based on epoxy plate positioning as free-free ends conditions i.e hung by bungee and place on sponge. Results of these experimental setups will be compared to establish modal parameters on undamaged epoxy plate. However, before investigating modal parameters of damaged epoxy plate, establishing modal parameters of undamaged epoxy plate is highly essential for damage detection purpose.

Keywords : Damage Detection, Operation Modal Analysis, Composite Material, Vibration

1. INTRODUCTION

The study of a method that enable to detect and localize damages for monitoring status of engineering components or systems at the earliest stage is a topic that widely spread throughout civil, mechanical and aerospace engineering communities. This is due to deterioration of engineering components over its service period. Beside that, damage detection is also necessary to ensure the integrity of engineering structure. Environmental variation effect such as heat, wind and earthquakes could accelerate damage of a structure and could possible cause major disaster.

Current damage detections mainly still rely on visual inspections and local nondestructive evaluations such as ultrasonic, magnet field, thennal field, acoustic, radiographic or eddy-current methods [1]. These methods can only detect damages on or near the surface of a structure. Therefore, global damage detection is needed which could apply to more complex structures. Previous studies of damage detection have led to the development methods that could examine changes in the vibration characteristics of structures. Damage detection method base on vibration test is becoming popular topics and

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received consideration in literature last three decades [2]. The standard approach of damage detection based vibration is by utilizing modal parameters obtained from modal testing [3,4]. This method can be utilized by changes in its dynamic properties. Modal parameters such as frequencies, mode shapes and modal damping are functions of physical properties will cause changes of modal properties.

Composite materials are well known to be light weight and high strength compare to metal and metal alloys. For example, thennoset epoxy is well known for their better adhesion, chemical and heat resistance, good mechanical properties and good electrical insulating properties [5-7]. Hence, there are wide range of epoxy applications including fiber-reinforced plastic materials and general purpose adhesives. In aerospace industry, epoxy is used as a structural matrix material which reinforced by fiber. Epoxy is also function as structural glue. In industrial tooling, epoxy use to produce molds, master models, laminates, castings, fixtures and other industrial production aids. Epoxy replaces metal, wood, and other traditional materials because it improves production efficiency, lower cost and shortens process lead time.

Despite the advantages, composite materials including epoxy are prone to subsurface damage mechanisms such as delaminations, core cracking, and disbonds [8,9].Subsurface damages as per mention are usually undetected in visual inspections. These damages can only be detected and monitored by observing localized nonlinear vibrations.

Vibration can be analyzed by changes of frequencies, mode shapes and damping on undamaged and damaged composite materials. In this study, Operation Modal Analysis (OMA) will be implementing experimentally to obtain the require infonnation. Operation Modal Analysis (OMA) or also named as ambient modal analysis, utilizes only response measurements of the structures in operational condition to identify modal characteristics [10]. Operation Modal Analysis (OMA) is simple with no elaborate excitation equipment and no specific boundary conditions definition is needed [11]. The advantages of OMA technique are beneficial for aerospace and mechanical engineering.

The aim of this study is to develop a model of global damage detection in composite materials mainly in thennoset epoxy material. The objectives of this study namely, to develop dynamic model for vibration response in composite materials with damage, to verify experimentally the effectiveness of developed dynamic model by establishing

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variation type of damages in composite materials and to obtain the dynamic characteristics on a healthy and damage composite materials. But, this paper will only present OMA technique on undamaged epoxy plate to determine modal parameters.

One of parameter used to detect damages in composite materials is frequency variations. Despite that, natural frequency changes are not sufficient to identify location of damage structure [12]. This is due to cracks associated with similar crack lengths but at two different locations may cause same amount of frequency change. However, G.M. Qu et al. [13] found strain mode shape is more sensitive parameter as compared to natural frequencies and displacement mode shape in identifying damage locations on piezoelectric composite plate.

2. EXPERIMENTAL SET-UP

In this study epoxy plate was use as undamaged specimen as shown in Fig 1. The specimen was prepared from a mixture of Morcote BJC39 with a ratio of base and hardener of 3:1 left to harden on a special mold. The undamaged specimen with made to size of 210 mm long, 250 mm width and 3 mm thick allow to cure in room temperature for minimum 8 hours. No further process is required after this.

As for the boundary conditions for the experimental setup, a free-free ends condition was used. To realize this two ways of attachment was used as shown on Fig. 2 and Fig. 3 which is hung by bungee[l4] using rubber band and placing on sponge[IO].

Fig. 1: Test specimen of undamaged epoxy plate

Fig. 2: Test specimen hung by bungee

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Fig. 3: Test specimen placed on sponge

The OMA test was applied on both of the experiment set­ups. The measurements were taken using Bruel and Kjaer PULSE. Vibrational excitation was done on both experiment set-ups. Random excitation as for Fig 2 was provided by pen tapping on the test specimen, whereas random excitation on Fig. 3 was provided by hand tapping on the test specimen. Both experiment set-ups represent as free all sides of undamaged epoxy plate.

The grid layout for the frequency response measurement was mapped on the epoxy plate surface to ensure right result has been taken. There were 40 points of measurement and node spacing was identified to locate the point of accelerometer as shown on Fig.I.

In both experiment set-ups, 2 accelerometers were used. One was set at fixed location on point number 1 as reference and the accelerometer was roved on the other 39 points as show on Fig.3.

Peak-Picking was performed using Frequency Domain Decomposition (FDD) to extract frequency [Hz] data and mode shape of each peak. Peak-picking was also performed using Enhanced Frequency Domain Decomposition to extract damping ratio [%].

An Operational Modal Analysis (OMA) was also done on fiber glass reinforced plate (350mm long,350 mm wide and 4mm thick).2 sets of measurement was done on the plate which is before and after 1 mm drilled hole was made on the plate. The plate was placed on a bun gee as shown on figure 2.There were 42 points were of measurement and 2 accelerometers were used. One accelerometer was set at fix point number 1 as reference and the other accelerometer was roved on the other 41 points.

3. RESULTS AND DISCUSSION

3.1 Results when epoxy plate hung on bungee Fig. 4 shows the Peak-Picking performed using Frequency

Domain Decomposition technique to extract frequency [Hz]. Fig. 4 also shows 6 peaks were chose and 2 peaks were found automatically and the other 4 were chose manually.

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Fig. 4: Peak-Picking on FDD (Plate Hung by Bungee)

The frequency [Hz] and damping ratio [%] of 1st mode until 4th mode are shown in Table 1 as per below table. In comment column, "Found Automatically" shows Peak­Picking were done automatically. Selection 1 st mode until 4th mode were done by validating the Mode shape of each frequency [Hz].Mode shapes of 1st Mode until 4th Mode were shown in Fig. 5 until Fig. 8.

Mode Frequency [Hz] Damping Ratio [%] Comment

1st Mode 80 5.118

2nd Mode 116 5.101 Found Automatically

3rd Mode 232 3.123

4th Mode 400 3.372 Found Automatically

Table 1: Frequency and Dampmg Ratio (Plate Hung by Bungee)

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Fig. 5: Mode Shape of 1st Mode (Plate Hung by Bungee)

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Fig. 6: Mode Shape of 2nd Mode (Plate Hung by Bungee)

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Fig. 7: Mode Shape of 3rd Mode (Plate Hung by Bungee)

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Fig. 8: Mode Shape of 4th Mode (Plate Hung by Bungee)

3.2 Results when epoxy plate place on sponge

As per below Fig. 9 shows Peak-Picking performed using Frequency Domain Decomposition technique to extract frequency [Hz].

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Fig. 9: Peak-Picking on FDD (Plate Place on Sponge)

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Table 2 shows frequency [Hz] and damping ratio [%] of 1 st mode until 4th mode. Same method was done exactly as mention before in selecting 1 st Mode until 4th Mode which was validating mode shape of each frequency [Hz] extracted. Fig. 10 until Fig. 13 show mode shapes of 1 st Mode until 4th Mode.

Mode Frequency [Hz] Damping Ratio [%] Comment

1st Mode 116 7.6

2nd Mode 152 6.339

3rd Mode 240 8.558

4th Mode 844 3.864 Found Automatically

Table 2: Frequency and Damping Ratio (Plate Place on Sponge)

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Fig. 10: Mode Shape of 1st Mode (Plate Place on Sponge)

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Fig. 11: Mode Shape of 2nd Mode (Plate Place on Sponge)

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Fig. 12: Mode Shape of 3rd Mode (Plate Place on Sponge)

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Fig. 13: Mode Shape of 4th Mode (Plate Place on Sponge)

Table 3 and Table 4 show comparison of damping ratio [%] and frequency [Hz] on each experimental setup.

Mode Dampin Ratio [%]

Difference[% ] Hung by Bungee Place on sponge

1st mode 5.118 7.6 48.50 2nd mode 5.101 6.339 24.27 3rd mode 3.123 8.558 174.03 4th mode 3.372 3.864 14.59

Table 3: Difference of damping ratIO [%] between 2 experimental set-ups

Mode Frequency [Hz]

Difference[%] Hung by Bungee Place on sponge

1st mode 80 116 45.00 2nd mode 116 152 31.03 3rd mode 232 240 3.45 4th mode 400 844 111.00

Table 4: Difference of frequency [Hz] between 2 experimental set-ups

As show in Table 3 and Table 4, overall frequency [Hz] and damping ratio [%] values clearly are bigger percentage when epoxy plate place on sponge than when epoxy plate hung by bungee. In addition there are major difference on 3rd mode of

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damping ratio [%] values and 4th mode of frequency [Hz] values.

3.3 Results before and after drilled hole fiber glass reinforced epoxy plate

As shown on Table 5 below is changes of frequency and damping ratio before and after fiber glass reinforced plate were drilled to manufacture a hole.

healthy sample drilled hole sample

Mode

shape frequency Damping

shape frequency Damping

[Hz] Ratio[%] [Hz] Ratio[%]

1 torsional 39.5 3.475 torsional 40.5 2.693

2 flexural 78 2.456 flexural 82.5 2.47

3 torsional 101 1.984 torsional 102 1.454

4 torsional 240 3.349 torsional 242 2.652

5 torsional 519 1.142 torsional 525.5 1.102

6 flexural 604.5 0.6836 flexural 615.5 0.7513

Table 5: Parameter changes before and after fiber glass reinforced drilled hole

diff before and after

Mode drilled hole freqency Damping

[%] Ratio[%] 1 2.5 1.8 2 5.8 1.9 3 1.0 2.4 4 0.8 5.3 5 1.3 2.1 6 1.8 1.6

Table 6: Percentage changes before and after fiber glass reinforced drilled hole

Table 6 shows changes of frequency and damping ratio base on mode respectively. From this result overall data shows just small changes occurred before and after the plate was drilled.

4. CONCLUSION

A composites plate which epoxy plate was used in this research to determine undamaged epoxy plate base on two epoxy plate positioning set-ups hung by bungee and placed plate on sponge. By validating mode shapes on each experimental set-up, referring Fig. 5 and Fig. lO mode shape of 1 st mode, mode shape in Fig. 5 shown better 1 st mode shape compared to mode shape in Fig.lO. This shows positioning epoxy plate by bun gee is better than placing on sponge. This is due during vibrational excitation, epoxy plate on bun gee moves up and down rather than moving side by

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side when epoxy plate placed on sponge. This is also due to first torsional mode was not detected by the OMA technique because of insufficient structural energy excited around that frequency. However, base on Fig. 6 until Fig. 8 and Fig. 11 and Fig.I3, 2nd mode until 4th mode show similar mode shape.

Comparing Fig. 4 and Fig. 9 of FFD curve on both positioning set-ups, in the range of 1000Hz, Fig. 4 when plate hung by bungee shows consistency. However, Fig. 9 when plate base on sponge shows inconsistency starts at frequency above 400Hz.

Base on above assessments, positioning epoxy plate by hanging by bungee is more efficient than positioning plate on sponge in obtain modal parameters of undamaged epoxy plate.

As for result of drilled hole plate, there were no significant changes of frequency and damping ratio by comparing before and after the plate was drilled. Base on previous studies [2] [3] [8] [9], frequencies are significantly lower than expected on damaged structures. This is due to abnormal loss of stiffness. But in this case, the changes of frequencies are contradicted base on previous studies which is higher than expected due to the material stiffness used is stiffer than expected.

However, for future works, more conditions of damages are essential to conduct experiment as verification of available experiments results. Moreover, Fenite Element Method ( FEA) using Nastran as software are also important to verify and compare result of conducted experiments.

ACKNOWLEDGEMENT

The authors would like to acknowledge with gratitude the great support from Research Management Institute (RMl), UiTM and financial support from the Minister of higher Education Malaysia (MOHE) for the grant of Fundamental Research Grant (FRGS).

REFERENCES

[1] Michele Basseville, Albert Benveniste, Maurice Goursat, Laurent Mevel,"Subspace-Based Algorithms for Structural Identification, Damage Detection, and Sensor Data Fusion", EURASIP Journal on Advances in Signal Processing, 2006.

[2] Zhicun Yang, Le Wang, Hui Wang, Yan Ding, Xiaojuan Dang, "Damage Detection in Composite Structures Using Vibration Response Under Stochastic Excitation", ScienceDirect Sound and Vibration, 5 May 2009 ..

[3] Dionysius M. Siringoringo, Yozo Fujino, "Experimental Study of Laser Doppler Vibrometer and Ambient Vibration for Vibration-Based Damage Detection", ScienceDirect Engineering Structures, 2 May, 2006.

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[4] Andre Engelbrecht, "Structural Integrity Monitoring Using Vibration Measurements" , Department of Mechanical and Aeronautical, Faculty of Engineering, Built Environment and Information Technology, University of Pretoria, October 2000.

[5] http://www.wordig.comJdefinition/epox, 2010.

[6] http://en.wikipedia.orglwiki/epoxy , 2010.

[7] Ralph E. Wright, " Molded Thermosets, A Handbook for Plastics Engineers, Molders and Designers" 1991.

[8] Sara S. Underwood, Douglas E. Adams, "Composite Damage Detection Using Laser Vibrometry with Nonlinear Response Characteristics ", 18 October 2010,Proceedings of the IMAC-XXVIII.

[9] Ethan Brush, Douglas Adam , "Development of a Dynamic Model for Subsurface Damage in Sandwich Composite Materials", Proceeding of the IMAC­XXVII February 1-4 2010, Jacksonville, Florida, USA.

[10] Mehdi Batel, Svend Gade, Nis Moller, Henrik Herlufsen , "Ambient Response Modal Analysis On a Plate Structure", Bruel & Kjaer Sound & Vibration Measurement AlS, Denmark.

[ll] Lingmi Zhang, Rune Brinker, Palle Andersen, "An Overview of Operational Modal Analysis : Major Development and Issues", lmze@nuaa.edu.en.

[12] O.S. Salawu, "Detection of Structural Damage Through Changes in Frequency: a review", Elsevier Engineering Structures, Vol. 19, 31 May, 1995

[13] G.M Qu, Y.Y Li, L. Cheng, B. Wang , "Vibration Analysis of a Piezoelectric Composite Plate With Cracks" ,ScienceDirect Composite Structures, 19 December, 2004.

[14] Abd Halim Idris, "Dynamic Characterization of Fibre Reinforced Composite (S- Glass) In Rigid Armoured Vehicle" 2006, University of Technology MARA (UiTM).

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