Design and Calibration of a Multi-Axial Load Cell for Quasi-Static and Dynamic Testing

Design and Calibration of a Multi-Axial Load Cell for Quasi-Static and Dynamic Testing

Van der Laag, D.A.M.

Kaminski, M.L. (mentor)
Den Besten, J.H. (mentor)
Van Woerkom, P.T.L.M. (mentor)
Klein Breteler, R.F. (mentor)

Mechanical, Maritime and Materials Engineering

Marine and Transport Technology

Ship Hydromechanics and Structures

2016-07-07

In structural analysis, multi-axial loads are often treated as a superposition of individual load com- ponents. Although this proves to be sufficient for static analyses, for dynamic and especially fatigue assessments, this sometimes leads to extreme overestimation of loads. In turn this results in highly over-dimensioning of structures to prevent failure due to fatigue. To provide a means for multi-axial testing, a load cell is designed, for measuring combinations of both forces and moments up to excitations of fe = 30 [Hz]. As the load cell is intended for validation of theoretical and experimental research, its accuracy must be as high as reasonably possible. Therefore, the question to be answered is how a load cell is created such, that it measures three forces and three moments independently and at a high accuracy. The load cell consists of a steel hollow circular cross-section, with flanges and a bolt-hole pattern at both ends. The transition to the flanges is smoothed with a varying wall thickness to reduce stress concentrations. The external dimensions and geometry are dictated by the experimental set-up of the first tests after calibration and by the required fatigue properties. The load cell is instrumented with 32 strain gauges, connected into 8 Wheatstone-bridges. Additionally, 32 Fibre Bragg Grating (FBG) sensors are applied to the outer surface and another 14 FBG sensors are applied inside the load cell. Calibration of the load cell is performed for tension and compression only, Fz = ±1000 [kN]. The acquired data is analysed using both a linear and non-linear regression models, based on a maximum likelihood analysis. This data is used to define and assess the sensor sensitivity, non-linearity and the uncertainty (as a quantified direct pendant for accuracy). The results show a non-linearity in Fz measurements of less than 0:3% and an uncertainty of less than 0:4%, for the full working range of Fz = ±1000 [kN]. The tests show incredibly small transverse sensitivities, confirming good isolation of separate load components. For full commissioning of the load cell, however, further calibration of other load components is required. The results indicate as well that the accuracy of individual FBG sensors is met by strain gauges, only if applied in full Wheatstone bridges. Additionally, strain gauges show changing sensitivity and increasing non-linearity with increasing excitation frequencies, whereas FBG sensors appear unaffected. The immunity to electrical noise, the linear response and the sensor size make the FBG sensor a very promising measurement technique, which may be applied in many research areas. Further research to application and connection methods for FBG sensors may yield even higher accuracies and a means for quick application and testing.

http://resolver.tudelft.nl/uuid:d460e61d-4d29-4f30-8a90-86a104dc81ad

Student theses

master thesis

(c) 2016 Van der Laag, D.A.M.