Investigation of the impact behavior of fiber composite structures under planar, biaxial prestress - UniVo

• Study on the effects of impact loads on fiber composite structures
• Investigation of the interaction between planar preloads of the structure and introduced impact damage
• Test series under uniaxial preloads, biaxial preloads and with non-preloaded reference tests

For reasons of lightweight construction, fiber reinforced plastics (FRP) are increasingly being used for structural components in aviation. These are characterized by high density-specific mechanical strength and rigidity and are subject to various loads during use. Impact damage is particularly critical, as it significantly reduces the mechanical strength of the structure, but often cannot be detected by visual inspection. Such a barely visible to completely invisible damage is referred to as BVID (Barely Visible Impact Damage). BVIDs can be caused by birds, hail and rockfalls, for example, or by dropped tools during maintenance work. The resulting damage is internal to the structure and is categorized into failure within each laminate layer and damage between the laminate layers. The second type of failure in particular, also known as delamination, can become safety-critical for a structure, since it can significantly reduce the local bending stiffness, compressive strength and fatigue strength of the structure.

In order to assess the influence of impact damage and to be able to plan for reliable safety when designing structures, such a damage event is simulated using standardized test procedures. Test specimen plates are pre-damaged in a defined manner by means of a transverse impact and the residual compressive strength after such an impact stress is determined using CAI (Compression After Impact) tests. However, the tests completely disregard the fact that multiaxial stress states are usually already present in impact-endangered, load-bearing components during a corresponding impact stress. In the case of aircraft structures, these arise, for example, from the structural and/or aeroelastic stresses. The fact that such previous stresses have an influence on the damage behavior and extent has already been fundamentally proven in various works and publications. However, the question currently remains as to what influence and what influencing variable different load combinations, in particular biaxial load states, have on an impact event and the damage that remains. However, such information is absolutely necessary for the most realistic possible dimensioning of preloaded component structures.

It is precisely this question that is to be investigated within the framework of the UniVo project. For this purpose, the impact behavior of fiber composite materials under planar, biaxial preloads is to be investigated. The focus of the work is on impact loads in the low-speed range and structures made of carbon fiber reinforced plastic (CFRP). Impact loads in the low-velocity range or low-velocity impacts often cause damage in the BVID category and are particularly safety-critical due to the possible undetectability of the damage. CFRP is used for load-bearing structures due to its high mechanical resilience, and the precise assessment of the extent of damage in the event of an impact load is essential for a more reliable design and evaluation of the structure. As part of the project, the effect of prestressing on the impact behavior should finally be made assessable using key figures. For this purpose, impact tests should be carried out under a combination of different load conditions. The assessment of the severity of the damage in the impact tests is primarily based on the damage projection areas determined using the ultrasonic method and the energies absorbed during the damage process.