GT2017-65168

STUDY OF INTERLAMINAR FRACTURE PROPERTIES OF CERAMIC MATRIX COMPOSITES AT ROOM AND ELEVATED TEMPERATURES Rabih Mansour The University of Akron Akron OH USA Yogesh P Singh The University of Akron Akron OH USA Manigandan Kannan The University of Akron Akron OH USA Gregory N Morscher The University of Akron Akron OH USA Frank Abdi AlphaSTAR Corporation Long Beach CA USA Jalees Ahmad AlphaSTAR Corporation San Diego CA USA Cody Godines AlphaSTAR Corporation Long Beach CA USA Saber DorMohammadi AlphaSTAR Corporation Long Beach CA USA Sung Choi NAVAIR Patuxent River MD USA ABSTRACT Interlaminar fracture properties play an important role in predicting failure of structural components for CMC materials In engine applications components are subject to large thermal gradients which induce interlaminar stresses One of the main challenges in evaluating interlaminar fracture toughness at room and elevated temperatures is the development of an experimental setup that provides ease for testing and allows for in situ monitoring of the interlaminar crack growth Therefore a wedge loaded DCB testing method is developed The method utilize electrical resistance to monitor crack growth and was applied to a woven polymer infiltrated pyrolysis PIP SiC SiNC composite Post testing inspection was carried out using optical microscopy of polished cross sections showing crack morphology It was found that crack growth rate at room temperature is double the one at 815 C for initial tests in this composite system Estimates of Mode I energy release rate suggests flat R curve behavior at room temperature in comparison to rising R curve behavior at 815 C NOMENCLATURE Aarm1 2 cross sectional area of arm1 and arm2 respectively Ax y cross sectional area of damaged section of the sample corresponds to x or y a crack extension length a total crack length notch length crack extension length a0 crack length correction factor due to material anisotropy E Young s modulus GI mode I energy release rate h half the sample thickness h1 2 arm1 and arm 2 thicknesses respectively L distance from ER probe to notch tip MOD mouth opening displacement P interlaminar load R measure electrical resistance at room temperature R measure electrical resistance at room temperature ROP resistance due to current flow in the out of plane direction t sample thickness w sample width x y distances from crack center to sample edge 2 wedge head angle electrical resistivity of the un damage composite effective electrical resistivity at high temperature INTRODUCTION With LEAP 1A engines entering service in 2016 the focus on ceramic matrix composites CMCs has increased significantly since these materials provide better thermal stability at higher temperature compared to conventional superalloys allowing for an increase in engines operating 1Copyright 2017 ASME Proceedings of ASME Turbo Expo 2017 Turbomachinery Technical Conference and Exposition GT2017 June 26 30 2017 Charlotte NC USA GT2017 65168 This work was authored in part by a U S Government employee in the scope of his her employment ASME disclaims all interest in the U S Government s contribution temperatures 1 2 Currently the most advanced ceramic composites are SiC based CMCs which consist of SiC fibers and a predominantly SiC matrix 3 However the complex architecture of these composites systems especially the ones with 2D or 3D woven fibers makes the understanding of internal damage mechanism more difficult The presence of porosity due to processing increases the susceptibility of interlaminar crack initiation since each pore serves as a stress concentration point for interlaminar cracks to initiate The understanding of interlaminar properties is therefore important primarily at elevated temperatures where interlaminar failure is likely to occur at the weak fiber matrix interface There are a number of challenges associated with the study of interlaminar fracture toughness of CMCs at high temperatures First of all during high temperature testing it is not possible to have direct measurements of crack lengths simply because the sample is concealed inside a furnace It has been found that electrical resistance ER provides a sensitive measure of internal damage in CMCs 4 5 Therefore an electrical resistance based method has been developed to indirectly monitor crack growth The method has shown good validity during room temperature testing The second challenge for interlaminar fracture testing is the lack of standardized testing method for determining interlaminar properties Common fracture testing techniques require the use of adhesive bonding 6 7 and while this is applicable at room temperature adhesive bonding is a limiting feature at high temperature As a result there is a need for testing technique that does not require the use of loading pins In previous work the authors introduced a wedge loaded double cantilever beam DCB testing technique that can be employed at room and high temperature testing 8 The presented work aims at establishing a methodology to compare interlaminar fracture properties for non oxide CMC materials at room and elevated temperatures Change in electrical resistance will be used to determine crack growth during interlaminar testing The difference in crack growth rate and the fracture toughness of the material at both room temperature and 815 C are discussed in detail in this paper EXPERIMENTAL PROCEDURES Material The material tested in this study was a commercial SiC SiNC system S200 fabricated by COI Ceramics Inc San Diego CA with ceramic general woven silicon carbide CG NicalonTM fibers in a matrix of silicon nitrogen carbon that was processed via a polymer pyrolysis process PIP The fibers were coated with boron nitride and their architecture consisted of sixteen plies of balanced 2 D woven eight harness satin Five samples were tested in this study two at room temperature and the other three at 815 C The samples were 70 mm in length 10 mm in width and 5 2 0 2 mm in thickness same as panel thickness A notch was machined in each sample in the mid plane with a thickness of 0 4 and a length of 23 mm For mechanical testing a ceramic wedge polycrystalline silicon nitride GN10 from Allied Signal Torrance CA with an 18 head angle 2 was used to create a splitting force by inserting the wedge in the notch forcing the arms to separate and creating an interlaminar crack around the notch tip An Instron 5582 machine with a load cell of 500 N was used for mechanical loading using a constant displacement rate of 1 mm min For room temperature testing the wedge was held in the top grip of the Instron while the sample was kept stationary at the bottom For high temperature testing a single zone furnace 38 1 mm total height with a temperature profile as shown in Fig 1 was used Measurements of temperature profile were taken throughout the height of the furnace using an R type thermocouple As shown in Fig 1 the hot zone for 815 C is 6 mm However due to the chimney effect there is a significant drop in temperature above the hot zone Therefore in order to maintain the area of interest of the sample notch and crack tip in the hot zone throughout the test the sample was held in the top fixture of the Instron while the wedge was kept stationary at the bottom grip The high temperature test started with the notch tip in the hot zone As the sample moved down towards the bottom of the furnace the crack propagated in the opposite direction As a result the crack tip was maintained inside the hot zone during the test Figure 1 Furnace Temperature profile at 815 C Electrical Resistance Electrical resistance ER was measured by four point method using a Keithely 2450 SourceMeter During the measurement a direct current 10 mA was applied through the outer probes and the voltage was measured across the two inner probes to monitor the resistance of the material during testing The advantage of this procedure is that it minimizes the effect of contact resistance on the measurements since the current flowing through the inner probes must be near zero For probe attachments thin copper wires were glued to the material using silver epoxy All probes were attached to the arms forcing the current to flow from one arm to the other around the notch crack tip This ER configuration has been shown to be an effective way for monitoring crack growth 4 8 and it will be utilized in this work to provide measurements of crack length 2Copyright 2017 ASME RESULTS AND DISCUSSIONS Crack Length Measurements Using ER Figure 2a shows a picture of a sample prior to testing with one surface coated with white paint to elucidate the crack Figure 2b represents a schematic of the test setup with an interlaminar crack that propagated a certain distance As the crack propagates during room temperature testing the current has to travel a longer distance from one arm to the other This increase in travel distance contributes to an increase in ER and can be expressed using the following equation 1 2 1 where is the axial electrical resistivity of the undamaged composite L is the distance from the inner lead to the notch tip a is the crack length Aarm1 Aarm2 is the cross sectional area of the arm1 arm2 respectively Ax Ay are the areas created by crack propagation and ROP is a resistance term related to the out of plane resistivity of the sample By substituting the dimensions h arms thickness t specimen thickness and w specimen width for the appropriate areas the equation for the estimated crack length a at room temperature becomes 1 2 2 a b Figure 2 a Picture of untested sample showing initial notch and the outer fibers layout direction b Schematic of test setup showing the different geometries During high temperature testing on the other hand the measured ER depends on two factors interlaminar crack growth and the temperature variations along the current path Due to the furnace profile and heat conduction through the sample the current path had a varying temperature and therefore varying electrical resistivity Thus in order to estimate crack growth during high temperature testing the contribution of temperature gradient to ER should be eliminated Figure 3 shows the change of ER during heating up to 815 C with a heating rate of 50 C min Since SiC is a semi conductor its electrical resistivity decreases significantly with temperature and the measured ER between the two arms decreased by 75 These values are in line with data reported in literature 5 9 Interestingly after an increase in temperature above 300 C the measured ER data suddenly became more stable this is probably due to reduction in the contact resistance of the silver epoxy because of temperature Figure 3 Change of ER as a function of temperature during heating up to 815 C After heating up and testing the sample at 815 C the effect of temperature gradient on ER was estimated by repeating the test lowering the tested sample inside the furnace in the absence of the wedge In this case the change in ER was only due to temperature gradient after a crack was propagated in the sample Figure 4 illustrates the displacement dependence of ER for actual test and simulated loading without inducing a new crack after testing The ER values at the end of the displacement in both cases agree very well This suggests that the change in ER is a sum of ER increase due to crack growth and due to temperature gradient effect The difference in ER at zero displacement is associated with the presence of the interlaminar crack after high temperature testing Figure 4 Measured ER as a function of displacement for actual high temperature testing and high temperature test simulation with further propagating the interlaminar crack 3Copyright 2017 ASME Therefore to determine the contribution of crack growth to the increase in ER we first estimate the ER change due to temperature gradient without a crack by subtracting the crack effect on ER from the ER change due to temperature gradient with a crack The new curve is then subtracted from the total ER change during high temperature testing to obtain a relationship between ER change and crack growth during high temperature testing as shown in Figure 5 Figure 5 ER change due to crack propagation during high temperature testing To calculate crack growth during the high temperature test using Eq 2 one must find first the resistivity of the material throughout the crack path However this task is challenging due to temperature gradient and its effect on resistivity Therefore one can find an effective resistivity instead To do so Eq 1 will be used at the beginning and the end of the high temperature test where in both scenarios the crack length a is known a can be measured at the end of the high temperature test 1 2 3 1 2 4 By subtracting the previous two equations we can get an expression for effective resistivity as follows 5 where R is the change in ER during the high temperature testing due to crack growth as shown in Fig 5 a is the total crack growth during high temperature testing Mechanical and Electrical Behavior Figure 6 shows the mechanical electrical behavior and crack length during interlaminar testing at room temperature for one of the samples Damage initiation can be observed clearly as a slight deviation in the slope of the mechanical data This onset can also be seen as an increase in ER The data shows that after the crack propagates a certain distance there is a continuous decrease in load which indicates minimum crack growth resistance Figure 6 Mechanical electrical behavior and crack length during room temperature interlaminar testing The mechanical behavior during high temperature testing is presented in Fig 7 Similar to Fig 6 the increase in ER is in line with damage onset However for this case there is a continuous increase in load in respect to crack propagation This indicates crack growth resistance is greater during high temperature testing Figure 7 Mechanical electrical behavior and crack length during high temperature 815 C interlaminar testing It is important to mention that the load reported in Fig 6 and Fig 7 include the load required to initiate and propagate the interlaminar crack in addition to the friction contribution between the wedge and the sample arms The friction force depends on the coefficient of friction between the sample and the wedge The coefficient of friction between silicon nitride the wedge material and silicon carbide the specimen material has been found to be 0 26 10 However this coefficient of friction changes between room and high temperature testing due to the wedge exposure to temperature To the best of our knowledge there is no data in open literature on the effect of temperature on the coefficient of friction between Si3N4 and SiC The reported data for self mated Si3N4 4Copyright 2017 ASME or SiC show minimum effect of temperature on the coefficient of friction 11 12 Assuming small change in the coefficient of friction between room and high temperature testing the crack growth resistance at high temperature is greater than that at room temperature Even looking at the overall mechanical response between room and 815 C we see clear decrease in load during crack propagation at room temperature compared to slight increase in load for high temperature testing Fig 6 Fig 7 This indicates that the interlaminar crack growth resistance is greater at 815 C compared to room temperature When comparing crack growth rate between the two tests the crack growth rate at room temperature is double the rate at high temperature Fig 8 This is speculated as being due to slight softening of the matrix as a result of heating Matrix softening would cause the interlaminar crack to propagate slower through the matrix Also it would allow the arms to be more compliant therefore bending more before forcing the crack to propagate This claim can be supported by the fact that strain to failure increases significantly with temperature for this material by 40 during testing at 1000 C 13 Figure 8 Crack growth rate at room temperature and at 815 C Crack Morphology Samples were examined under optical microscopy post test after polishing them near the surface Figure 9a shows the crack morphology after room temperature testing The crack appeared to initiate at the notch tip near the adjacent transverse fiber tow Then it propagated to the nearest porosity and then it continued growing along a longitudinal fiber tow going through the outer surfaces of transverse tows and not through them It is worth mentioning that from this micrograph it seems that only one dominant interlaminar crack propagated through the sample For crack propagation in the sample tested at 815 C Fig 9b two cracks initiated at the notch tip around the neighboring transverse tows the cracks then propagated to the close longitudinal tows then intersected to become one dominant crack Interestingly a secondary crack was found near the outer surface of the sample It is difficult to know if this crack existed prior to testing or if it occurred during heating high temperature testing a b Figure 9 Crack morphology near the surface post a room b high temperature interlaminar testing in the X Y plane Figure 10 displays the crack path across the width of the sample shown in Fig 9a in a plane perpendicular to the one in Fig 9a The micrograph shows that the crack propagated through the outer surface of the transverse tows in the mid section of the sample and along the longitudinal tows only near the edges Figure 10 Crack morphology across the width post room temperature testing in the Y Z plane Energy Release Rate The energy release rate can be evaluated using beam on elastic foundation model 14 12 2 2 2 3 11 1 0 64 2 6 where P is the load inducing interlaminar crack initiation and propagation and h is half the sample thickness assuming a sample with identical arms and a notch with negligible thickness Since this is not the case in this study h will be consider equal to x distance from th