Best Practice Machining of Composite Materials

We are often asked about the best practice for machining of composite test specimens . Machining handbooks generally do not cover composite materials, so it’s not possible to look up the correct machining tools or feed rates. This lack of information can lead to the machining of composites being a bit of a black art. With this in mind, what effect does poor machining have on test results? And, what pitfalls should you watch out for when machining composites?

In a previous blog I have alluded to the most important aspect of machining; alignment with respect to fibre orientation and its effect on test results. But what other considerations need to be included to guarantee reliable testing?

Heat dissipation issues

Typically, when machining metals, the waste material chips away, but with composites there isn’t so much of a chipping effect, as a shattering process. Rather than shearing the material away, the impact of the cutting tool edge fractures the carbon fibres. Because the material doesn’t chip away and due to the low thermal conductivity of composite materials, there are significant difficulties in dissipating heat when machining, leading to high local temperatures. This can lead to local damage and distortion of the composite as the matrix melts. With this in mind, it is highly recommended that machining operations are water cooled. Alongside the heat dissipation benefits, the water prevents the fine carbon fibre dust becoming airborne and the associated health effects that would bring.

When wet cutting is used however, steps must be taken to ensure that specimens are adequately dried and that no water damage is introduced. Moisture can damage composites by reducing the strength of the interface between the fibres and the matrix, increasing the residual stresses within the matrix and in the case of glass fibres, leaching salts out from the fibres.

What effect does surface finish have on the measured test results?

While there can be a small advantage in having a rough surface finish for composites to prevent extensometer slips, consideration must be given to the notch sensitivity of composite materials. Therefore, in general, the rougher the surface finish, the lower the measured result, in terms of strength or ductility.

In “Mechanical Testing of Advanced Fibre Composites”, J Hodgkinson  investigated the effect of surface finish on tensile strength of carbon fibre using three different surface finishes. Samples were cut using a diamond blade with a 600 grit finish. Samples were then abraded with 80 grit aluminium oxide paper until all visible cutting marks were removed. It can be seen in Figure 1, that by performing this abrasion, the tensile strength increased by 20%. Any edge polishing must be carried out carefully in order to avoid radiusing the edge of the specimen, or introducing localised width variations. Unfortunately, details of the grinding operation are not given by the author, but it can be seen that ground samples exhibit a greater tensile strength, however they also show a greater degree of standard deviation.

 

 

With this in mind it is generally accepted that machining with a diamond blade is optimal. Rough machining could be carried out by band saw, however this can damage fibres and gives a rough finish. Early versions of ASTM D3039 used to recommend cutting test pieces 3mm wider than required, before grinding them back to the required finished width, however the latest version of the standard requires that the final dimensions are reached using water-lubricated sawing, milling or grinding.

 

Waterjet cutting is now a popular technique, and it allows for the machining of more complex shapes. Samples can be machined out of a laminate in such a way that they are easily traceable to their location within the original laminate. If the laminate has been analysed by ultrasonic c-scan prior to machining, it is then possible to determine whether  a weaker specimen within a test batch is due to a defect within the laminate in that location.

Great care must be taken with waterjet cutting to tailor the method to the requirements of composite materials. A poor waterjet cutting method can lead to fibres being knocked out of the laminate, resulting in a poor surface finish, even delamination, giving significantly lower results. A further drawback of waterjet cutting, is that it leaves a tab of material attached to the sample, which will need some post-processing to remove. An example of a poorly cut waterjet specimen can be seen below. Such a sample should be discarded due to the presence of obvious notches and machining damage, which would give unrepresentative results. Waterjet cutting can’t always deliver the same level of precision as general machining tools, which can be critical with some composite specimens.

 

Accuracy of machining – compression specimens

The accuracy of cutting of test samples is obviously a key requirement. This is especially true for compression test specimens, where there is a tolerance of ±0.02mm on the dimensions. With ASTM standards there are also strict requirements on the parallelism and perpendicularity of test specimens. This means that machining must be highly accurate. The easiest way to achieve high accuracy is by cutting the samples to near net shape and then using a surface grinder to achieve the final specimen geometry.

In recent years there has been a significant amount of development in the field of CNC plate saws, by manufacturers such as Extec and Sharp & Tappin These saws allow for geometries to be programmed into the machine and then they automatically machine samples to the correct geometry, with the claim that no post processing is required. This facility would allow a lab to increase its workshop throughput, while also increasing operator safety. However they do have a sample size limitation, meaning an element of pre-processing would be required in order to ensure that the laminates for machining would fit on the machine bed.

Other machining considerations, notches and drilled holes

Machining quality for composite test samples is not limited to simple straight edges; the quality of machining of notches for v-notched shear testing can have a significant effect on measured shear strength. Similarly the quality of drilled holes can also have a major effect on what? Because delaminations can be introduced by drilling.

 

It is also difficult to prevent breakout where the drill bit enters and exits the laminate. It is worth noting that it is difficult to drill holes in unidirectional laminates without causing longitudinal splitting. A smaller drill point angle tends to produce better exit quality. However, too small a point angle can give the tool poor edge strength. For CFRP, the optimal compromise seems to be a point angle of 90 degrees. By comparison, a more typical point angle for drilling metal is 135 degrees (meaning the composites drill is “pointier”).

Entry defects can often be reduced by increasing the feed rate—although this poses the danger of exacerbating exit defects. The optimum feed rate balances quality on both the entry and exit sides. A solution that we have developed here at R-TECH is to use a sacrificial piece of composite material both above and below the laminate which needs a hole drilling in it. This leads to entry and exit defects being contained within the sacrificial samples. As with machining, it is important that coolant is used during the drilling process in order to prevent undue damage to the composite samples.

At R-TECH we have a machine shop manned by highly experienced and skilled operators, who are able to carry out composite machining operations to the highest quality, giving you confidence in our test results. To find out more about our composites preparation capabilities, from laminate manufacture to sample polishing for microsection analysis, click here.

References

• Hodgkinson, J. (2000). Mechanical testing of advanced fibre composites. Cambridge: Woodhead Pub.
• ASTM D3039M-17 Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials. (2017). ASTM International.
• Zelinski, P. (2018). How To Machine Composites, Part 1-5 — Waterjet Cutting. [online] Mmsonline.com. Available at: https://www.mmsonline.com/articles/how-to-machine-composites-part-5—-waterjet-cutting-of-composites [Accessed 17 Aug. 2018].
• Sandvik Coromant (2010). User’s Guide Machining carbon fibre materials
• Hull, D & Clyne, T. (1996). An Introduction to Composite Materials. 2^nd Cambridge University Press