Damage detection by direct potential measurement of carbon fiber reinforced polymers (CFRP) is a promising application for structural health monitoring. The unidirectional ply has a highly anisotropic conductivity, due to the high electrical conductivity of the carbon fibers and the isolating properties of the polymer matrix. The high conductivity in fiber direction is due to the direct connection through the fibers, while the low conductivity transverse to fiber direction is due to contacts between fibers. This anisotropy is reduced in the laminate depending on fiber volume fraction, direction and ply thickness. A material with a quasi-isotropic lay-up, very thin plies and high fiber volume fraction has therefore a quasi-isotropic conductivity. Additionally, this quasi-isotropy is increased when woven instead of unidirectional plies are used.
An additional property of the composite material is its piezoresistivity. This means that the resistivity of the material repeatably changes, when the material is strained. The effect is dependent on the direction and more pronounced transverse to the fibers, since the number of contacts between fibers decrease with increasing strain.
In this work the conductivity of anisotropic CFRP laminates made from unidirectional plies is investigated. The electrical properties of unidirectional laminates are characterized, both the absolute conductivity and the piezoresistivity are experimentally determined. The electrodes for current injection and voltage measurement are a challenge for this material, since the contact resistances of the electrodes are in the same range as the measured values. For damage detection, the level of the contact resistance is not of that high importance, since usually the potential change is evaluated and not the exact values. In this case it is essential, that the contact resistances are stable and do not change over time. Therefore, rivets are investigated. They are more stable than wires applied with silver epoxy and are also more relevant for practical application. The damage detection potential of the method is investigated by drilling holes into the material and measuring the resulting potential change. These measurements are compared to finite element results.