Damage mechanisms and energy absorption in low velocity impact phenomenon on composite laminates

Low velocity impact is one of the most critical load condition for composite materials in particular in aeronautics: due to the weak bonds between the plies, even small energies can result in hardly detectable damages, causing considerable strength losses. Generally, the earliest observable damage on a laminate subjected to low velocity impact is delamination, mainly responsible for the compression strength decay. A lot of research works are about the mechanisms of delamination initiation and growth [1-3]. Most of the studies performed [4-7] concern the domain of low energies, where internal damage can occur without any evident surface modifications so one of the most potentially dangerous aspect is the difficulty to detect damage by visual inspection. The only external indication of an impact is the indentation, brought to the concept of "Barely Visible Impact Damage" (BVID) adopted in the design of aeronautical structures: for an adequate safety, it is required for the laminates an assigned minimum strength in the presence of a barely visible indentation. But there is not a general agreement about the meaning of BVID. Much work has also been carried out to predict the residual material properties from the indentation depth so a powerful tool in the inspection of composite structures would be supplied.Since the residual material properties after impact are of primary concern in applying damage tolerance concepts, efforts were also done to correlate analytically or experimentally the residual strength with impact energy and damage mechanisms. Parameters like maximum force or penetration energy, are of interest too. At the light of what above asserted, a big number of experimental results obtained after low velocity impact tests performed on CFRP specimens of various thicknesses, lay ups and stacking sequences, were analysed and organised to obtain useful information at the aim to predict the low velocity impact damage. An instrumented drop weight apparatus, CEAST Fractovis Mk 4, were used impacting the laminates simply supported on steel plates with hole 40 or 50 mm in diameter, with cylindrical steel impactors with hemispherical nose different in diameter. The mass and the drop height were varied at the aim to obtain different impact energies up to penetration. Some models were verified also on GFRP system. After impact tests, each specimen was inspected to ascertain eventual visible damage, and the indentation resulting from the impactor-material contact was measured by a micrometric dial gauge following the EN6038 Standard. Then, the samples were ultrasonically scanned by a Krautkrämer USD 10 ultrasonic system, in order to gain information on delamination presence and extent. A number of static tests were carried out simply supporting the samples on a steel plate having a circular opening 50 mm in diameter, and loading them at the centre by a hemispherical steel indentor on an Instron 1251 servo-hydraulic testing machine in displacement control. The displacement rate was 5 mm/min.