Metal Structural Integrity Monitoring via Optical ... - ScienceDirect.com

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ScienceDirect Energy Procedia 75 (2015) 2061 – 2067

The 7th International Conference on Applied Energy – ICAE2015

Metal structural integrity monitoring via optical response of quantum dots-epoxy resin Ziming Zhao, Shaofeng Yin, Weiling Luan*, Shan-tung Tu Key Laboratory of Pressure Systems and Safety (MOE), School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, P.R.China

Abstract

To avoid the loss and accident caused by energy equipment failure, structural integrity monitoring requires precise sensors for measurement of stress and strain. Quantum dots (QDs)-epoxy resin composite shows variation of photoluminescence (PL) intensity under stretching. This paper presents three different phenomena of PL intensity during cyclic stretching. QDs-Epoxy Resin blank stretching and ANSYS calculation provide PL intensity tendency with strain transforming. Continuous cyclic loading after the PL intensity change becomes stable, illustrating a non-synchronized strain between metal and coating. The synchronous strain can be achieved in a wide application with the development of sensitive optical stress-strain sensors via optimization and characterization of QDs-Epoxy Resin. © 2015 Published by Elsevier Ltd. This © 2015The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of Applied Energy Innovation Institute

Keywords:structural integrity monitoring, photoluminescence, synchronous strain, quantum dots

1. Introduction With the improvement of material processing ability and system monitoring level, petrochemical industry and nuclear power plant are developing towards large-scale structure, high process parameters and production. To avoid the loss and accident caused by equipment failure, the safety and reliability of the component put forward high requirements. Strain measurement has been the most reliable method for structural integrity monitoring. Due to the minute changes of the service conditions, the safety of the mechanical equipments or components could hardly be achieved merely based on assessment prior to operation [1]. Hence, stress-strain detecting technique is the most important issue for structure monitoring since the creep strain accumulation is the main cause of failure mechanism. There have been several

* Weiling Luan. Tel.: +86-21-6425-3513; fax: +86-21-6425-3513.. E-mail address:[email protected].

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.291

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established methods for strain measurement, such as the bond electrical resistance strain gauge[2], Bragg grating strain sensor[3] and holographic interferometry technique[4]. Although these methods have extensive applications with high resolution, they are limited by response time and destruction of samples. Quantum dots (QDs)-epoxy resin composite utilizes the optical response of fluorescent QDs is therefore proposed to satisfy this requirement. QDs, fluorescence nano-semiconductor materials, have extensive application in biological probe, solar cells and light emitting diodes[5-7]due to its unique quantum effect property. Recently, luminescent nanocrystal stress-strain gauge has been developed utilizing QDs luminescence characteristics under different pressure. QDs with different shape, including dots, nanorods and tetrapods, present a wavelength blue or red shift under hydrostatic or non-hydrostatic pressure[8-10]. The experiments showed the optoelectronic properties of all these nanocrystal morphologies are affected by strain under gigapascals. In addition, CdSe@CdS tetrapod QDs were embedded into polymer matrices, acting as an in situ luminescent stress probe for the mechanical properties of polymer fibers. The mechano-optical sensing performance can be enhanced with increasing nanocrystal concentration, while cause minimal change in the mechanical properties[11].These discoveries have been utilized to detect stress or strain by investigating pressure-induced wavelength shift of QDs, which could either be used as themselves or incorporated into polymer. However, instead of wavelength shift, the photoluminescence (PL) intensity variation of QDs under pressure is easier to be captured. The optical response to stress is sensitive to pressure that is less than 1 MPa[12]. This paper investigates the PL intensity properties changing by stretching the metal matrix coated with CdS@ZnS QDs-epoxy resin. A nanocrystal stress-strain gauge with optical readout can be utilized for structural integrity. 2. Experimental Core/shell structured CdS@ZnS was prepared via microreaction technology with an average size of 5nm[13-16]. A bisphenol-A type epoxy resin and modified amine curing agent are selected as carrier due to their transparency, strong adhesion to the metallic matrix and distinct fluorescence wavelength different from CdS@ZnS under UV light. The CdS@ZnS mixture epoxy resin is spin-coated on the surface of the standard flat tensile test specimen which is commonly used in the uniaxial tensile tests. The coating thickness is carefully controlled by the spin-coating speed. The specimen has a rectangular crosssection and shoulders with a through hole designed for pinned grips to assure good alignment, as shown in figure 1. The curing of composite-epoxy is accelerated by vacuum drying, which ensured a flat surface without air bubbles. (a) (b)

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Uniaxial tensile tests are performed on the 304 SS specimen coated with QDs-epoxy resin. The strain response was characterized by load control using a universal testing machine (MTS-SANS, CMT5504).The tensile test specimen was connected to the pinned grips at both sides and applied tension by means of the top grip moving up through the movable crosshead, which is controlled by an electronic motor. The load was increasing 0.5 KN per 5 s and then held constant at every 0.5 KN for 5 s. The optical fiber spectrograph (Ocean Optics, QE65-Pro-FL) is utilized to measure PL intensity variation. 3. Results and discussion Figure 2 shows the PL intensity of QDs-epoxy resin film while working at uniaxial tensile test. Each set of experiments includes three cycles from 0 KN to 5.5 KN then back to 0 KN. Single cycle is divided into two steps containing loading and unloading. As shown in figure 2a, the PL intensity presents a linear increasing as metal strain increases at the first loading. In the section of uninstalling, the PL intensity continues a linear rise, which is similar to the first cycle. For the second and third cycle, the PL intensity variation follows the first cycle. The PL intensity shows an accumulative effect for the later three cycles, but the magnitude of change is decreasing with cycle times. Comparing with figure 2a, figure 2b presents a same rising tendency in the section of uninstalling and accumulation effect of PL intensity. Also, the amplitude of PL intensity decreases gradually in each cycle. The only difference is that the PL intensity does not increase or even decline along with the strain increase in all three cycles. However, we have also observed another phenomenon and shown in figure 2c. PL intensity increases with the strain and decreases with strain decreases. Furthermore, the variation of PL intensity shows a good reproducibility in the second and third cycles.

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Fig.2 Spectrum diagrams of PL intensity variation in the process of three times cyclic uniaxial tensile testing under three situations: (a): PL intensity increases with strain rising and decreases with strain declining; (b): PL intensity maintains decreases with strain rising and increases with strain declining; (c): PL intensity increases with strain rising and decreases with strain declining.

Unlike the previous experiments in which wavelength shifted under hydrostatic pressure[8-10], the uniaxial tensile testing displays PL intensity changing. Also, there is no significant change in the PL intensity of QDs with the increase of hydrostatic pressure, as shown in figure 3a[12]. Therefore, we

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regarded that the intensity variation effect was caused by interaction between the QDs and epoxy resin. Figure 3b shows stress-strain curves of QDs-epoxy resin blank. The epoxy resin is non-linear elastic material and produces a residual strain after stretching. With the increaseing load of specimen fracture, the intensity exhibits monotonically decrease in the overall trend, as shown in figure 3c and figure 3d. However, from figure 3c, we find a brief rise in the case of preminor strain. This phenomenon is widespread while blank stretch repeatedly due to the uncertainty in concentration of QDs stock solutions, settling, and changes in QDs activity. The analysis of ANSYS calculation presents QDs-epoxy resin coating stress under a load of 5.5 KN, as shown in figure 4, where the coating stress is less than 1 MPa. It also explains why the intensity increases or decreases with loading, shown in figure 2a and figure 2b. The residual strain of epoxy resin determines the amplitude of PL intensity increase. (a)

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Fig.4 ANSYS calculation pattern of standard flat tensile test specimen with QDs-epoxy resin coating, the red region show stress distribution of coating with the standard flat tensile test specimen stretching in 5.5 KN

After repeat of cyclic load, the PL intensity decreases gradually to a stable level. On the basis of 0-5.5 KN, we add 5.5-7.5 kN and 5.5-9.5 KN cyclic tensile test. Figure 5 displays PL intensity change in the added cyclic loading segment. The new variation of intensity in 5.5-7.5 KN segment is close to 5000 Counts and more than pre-stabilized status in 0-5.5 KN segment, seeing figure 5a. Meanwhile, figure 5b illustrates different slope of PL intensity change between 0-5.5 KN segment and 5.5-9.5 KN segment. And point 5.5 KN is an inflection point of two different slopes. Throughout the stage, the metal specimen has been in the elastic range. There is a non-synchronous strain between QDs-epoxy resin coating and metal specimen, therefore the interface displays a slight delamination. The continued rising intensity in the unloading of figure 2a and figure 2b may be also caused by the delamination. We surmise the structure of delamination areas has been taken placed in the unloading segment. 39500

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Fig.5 PL intensity spectrum diagrams under variable cyclic loads.(a): PL intensity variation in 5.5-7.5 KN segment. The green region show amplitude variation of PL intensity between 0-5.5 KN segment and 5.5-7.5 KN segment; (b):PL intensity variation in 5.5-9.5 KN segment. The green region showed different slope of PL intensity changing between 0-5.5 KN and 5.5-9.5 KN segment

The discussion above illustrates that the QDs-epoxy resin coating will have synchronous strain with metal specimen while there is no slight delamination in the interface. The intensity will increase or decrease linearly with strain, simultaneously. The characterization and optimization of QDs-epoxy resin composite has large potential application in the development of sensitive optical strain sensors. Through repeatable experiments, the change of relative intensity range was presented as figure 6.

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4. Conclusions In summary, the stretch of the standard flat tensile test specimen results in a slight strain on a CdS@ZnS-epoxy resin composite coating extended along with the standard flat tensile. The intensity of coating has been demonstrated a sensitive optical response. The blank spectrum exhibits PL intensity change which comes from uncertainty in concentration of QDs stock solutions, settling, and changes in QDs activity due to the increase of load segment. Experimental results of the accumulation effect of PL intensity in several cycles is assumed be caused by the interface micro delamination. Meanwhile, the unknown structural transformations of delamination areas result in the intensity continuingly rise in unloading segment. The residual strain of epoxy resin determines the amplitude of PL intensity increase. Furtherly, ensuring synchronous strain with metal specimen makes it possible for QDs-epoxy resin composite as the structural integrity monitoring of a new optical strain gauge. Acknowledgements The authors gratefully acknowledge the National Natural Science Fund of China (51172072, 51475166) and the National Basic Research Program of China (2013CB035505) for the financial support. References [1] S. T. Tu. Life prediction and monitoring of criticalindustrial equipment Symp. Transferability andApplicability of Current Mechanics Approaches ed G C Sihˈet al (Shanghai: East China University Science andTechnology Press) 2009, pp:13–22 [2] R. B. Watson. Calibration techniques for extensometry-possible standards of strain-measurement. Journal of Testing and Evaluation, 1993, 21(6): 515-521 [3] Y Tu, S. T. Tu. Fabrication and characterization of ametal-packaged regenerated fiber Bragggrating strain sensor for structural integritymonitoring of high-temperaturecomponents. Smart Materials and Structures, 2014, 23, 035001 [4]M. S. B. Fernandez, J. M. A. Calderon, I. I. C Segura,et al.Stress-separation techniques in photoelasticity: A review. Journal of Strain Analysis for Engineering Design, 2010, 45(1):1-17


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Biography Weiling Luan received her doctorate in material science and technology in 1998 at Shanghai Institute of Ceramics, CAS. Then she was a postdoctoral researcher at Utsunomiya University in Japan. Now, she is a professor at East China University of Science and Technology. Her research interest includes Micro chemical-mechanical system and the sustainable energy.

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