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Propagation of a shock discontinuity in an elasto-plastic material: constitutive relations

100%

Dequiedt J. L. , Stolz C.

tom Vol. 56, nr 5

391-410

The shock discontinuity problem is analyzed in the case of elasto-plastic materials; the jump relations for internal state variables cannot be exhibited directly. For this purpose, we solve the internal shock structure problem, assuming that the shock front is a continuous transition in a thin layer, taking account of dissipative effects. The shock generating function P is introduced. The canonical equations of the shock structure are determined in the general case when the evolution of plasticity is derived from a pseudo-potential of dissipation D. The plane wave is analyzed for an isotropic material obeying a von Mises criterion, assuming that inside the shock the material is under pure axial compression: the existence and uniqueness results are established.

The development of nutrient contents on a new conservation area in the far North of Germany concerning different types of use. a proposal for a sustainable development in nature conservation practice

61%

Stolz C. , Deppe U. , Kammer G. , Kuhwald M. , Nass D. , Sonnichsen L.

tom 51

nr 1

The present study analyzes a short-term observation of a newly created nature conservation area. The aim was to investigate different effects of grazing by cattle and, otherwise, the frequent mowing on the development of nutrient contents of soils. The results are typical for the strong sandy Weichselian outwash plain in the north of Central Europe (Schleswig-Holstein, Germany). Two neighboring testing areas of different use and sowed with an agricultural grass were observed for more than two years. The first area had been grazed intensively by cattle and the other one had been mowed twice a year. During this time, several nutrients and other soil parameters have been analyzed at regular intervals. Furthermore, we made observations about soil compaction and the succession of vegetation. The results show a stronger reduction especially of nitrogenous nutrients on the mowing area (-25%). In contrast, on the grazing area, the contents of nitrogen doubled during two growing seasons (+125%). However, a high atmospheric input of nitrogen strengthened the result. Less conclusive were the results about the contents of potassium, phosphorus and SOM. Therefore, mowing could be advised, if quick results are required concerning the impoverishment of soils. Furthermore, the development of succession vegetation was quite different on both areas with the number of plant species 12% higher on the grazing area. However, long-term but non-intensive cattle-grazing must still be rated as an excellent method of maintenance for this type of cultural landscape. It corresponds with the centuries-old land-use practice in this type of landscape and promotes high level of biodiversity. Therefore, there is nothing that speaks against non-intensive grazing from the beginning on a newly created nature-protection area within a long-term conception of nature conservation. The removal of A horizons should be avoided as it damages Holocene soil profies and has only short-term effects.