Developing Failure Criteria for Application to Ship Structures Subjected To Explosive Blast Loadings

A research programme is being undertaken at TNO to investigate vulnerability reduction on warships. In this framework, studies have been performed regarding the structural damage due to both an internal missile explosion and close in explosions. Due to the severity of the explosive loadings the structural deformation is considerable and an accurate method to predict both the initiation and progression of material damage would be of significant value to assist in the design of more blast resistant structures. The ability to model material failure is available with many of the material models in the explicit finite element code LS-DYNA although further development is required in order to correlate to observed experimental results. In the material models, the initiation of failure is typically defined by the uni-axial failure strain ef. This parameter is not an independent material constant and the failure characteristics will vary depending upon the applied stress state (for example sensitivity to tri-axiality), the temperature and rate of loading. For ship plate steel the manufacturing process of rolling may introduce anisotropic failure characteristics which differ in the rolling and transverse directions. The failure strain needs to be adjusted for coarser meshes which are unable to represent local strain gradients. This paper describes some of the work that has been done to describe the failure properties of typical ship plate steel with the development of a user defined material model to predict and further understand the failure characteristics.