Fracture and Deformation Division National Bureau of Standards Boulder, Colorado

Transcription

1 INFLUENCE OF DAMAGE ON MECHANICAL PERFORMANCE OF WOVEN LAMINATES AT LOW TEMPERATURES Ronald D. Kriz Fracture and Deformation Division National Bureau of Standards Boulder, Colorado Walter J. Muster Metals Division Swiss Federal Laboratories for Materials Testing and Research (EMPA) Dubendorf, Switzerland ABSTRACT Large quantities of nonmetallic woven composites will be used in magnetic fusion energy structures at low temperatures. We predicted and measured the influence of crack formation on the mechanical performance of standard glass/epoxy laminates (G-lCR, G-llCR) at low temperatures. From experiments with tension loads, we studied the formation of damage as a collection of fiber breaks, fiber bundle cracks, and delaminations between adjacent fiber bundles. We measured fiber bundle cracks in the laminate interior and individual fiber fracture at the laminate edges. We discovered that the sequence and type of damage control the discontinuities ("knees") in the load-deformation (stress-strain) diagrams. We found that G-llCR has two knees and three distinct moduli, whereas G-lCR has only two moduli and a single knee at a lower strain than G-llCR. Decrease in moduli measured near the knees compared well with predictions from a finite element model. INTRODUCTION Magnetic-fusion-energy (MFE) power plants may use large quantities of nonmetallic composites. 1 There are a variety of applications where woven glass/epoxy laminates can provide thermal insulation, electrical insulation, and structural support. 2 In the structural design of super conducting magnets at 4 K, nonmetallics are used primarily as electrical insulation where only small mechanical loads occur. However, when cooled to liquid helium temperature (4 K), large internal stresses can occur within these structures because of large differential thermal contractions between metals and nonmetals. When these thermally induced stresses are combined with stresses induced by small mechanical loads (magnetic loads), microcracks may initiate at applied loads well below the yield strength. The formation of large numbers of microcracks could limit the electrical, thermal, and mechanical usefulness of these materials for MFE applications. The coalescence of R. P. Reed et al. (eds.), Advances in Cryogenic Engineering Materials Plenum Press, New York

2 NX- -NX 9 - Ply Crack O s= '-; V NX- 9 t -NX O?-:- Fig. 1. Repeating pattern for a two-layer woven laminate with an interior fill resin crack and a /9 laminate (8 = ) with a 9 ply crack. microcracks and voids could also limit their usefulness as barriers for containment of cryogenic liquids and gas. In this study, we measured the variability in mechanical performance caused by the formation of damage in a unit cell of plain weave. THEORY The unit cell of a woven structure was defined by Ishikawa3 as the smallest repeating structural unit. The four sides or cross sections (see Fig. 1) of a plain-weave unit cell are identical. 4 Hence, from a thin slice of the common cross section we ftpproximate the three-dimensional stress state by a finite element model (FEM) that assumes generalized plane strain. Details of this analysis are given in ref. 4. Damage in this model is limited to transverse cracks in the resin of the internal fill fiber bundles (see Fig. 1). Hence, only 5 percent of all possible fill resin cracks were modeled. To study the significance of fill resin cracks with weave geometry, we varied e (Fig. 1) from to 45. The condition of e = in Fig. 1 represents a special limiting case for the woven laminate, where the fill resin crack becomes a ply crack in a /9 laminate. From experiment, the laminate load, Nx = 175 N/mm, near the knee was used to calculate the strains EX =.11 and Ey =.17. Young's moduli in the damaged and undamaged state were calculateo using these strains. A worst-case thermal load of T = -318 K was chosen for the operating temperature of 76 K, and a strain-free state was assumed at 395 K. Under these load conditions, we predicted the influence of weave curvature and fill-crack damage on the elastic behavior of a unit cell at 76 K. Results of these calculations are compared with experimental results in the following section. EXPERIMENT Ten G-lCR and ten G-llCR tensile specimens were cut by shear from panels of cryogenic grade laminates. 2 For all specimens, the warp fiber bundles were aligned with the load axis. Both panels were.5-mm thick with three layers of a plain weave. All specimens were 5.-cm long and 1.77-cm wide. The edges of four specimens of each material were polished so that edge damage could be recorded by the replication technique described in 138

3 ref. 5. There are four test groups: equal numbers of specimens of G-1CR and G-11CR were tested at 295 K and 76 K. All specimens were quasi-statistically loaded in tension at 2 N/s. The first three specimens from each group were loaded in tension to ultimate fracture strength. Two specimens with polished edges were loaded in increments of 2 N. Damage at the edge of these specimens was recorded by replication5 at room temperature after each load increment. For 76-K tests, a warm-up period was required before replication of edge damage at room temperature. Densities of fill cracks were determined by observation through the thicknesses of the semitransparent thin laminates. RESULTS AND DISCUSSION A summary of experimental results is given in Table 1. Our results compare well with the previous studies. 2,6 We report two additional Young's moduli (E2, E3) and a strain ( +) that locates the first knee in the stressstrain diagram. A second knee was observed for G-11CR, but is onlyrepol'ted here as a change in modulus between E11 and E11. From experimental observation we correlate individual damage events to nonlinear behavior in the stress-strain diagrams (see Figs. 2-7). New parameters for these nonlinearities are defined geometrically at 76 K in Fig. 4 for G-1CR and in Fig. 7 for G-11CR. At room temperatvre (295 K), bimodulus behavior does not exist (see Fig. 2). Other studies7,tl reported similar bimodulus results at low temperatures, but no explanations are given for this behavior. In this study, we examine relationships between damage accumulation and these nonlinearities. The stress-strain response of a bisphenol A epoxy resin, representative of resin used in G-1CR and G-11CR,4 is included in Figs. 2, 4, 6, and 7 for comparison. The failure strain of the resin at 295 K is larger than those of G-1CR and G-11CR. At 76 K, the opposite is true: we observed that the resin failure strain, ein' was much lower than laminate fracture strains. We also observed that T IS slightly lower than resin' This observation implies that the first knee at 76 K could be related to a resin failure in the laminate. Because the resin is constrained in the fiber-reinforced laminate, we predict that the resin will fail at a lower strain ( + < resin)' At 76 K we observed several epoxy failures in the fill fiber bundles of G-1CR Table 1. Summary of experimental results. Moduli E1 (GPa) E2 (GPa) E3 (GPa) 295 K 26.9 G-1CR K 29.9 G-11CR 76 K Strains 4 (%) UTS (%) Strength UTS (MPa) UTS indicates ultimate strength. Subscripted values defined in Figs. 4 and

4 Fig. STRESS, N/mm Q 5 I I I I 4' 32 Z 8 (/) (/) G-1CR 295 K W -- Strain W III 3 > Cracks in" fill 24 W 6 II.. )( "ber resin III a Crack in II.. #. Q. Z warp fiber..j..j < 2 16 II.. 4.: I-.: E uj 1 2Q 3 LOAD, N E < u... u (/) (/) < < 2. Correlation between formation and stress-strain response of G-1CR in tension at 295 K. Note: stress-strain curve is inverted. G-1 CR 295 K Fig. 3. Edge damage corresponding to loads of Fig. 2. Note: Large crack opening displacements between broken fiber ends at 185 N in the region of final failure. 14

5 o 4Ir------r ,_ z c( a: I en Fig. 4. Same as Fig. 2 except at 7.6 K with geometric parameters defined. Top View G-1 CR 76 K Side View Fig. 5. Top and edge views of damage corresponding to loads of Fig. 4. Damage in top view is fill resin cracks often observed as shadows below the surface. 141

6 and G-11CR prior to the knee (see Figs. 4 and 7). In all cases before fracture we observed only cracks in the resin of the fill fiber bundles (fill resin cracks) and individual fiber fractures in the warp fiber bundles. After the initiation of damage, we measured the growth of this damage with increasing load. To correlate the damage events with stress-strain nonlinearities, we inverted the conventional stress-strain format and superposed the damage growth on the same diagrams with load as the common abscissa (see Figs. 2, 4, 6, 7). In all cases damage increased exponentially with increasing load. The nonlinearities shown in Figs. 2 and 4 correlated well with the growth of fill resin cracks in G-1CR at 76 K and 295 K, but the growth of warp fiber fractures at much lower loads had no influence on these nonlinearities. The edge damage state of G-1CR at 295 K in Fig. 3 revealed unusually large displacements between broken fiber ends only within the fractured region. Also, fewer fiber fractures were observed on the edge of G-11CR than in G-1CR at 76 K. The bimodulus behavior of G-1CR at 76 K is compared in Fig. 8 with the change in modulus predicted by FEM where only 5 percent of fill fiber bundles were modeled with resin cracks. From experiment, we observed resin cracks in all fill fiber bundles (1 percent). Hence, the decrease in modulus for G-1CR in Fig. 8 is larger than that predicted by the FEM. For G-11CR the increase in modulus at the second knee in Fig. 7 could be caused by the straightening of warp fiber bundles after multiple resin cracks in the fill fiber bundles. Unfortunately, there is no evidence of warp bundle straightening at 76 K because replicas were taken at reduced loads after warm-up to prevent formation of new damage. The two-dimensional FEM4 approximation appears to be a serious oversimplification, which models only a portion of the woven structure. A complete three-dimensional model that includes fiber straightening would provide a more accurate prediction. For both G-1CR and G-11CR, only small delaminations were observed after fracture. Before fracture no delaminations occurred at either 295 K or 76 K. Previous studies9 for nonwoven laminates showed that delaminations STRESS,N/mm I 32 Z 8 en en? w a: Bisphenol A I a: w Epoxy I / a: III 3 :? 24 w 6 II.. }' /, III - Il.. a: z j...j c( c(...j a: II.. r:: I- G-llCR 295 K r:: en,/ J /'.- E -- Strain E Cracks in fill en /. fiber resin / en :II: I :II: c( c( a: a: Fig. 6. Correlation between damage formation and stressstrain response of G-11CR in tension at 295 K. Note: stress-strain curve is inverted. 142

7 STRESS, N/mm I I I 1 8 Z en en a: G-11CR76K w W -Strain a: m 3 d 24 a: 6 II Cracks in fill I W fiber resin I m a. I II: I --- Crack in warp II. c( Z fibers c( I: a: II. l- I: en E... E en c( c( II: a: Fig. 7. Same as Fig. 6 except at 76 K with geometric parameters defined. can originate from the tips of transverse ply cracks at the inte41ayer interface. For woven laminates with e = 45, we predicted by the FEM a decrease in delamination stresses when the fill resin crack occurs and when the temperature decreases to 76 K. Hence, in this study we verified that delaminations are prevented for moderate to large weave curvatures by the formation of fill resin cracks at low temperatures. SUMMARY From experiments at 76 K and 295 K we observed that damage in specimens of G-lCR and G-llCR accumulated as resin cracks in fill fiber bundles and iridividual fiber fractures in warp fiber bundles. Only small regions of delamination were observed between warp and fill fiber bundles after laminate fracture. For both materials, larger loads were required to initiate damage at 76 K. Fiber fractures had no influence on the appearance of the stress-strain diagrams. For G-lCR at 295 K, large numbers of fiber fractures occurred randomly and large crack opening displacements were observed in the region of laminate fracture. At a lower temperature we observed fewer fiber fractures and measured a higher laminate fracture strength. From theory we predicted that a high density of fill resin cracks would result in a reduced modulus, or a knee, in the stress-strain diagram. In G-lCR and G-llCR at 76 K, a high density of fill resin cracks was observed at a strain similar to the failure strain of the epoxy resin. At 76 K we observed two knees and three moduli for G-llCR where the third modulus increased in value. For G-lCR at 76 K, we observed only one knee at a strain lower than that of the first knee of G-llCR. Most of the damage and nonlinearities in the stress-strain diagrams occurred at strains well below fracture. This is an important consideration for design applications where these materials are only loaded to small strains. From both theory and experiment, we conclude that the weave geometry is beneficial at low temperatures: when fill resin cracks occur delaminations are prevented at the fill-warp interface. 143

8 r---...,..., ,., 5x 1 6 psi to Q. X w rji ::l...j ::l ::t CI) C, Z ::l > I '8=11 76 K J5J 8, degrees 4 2 Fig. 8. Influence of internal fill cracks on Young's modulus of a woven unit cell: a comparison between FEM and experiment. ACKNOWLEDGMENTS This study was sponsored by the U.S. Department of Energy, Office of Fusion Energy. Material was supplied by Spaulding Fiber Company. The cryogenic tension fixture was machined by Mr. Dale Thoel at NBS. REFERENCES 1. M. B. Kasen, Composite laminate applications in magnetic fusion energy superconducting magnet systems, in: "Proceedings of the 1978 International Conference on Composite Materials," AIME, New York (1978), pp M. B. Kasen, G. R. MacDonald, D. H. Beekman Jr., and R. E. Schramm, Mechanical, electrical and thermal characterization of G-1CR and G-11CR glass/epoxy laminates between room temperature and 4 K, in: "Advances in Cryogenic Engineer ing--mater ials," vol. 26, Plenum Press, New York (198), pp T. Ishikawa, Anti-symmetric elastic properties of composite plates of satin weave cloth,.fiber Sci. Technol. 15: (1981). 4. R. D. Kriz, Mechanical-damage effects in woven laminates at low temperatures, in: "Materials Studies for Magnetic Fusion Energy Applications at Low Temperatures--VIII," R. P. Reed, ed., NBSIR , National Bureau of Standards, Boulder, Colorado (1985), pp D. O. Stalnaker and W. W. Stinchcomb, in: "Composite Materials: Testing and Design (Fifth Conference)," ASTM STP 674, American Society for Testing and Materials, Philadelphia (1979),pp H. M. Ledbetter, Dynamic elastic modulus and internal friction in G-1CR and G-11CR fiberglass-cloth-epoxy composites, Cryogenics, 2: (198). 7. H. Benz, I. Horvath, K. Kwasnitza, R. K. Maix, and G. Meyer, Worldwide cryogenics--switzerland cryogenics at BBC Brown, Boveri & Co., Ltd., Cryogenics 19:3-15 (1979). 8. M. B. Kasen, Mechanical and thermal properties of filamentaryreinforced structural composites at cryogenic temperatures 1: Glassreinforced composites, Cryogenics, 15: (1975). 9. A. L. Highsmith, W. W. Stinchcomb, and K. L. Reifsnider, Effect of fatigue-induced defects on the residual response of composite laminates, in: "Effects of Defects in Composite Materials," ASTM STP 836, American Society for Testing and Materials, Philadelphia, (1984), pp