Fiber reinforced polymers are used in a wide variety of applications, especially in light weight applications where high stiffness and strength are required while maintaining low weight. Since delamination often occurs in fiber reinforced polymers and can result in the failure of the component, it is important to gain deep insight into their fracture mechanical behavior. It is still state of the art to observe the growing crack length with a microscope or camera system and manually determine the initiation and propagation of the crack, which can lead to subjective and inaccurate results. The use of acoustic emission (AE) analysis could be a solution to automate the crack initiation and crack length detection.
The aim of this thesis was to determine the delamination behavior of glass fiber (GFRP) and carbon fiber reinforced polymer (CFRP) laminates with the same epoxy matrix system. For this purpose, the critical energy release rate was determined for both materials with different test setups. Mode I, mode II and mixed mode I/II tests were performed according to different standard methods. For mode II and mixed mode I/II two standard methods were tested in order to compare the results to each other. To gain a deeper insight into the delamination behavior, a special focus was given to the method of AE. The crack initiation was determined by the use of AE and compared to the initiation values according to standard methods. Furthermore, AE was used to perform a localization of the crack front. The detected crack front via AE was compared to the results of the visually observed crack front. Microscopy was performed for all specimen in order to gain insights into differences of fracture surfaces depending on the loading mode.
The determined critical energy release rates for all tested modes showed similar trends for both materials. Mode I tests resulted in the lowest values for the critical release rate, followed by mixed mode I/II tests and the highest values were measured during mode II testing. The initiation values were lower than the propagation values and the onset obtained with AE resulted in the most conservative initiation criterion. It was shown that AE can be used to determine the onset of delamination. The critical energy release rate for GFRP was approximately two times higher than for CFRP.
The analysis of the fracture surfaces showed differences depending on the tested mode. The surfaces of mode II and mixed mode I/II tested specimens showed shear hackles, which are typical for mode II fracture surfaces. Surfaces of mode I tested specimens did not show any sign of shear forces, but evenly distributed fibers with no loose matrix material in between.
The localization of the crack front via AE yielded good results, which were in accordance with the detected crack front via the camera system. The localization of the AE signals showed a band with high amplitude AE signals. For mode I tests, this band was wider distributed for GFRP which is a sign for fiber bridging, which could be observed during the tests as well. This fact could be seen in the increasing resistance curve of mode I GFRP tests as well. Another reason for the wider distributed AE signals for GFRP laminates is that due to the lower velocity of sound of GFRP compared to CFRP the localization is less accurate and leads to more scattering of the results.
In conclusion, all tested methods yielded reasonable results and both materials showed the same trends. It was shown that AE can be used as a helpful method for the investigation of the delamination behavior of composite materials.