Analytical and experimental determination of the specific penetration resistance. Description of the facial and rear weakening free-surface effect

In the present paper, the main force parameter is estimated under the static tests. This parameter is the specific penetration resistance and it is generally believed that the parameter is a constant in case of static penetration. However, a number of experimental and analytical data illustrate varying the specific penetration resistance according to the current depth of penetration. It is noted that the weakening free-surface effect decreases the specific penetration resistance near the facial or rear edge. Consequently, the relevance of the topic is emphasized by the influence of the facial and rear weakening free-surface effect on the penetration parameters detected in the experimental studies and engineering calculations. The refined approximation of the specific penetration resistance presented in this paper is taking account of the penetration of the sharp indenter into the plate of middle thickness within the framework of the viscous crater formation and the facial and rear weakening free-surface effect. Also this article contains data processing technique. For carrying out the tests a number of experimental samples were made. It is plates of different thicknesses, it must be emphasized that test sample materials are technical plasticine, plumbum and Wood's metal. It should also be noted that for the static tests three cone-nose indenters were made. Indenter sizes: the diameter of the cylindrical part is 7 mm in all cases and the lengths of the conical nose are 3.2 mm, 5.6 mm, and 8.4 mm. The test were carryed out on a testing machine Zwick/Roell Z-250. The key parameters derived from the experiment are the specific penetration resistance of the deep layers, the friction coefficient and the parameters of the weakening free-surface effect. The results obtained in the experiment lead to the approximation of the resistance force from more general parameters. These parameters are the specific penetration resistance of the deep layers and the friction coefficient of a sample, geometric parameters of indenter and plate. An approximation error does not exceed 25 % for the technical plasticine, 16 % for the Wood's metal, and 25 % for the plumbum. These errors are given for sharp (the length of the cone-nose is 8.4 mm) and middle (the length of the cone-nose is 5.6 mm) indenter because of a problem has been in depth considered in the investigation. This problem is that penetration of the blunt indenter is not follow to condition of viscous crater formation. Therefore, different versions should be used to describe the penetration process (for example, plugging mechanism). It is proposed in penetration models for the estimation of the penetration resistance force of sharp indenters into the plate of middle thickness.