PSI - Issue 34

Sigfrid-Laurin Sindinger et al. / Procedia Structural Integrity 34 (2021) 78–86 S.-L. Sindinger et al. / Structural Integrity Procedia 00 (2019) 000–000

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corporate the di ff erentiation (Tsai and Wu, 1971). The simplified approach was adopted at present out of inavailability of compression strength data, a shortcoming that shall be addressed in future work.

3. Results 3.1. Mechanical Material Properties

Fig. 3 depicts the mechanical material properties obtained directly from the on- and indirectly from the o ff -axis tensile coupons in color and gray-scale, respectively. While the results of Young’s modulus and Poisson’s ratio have been already discussed in a recent preceding publication (Sindinger et al., 2021b), the remaining data is presented herein for the first time. All parameters exhibit a directional material response. In case of Young’s modulus and ultimate tensile strength, values obtained in fiber direction ( x ) are considerably higher than the two transverse directions, which show a less pronounced di ff erence. Regarding Poisson’s ratio, the especially low value of ν zx compared to the other orientations stands out. This, however, is expected since in that configuration the x -oriented fibers restrict transverse strain. For the derived shear parameters, in-plane behavior scored considerably higher values than the two transverse directions, for which only minor di ff erences were disclosed.

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Fig. 3: ( a - e ) Material behavior over thickness via continuous models (see Eq. 2) fitted to experimentally determined means (markers) with standard deviations (errorbars). ( f ) Bars depicting the maximum relative di ff erence over thickness within each coupon orientation for all parameters.

The experiments furthermore revealed that values tend to decrease with decreasing specimen thickness. This was observed in all parameters and for all orientations except ν xy , which overall showed the smallest variation over the thickness as apparent from Fig. 3f. Therein, it can be seen also that the ultimate strength parameters σ ∗ i ( t ) and τ ∗ i j ( t ) exhibit the severest span, with di ff erences exceeding 40 % between minimum and maximum coupon wall-thickness.

3.2. Three-Point Bending Test and Numeric Simulation

Fig. 4a shows the instant of failure by fracture during three-point bending of a thin-walled ribbed beam structure as captured by digital camera during the experiment. In the detail view, it can be seen that the first ribs that fractured are the bottom skin, left of a region of intersecting ribs as well as the adjacent horizontally oriented rib, also at the