The Static tensile testing did not indicate any significant differ- ence in mechanical response between CM and AWJM specimens (strength and stiffness), as both specimen types had similar ulti- mate tensile strengths and  strains。

For the fatigue tests the main results observed are the following:

Fig。 13。 Temperature gradient vs。 cyclic stress for (a) CM and (b) AWJM specimens。 Endurance limit obtained by     intersecting two linear fits。

analysis, we can advance that the type of damages generated by machining processes as shown in Figs。 5 and 6 may be  the  cause of this variation in the endurance limit values of the composite specimens。 On average, the endurance limit of AWJM specimens was about 10% higher than that of CM specimens。 Similar results are obtained during fatigue tests of pin loading specimens (com- pression-compression) conducted by Persson et al。 [6] on speci- mens drilled by two processes of machining。 The authors have observed that, specimens drilled with unused PCD drill (conven- tional machining) present failure strength around 19% lower com- pared to the specimens drilled with KTH method (orbital milling) using a diamond grain-coated mandrel。 These results clearly show that the signature of the machining process has an important role on the mechanical behavior of a machined composite   structure,

4。Conclusions

In this paper, a Thermographic Damage Criterion (TDC) based on heat dissipation was developed in order to assess the effect of two types of machining processes (conventional machining vs。 abrasive water jet machining) on the mechanical behavior of CFRP plates。 CF/Epoxy specimens with [±45°]2S stacking sequence were investigated for surface damage at the hole wall after drilling by each technique, and were subsequently loaded under static and fa- tigue tests to evaluate the influence of hole surface damages on the mechanical performance of the CFRP  specimens。

The hole surface of CM plates show visibly distinct regions of fi- ber pullout and epoxy matrix degradation, while those of AWJM plates only show striation marks in the direction of water-jet。 No

–

Both specimens have shown similar damage for loads less than 46% UTS。 For loads above this value, the damage is more impor- tant for the specimens drilled using conventional   machining。

–The maximum temperature profiles in the area surrounding the hole (i。e。, studied area) follow the same evolution as the damage profiles for both machining processes。 However, the presence of damages induced by the cutting tool in conventional machining (pullout of fibers and degradation of the resin) provoke more heat dissipation than ridges and craters damages observed on the walls of the holes machined by abrasive water jet (Fig。 6)。

–The endurance limit for specimens drilled with abrasive water jet is 10% more than those drilled with conventional machining。

–The damage rates calculated from the heat dissipation (TDC) are larger than those calculated from the decrease of rigidity。 TDC further confirms the difference between AWJM and CM specimens。

–SEM observations of the fractured specimens confirm that the fiber pullout and matrix degradation areas in CM specimens   act as a stress concentrator for crack initiation and propagation on the machined  surface。

–IR thermography can be effectively used for accessing the effect of different machining techniques on the mechanical behavior of CFRP during fatigue testing。

Finally, all observations indicated that the choice of the machin- ing process has an important impact on the mechanical behavior of CRFP。

Acknowledgments

The author expresses sincerely thanks to JEDO TECHNOLOGIE industry, Toulouse, France for technical support。 This project is supported in part by NSERC-DG。 This project is supported in part by NSERC。