PSI - Issue 38

S. Häusler et al. / Procedia Structural Integrity 38 (2022) 230–237 S. Häusler et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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p attern on the specimen’s surface. The illumination and a camera were facing the speckled side of the specimen, to take the images for the strain analysis, Figure 1 (a). Another camera taking pictures of the transmitted light was positioned on the back of the specimen, together with a thermal camera for monitoring the temperature. In order to avoid overheating, in all experiments the specimens were cooled with a fan or if necessary with compressed air. Figure 1: (a) side view on the experimental setup, with camera and illumination on the left for strain measurement and camera and thermal camera on the right for temperature monitoring and backlight images. The examination area is highlighted in red and the dimensions are given in mm. (b) area of examination with edge lengths in mm and schematic digital extensometer displayed as dashed orange line.

Table 1: Parameters of the experiments Purpose

Fibre angle

R -value

Quantity

0.1

9

90°

-1

10 12

Stiffness degradation

0.1

±45°

-1

7

2.1. Specimen and material The rectangular shaped specimens (Fig. 1) have a four-layer setup with an average fibre volume fraction of around 52-55%. While the ones with a fibre angle of [90°] 4 are unidirectional, the specimen for shear testing have a [±45°] 2S layup in symmetrical order. The layers have been arranged manually and soaked in resin with vacuum infusion processing, similar to rotor blade manufacturing proceedings. Afterwards they got cured in an annealing cycle. The material is a glass-fibre-epoxy commonly used in the wind industry, consisting of a unidirectional fibre mat provided by Saertex® and the resin system Epikote RIMR 035c with Epikure RIMH 037 provided by Hexion®. To avoid geometric influences, the same geometry was used for all specimens. Therefore, a compromise between the size of the testing area, machine force limits and buckling in compression testing had to be made. The resulting geometry is shown in Figure 1 (a) and (b), as it is also recommended in the Optimat project, summarised in Nijssen (2006) and Wedel-Heinen et al. (2006). The specimens have a thickness of 3.6 mm, with ±45° tabs of the same material. Before testing, four stripes of speckle pattern are applied near the edges of the examination area, Figure 1 (b), as a compromise between free space for the backlight analysis and enough speckle area for strain measurement. 2.2. Measurement system The strains are determined by calculating the ratio between the horizontal displacement and the initial length of the solid vertical orange lines in Figure 1 (b). Therefore, each vertical line is divided into 200 points. The strain is calculated from the displacement of opposite points along the two parallel lines. This basically creates 200 clip gauges equidistantly distributed over the width of the specimen, an example is illustrated as dashed orange line. For the lateral strains the same procedure is conducted, but with horizontal lines. Thus, the strains are averaged over the