Identification and Repair of Low-Velocity Impact Damage in Composite Structures of Military Vehicles

Abstract

The use of composite materials in military ground vehicle hulls offers significant decrease in the weight instead of metal structures, without compromising the safety. However, composite structures are more prone to damage due to a low velocity impact than metals. LVI damage may occur as barely visible, or completely internal and cannot be detected with bare eye. This makes non-destructive testing (NDT) methods necessary and useful. NDT methods make the inspection of both internal and external damage possible without the need of damaging the structure. The most common repair types for composite structures are bonded, bolted and riveted repairs. These are challenging to execute in field environment and require a trained and skilled expert. In the present thesis, the aim of the experimental part was to manufacture specimens that simulated ground military vehicle hull structure, to inflict external and internal damage to them at low velocity and to inspect the occurred damage with varying NDT methods that would be suitable for in-field inspection. In-field inspection refers to inspection performed at the operational location, outside of a controlled workshop location. The aim was to find suitable methods for inspection of damage in ground military vehicle composite structure at in-field conditions and to research how the damaged military vehicle structures can be repaired at the in-field conditions. Around 50 specimens with chosen multi-layered sandwich composite structure were manufactured and damaged with low velocity. NDT methods that were used to inspect the damage gained were visual inspection (VI), tap testing, infrared thermography (IRT) and ultrasonic testing (UT). The NDT experiments were done by different experts at different times, and it was challenging to find a method that could inspect especially the internal damage within the structure. UT was finally stated to be the most suitable option for locating both internal and external damage. There are limitations for using this method for in-field inspection of used structure, such as damping of signal in foam core material and need for a highly skilled technician in NDT. In addition, it was stated that simple methods such as VI and tap testing are suitable methods for the inspection of damage at the first stages. Further research is advisable to study the applicability of different NDT methods at in-field conditions for inspection of composite structures.