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Comparison of polymer composites behavior to low-velocity impact and quasi-static indentation

Bieniaś J., Jakubczak P., Surowska B.

Composites Theory and Practice

2013

R. 13, nr 3

155--159

Fibre Reinforced Polymers (composites) are widely used in the aerospace industry due to their excellent quasi-static mechanical properties in relation to density. However, it is known that polymer composites do not have good resistance to dynamic loads, especially to low-velocity impact phenomena, which is one of the most important issues for composite structures, particularly in aerospace due to the effect it has on material structures. The purpose of this study was to investigate the differences in polymer composite behavior between low-velocity impact and the similar (the same boundary conditions) quasi-static indentation. The composites used in this study were: Carbon Fibre Reinforced Polymer (CFRP) and Glass Fibre Reinforced Polymer (GFRP) manufactured by the autoclave method (materials used in aerospace technology). Impact tests were carried out according to the ASTM D7136 standard. Quasi-static indentation was performed according to the ASTM D6264 standard. After the tests, the samples were subjected to non-destructive and microscopic testing methods to investigate the damage size and failure character. It was noted that low-velocity impact causes significant damage to both kinds of composite structures, while the quasi-static indentation under the same impact force level results in some internal degradation of the laminate structures (barely visible damage). However, the size of it is extremely different to the case of low-velocity impact. The failure types of composite structures after static and dynamic loads are similar. The major failure type in composites after static and dynamic loads are matrix cracks, delaminations, and in the case of impact fibres-cracks. To obtain similar damage character and size (as in the impact effect) in the composite structure on account of quasi-static indentation, a much higher force level in comparison to dynamic loads is necessary.

Polimerowo-włókniste materiały kompozytowe są szeroko stosowane w technice lotniczej z uwagi na korzystne właściwości mechaniczne w odniesieniu do gęstości. Jednak, materiały te charakteryzują się niską odpornością na obciążenia dynamiczne, szczególnie na uderzenia dynamiczne o niskiej prędkości, co jest jednym z najważniejszych zjawisk eksploatacyjnych w technice lotniczej z uwagi na uszkodzenia, jakie może powodować w strukturach kompozytowych. Celem przeprowadzonych badań było porównanie reakcji polimerowo-włóknistych materiałów kompozytowych na obciążenia dynamiczne o niskiej prędkości oraz na analogiczne (te same warunki obciążenia) statyczne wciskanie wgłębnika. Do badań wykorzystano laminat wzmacniany wysokowytrzymałym włóknem węglowym w osnowie żywicy epoksydowej oraz laminat wzmacniany wysokowytrzymałym włóknem szklanym w osnowie żywicy epoksydowej (materiały stosowane w technice lotniczej). Obciążenia dynamiczne zostały przeprowadzone zgodnie z normą ASTM D7136. Próba statycznego wciskania wgłębnika w kompozyt została przeprowadzona zgodnie z normą ASTM D6264. Po badaniach próbki poddano nieniszczącej oraz mikroskopowej ocenie stanu struktury w celu określenia rozmiaru i charakteru powstałego uszkodzenia. Zaobserwowano, że obciążenia dynamiczne o niskiej prędkości powodują znaczące uszkodzenie struktury obu rodzajów kompozytów. Analogiczne obciążenie statyczne przy tej samej sile wywieranej przez wgłębnik na materiał powoduje jedynie makroskopowo niewidoczne uszkodzenie wewnętrzne. Rozmiar uszkodzenia jest skrajnie różny w przypadku obciążeń dynamicznych i statycznych, przy czym charakter uszkodzenia jest zbliżony. Dominującym rodzajem degradacji kompozytów są pęknięcia osnowy i rozwarstwienia oraz pęknięcia włókien w przypadku obciążeń dynamicznych. W celu otrzymania podobnego uszkodzenia laminatów przez statyczne wciskanie wgłębnika niezbędna jest znacznie większa siła wywierana przez wgłębnik na materiał w porównaniu do obciążeń dynamicznych.

The modelling of transfer influence vehicle from ground surface on objects in placed

Kuczmarski F., Bartnicki A., Sprawka P.

Journal of KONES

2007

Vol. 14, No. 1

245-253

This paper describes the initial model of the load-mine-ground relation and presents the preliminary results of a computer simulation. Pressure mine-clearing devices, with discs loosely embedded on the axis of the minesweeping section, (disc mine-clearing devices) cause deformation to the mine 's firing mechanism, which, through the actuation of the fuse, leads to the detonation of the inner-placed explosive. It is assumed that under the load of a mine clearing vehicle, the active part of the mine's cover will be vertically displaced by 6-10 [mm] actuating the fuse of the mine. Although the construction parameters of pressure mine-clearing devices are known, there is a shortage of instructions and data requisite for proper construction and effectiveness examination of dynamic mine-clearing vehicles. In fact, during mine clearing with a dynamic mine-clearing device, the force exerted upon the mine (Ngr) is that of the pressure by the disc (an element of the device). This force differs from a static load and the differences result from the following factors: mine placement type of soil as well as its physical and mechanical properties construction properties of a mine-clearing device pace of mine clearing The issue of influence transferred from the surface of the ground onto the mine placed inside is not present in literature and it constitutes an interesting and important scientific issue in terms of selecting parameters for dynamic minesweeping devices. Similarly to a static load, shock waves propagating within the ground cause its volumetric and structural deformations. The main cause of the differences lies in the briefness of a dynamic load. The gradual growth of static loads (clenching) causes displacement of air and water in the pores and their partial squeezing out. In the case of a static load applied to sandy soils, the process lasts from a few minutes to a few hours, whereas in the case of clay and loessial ones, it extends to a few days, weeks or, sometimes, even months. Under static and dynamic loads, the ground reacts similarly to a three-component centre with a changing-with-time amount of air and water. The action time of a percussive load extends from a few to between ten and twenty milliseconds. Due to air and water inertia, this is too short a period of time to squeeze them out of the pores of the soil, which reacts in a way similar to a three-component centre with a constant amount of air and water. Since in unhydrated soils the main part of the pores' volume consists of air, both static and dynamic loads are accompanied by the absorption of the main stresses by the skeleton. With static loads exerted upon the hydrated soil, both water and air flow loosely out of the pores without the absorption of the loads. In terms of strength the only working element here is the ground's skeleton. In the case of short-lasting, intense loads, water (with a small amount of trapped air) will not flow out simultaneously with the skeleton and, as in the case of mean pressures, it may absorb the load to a larger extent than the skeleton itself. The way the hydrated ground reacts to low pressures - below 1 MPa - is therefore largely determined by the amount of air in the pores. Therefore, the ground with a mine placed inside is a multiphase center with changeable properties - the semi-limited area with mechanical properties changing with each cycle of dynamic influence put upon its surface. Although the issue of the ground pressure measurement is dealt with in various publications, there is a lack of data concerning how it changes under the mechanical influence of moving vehicles and under that of dynamic minesweeping devices - the problem which is essential for a safe minefield crossing. The effectiveness of percussive minesweeping devices can be evaluated upon the basis of the experimental data obtained by means of measuring equipment. The elaboration of a reliable ground-mine relation model for short-time (percussive) influences transferred to the ground surface should accelerate the process of selecting parameters for dynamic mine clearing in terms of obtaining the maximum destructive impact, either directly upon the mine or through the actuation of its firing mechanisms for various types of mines, different depths at which they are buried as well as for various soils with their different properties. The above task is the aim of further works focusing on the subject presented in this paper.