PSI - Issue 12

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2 0 Strain [%]

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 0 Strain [%]

300 P.M. Giuliani et al. / Procedia Structural Integrity 12 (2018) 296–303 P.M. Giuliani, O. Giannini, R. Panciroli / Structural Integrity Procedia 00 (2018) 000–000 Figure 2. Left: Typical cyclic curve for the flax/epoxy specimens. Right: Accumulated inelastic strain versus the maximum strain attained at each load cycle. 5

100 150 200 250 300 350 400

Stress [ MPa ]

0 0 . 15 0 . 3 0 . 45 0 . 6 0 . 75 0 . 9 1 . 05 1 . 2 1 . 35 1 . 5 0 50

Strain [%]

Figure 3. Cyclic test with removal of the inelastic strain accumulated at each load step (solid line) with superposed chord modulus (dashdotted line) and fitting of the peak stresses (dashed line). ig. 3. Cyclic test with removal of the inelastic strain ccumulated at each load step (solid line) with su erp sed chord modulus (dashdotted line) and fitting of the peak stresses (dashed line).

A second experimental campaign has been conducted on flax/SuperSap specimens. Results are not detailed for brevity, but we comment that we found similar repeatability, but elastic modulus and final strength were found to be approximetely 50% lower than for the flax/epoxy specimens, indicating that the SuperSap resin has drastically lower mechanical properties than traditional epoxies. The research then moved to the second flax fibers (BioTex). These have been impregnated with the SuperSap Resin only. Results for the BioTex/SuperSap specimens show a high dispersion in terms of both initial modulus E 1 and final modulus E 2 , as clearly observable on the left chart of Figure 5. Such behavior, which is not in line with the results obtained using the other kind of flax fibers, is found to be strictly related to the location of the specimen on the original laminate. It is found that, moving transversally to the fibers, the elastic modulus increases while approaching the center of the laminate, to later decrease again to the original value. We comment that the laminate has been produced utilizing a fiber roll 250 mm with, transversally cut every 300mm, and packing the laminae one on top of the other. The external fibers of the roll are thus always located on the external portion of the laminate, and the center of the roll always corresponds to the center of the laminate. It has been then found that the density of the fibers varies moving along the width of the roll, with higher density at the mid-span. Whereby we can not give an exact chart in Figur 2, which shows the i el stic accumulated strain gainst the maximu strain attained at each cyc . Our resul s further supports other findings in the liter tu , which stated that the inelastic strain appears above 0.1% of applied strain. The inelas ic strain is never recovered, and it is found that removing the inelas ic strain at th beginning of each cycle, hence t anslating all the loading cycles o the rigin, the nitial portion of the stress-strain curve at the following loading cycle always presents an initial elastic modulus equal to E 1 . T e initial modulus is pr served until the transition point, which appears to preserve its position, and it follows an increase of th second elastic modulus E 2 , until the previous peak stress is attained. It then follow a l st portion of the urve tracing the monotonic trend, hence the original E 2 . Results thus how tha t the overall modulus increases at each load cycle. Figure 3 shows an example of is beh vi r. The chart also rep rts a cu ve fitting the p ak loads attained at each cycle, and the chord modulus E 1 (dash-do ted line). 6 M. Giuliani, O. Giannini, R. Panciroli / Structural Integrity Procedia 00 (2018) 000–000 20 25

15

E 1 [ GPa ]

10

5

0

1 2 3 4 5 6 7 8 9

Specimen

Figure 4. Variation of E 1 moving along the Biotex/SuperSap laminate. Specimens are numbered from left to right moving transversally to the fibers direction. i . 4. Variation of E 1 moving al ng the Biotex / SuperSap laminat . pecimens r numbered from l ft to right moving transversally to the fibers direction.

estimation of the fiber density variation, it can be stated that the variation is in the order of 15%. This leads to remarkable variations between the elastic moduli, as summarized in Figure 4, where the specimens are numbered left to right moving along the laminate transversally to the fibers. A second experimental campaign has been conducted on flax / SuperSap specimens. Results are not detailed for brevity, but we comm nt that we found sim lar repeatability, but elastic modulus and final strength were found to be approximetely 50% lower than for the flax / epoxy specimens, indicating that the SuperSap resin has drasti ally lower mechanical properties than traditional epoxies.

225 250 275

1 . 2 1 . 4