M。 Saleem et al。 / Composites: Part A 55 (2013) 169–177 173

(a) (b)

Fig。 6。 SEM photographs comparing the hole surfaces machined with two different techniques。 Circular hole machined with (a) conventional cutting tool and (b) abrasive waterjet。

Fig。 7。 SEM micrograph showing cross section of circular hole of carbon/epoxy composite, circular hole obtained with CM。 (For interpretation of the references to color in this figure legend,  the reader  is referred to  the  web version of this  article。)

Fig。 8。 SEM micrograph showing cross section of circular hole of carbon/epoxy composite, circular hole obtained with AWJM。 (For interpretation of the references to color in this figure legend,  the  reader is referred to  the  web version  of this  article。)

Although the average surface roughness is similar for both types of specimens (Ra = 3。93 lm for AWJM and Ra = 3。5 lm for CM), the

fatigue behavior for these specimens is different。 One can conclude that  unlike  metallic materials,  the  criterion used  for  quantifying

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

the quality of machining based on the average roughness (e。g。, Ra, Sa, etc。) is not suitable for composite materials。 Similar observa- tions on the hole’s wall damages are confirmed by SEM micro- graphs as shown in Fig。 6。 The hole wall surface features are captured in Table 2。

Surface roughness is a measure of surface texture that is quan- tified by the deviation of a real surface from its ideal form。 Average surface roughness Ra measures the average vertical distance of the mean roughness profile line from all measured data points (includ- ing peaks and valleys)。

Maximum profile valley depth Rv is a measure of the vertical distance of the mean roughness profile from the deepest ‘valley’ data points only。 Both are good predictors of fatigue performance, since higher surface irregularities increase the probability of nucle- ation sites for cracks。 Skewness Rsk is a dimensionless parameter measuring the asymmetry of the probability distribution of rough- ness profile vertical distances from all data points, with negative values indicating that the bulk of the data fall on the right side of the probability density function curve。 Grzesik et al。 [37] have shown that negative Rsk values indicate surfaces with good bearing properties。

At this stage, the following question arises: Does the machin- ing-related damages observed in Figs。 5 and 6 influences the   limit

tion of the temperature is due to the thermo-elasticity of the mate- rial and the friction between layers (i。e。 fibers/fibers and/or fibers/ matrix), whereas, in second stage, the temperature reaches a bal- ance that is due to  saturation  in the  damage。  With the  increase  in the loading, the rate of the damage and the frictions become more important。 This stability is followed by  an  abrupt increase  of the damage and temperature of the specimen corresponding     to the rupture [34,35]。

The thermal dissipation and damage accumulation results dis- cussed in this section indicate that damages produced in CM holes (fiber pullout and matrix degradation on the hole wall, Figs。 5a and 6a) tend to stimulate more damage under long-term cyclic loading than the damages in AWJM holes (striations  on  the  hole  wall  Figs。 5b and 6b)。

At this stage of the current investigation, we will try to obtain a correlation between the thermographic analysis and the endurance limit。

3。2。3。 Thermographic Damage Criterion