# Measurement of Super-Pressure Balloon Deformation with Simplified Digital Image Correlation
## 1 Introduction  
A super-pressure balloon (SPB) is a vehicle that can fly at a constant altitude for an extended period to perform scientific observations at a fraction of the cost of using a satellite. The SPB maintains its internal gas at a pressurized state, which suppresses buoyancy fluctuation when the balloon volume changes due to atmospheric temperature variations between day and night [1; 2]. JAXA has been developing a lightweight, high strength balloon made of thin polyethylene films and a diamond-shaped net with high strength tensile fibers. Previous research shows that the tensile strength of the net meets requirements on SPB member strength, though the nets covering the SPB sometimes become damaged during the inflation test [3; 4; 5]. This may be due to non-uniform expansion, which causes stress concentration, although no method exists to confirm this hypothesis [6; 7; 8]. Contact measuring devices like strain gauges are not suitable because the SPB is too large to monitor the whole balloon and because they can deform the balloon surface during the contact measurement. Conversely, non-contact measurement methods such as the Digital Image Correlation method (DIC) can be efficiently used for this application.  
DIC--an optical method to measure changes in images--usually requires the use of patterns to be applied onto the specimen surface. This method is used not only for measuring the deformation of a test piece in a tensile test but also in fracture mechanics problems and bioengineering applications [9; 10; 11]. This method may be able to detect the stress concentration on the SPB [12]. However, it is not suitable to study the shape of SPBs, as ink spots on the thin film may affect its strength and weight properties. To measure the deformation as accurately as possible, it is necessary to spray the particles evenly and as finely as possible to a wide range. However, if we do this, a large amount of ink will be applied to the surface, not only will it weigh more but also the polyethylene film will not stretch uniformly due to curing of the ink.  
To overcome this problem, a Simplified DIC (SiDIC) using intersection detection technology was developed, which allowed us to track the diamond-shaped weave of the net so that we could measure the deformation of the SPB during the pressurization process. In this study, we developed SiDIC and verified the measurement accuracy, using a rubber balloon and diamond-shaped plastic net. First, the measurement accuracy of DIC using a patterned rubber balloon was confirmed and the deformation size measured by DIC was consistent with the rough calculations. Next, the accuracy of SiDIC was tested using a rubber balloon with random spray patterns and covered by a diamond-shaped plastic net. The pictures taken before and after deformation were analyzed using DIC and SiDIC, and the results were compared. DIC and SiDIC measured very similar deformation fields. The two methods were then tested using a rubber balloon covered just with the net. It was found that SiDIC entailed a smaller amount of data although it measured the deformation more accurately than DIC. In addition, assuming the stress concentration, the net was bonded to the balloon to restrict the deformation. Remarkably, SiDIC could properly identify the undeformed region. In summary, SiDIC is a simple and efficient method for measuring the SPB's deformation field.