PSI - Issue 19

Sara Eliasson et al. / Procedia Structural Integrity 19 (2019) 81–89 Sara Eliasson / Structural Integrity Procedia 116 (2019) 000 – 000

84

The shear force bearing part is the adhesive in the specimen. Hence, this is a vital part of the specimen and there is a need to assure that no adhesive failure occurs. Fatigue properties of the adhesive will also be of a high importance. The shear load in the adhesive was estimated according to equation 1. ℎ = 2 = 2 (1) Where τ adhesive is the shear stress in adhesive layer (MPa), F is the force applied to specimen (N) and A tab is the clamping surface area of the tabs (mm 2 ). The clamping force can contribute to relieving the adhesive of stress, but it can also contribute to debonding of the adhesive component due to the applied pressure from the grips, which at the same time can result in crushing or pre damaging the specimen. One approach to avoid this is to increase the area of the tab. In the current investigation, different force magnitude applied has been tested instead of changing the area to achieve an optimal clamping force, to avoid slippage and above mentioned failures. The tab length is limited to the depth of the grips; 50 mm. The value is chosen to keep the tab completely clamped inside the grips. Furthermore, the temperature was monitored during testing using a temperature scanner Flir i7. The temperature was monitored in the tab area of the specimen and in the gauge length to assure that the temperature change did not increase more than 10°C, to avoid changing the material characteristics, according to ASTM D3479/D3479M-12 (ASTM International, 2012). 3. Tensile and fatigue testing The material was characterized with regards to its static and fatigue properties. The static tests were performed on a static testing machine Instron 4505 with a calibrated 100 kN load cell. The fatigue testing machine used was a servo hydraulic MTS with 100 kN capacity and at a testing frequency of 5 Hz and at an R-ratio of 0.1. The run-out number of cycles was set to 2 million. The fatigue testing was performed at a maximum load level of 70 – 90% of the Ultimate Tensile Strength (UTS) of the material due to the flat nature of S-N curve for CFRP materials (Harris, 2003). Below 70% of the UTS it is common that the tests results in run-outs. Table 2 shows a summary of the material properties from the static testing. Stiffness is measured with two different methods. These results are compared and used to evaluate the use of method for upcoming research by the authors. Figure 3 shows a summary of the preliminary fatigue test results for the final choice of specimen configuration. It is recommended to carry out more fatigue tests with the chosen specimen geometry for a higher validity of the chosen method. However, these first results indicate that the choice of specimen geometry will work for upcoming test program.

Table 2: Summary of static testing characterization of CFRP material. N Total Mean Minimum

Standard Deviation

Median

Maximum

Stiffness DIC (GPa)

5 6 5 9

105.9 109.9

11.41

94.4

101.52

124.3 124.5 0.34 1427

Stiffness Extensometer (GPa)

8.77

100.7 0.33 1172

109 0.34 1297

Poisson’s ratio

0.34

0.0045

UTS (MPa)

1279.3

89.73