Study on Collapse Characteristics and CSRF Method for Cylinder-to-Cylinder Intersections by Plastic Analysis

It is known that the collapse strength of complex three-dimensional structures is hard to be evaluated accurately with elastic analysis, and more accurate results require the use of inelastic analysis. A typical example is cylinder-to-cylinder intersections. These are basic structures of process plant, and can be seen in the branch piping and nozzles of pressure vessels. In the previous work, the authors proposed CSRF (: Collapse strength reduction factor) method which is an alternative to the SIF (: Stress Intensification Factor) method [1]. The CSRF in the previous work was based on the limit load analysis and was defined as the ratio of the simple cylinder limit pressure to the cylinder-to-cylinder limit pressure, which was represented as CSRF^L in the paper. The SIF is based on the elastic analysis and is defined as the ratio of the local primary membrane stress (PL) to the general primary membrane stress (Pm). Comparing the SIF with CSRF^L, it was revealed that the proposed concept of CSRF method provides more reasonable design than SIF method. The CSRF in the previous work (CSRF^L) was, as mentioned above, based on the limit load analysis, and elastic-perfectly plastic constitutive material model was applied. Therefore effect of the material constitutive material model as strain hardening behavior after yielding on the CSRF has not been investigated. In this paper, the concept of CSRF method was extended to general plastic materials in order to confirm the robustness of this method. The elastic plastic analyses were conducted on the cylinder-to-cylinder intersection model considering material and geometric nonlinearity. The values of CSRF^PP and CSRF^LP were newly proposed. CSRF^PP is purely the strength ratio between an intersection and a simple cylinder. CLRF^LP is defined as the strength ratio to the simple cylinder based on limit load analysis. The results showed that the collapse pressure of intersections considering material characteristics is larger than that of limit load analysis. As a result, CSRF^PP and CSRF^LP are smaller than CSRF^L in almost every case. Therefore, independently of material characteristics, CSRF^PP and CSRF^LP methods give more accurate design than CSRF^L method.