Seminarium Instytutowego Seminarium Mechaniki im. W. Olszaka i A. Sawczuka

Strength enhancement of cellular structures under impact loading

prof. H. Zhao

poniedziałek, 9 czerwca 2003, godz. 10:00, sala Aula (II p.)

The presentation aims at the understanding of the strength enhancement of cellular materials under impact loading. It tends to answer whether or not (and if yes, why?) there is a structural effect responsible for this enhancement, which is not due to the base material rate sensitivity. A model structure (a Cu-Zn alloy square tube) is experimentally, numerically and theoretically studied. Actually, the observed crushing mode of this square tube is a progressive folding process, the same mode as for the cellular structures such as IFAM aluminium foam and honeycombs.

Static and dynamic compressive tests on the specimen of small dimension cut from this brass tube is performed to prove the rate insensitivity of the base materials. Afterwards, experimental compressive crushing tests of this square tubes are performed under static and dynamic loading. A large scale direct impact Hopkinson bar test allows for a reliable measurement of the force and the displacement history under impact loading (7-15 m/s). It shows that there is a significant enhancement of progressive peak crushing forces.

On the basis of this experimental observation, it is clear that the augmentation of the crushing resistance of the square tube (with a progressive folding mechanism) have an origin other than the rate sensitivity of the base material. Numerical reproductions of those crushing tests under static and impact loading is realised in order to examinate the local information such as the time history of stress and strain fields in the whole tube, which is difficult to measure especially under impact. It shows that the regions around the four ridges of square box column remain straight while the middle plan plates is already in a deep bending. Furthermore, these corner line zones support the most part of the global crushing loading. The buckling of these ridge-areas determines the progressive peak load in a folding process and the lateral inertia effect is the main reason of the enhancement of this progressive peak load.

Theoretic analysis of an idealised square box column model leads to a pretty good predictions of initial and especially progressive crushing peak loads. Post mortem microhardness measurement confirms such numerical observation and gives besides a remarkable fitness between experiments and simulations. It provides then a convincing validation of such a inertia effect theory.