2894 2895 2896 4891 Resin KIC (MPa·m ) 1.74 2.07 1.86 1.41 Minimum Viscosity (mPa·s) 35 53 62 75 Resin Tensile Modulus (GPa) 3.2 2.8 2.7 4.1 Tensile Strain at Failure (%) 7.7 9.0 7.0 4.4 Ultimate Tensile Stress (MPa) 96 82 84 101 Tg by E' Onset (°C) 155 166 179 186 Tg by tan d Peak (°C) 166 177 189 197 1/2 Table 2. Neat resin properties. appropriate standards and conditioned according to ASTM D5229 Procedure 'D' for dried specimens and Procedure 'BHEE' for wet specimens. "Dry" specimens were conditioned at 70°C and "Hot/ Wet" specimens were conditioned at 85°C/85% relative humidity. The ultimate compressive strength of the composite Panel B specimens was determined according to ASTM D6641/D6641M (Combined Loaded Compression - "CLC"). Untabbed specimens were utilized according to Procedure A of the standard. The ultimate interlaminar shear strength (short beam shear - "SBS") of Panel B specimens was determined according to ASTM D2344/ D2344M. The ultimate open hole compression strength ("OHC") of the Panel C specimens was determined according to ASTM D6484/D6484M, Procedure B. For elevated temperature measurements, the test temperature was recorded using a calibrated digital thermometer with a 'K' thermocouple affixed to the test fixture near the specimen gage area. The mode I fracture behavior ("GIC") was determined for unconditioned Panel B specimens according to ASTM D5528. Values reported were calculated using the compliance calibration method for propagation. The mode II fracture behavior ("GIIC") was determined for unconditioned Panel B specimens according to a 3M internal procedure based on ASTM D7905. Values reported were calculated using the fivepoint compliance calibration method. Specimens were tested at a rate of 2 mm/min initially followed by 5 mm/min to final break. Compression after impact ("CAI") was measured according to ASTM D7136/7137. Panel A specimens were impacted with a standard impactor with a target energy of 35 J. The compression strength after impact was measured according to ASTM D7137 with four strain gauges affixed to the specimens to ensure even application of the compressive load. All specimens exhibited a lateral failure through the middle of the damage area. w w w. s a m p e . o r g Figure 1. Glass transition temperature of neat resin castings by differential mechanical analysis. RESULTS As described in the introduction, 3M™ Matrix Resin 2894 is an example of the class of RTM epoxy resin systems with high impact resistance and modest service temperature and serves as a baseline for this study. Resin 2894 has a very respectable KIC fracture toughness of 1.74 MPa·m1/2 and tensile strength of 96 MPa and 7.7% strain to failure as is shown in Table 2. In this study, two additional resin systems, Resins 2895 and 2896 have been studied with somewhat higher fracture toughness and tensile properties, and with sequentially increasing glass transition temperatures of 177°C/166°C (tan d peak/E' inflection) for 2895 and 189°C/179°C for 2896 as shown in Table 2 and Figure 1. Matrix Resins 2895 and 2896 have somewhat higher KIC values of 2.07 and 1.86 MPa·m1/2 respectively. The neat resin tensile strengths are somewhat lower than 2894 at 82 and 84 MPa respectively. The modest service temperature of 2894 is partially the result of the lower glass transition temperature of 166°C/155°C. Resin 2894 shows higher composite compres- J U LY/AU G U S T 2 0 2 0 | SAMPE JOURNAL | 41http://www.sampe.org