Issue 29

M. Scafè et alii, Frattura ed Integrità Strutturale, 29 (2014) 399-409; DOI: 10.3221/IGF-ESIS.29.35

The strain gauge configuration allows the determination of sample bending during the test time, by means of the following factor:







1 2 1 2   





Bending

[%]

100

(8)

where ε 1

and ε 2 are the longitudinal strains measured respectively by strain gauge 1 and 2 [2].

The percent bending, as calculated in eq. (8), provides a reasonable indication of Euler buckling. Failure and midpoint bending are reported in tables from 2 to 7 as requested by ASTM D 6641. The latter is determined at the midpoint of the strain range used for chord modulus calculations [2].

% Bending

Material E

σ max

E SG1chord

E SG2chord

ΔE %

ν

Midpoint

Failure

E1 E2 E3 E4 E5 E6

71.4 79.4 71.9 77.1 75.8 79.3 10.6 13.4 100.0

97.0 98.8 93.7 99.8 95.2

100.0

3.0

100.0

2.4 5.3 3.6 7.0 2.7

-8.1

94.4 95.1 99.5 92.5

-4.6

91.1 81.2

1.2 4.7 3.4 0.4

1.5

-0.3 -2.8

- - -

-

-

-

- - - -

- - - -

Mean Value

96.9

96.3

-0.6

90.8

Std. Deviation

2.5 2.6

3.3 3.4

- -

9.4

CV %

-

Table 6 : Compressive test results of E samples (normalized results).

% Bending

Material F

σ max

E SG1chord

E SG2chord

ΔE %

ν

Midpoint

Failure

F1 F2 F3 F4 F5 F6

21.0 19.9 20.5 20.8 20.6 21.2 20.7

4.9 5.2 5.0 4.7 4.7 4.9 0.2 4.1 -

5.2 4.9 5.0 5.3 5.1 5.1 0.2 3.9 -

5.6

6.7 5.0 5.5

18.0

- - - - - - - - -

-7.3 -0.8 11.9

5.7 8.4 9.4 9.0

- - -

8.8

-

- - - -

Mean Value Std. Deviation

3.6

5.7 0.9

0.5 2.4

- -

CV %

-

Table 7 : Compressive test results of F samples (normalized results). Tab. 8 shows the compressive strength of UD material calculated by means of classical lamination theory. The four back out factors (2 for cross-ply and 2 for angle-ply), determined through Eq. (3) and (4), are:

 BF cross-ply A = 3.98  BF cross-ply B = 2.22  BF angle-ply C = 3.41  BF angle-ply D = 2.04 The compressive stiffness (E x

and E y ) for BF determination were obtained averaging the mean values listed in Tab. 6 and

7. The 0° lamina compressive strength values obtained by Eq. (1) are reported in Tab. 8.

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