PSI - Issue 19

Christian Schneider et al. / Procedia Structural Integrity 19 (2019) 370–379 Ch. Schneider, et al/ Structural Integrity Procedia 00 (2019) 000 – 000

374

5

In the second, 90°-ply (see Fig. 3 (right)), the absolute differences of the risk parameter between the different versions of the model are smaller than those in the first ply (see Fig. 3 (left)) due to the shielding effect of the first ply. Also the effect of the material is reversed in the second ply for the tabs without taper angle (GFRP90 and ALU90). Detailed investigation of the local stress state showed, that this is due to stress concentrations caused by a local bending moment near the edge of the tab. This effect disappears when a taper angle is used since as no peak can be observed around the end of the tab for GFRP14 and ALU14. The difference between ALU14 and GFRP14 of 1.4 percent at X = 0 mm is negligible. The biggest value of the normalized risk parameter of 1.07 is reached for GFRP90. Considering the results of the adhesive layer, Fig. 4 shows, that for ALU90 the Mises stress in the center leads to a maximal stress of 63.5 MPa compared to 16.8 MPa for GFRP14. The difference between the results at the center and the edge with an average of 7.0% is small. GFRP90 and ALU14 lead to a similar stress distribution. GFRP14 distributes the stresses most evenly across the length of the tab.

Fig. 4: Nominal Puck 3D risk parameter in the adhesive layer over X-distance from tab for all models. Based on the comparison of the results, the optimal tab configuration is GFRP with a taper angle of 14°. For this model, the failure is expected to start in the 90° plies since the absolute value for the risk of failure is at least 37% higher in the 90° plies than at the peak in the 0° plies. Furthermore, the risk of failure in the 90° plies is higher in the gauge section than at the end of the tabs such that failure in the gauge section can be expected. Because of this reasons the tab configuration with GFRP and a taper angel of 14° was used for testing.

3. Materials

For the experimental tests two different kinds of plates made of carbon fibres and epoxy resin were produced. On the one hand, unidirectional (UD) lamina at angles of 90° (specimen UD90) and on the other hand, embedded lamina consisting of UD 0°- and UD 90°-layers built in a symmetric way referring on the middle plane (specimen EMB090). UD90 plies consisted of 10 layers and the embedded ones were made of 8 layers. The exact layup of the two types of specimen is shown in Table 5.

Table 5: Layup of the two different specimen types. Name Shape

Layer definition

Thickness

Specimen EMB090 Specimen UD90

Long strip Long strip

[0° (1) /90° (3) /symm.] 1.6 mm

[90° (5) /symm.]

2 mm