PSI - Issue 17

Trevor Sabiston et al. / Procedia Structural Integrity 17 (2019) 666–673 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The continuous curvature through the gauge section reduces the stress concentration at the end of the gauge section. The reduction in cross sectional area in the gauge section reduces the load required to achieve a given stress level in the modified Waterloo geometries compared to a strip geometry, reducing the chances of buckling during compressive loads. The main concern with using the 15 mm strip specimen is that there is a high likelihood of buckling during the fully reversed fatigue test. A summary of the tensile and compressive material properties of the NCF material considered are given in Tab. 1. The tensile properties were obtained using the 6.4 mm wide modified Waterloo geometry and extensometers for strain measurement. Both tensile and compressive testing were conducted on an MTS servo hydraulic test frame with a cross head speed of 1.5 mm/min. The compressive tests were conducted using the 10 mm width modified Waterloo geometry specimens, and DIC was used for strain measurement at room temperature and extensometers were used at 130 ºC. For tensile testing 5 specimens were used at 130 ºC and 3 for room temperature. For the compressive testing 3 specimens were used at both room temperature and130 ºC.

Tab. 1. Tensile and compressive material properties of the NCF at room temperature and 130ºC

Property [GPa] [MPa] 553 ±5.66 [GPa] 40.7 ±4.70 [MPa] 384 ±7.37 43.9 ± 0.86

37.6 ±4.95 460 ±12.4 32.5 ±1.08

Room Temperature 130 ºC

192 ±10.8 The compressive properties are lower than the tensile properties at both temperatures. The properties at 130ºC are lower than they are at room temperature with the most significant reduction being in the UCS. For the other properties there is approximately a 20% drop in the property from room temperature to 130ºC. The variability of the testing was also higher at 130 ºC than at room temperature except for the compressive modulus. The four specimen geometries were tested under fully reversed fatigue loading to evaluate how the specimen geometry and preparation method affected their fatigue test performance. The 15 mm wide strip was cut using both CNC and a saw, the 15 mm width modified Waterloo geometry was cut using both CNC and waterjet cutting. A plot of the trial fatigue tests is shown in Fig. 3. The 15 mm width strip geometries fail due to buckling an delamination at orders of magnitude fewer cycles than the modified Waterloo geometry specimens. Two of the strip geometries survive to higher cycle counts however they are not recommended for use under fully reversed loading due to buckling. The 15 mm width modified Waterloo geometry requires larger loads during the fatigue tests to reach a given applied stress level than the other specimen geometries considered. The waterjet cut specimens have much more variability and tend to fail prematurely compared to the CNC cut specimens of the same geometry. This is likely due to the differences in through thickness surface quality achieved using these methods. The 6.35 mm width specimen was found to exhibit more variability in the preliminary fatigue testing likely due to amount of stitching yarns within the gauge section of the specimens. Due to the increased load required for the 15 mm modified Waterloo geometry and the increased variability in the results for the 6.4 mm width modified Waterloo geometry, the 10 mm wide modified Waterloo geometry was selected for the evaluation of the fatigue properties of the NCF composite. The modified Waterloo geometries did not fail due to buckling and delamination in the same manor that the strip geometries did as shown in Fig. 4. 3.2. Trial Fatigue Test Results