Methodology for Validation of Finite Element Analysis Utilizing Strain Gauge Measurements

Abstract

As analysis utilizing Finite Element Method (FEM) has become widely adopted in engineering practices and incorporated into governing standards, physical validation of these analyses is often forgone. Physical validation gives insight into the validity of assumptions and simplifications commonly used to efficiently process FEM simulations. This paper proposes that one reason physical validation is commonly forgone is a lack of knowledge of a general end to end methodology for the physical measurement, processing, and comparison of data. This paper presents such a methodology for the comparison of structural mechanical finite element analysis against strain gauge measurements utilizing the test case of a pressure vessel. Rectangular, three-axis, 45° strain gauge rosettes have been used to obtain normal strain inputs. The limitations and pitfalls of employing strain gauges with less than three measuring directions are briefly discussed. A procedure is provided for converting the three measured normal strains into three principal strains, von Mises equivalent strain and maximum shear strain. The principal directions, as well as an algorithm needed to resolve the ambiguity of the angle between the principal directions and gauge axes, are provided as well. Then, the strains are converted into principal stresses, von Mises equivalent stress and maximum shear stress. The post-processed strain gauge readings are visualized by employing 3D Mohr’s Circle for stress and strain. The visualization provides clear proof that the maximum shear lies on a plane different from the one on which the gauge has been attached. Using the described methodology, comparison shows that the difference between the FEA results and the post-processed strain gauge readings is less than 5%. The magnitudes of principal stresses and strains, the equivalent stress and strain, as well as the maximum shear stress and strain are compared. Besides the magnitudes of stresses and strains, the principal directions are compared and scrutinized, revealing the corroboration between the FEA and the physical measurements. This corroboration gives validity to both the methodology and assumptions, such as plane stress, used.