Session: 01-08-01: Nondestructive Evaluation and Structural Health Monitoring, 01-11-01: Wind Energy

Paper Number: 120842

120842 - Force Stiffness Technique for Non-Destructive Evaluation of Buckling Load of Oblate Ellipsoidal Shells 

Buckling is a critical mode of failure that governs the design of thin-walled shell structures. The buckling load of shells is sensitive to geometric imperfections, the effect of boundary conditions, or eccentricity in the load application path. Non-destructive evaluation techniques are widely used for estimating the buckling load of structures. Force-stiffness (F-s) technique is one such method used for the estimation of buckling load without subjecting the structure to failure [1]. This paper focuses on the evaluation of the buckling load of oblate ellipsoidal shell structures subjected to external pressure using the F-s technique. Experiments are carried out on scaled ellipsoidal shells fabricated using steel with similar R/t values. Necessary fixtures for applying hydraulic pressure externally on the dome are designed and realized. The experiments are performed on five domes until buckling and the strains under pressure loading are measured using strain gages. Graphs of the pressure-to-strain ratio versus applied pressure areplotted for the five scaled-down domes. It is observed from these plots that the F-s technique can predict the buckling load with 95% accuracy in all the scaled-down domes using strain data up to 80% of the buckling load. All the domes buckled within a range of 2.8 to 3.5 MPa. With this confidence, the F-s technique is applied to a full-scale dome structure, almost 3 times the size of the scaled-down dome, subjected to external pressurization. From the strain data obtained, the F-s technique estimates the buckling load of the full-scale hardware to be 2.15 MPa which is close to the experimental estimate of 2.04 MPa. Furthermore, the application of the F-s technique numerically to predict the buckling load of oblate shells is also explored.

Reference:

1.     Jones, R. E. and Greene, B.E., The force stiffness technique for non-destructive buckling testing, 15^th Structure, Structural Dynamics, Material Conference - AIAA (1974)

Presenting Author: Gopikrishna Rangarajan IIT Hyderabad & DRDL

Presenting Author Biography: Currently working at the Defence Research & Development Laboratory as a Scientist and a part-time researcher at IIT Hyderabad. Area of work is Structural testing of aerospace vehicle structures, thermo-structural qualification of airframe and pressure testing of pressure vessels. I completed my under-graduation from Madras Institute of Technology India, Masters at IIT-Madras, India. I have worked in the area of health monitoring using Acoustic Emission Technique. During 13 years of work experience, I have received organisation level Young Scientist Award in 2018 and also been a part of group awards at the laboratory level. Current area of research includes non destructive methods for estimation of buckling loads of dome structures subjected to external pressure.