## 4 Results
### Failure Behavior Observation  
Using the developed PFA model, the failure behavior of the composite laminate was observed. Figure 18 shows a magnified load-displacement curve for the type-1, one-open-hole tensile PFA result with the calculated damage variable contour. If the calculated damage variable is \"1,\" it implies that it is fractured completely at the corresponding failure mode. Figure 18a-d are pointed on the graph according to the time when each damage variable became 1 or greater and when final fracture occurs. First, it was observed that the shear failure damage variable becomes \"1\" or more before the displacement reaches 1.6 mm. As the displacement exceeds 1.6 mm, the damage variable is \"1\" at the matrix tension failure mode. At the fiber tension failure mode, the damage variable became \"1\" after the maximum load; thus, resulting in the complete fracture of the composite specimen. Fracture was initiated from the open-hole edge because of the stress concentration. Figure 19 shows a load-displacement curve for the type-1, two-open-hole compressive PFA result with the calculated damage variable contour. Figure 19a-c was selected to observe the damage behavior during the analysis. Figure 19a,b is the point where the shear and matrix compression failure damage becomes \"1\" or greater, respectively. Figure 19c is a point where the final fracture occurs. Compressive damage behavior displays some different behavior from the tensile analysis result. Tensile failure behavior shows the final load drop with fiber failure damage; however, compressive failure behavior shows the final load drop occurred with shear and matrix failure damage. After the maximum compressive load point, the shear and matrix compression failure damage variable became \"1\" or greater; hence, final fracture occurred as the displacement increased. At the final load drop state, the fiber compression failure damage variable did not reach \"1\". These behaviors can also be observed with the test specimens. Figure 20 shows the fractured test specimens for the tensile and compressive tests, respectively. Figure 20a demonstrates that the specimen was completely fractured in two pieces while Figure 20b illustrates that the fractured specimen is in one piece. It is because at the final fracture of the compressive test, the fibers did not completely fracture. The fracture was initiated from the open-hole edge because the stress concentration was similar to the tensile cases. The side view for the tensile specimen exhibits catastrophic fracture, as evidenced by the fiber breakage shown in Figure 20a. However, as shown in Figure 20b, the compressive test specimen was mainly fractured in the shear failure mode [25; 34; 35], and the fibers were not completely broken. These results confirm again that the developed PFA model can accurately predict the failure of the composite laminates.  
Figure 17: Location of attached strain gauges and longitudinal strain (\(\varepsilon_{xx}\) ) value comparison for two-open-hole compressive tests: (**a**) Location of strain gauges, (**b**) strain comparison results for location 1, and (**c**) strain comparison results for location 2 and 3 of the specimens.  
Figure 19: _Cont._  
Figure 18: Load-displacement curve for one-open-hole tensile analysis result with each damage points and the corresponding calculated damage contour when the (**a**) shear failure, (**b**) matrix tension failure, and (**c**) fiber tension failure damage variable become \(1\) or greater, and (**d**) final fracture occurs.