Structural Integrity of Composite Laminates with Embedded Microsensors

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1 Strutural Integrity of Composite Laminates with Embedded Mirosensors Yi Huang, Sia Nemat-Nasser Department of Mehanial and Aerospae Engineering, Center of Exellene for Advaned Materials, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA , USA ABSTRACT The study of the mehanial interation among the host, interfae, and a devie embedded within a laminated omposite is important. Embedding miro-sensors in omposite laminates produes material disontinuity around the inlusions. This in turn produes stress onentrations at or near the inlusions. Both D plane strain and 3D FEM models are developed to analyze the stress/strain state surrounding the embedded miro-sensors within a unidiretional omposite laminate. The objetive of the present numerial effort is to take into aount the observed resin-rih areas aused by embedment, and to determine their effets on the loal stress field around the embedment and the orresponding potential failure modes. Keywords: Strutural Health Monitoring, Stress/Strain Conentration, Finite Element Analysis, Matrix Miro-raking, Composites. INTRODUCTION Adding health monitoring funtionalities into omposites brings out strutural integrity onerns about the effet of the inlusions on the load arrying apability, strutural life and failure mehanisms. The presene of the embedded devies auses material and geometrial disontinuities whih are responsible for unwanted peak values of stress/strain onentration with onsequenes on the stiffness redution and the overall material performane. These bring the interest of studying the mehanial interation among the host, interfae, and the embedded devies. Experimental studies [-6] have been onduted on the strength and failure of omposite materials with embedded devies suh as silion hips, PZT sensors, or fiber opti sensors, et. From these referenes it appears that in ases where the thikness of the inlusion did not alter signifiantly the through-the-thikness geometry of the host material, the loal stress onentration values remained very small thus negligible effets on the material integrity were observed. However, in ases where the integrated devies were of not negligible size, the redution of the strength of the embedded material as well as the damage mehanisms had to be quantified and investigated. With regard to the effets on loal stress field and failure mehanisms, a number of numerial studies [7-0] have been performed on embedded optial fiber sensors in omposite laminates. Among the few numerial studies on embedded retangular implants, Chow and Graves [] investigated the stress and displaement fields near a soft implant inside a laminated omposite material. The implant was embedded with utting several plies of the omposites. Their results indiated that the interlaminar stresses are an order of magnitude lower than the stress representing the applied far field load and the stress onentration fator was dependent on the staking sequene. Singh and Vizzini [] analyzed the interlaminar stress state surrounding an interlaed, ative piezoerami atuator embedded within a unidiretional omposite laminate. Their FE model appeared to be not sensitive to the transition resin layer thikness and the resinpoket lengths (in ase it is greater than seven times of the ply thikness). They onluded that interlaing inreases the strength of the omposite strutures with embedded atuators by redistributing the load around the inlusion and the Sensor Systems and Networks: Phenomena, Tehnology, and Appliations for NDE and Health Monitoring 007, edited by Kara J. Peters, Pro. Of SPIE Vol. 6530, 65300W, (007) X/07/$8 doi: 0.7/.75 Pro. of SPIE Vol W-

2 host/inlusion interfae. They also stated that the ourrene of delamination within the host is virtually independent of whether or not the inlusion is ative. In our study, a retangular-shaped hip resistor was embedded into a omposite laminate as a simulated miro-sensor. The omposite laminate was made from S/BT50E-LV unidiretional glass-epoxy prepreg. The simulated mirosensor was plaed with its length aligned with the fiber diretion (Figure ). The present paper investigates the impat on the loal stress/strain field under the tensile loading ondition of the resin rih area and the fiber distortion due to the embedment of the retangular-shaped implant. FEM models are developed to analyze the stress/strain state surrounding the embedded miro-sensors. In addition, failure riteria are applied to identify and loate the damage initiation. Figure The embedding onfiguration. FINITE-ELEMENT MODEL The presene of an embedded simulated retangular-shaped sensor in a fiber reinfored omposite reates a resin poket (Figure ). The sizes and the shapes of the resin pokets were measured, and the dimensions were averaged for our numerial study. D model ontaining omposite, resin, and senor areas was set up to study the loal stress/strain field (Figure 3). Due to the symmetry of the idealized model, only one quarter of geometry has been onsidered. A plane strain state has been assumed. A 3D model has also been studied and the results ompared with those given by the D plane strain model here desribed but no signifiant improvement has been found in terms of the stress and strain results. Four node quadrilateral plane strain elements are mostly used in this numerial analysis. In addition, three node triangular elements are used to model the tip of the resin poket. Figure 4 shows the loal finite mesh around the resin poket. It an also be notied that a finer mesh is used to disrete the domain around the sensor and partiularly the resin poket root, where maximum stresses are expeted. The elements shape in these ritial areas is arefully hosen to better approximate the ply distortion next to the inlusion due to its embedment within the omposite. Symmetrial boundary onditions are applied to the model with respet the displaement in the x and y diretions. A uniaxial tensile load is applied at the end of the model in the form of a uniform displaement in the longitudinal diretion. Pro. of SPIE Vol W-

3 Figure Mirograph of a setion of S-glass/epoxy omposite laminate with embedded simulated miro-sensor Figure 3 Idealized d model Pro. of SPIE Vol W-3

4 Figure 4 Loal finite element mesh of D FEM model 3. RESULTS AND DISCUSSION From the output strain field, it appears that the maximum longitudinal and shear strains our at the sensor orners within the resin-oating interfae (resin rih region). Among the three strain omponents, the shear strain has the maximum value. Sine the neat epoxy resin an arry the least strain among all the material omponents in the omposite, shear debonding at the resin-sensor oating interfae is expeted to be the main ause of failure initiation. The three stress omponents along the resin-omposite and resin-sensor oating interfaes generally have the same trend. However, in the latter ase, the magnitude of stress is higher. The maximum longitudinal and transverse stresses our at the sensor orner area. At the end of the resin poket, the transverse stress inreases. However, this small inrease is not enough to ause fiber-matrix splitting. The shear stresses along both interfaes show sharp rises at the sensor orners, dereasing monotonially away from the singular point. Based on the stress distribution results from, the values of the stress applied at the far end that ause the initiation of the failure were alulated both for the omposite area and the neat resin area separately. For the omposite domain, maximum stress riterion is applied. Failure is expeted to our when at least one stress omponent along one of the prinipal material axes reah its orresponding strength. The riterion is defined as follows: Tension failure Compression failure Shear failure or (a) = F t = F = F t or (b) 6 = F 6 = F () Here, the subsripts and refer to prinipal material axes of the omposite material. Pro. of SPIE Vol W-4

5 For the neat resin area (inluding the transition resin layer and the resin poket), von Mises riterion is applied. Failure is expeted to our when the stress omponents satisfy the hosen failure riterion. The von Mises riterion is defined as follows: (a) F r t = ( 3J ' / ) ' J = / 6[( ) + ( ) + ( ) ] (b) 3 3 Using these riteria and the strengths of the onsidered material, the failure initiation stress is predited and the site of failure initiation is loated. The predited value shows that miro-raks may initiate around the embedded sensor at an early stage of the loading proess. 4. CONCLUSIONS The presene of the embedded miro-sensors in laminated omposites is predited to initiate premature failure. Complex miromehanial interations our at the fiber glass/epoxy laminate and the embedded simulated sensor interfae. FEM analysis has been onduted to study the effet of the embedded simulated miro sensor on the stress/strain fields and failure mehanisms within the host omposite laminate. Under tensile loads, the initial failure is expeted to be matrix raking at the sensor orners in the resin-sensor oating interfae. The failure initiation load is predited using von Mises riterion. REFERENCES. D. J. Warkentin, E.F. Crawley, 99, Embedded Eletronis For Intelligent Strutures, AIAA Journal, K. S. Kim, M. Breslauer, G. S. Springer, 99, The Effet of Embedded Sensors on the Stength of Composite Laminates, Journal of Reinfored Plastis and Composites, Vol., J. S. Sirkis, H. Singh, A. Dasgupta, C. C. Chang, 99, Experimental determination of damage and interation strain fields near ative and passive inlusions embedded in laminated omposite materials, presented at the ADPA/AIAA/ASME/SPIE Conf. on Ative Materials and Adaptive Strutures, J. P. Hansen, A. J. Vizzini, 000, Fatigue Response of a Host Struture with Interlaed Embedded Devies, Journal of Intelligent Material System and Strutures, Vol., S. Mall, J. M. Coleman, 998, Monotoni and fatigue loading behavior of quasi-isotropi graphite/epoxy laminate embedded with piezoeletri sensor, Smart Mater. Strut. Vol. 7, C. A. Paget, K. Levin, 999, Strutual Integrity of Composites with Embedded Piezoeletri Cerami Transduer, Part of the SPIE Conferene on Smart Struture and Integrated Systems, Newport Beah, California, SPIE Vol. 3668, A. Dasgupta, Y. Wan and J. S. Sirkis, 99, Predition of resin poket geometry for stress analysis of optial fibers embedded in laminated omposites, Smart Mater. Strut. Vol., K. Levin, S. Nilsson, 994, Analysis of the loal stress field in a omposite material with an embedded EFPI-sensor, presented at the Seond European Conf. on Smart Strutures and Materials, Glasgow, N. C. Eaton, R. C. Drew and H. Geiger, 995, Finite element stress and strain analysis in omposites with embedded optial fiber sensors, Smart Mater. Strut. Vol. 4, K. Shivakumar, A. Bhargava, 005, Failure mehanis of a omposite laminate embedded with a fiber opti sensor, Journal of Composite Materials, Vol. 39, No.9, W. T. Chow, M. J. Graves, 99, Stress Analysis of a Retangular Implant in Laminated Composites Using -D and 3-D Finite Elements, AIAA Journal, D. A. Singh, A. J. Vizzini, 994, Strutural integrity of omposite laminates with interlaed atuators, Smart Mater. Strut. Vol. 3, Pro. of SPIE Vol W-5