Strain Field Measurements Using Digital Photogrammetry for Large Inflatable Structures

Strain measurements are an important tool to evaluate the safety and reliability of large inflatable structures. However, some problems exist when using known measurement techniques in engineering applications. In this work, a method based on digital photogrammetry (DP) and Delaunay triangulation is proposed to measure the strain fields. This method takes into consideration the complex surface profile and geometrical nonlinearity and is suitable for large inflatable structures for which strain fields can be obtained using only photogrammetry. The coordinates of the discrete points on inflatable structure are determined by digital photogrammetry, and then, these points are turned into a triangular mesh using the Delaunay triangulation method. The strains can finally be calculated using the finite element method (FEM) by using the changes in the coordinates. In this work, a bulge test for a circular fabric sheet is performed to verify the validity of the proposed method. The strain fields calculated from this method are compared with results from digital image correlation (DIC). Good agreement is found between the photogrammetry and DIC measurement results. The comparison indicates that the concept of using photogrammetry to measure a strain field is valid. Furthermore, the advantages and disadvantages of the proposed method are discussed.