PSI - Issue 28

F.W. Panella et al. / Procedia Structural Integrity 28 (2020) 1709–1718 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Additional fatigue tests, denoted with B type were conducted with four-point-bending method on the second sample series constituted by CFRP bars of rectangular cross section [dim. 130 × 39 × 7 mm as shown in Fig. 3a] and lay-up [+45, -45,0,90] 4s (Dattoma et al. (2019)). After preliminary procedural tests, new specimens are tested for complete and extensive experimental overview of fatigue bending duration and general data evaluation for damage modes and detection due to test variability (Dattoma et al. (2019)). Fatigue test results of both CFRP samples are reported in Table 1, with amplitude and maximum normalized loads, with respect to maximum static resistance F static ; in addition, relative fatigue failure mode and fatigue strength of both test type were examined in accordance with ASTM D 3479 and ASTM D7264M – 07 (for flexural properties of composites), for CFRP beam sample.

Table 1. Scheduled experimental results for CFRP fatigue tests. ID Fatigue test F max_adim [%] F Amp_adirm [%] N. cycles [N f ]

Damage zone and type

W1 36.86 W2 35.46 W3 32.84 W4 26.8 B1 58.30 B2 53.39 B3 52.76 B4 49.96 B5 58.96 B6 54.98 B7 47.81 B8 51.39 B9 52.59 B10 60.56 B11 55.54 B12 50.20 B13 64.54

17.42 16.85 14.74 12.73 27.70 25.34 24.07 22.87 28.68 26.29 22.07 23.74 23.90 29.48 24.02 23.84 30.60

101617 370173 536825 71798 78316 204000 142677 442561 185343 73696 13289 66911 307071 15824 18930 25800

Crack 1° layer; delamination 6° layer Crack 1° layer; delamination 6° layer Crack 1° layer; delamination 5° layer

CFRP Wrinkle samples

2000000 Undamage

Crack at upper left support; delam. in 6° and 16° plies

Delamination in 5° and 16° plies Delamination in 6° and 24° plies

Crack on 1° ply; delam. in 2°, 16° and 30° plies

Delamination in 10° ply Delamination in 10° ply

CFRP Beam samples

Un-damage

Delamination in 6° and 8° ply Delamination in 6° and 10° plies

Delamination in 10° ply

Delamination in 6° and 10° layers

Delamination in 10° layer

Delamination in 6°, 8° and 10° layer

a

b

Fig. 3. (a) Experimental configuration of CFRP beam sample; (b) fatigue curves’ diagram of CFRP wrinkle samples.

Figures 3b and 4a illustrate Wohler curve trends, described with maximum, amplitude and mean test loads, normalized to relative static results as function of fatigue cycles’ number, where linear behaviour is approximately identified in double logarithmic plot despite short number of available samples. New CFRP bending tests increases correlation coefficient of 3.15 % respect to previous result (Dattoma et al. (2019)), while few wrinkle data show linear trends with same correlation coefficient values for all curves. In this article, the added tests value is given by combination tests with inspection experimental methodologies, described in next chapter, in order to monitor fatigue behaviour during life and detect initial delamination phase and successive evolution under bending fatigue loads and