region at the ith cycle, and Atotal is the area of the overall sample re- gion under study。

By using of the Eq。 (3) we can plot the TDC over each fatigue test for the two specimens made of AWJ and one specimen made of CM (cf。 Fig。 12)。

From Fig。 12 we observe that, for all applied loads, TDC of spec- imens machined with AWJ are lower than TDC of specimens drilled with CM。 These results are in good agreement to that observed with extensometer (cf。 Fig。 3)。 However, regardless of the machin- ing process used and for the same load, the damage rates calcu- lated from the heat dissipation (Eq。 (3)) are larger than those calculated from the decrease of rigidity (Eq。 (1))。 Again, this is con- sistent with the findings of Toubal et al。 [35] where a direct corre- lation was observed between accumulated damage and surface temperature。 The TDC can thus be used to predict damage accumu- lation under a given fatigue-loading  scenario。

3。2。4。Endurance  Limit Investigation

In general, the endurance limit is obtained by Whöler curves (stress vs。 cycles)。 This endurance limit can also be obtained from the  temperature  stabilization  curves  [39,40]  by  intersecting the

two straight lines that interpolates the stabilization temperature (DT = Tf — T0) and the corresponding stress level。 The profile of DT at 5000 cycles for different loads for CM and AWJM   specimens

  is shown in Fig。 13。 As can be seen, the endurance limit of the spec-

M。 Saleem et al。 / Composites: Part A 55 (2013) 169–177 175

Areas considered for analysis

(a)(b) (c)

Fig。 9。 Temperature contours for AWJM specimens at 5000 cycles under (a) 3 kN, (b) 8 kN and (c) 11 kN cyclic loads。 Highest temperatures are in pink/violet and lowest are in black/dark blue。 The area around the hole within the rectangle is considered for investigation。 (For interpretation of the references to color in this figure legend, the reader is referred  to the  web version  of this article。)

(a) (b)

failure

(c)

Fig。 10。 Temperature contours for CM specimens at 5000 cycles under (a) 3 kN, (b) 8 kN and (c) 10 kN cyclic loads。 The area around the hole within the rectangle is considered for investigation。  (For interpretation of the  references  to  color in this figure legend,  the  reader is referred  to the  web version  of this   article。)

Fig。 11。 Curves of damage accumulation and temperature for an AWJM specimen over a lifetime of 5000 cycles。 (For interpretation of the references to color in this figure legend,  the reader  is referred to  the  web version of this  article。)

Fig。 12。 Thermographic Damage Criterion (area fraction of the region of maximum temperature) at each cyclic loading level from 3 kN to failure。 (For interpretation of the references to color in this figure legend, the reader is referred to the web version of  this article。)

imen drilled using CM (Fig。 13a) is  lower  than  that  of  AWJM  (Fig。 13b)。 The values of endurance limit for five conventionally

drilled   specimens   and   five   abrasive   water-jet   specimens  are

83。3± 4 MPa and 93。5 ± 0。76 MPa respectively。 Based on the above

176 M。 Saleem et al。 / Composites: Part A 55 (2013)   169–177

regions of fiber pullout or thermal matrix damages were evident   in the AWJM holes。 Surface roughness parameters (Ra and Sa) indi- cated that AWJM and CM holes have the same surface roughness values, although the type and the form of the damage generated   by each machining process is completely   different。