KEYWORDS: Abrasive waterjet, machining, depth, tolerance, model, integrity.
The waterjet technology appears to be the most convenient for composite part ...
Abrasive waterjet machining of composite materials F. Cénac 1,2, R. Zitoune 1, F. Collombet 1, and M. Déléris 2 1
Laboratoire de Génie Mécanique de Toulouse, équipe Pro²Com, Université Paul Sabatier, 31077 Toulouse cedex 4, France 2 JEDO Technologies, BP 78204, 31682 Labège cedex, France
KEYWORDS: Abrasive waterjet, machining, depth, tolerance, model, integrity The waterjet technology appears to be the most convenient for composite part cutting. Indeed, it leads to low induced force and temperature on the material which is particularly adequate for machining. However, the use of the abrasive waterjet technology (AWJT) requires a great control of the machining parameters in order to avoid delamination. Different research programs already aimed the confrontation between AWJT and composites [1, 2]. But they generally concern one or two materials and deal more with the machining process than with the composite specificities. This study intends to specify the range of application of the AWJT for blind machining of long-fiber polymer-matrix composites, and to link the result to the structure of the material (matrix, reinforcement and manufacturing process). For this reason, the study concerns eight composite materials frequently used in industry. This includes: -
Carbon / epoxy Hexply UD T700 268 M21 34 % (autoclave) 20 plies quasi-isotropic
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Carbon / epoxy Hexply UD T700 268 M21 34 % (oven) 20 plies quasi-isotropic
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Carbon 3K / epoxy DBF (RTM) 20 plies plain weave
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Glass / epoxy HexFIT (autoclave) [0°/90°]4
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Glass / epoxy HexFIT (oven) [0°/90°]4
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Glass / phenol HexPLY 260 (oven) 8H 30 plies crowfoot satin
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Glass 20860 / epoxy DBF (RTM) 20 plies plain weave
- Glass 7781 / epoxy DBF (RTM) 20 plies Within the first stage of the work, we studied the influence of the AWJT parameters (Pressure (P), in-feed speed (V), abrasive mass flow rate (Da), and standoff distance (s)) and the material manufacturing process on the average machining depth (h) and the machining quality. Three materials (the M21T700 and the two HexFIT) were machined during this first step. The jet scanned the sample with several adjacent passes. A mask (stencil) was used to limit the area of the machined surface to the desired shape (Fig. 1). Those tests were developed to respect the Doehlert experimental design, leading to forty-one machining operations per sample. For the analysis, the pockets were divided into two groups: a first one contains the unsuitable pockets which involved delamination or excessive striation. Those samples were used to define the AWJT range of application for composite machining. The second group of pockets contains the clean samples. For each of them, twenty-one depth measurements were obtained using a 3D feeler in order to inform about the average depth and the tolerance. At the end, those values were used to identify a penetration forecasting model (equation (1)) which
matches the common literature. The five constants were identified from the experimental design results.
hep = a0 ep .P
a1ep
.V
a2ep
.s
a3ep
a4ep
.Da
(1)
In Fig. 2, the average machined depth obtained for the three material samples for the different machining settings is presented. It appears that the evolution of depth versus machining parameters is similar is similar for each material. Furthermore, the relative deviation between the depths obtained for the three materials is almost constant. This observation leads to only use one parameter (a0) to define the material (this is named workability [3]) for scan machining average depth forecasting models. Besides, the AWJT range of application differs from a material to another one: several materials (like the HexFITs) delaminate more easily than the others (like the M21T700). Concerning the technology, the ratio between the abrasive mass flow rate and the hydraulic energy (pressure) in one hand, and the in-feed speed in the other hand, appear to highly control the material integrity. Their limit values also directly depend on the material characteristics. The materials and their manufacturing process (autoclave, oven, RTM…) also influence the pocket roughness and tolerance as far as they imply a size and a distribution of the heterogeneities (porosity, reinforcement mesh sizes…) Machining tolerance and roughness models are being developed in order to complete the reachable results forecasting. The set of materials that is presented above is being machined in order to follow a five hundred point experimental design. Many samples will be equipped with Bragg network (optical fibbers) then mechanically tested to follow the material reactions before, during and after the waterjet machining operation.
10 mm
Fig. 1: M21T700 machined pockets of the experimental design
Average depth of penetration (mm)
HexFIT autoclave HexFIT oven M21T700
Pocket number
Fig. 2: Average machining depth (mm) for the twenty-six samples
REFERENCES 1. Wang J., “Abrasive Waterjet Machining of Polymer Matrix Composites”. International Journal of Advanced Machining Technology, Vol. 15, pp 757-768, 1999. 2. Hashish M., Status and potential of waterjet machining of composites, Proceedings of 10th American Waterjet Conference, Huston, Texas, paper 64, 1999. 3. Momber A.W., Kovacevic R., Principles Of Abrasive Water Jet Machining, Springer, 1998. 4. Wang J., “Predictive depth of jet penetration models for abrasive waterjet cutting of aluminia ceramics”. Int. J. of Mechanical Sciences, Vol. 49, pp 306-316, 2007.
