Abrasive waterjet machining of composite ballistic materials (Kevlar)

Abstract

Recently, there has been a development of modern ballistic armor due to the creation of high performance composite materials based on aramid fibers with better weight reduction, higher strength and toughness. The use of metal for many critical applications has dramatically decreased since the new composite materials such as aramids provide excellent protection against handgun level threats. However, aramid fibers (Kevlars) are very difficult to machine by conventional machining techniques which cause various forms of material damage such as delamination and fiber pullout that normally occur during the machining of composite materials. The processes also result to high tool wear rates and poor surface roughness which ultimately may cause costly secondary rework as well as part rejection. As the use of these composite ballistic materials is anticipated to be increasing in large volume, the processing costs will be an important factor in order to provide cost-effective components and to gain competitive advantages over other materials. In this thesis, abrasive water jet (AWJ) machining of a representative ballistic material namely Kevlar-reinforced phenolic, an aramid fibers and a product of Du Pont was used to conduct the machining experiments in order to determine the viability of the machining process for manufacturing protective components with the above materials. Design of experiments (DOE) and analysis of variance (ANOVA) were used to measure systematically the various parametric combinations of pressure, standoff distance, traverse rate and abrasive flow rate on the kerf taper and the changes of surface roughness as a function of cutting depth for the Kevlar composite specimens. Stylus profilometry was used to measure the surface roughness and a visual inspection including scanning electron microscopy (SEM) was conducted. It was found that a smoother surface is obtained at the mid region along the depth of the specimens compared to that at the region of jet entry and jet exit. It was also observed that a higher jet pressure and a low traverse rate produces a smoother surface. Width of the kerf both at the jet entry and the jet exit was found to have a decreasing tendency with increase in traverse rate. Mathematical models were formulated to predict the surface roughness and kerf taper in terms of the selected cutting parameters mentioned earlier for the Kevlar composite to the cutting depths of 9.2mm. The cut surface of this material using a band saw machine was compared with that of the water jet in terms of surface roughness and damages incurred.