Issue 49

R.V Prakash et alii, Frattura ed Integrità Strutturale, 49 (2019) 536-546; DOI: 10.3221/IGF-ESIS.49.50

elastic regime causes cooling of the specimen) of the CFRP specimens is not significant when compared to GFRP [13] or metallic materials as the carbon fibers have negative coefficient of thermal expansion.

% Loss in Stiffness *

Input Impact Energy (J)

Fatigue Test Condition

Residual Strength (kN)

% Damage Volume Fraction

S. No

1

Un-impacted

-

-

<0.5

53.67

1

23 J

Constant Amplitude

30.20

4.83

53.67

Lo-Hi Prog FALSTAFF Hi-Lo Prog FALSTAFF Lo-Hi Prog FALSTAFF Hi-Lo Prog FALSTAFF Lo-Hi Prog FALSTAFF

Specimen failed in fatigue

2

23 J

5.92

1.88

35 J

26.72

3.77

3

18.18

35 J

37.64

4.09

4

11.83

Specimen failed in fatigue

5

51 J

52.59

5.60

6

51 J

55.51

7.92

15.14

* The loss in stiffness is estimated w.r.t the un-impacted specimen stiffness (~ 27.02 kN/mm).

Table 2 : The damage volume fraction of post-impacted specimens Vs. loss in stiffness.

Fig. 3 (b) presents the plot of average temperature observed for the three area segments during tensile loading. It is noted that the highest temperature of 54.3 ° C is seen in area A1, followed by area A2 (42.4 ° C) and subsequently by area A3 (39.2 ° C). This peak temperature corresponds to specimen failure at the peak load. However, it is interesting to note that the temperature-time response for area A3 alone shows some spikes at ~ 70 seconds and once again at ~ 90 seconds) which was not seen in temperature response in other two areas of measurement. To verify if this has some correlation to damage progression, data of force-time response and temperature-time response was plotted.

(a) (b) Figure 3 : (a) The temperature variation within the pristine specimen during the tensile test; (b) The position of area segments A1, A2 and A3 marked in a specimen under no load condition. Fig. 4 presents the correlation between temperature rise and tensile load during two ply-drop failures. The ply-drop failures in area A3 have resulted in fiber-matrix debonding and fiber breakage, and as a consequence, the final failure of

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