PSI - Issue 28

Thomas Bergmayr et al. / Procedia Structural Integrity 28 (2020) 1473–1480 C.V. Thomas Bergmayr et al. / Structural Integrity Procedia 00 (2020) 000–000

1478

6

y

y

x

x

Fig. 6. The figure shows the strain of the submodel of the idealized spoiler model with respect to the global x -axis. Whereas the left side( x < 0) shows the DIC results and the right side ( x > 0) shows the numerical results.

leads to a change in the strain-state and values, which cause a deviation from the zero base-line to a finite value. With increasing distance from the FOS to the damage the strain along the ZST is converging to zero. Nevertheless, the results of the horizontal ZST depicted in Fig. 8b show a disadvantage of strain measurements via FOS sensors without local resolution. This is given due to the fact that one gets the averaged strain along a FOS without local resolution as a sensor output, which can yield to situation where the strains along ZST with di ff erent signs are canceling each other out .As a consequence, measurement values might be very small or even zero, pretending a undamaged state. To solve this problem fiber Bragg grating (FBG) sensors with multiple sensing elements or continuous distributed fiber optic sensors can be used. Considering, e.g., a FBG sensor several strain results can be collected along a ZST, yielding the question, how many measurements are needed to allow damage evaluation. For the considered submodel area with debonding, a number of at least six strain measurements was found to be adequate to approximate the whole strain profile along the ZST by a simple polynomial fit. Fig. 8b shows assumed local strain measurements as points along the simulated strain profile, which could be collected by FBG sensors. The marked points depicted at the strain profiles are representing such a Bragg grating position. A simple five order polynomial fit calculated with the six measurement points is made, which approximates the whole strain profile adequately to evaluate the damage. A lower number of strain outputs could still be adequate for simple damage detection and propagation monitoring. However, with further decreasing number of measurement points the approximation of the strain profile along the ZST is rapidly deteriorated. Nevertheless, the exact placement of the FOS is a big challenge because zero-strain directions depend on the load case. However, the numerical results, depicted in Fig. 9 indicates that the directions of the ZST are very sensitive and are well suitable for damage detection and monitoring propagation of damages like the considered face layer debonding. Fig.9a as well as Fig. 9b shows the di ff erence of the strains between the pristine model and the model of the first damage state. The Fig.9a shows the di ff erence ε xx , d − ε xx , p with respect to the x -axis, whereas 9b show the results of the horizontal zero-strain results ε β 1 , d . This figure clearly shows that the strains in zero-strain directions are much more sensitive for monitoring damages that, e.g., the simple x-direction, due to their superior deviations close to the damage but also at some distance to it. Consequently, for the considered sandwich debonding, strain measurements along ZST would allow a less dense sensor network compared to, e.g., measurements in x-direction.

6. Conclusion

This paper presents a study of the strain-based ZST-SHM method on a idealized aircraft spoiler by means of a experimentally validated numerical model. The numerical results of the investigated sandwich structure with face