Improving residual strength of unidirectionally reinforced plastic laminates by metal layering

Kurzfassung

Composite laminates are adjusted to the expected scenario by aligning their reinforcing fibers in load direction. Especially, when high specific uniaxial mechanical properties are aspired, notch and impact sensitivity properties, however, limit the fiber fraction in load direction drastically. Additionally, the thickness of equally orientated layers is limited to reduce crack distribution. Compression loading of a laminate, which have previously been impacted at a low impact energy, is particularly critical. Therefore, the laminates suffer a loss of their lightweight potential due to residual strength evaluation. However, these laminates generate the reference for comparison with other material groups, although composites are often advertised with means of their unidirectional properties. Recognizing these limitations, a new lay-up to increase the degree of capacity utilization of CFRP is analyzed in this thesis. The so-called UD-CFRP-steel laminates use a lay-up of CFRP and steel layers with a thickness below 0.08 mm to reduce the aforementioned disadvantages. At first, it is shown theoretically that when using less than 12% steel, a higher specific stiffness than by taking the CFRP reference is achievable. However, this only proves to be beneficial if the residual strength after an impact is comparable with the residual strength achieved by the CFRP reference after an impact of the same energy. In the majority of applications, the exclusive consideration of uniaxial properties is not sufficient. Hence, struts under longitudinal compression load are regarded. When considering compression strength, local and global buckling, a strut provides a simple example in which not only the laminate’s longitudinal strength and stiffness play an important role, but also the laminate’s transverse stiffness and flexural bending stiffness. Therefore, a parameter study on different struts with different loads is performed to achieve the ideal cross section for each material system and loading scenario. For certain scenarios a weight-saving potential is realized by the UD-CFRP-steel lay-up compared to the reference laminate. Before manufacturing test specimens, the surface treatment is regarded and a variety of pickling and grit blasting procedures are tested. Finally, a vacuum blasting machine is developed and used for the pre-treatment of the steel foils. Additionally, manufacturing experiments are performed to estimate the thermal residual curing stresses. In addition, methods for their reduction are presented. The subsequent compression testing shows that the compression stiffness and strength is increased essentially in comparison to the CFRP reference. Thereby, the specific compression modulus is also increased. Although the specimens indicate larger damage after being impacted at 9, 12 and 16 J, the residual compression strength is comparable to the CFRP reference. Hence, the specific residual compression strength of UD-CFRP-steel is lower. Additionally one can conclude from the results, that the arrangement of a ply influences the dent depth and the load-displacement of the specimens. Damaged specimens with the metal layers oriented more to the outside, achieve a more ductile compression failure than CFRP or specimens with the metal layers orientated preferably more to the center of the specimen.