Sensing delamination in composites reinforced by ferromagnetic Z-pins via electromagnetic induction

This paper investigates a novel technique for sensing delamination in through-thickness reinforced composites based on electromagnetic induction. This sensing technique features ferromagnetic Z-pins and a pair of coils attached to a laminate; the first coil creates a magnetic field that is intensified by the ferromagnetic pins, whilst the second coil detects the magnetic flux change that is caused by the pin motion relative to the coil pair when delamination happens. This approach avoids potential interferences due to contact electrical resistances that exist in electrical-based sensing approaches. The viability of this sensing technique is demonstrated by monotonic and cyclic bridging tests, involving Nickel/Iron alloy Z-pins embedded in E-glass/913 laminates under controlled delamination. A simplified electromagnetic finite element analysis is presented to help interpret the experimental results. The sensitivity of the magnetic-based sensing technique increases with loading rate. Both mode I and mode II delamination events can be detected by a voltage signal from the sensing coil, albeit there exists an initial "blind spot" at low loading rates. This sensing technique also allows monitoring the pin bridging status, e.g. the switch of pin pull-out side, without modifications to the architecture of a Z-pinned composite regarding expected mechanical response.