1359-835X/$ - see front matter © 2013 Elsevier Ltd。 All rights reserved。 http://dx。doi。org/10。1016/j。compositesa。2013。09。002

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

have shown that, during  drilling  with  high  spindle  speed  (40,000 rpm), the increasing of the feed rate does not cause the increasing of the damage size。 All these authors are in agreement for that, the delamination located at the hole exit is caused mainly by the thrust force of the drill。 For that various authors have been interested to the prediction of the thrust force analytically and numerically responsible of the damage induced at the hole exit [24,25,12]。

The damage arising from conventional machining (CM) can cause significant reduction in the tensile and compressive strength of a composite structure contributing to a significant economic burden as 60% of the components rejected during manufacturing are due to inferior hole quality [6,26–28]。 Possible damages in fi- ber–matrix composites include matrix cracking and thermal alter- ations [28], delamination at the hole entry and exit, degradation of the resin on the hole wall [7,9,10,27], fiber pull-out, burning and fuzzing, which compromises surface quality [5] – all causing reducing of the mechanical strength of the structural assembly。 These types of damage are strongly influenced by the tool geome- try and the machining parameters [28]。 Davim et al。 [29] observed that the cutting speed is a significant parameter, having the highest influence on the surface roughness whereas feed rate has the high- est influence on the delamination。 Zitoune et al。 [30] concluded that failure loads of specimens machined with a conventional drill are lower than those with moulded holes。 It was also concluded that specimens with drilled holes represent brutal fractures and specimens with moulded holes have progressive  fractures。

To avoid such problems, a non-traditional machining technique using an abrasive water jet is proposed in this paper。 Abrasive water jet machining (AWJM) is widely used for composites mate- rials。 With this process it is possible to reduce damages that are typical of CM techniques [7]。 AWJM does not produce heat-affected zones; hence there is no smoke, dust or work piece distortion [31]。 Currently only few studies have investigated the effect of the pro- cess of machining on the mechanical behavior of CFRP   [6,32,33]。

This paper describes in details the experimental tests conducted on the machining of CFRP stacks to investigate the influence of machining process on the mechanical behavior of carbon/epoxy composite plates with circular holes。 Static and fatigue tests are performed to analyze damage accumulation, thermal dissipation, and endurance limits。 In this study, assessment of surface micro- structure is done by means of standardized roughness parameters (Ra, Rv, Rsk, Sa, Sv)。 Furthermore, a new Infrared Thermographic Damage Criterion (TDC) is proposed, which relates the region of highest surface temperature with damage accumulation, thereby displaying the influence of applied machining techniques on the mechanical behavior of the composite specimens。 Therefore, the main objective of this study is to investigate the influence of the surface topology, created by non-conventional machining (AWJM) and conventional machining using twist drilling, on the mechani- cal behavior of the composite plates with circular   hole。

2。Materials and  methods

2。1。Specimen details

Unidirectional prepregs of 0。26 mm made of carbon/epoxy composites are used to manufacture CFRP specimens。 The raw material of laminated structures are provided by Hexcel compos- ites and are referenced as UD HexPly T700-M21GC with 58% fiber content and 1。6% void content。 The mechanical properties of car- bon/epoxy composite are given in Table 1 [12]。 The stacking se- quence of the composite panels was [±45°]2S which is chosen for the following reasons。 Firstly, since the main goal was to measure the temperature dissipated during the fatigue test, it is    important