Cure Monitoring and Stress-Strain Sensing of Single-Carbon Fiber Composites by the Measurement of Electrical Resistance

Transcription

1 Key Engineering Materials Online: ISSN: , Vols. 97-3, pp doi:1.48/ 5 Trans Tech Publications, Switzerland Cure Monitoring and Stress-Strain Sensing o Single-Carbon Fiber Coposites by the Measureent o Electrical Resistance Sang Il Lee 1,a, Dong Jin Yoon 1,b, Seung Seok Lee 1,c and Joung Man Park,d 1 Sart Measureent Group, Korea Research Institute o Standards and Science, DaeJeon, Korea Departent o Polyer Science & Engineering, Engineering Research Institute, Gyeongsang National University, Jinju, Korea a esang1@kriss.re.kr, b djyoon@kriss.re.kr, c sslee@kriss.re.kr, d jpark@nongae.gsnu.ac.kr Keywords: cure onitoring, stress-strain sensing, electrical resistance, residual stress, interacial shear strength (IFSS) Abstract. Cure onitoring and stress-strain sensing o single-carbon iber coposites were nondestructively evaluated by the easureent o electrical resistance. The dierence o electrical resistance beore and ater curing increased highest when gauge length o the specien was the sallest. As curing teperature increased, the electrical behavior o steel iber was dierent ro that o sei-conductive carbon and SiC ibers. Residual stress built in the iber was the highest at the iber axis direction. Whereas residual stress built in the atrix was relatively high at the iber circuerence and radius directions. Residual stress calculated ro the experient was consistent with the results ro the inite eleent analysis (FEA). The strain at low curing teperature was larger than that o higher teperature until the load reached axiu value. The apparent odulus o the electrodeposited coposites was higher than that o the untreated coposites due to the iproved interacial shear strength (IFSS). The electrical resistance was responded quantitatively with stress-strain behavior during the test. Electrical resistance easureent can be easible nondestructive techniques to evaluate cure onitoring and stress-strain sensing in the conductive iber coposites. Introduction A new evaluation technique o interacial properties as well as curing characteristics and the residual stress were investigated by the easureent o electrical resistance using various conductive iber reinorced coposites. The conductive iber reinorced coposites were studied as new sel-strain and/or sel-daage health onitoring sensors under teperature change and applied load [1]. Cure onitoring has been studied as an econoical new evaluation or curing characteristics and residual stress because conductive iber can act as a sensor in itsel as well as a reinorcing iber. The electrical resistance dierence and residual stress were investigated or single carbon iber coposite []. Residual stress in the iber-reinorced coposite occurred usually due to theral contraction o the atrix or the dierence o theral expansion coeicient (TEC) between iber and atrix. This stress ay aect on the interace between iber and atrix, and ay have a great inluence on the echanical perorance o icrocoposites. Residual stress can aect actually on the iber stress or the interacial shear strength (IFSS) [3]. Electrical resistivity with icroailure echaniss and nondestructive characteristics was recently studied in conductive iber coposites. To provide the correlation o interacial adhesion and electrical properties, the contact resistivity o carbon or steel iber/ceent atrix coposites was easured using single-iber electro-pullout test. The electro-icroechanical test can be deined as the siultaneous easureents o electrical resistance and echanical properties while echanical load applying [4]. This technique had been studied as econoically a new nondestructive evaluation (NDE) ethod or interacial sensing, teperature sensing, cure onitoring, strain-stress sensing and All rights reserved. No part o contents o this paper ay be reproduced or transitted in any or or by any eans without the written perission o Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-5/3/16,6:4:3)

2 Key Engineering Materials Vols icroailure daage sensing because conductive iber can act as a sensor in itsel as well as a reinorcing iber [5, 6]. In this work, cure onitoring and stress-strain sensing o carbon iber-reinorced coposites was evaluated by the easureent o electrical resistance. Curing characteristics and residual stress was investigated during curing process, and then strain-stress sensing was evaluated under cyclic loads. Experient Materials. Carbon iber o 8µ (Taekwang Industrial Co., Korea) in average diaeter was ainly used as conductive reinorcing aterials. SiC iber (Textron Co.) o 138µ and steel iber o 8µ were used to copare with carbon iber. Testing speciens were prepared with epoxy resin (YD-18, Kukdo Cheical Co., Korea). Jeaine D-4 and D- (Huntzan Petrocheical Co.) were used as curing agents. The lexibility o speciens was controlled by the ixing ratio o D-4 versus D-. Polybutadiene aleic anhydride (PBMA, Polyscience Inc.) was used as a polyeric coupling agent to iprove IFSS by electrodeposition (ED). Methods. 1) Preparation o Testing Speciens. Single carbon iber/epoxy coposite was prepared to easure electrical resistance, as shown in Fig. 1. In the specien, two pairs o narrow copper wires were ixed transversely on a old-released telon il with attaching guiding tapes, and then single iber was laid down in a longitudinal direction. The guiding tapes can prevent the overlow o resin and can deterine specien diension ater curing. A silver paste was used to connect electrically at the intersecting point between carbon iber and copper wires. Ater quantity o epoxy resin was poured into the silicone old, epoxy was precured at 8 o C or 1hour and then postcured at 1 o C or 1 hour. PBMA was diluted to.5wt.% concentration in deionized water. Ater the anode and the cathode were iersed siultaneously in an aqueous electrolyte solution, 3 voltages were supplied to both electrodes by power supply and typical coating tie was set up or 1in. Ater treated with ED, carbon ibers were dried at roo teperature without urther theral treatent. Current Contacts Voltage Contacts Silver Paste Cyclic Load HP 3441A Digital Multieter Ω 1 A B C D 1. Epoxy Carbon Fiber Telon Release Fil Silver Paste Carbon Fiber Epoxy Carbon Fiber Epoxy Matrix Fig. 1 Experiental systes or cure onitoring, stress-strain sensing and (c) FEA odel (c) ) Electrical Resistance Measureent. The electrical resistance was easured using a digital ultieter (HP3441A). For the curing onitoring, curing cycle was set up as three steps that were coposed o the precuring step at 8 o C or 1hour, the postcuring step at 1 o C or 1hour, and inally slow cooling step at roo teperature or 6hours. Ater the specien was placed in the oven, a testing coposite and the ultieter were connected electrically using very thin copper wires. Electrical resistance o the icrocoposite was onitored during curing process. For stress-strain sensing, stress-strain curve was easured by universal testing achine (UTM, Lloyd Instruents Co., U.K.). Testing speed and load cell were.5/in and 1 KN, respectively. Ater a testing specien was ixed into the UTM grip, the coposite and the ultieter were electrically connected using very thin copper wires. While cyclic loads were applied, the electrical resistance o the icrocoposite was siultaneously onitored with strain-stress changes, as described in Fig. 1. Strain-stress and electrical resistance were easured until the axiu load.

3 678 Advances in Fracture and Strength Electrical resistivity was obtained ro the easured electrical resistance, cross-sectional area o the conductive iber, A and electrical contact length, L ec o the testing iber connecting to copper wires. The relationship between electrical resistivity, ρ and resistance, R is as ollows: ρ = A R L ec Electrical resistance was easured by our-point probe ethod as shown in Fig. 1. Silver paste was used as electrically connecting glue at junctions A, B, C and D to aintain electrical contact between the iber and leading wires. The voltage was easured between junctions B and C, and the current was easured between junctions A and D. 3) Investigation o Residual Stress. It is coonly known that residual stress occurred by the dierent TEC between iber and atrix during curing and subsequent cooling process. The ollowing equation was designated to calculate the residual stress built in the iber, σ, σ R E = V ( α α ) ( V E + V E ) E T where E is the elastic odulus, α is TEC, T is teperature changes, and V is the volue raction. Subscript and ean the iber and atrix, respectively. The residual stress built in atrix was siply calculated ro TEC o each coponent and the odulus o SFC specien. Because the volue raction o atrix is too high in the SFC, the residual stress built in the atrix, σ R, can generally be estiated by the ollowing equation, σ R = ( α α ) T E( ε ) where E(ε) is the odulus which obtained ro the easured stress-strain curve o the specien. TEC o the iber is saller than that o the atrix, and ay be neglected in the calculating Eq. (3). The residual stress, resulted ro the experiental calculation, has been correlated with the value estiated ro FEA using coercial ANSYS 5.5. In Fig. 1 (c), the quarter section o the icrospecien was only odeled because geoetrical syetry was considered in the specien structure. The total nuber o nodes and eleent were 649 and 196, respectively. Nonlinear approach in FEA was used with the ulti-linear curve resulted ro true stress-strain curve. The relationship etween stress coponents and equivalent residual stress was designated by von Mises criterion, RF () (3) σ eq = 1 [( σ σ ) + ( σ σ ) + ( σ σ ) + 6τ + 6τ 6τ ] r θ θ z z r rθ θz + zr (4) where σ is the noral stress, τ is the shear, subscript r, θ, z are the iber radius direction, the iber circuerence direction, and the iber axis direction. Results and Discussion Cure Monitoring. Fig. shows the coparison o electrical resistivity change in a bare iber without atrix and single-iber ebedded epoxy coposite during curing process. As curing teperature increased, logarithic electrical resistivity o steel iber increased, whereas that o carbon and SiC ibers decreased. This could be due to the dierence o inherent electrical properties between steel

4 Key Engineering Materials Vols and carbon ibers based on the band theory. Electrical resistivity o the coposites ater curing was higher than that o the beore curing. It ay be explained that residual stress occurred by the dierent TEC between iber and atrix during curing. Fig. 3 shows the behavior o the electrical resistivity or single carbon iber/epoxy coposites with curing condition. As curing teperature increased, the dierence in electrical resistivity beore and ater curing increased. This ay be considered that relatively higher residual stress occurred or both the iber and atrix at higher teperature. In Fig. 3, the dierence in electrical resistivity beore and ater curing in the condition (3) with the optiu coposition was largest under sae curing teperature, whereas those o the condition and () were saller. Log [Electrical Resistivity (Ω c)] SiC iber/epoxy Bare SiC iber Carbon iber/epoxy Bare carbon iber -4.6 Teperature Steel iber/epoxy -4.7 Bare steel iber Tie (in) Fig. Logarithic elcectical resistivity o three ibers and their coposites during curing process Teperature ( o C) Electrical Resistivity (x 1-3 Ω c) Tie (in) 14 o C 1 o C 1 o C R Electrical Resistivity (x 1-3 Ω c) D4 : D = 3. : () D4 : D =.5 :.5 (3) D4 : D =.7 : Tie (in) Fig. 3 Electrical resistivity depending on curing teperature and curing agent coposition () (3) R σ r σ θ σ z τ rθ τ θz τ zr Equivalent Stress σ z Equivalent Stress o Fiber (MPa) σ θ σ r σ θ σ z τ rθ τ θz τ zr σ z Equivalent Stress σ r Equivalent Stress o Matrix (MPa) Z-Direction () Z-Direction () Fig. 4 Stress coponents and residual stress built in the iber and atrix by FEA The Measureent o Residual Stress. Fig. 4 shows all stress coponents and equivalent stress o von Mises criterion built in the iber and atrix by FEA. Residual stress on the iber axial direction is highest, whereas the other stresses is near zero and overlapped as shown in Fig. 4.

5 68 Advances in Fracture and Strength Residual stress o the iber axial direction, σz is trended to be siilar to an equivalent stress o z-direction. This could be considered that the stress o the iber axial direction has the signiicant eect on the total residual stress built on the iber. The stress o iber axial direction is very sall, whereas that o iber circuerence, σθ and radius direction, σr is relatively higher as shown in Fig. 4. Total su o the atrix stress in circuerence and radius directions is siilar to an equivalent stress o von Mises criterion. It ay be explained that two stress coponents with the circuerence and radius direction have the eect on the total residual stress built in the atrix. Stress-Strain Sensing. Fig. 5 shows stress-strain and electrical resistivity o carbon iber coposites as a unction o curing teperature under 5 cyclic loads. As curing teperature increased, apparent odulus increased and the elapsed tie until the axiu load is shorter Electrical Resistivity (Ω c x 1-3 ) Electrical Resistivity (Ω c x 1-3 ) (c) Electrical Resistivity (Ω c x 1-3 ) Fig. 5 Stress-strain and electrical resistivity as a unction o curing teperature under 5 cyclic loadings: 1 o C; 1 o C; (c) 14 o C Electrical Resistivity (Ω c x 1-3 ) Stress Strain Resistivity Electrical Resistivity (Ω c x 1-3 ) Fig. 6 Stress-strain curve and electrical resistivity: the untreated; the ED-treated Fig. 6 shows the behaviors o stress-strain and electrical resistivity o the untreated and ED treated carbon iber coposites under 3 cyclic loads. The elapsed easuring tie o the ED treated coposite is shorter than that o the untreated coposite. This could be due to the enhanced interacial properties. Fig. 7 exhibits the coparison o stress-strain and electrical resistivity between the untreated and ED treated carbon iber coposites under 1 cyclic load. The strain o the ED treated

6 Key Engineering Materials Vols carbon iber case is saller than that o the untreated coposite until the axiu load. This could be due to higher apparent odulus o coposite due to the iproved IFSS UT () ED () Electrical Resistivity (Ω c x 1-3 ) UT () ED () Electrical Resistivity (Ω c x 1-3 ) Fig. 7 Stress-strain and electrical resistivity o the untreated and ED-treated coposites Conclusions Interacial evaluation on teperature sensing, cure onitoring and stress-strain sensing or the single-carbon iber/epoxy coposites were investigated by the easureent o electrical resistivity. As curing teperature was elevated, logarithic electrical resistivity o steel iber increased, whereas those o carbon and SiC ibers decreased due to the band theory. With increasing curing teperature, the dierence in electrical resistivity beore and ater increased curing or carbon iber coposites. This could be considered that higher residual stress occurred in the iber and atrix at higher curing teperature. Residual stress built in the iber was highest at the iber axial direction, whereas that built in the atrix was relatively higher at the iber circuerence and radius directions. Ultiately, both residual stresses built in the iber and atrix increased with increasing curing teperature. As curing teperature increased, apparent odulus increased, and the elapsed tie was shorter until the axiu load. The higher atrix odulus was, the higher apparent odulus was. The elapsed tie o the ED treated coposite was shorter than that o the untreated coposite. This ay be due to the iproved interacial properties. The behavior o electrical resistivity was responded quantitatively against the change o stress-strain. While cyclic load was applied, the easureent o electrical resistance can be a useul ethod to investigated cure onitoring strain-stress sensing o carbon iber reinorced coposites as a easible NDE. Reerences [1] B. Frakhanel, E. Muller, T. Frakhanel and W. Siegel: J. Euro. Cera. Soc. Vol. 18 (1998), p. 181 [] J.M. Park, S.I. Lee, K.W. Ki and D.J. Yoon: J. Coll. Inter. Sci. Vol. 37 (1), p. 8 [3] M.S. Madhukar, M.S. Genidy and J.D. Russell: J. Copos. Mater. Vol. 34 (), p. 196 [4] X. Wang and D.D.L. Chung: Copos. Inter. Vol. 5 (1998), p. 77 [5] D.D.L. Chung: Theroc. Acta. Vol. 364 (), p. 11 [6] X. Wang, X. Fu and D.D.L. Chung: J. Mater. Res. Vol. 13 (1998), p. 381

7 Advances in Fracture and Strength 1.48/ Cure Monitoring and Stress-Strain Sensing o Single-Carbon Fiber Coposites by the Measureent o Electrical Resistance 1.48/ DOI Reerences [3] M.S. Madhukar, M.S. Genidy and J.D. Russell: J. Copos. Mater. Vol. 34 (), p /3MY-44XM-6WP8-WT7J