Prominence in Understanding the Position of Drill Tool Using Acoustic Emission Signals During Drilling of CFRP/Ti6Al4V Stacks

Introduction

A recent trend in the aerospace industry is an increase in the use of composites and aluminum and/or titanium alloys because of the outstanding mechanical properties that can be provided at critical- load load-carrying locations of the aircraft. The central wing box is a critical component made out of CFRP/Ti6Al4V stacks, located at the top of the fuselage; it forms the attachment point for both wings and all the engines. Similarly, CFRP/Ti6Al4V stacks are also used along the tail sections. Composites on the B787 account for 50% of the aircraft’s structural weight. Aluminum comprises only 12% of the mentioned weight; and titanium makes up a greater percentage than aluminum, namely 15%. Steel comprises 10%, and other metals share 5%(Alberdi,Artaza, Suárez, Rivero, & Girot, 2016).

Condition monitoring during drilling is aimed at understanding the state of the tool, by acquiring the different process parameters by direct or indirect methods. These data are then used to bring changes in either the machining parameters or to replace a worn tool. The timely replacement of tools can help in avoiding the rejection of parts with poor hole quality. The monitoring is done online, which means that the tool is inspected during the actual drilling process. The advantage of online inspection is that it is possible to influence the drilling process during the hole-making process—thereby correcting a potential hole quality problem before the hole is completed. Therefore, the timely replacement of worn tools can be achieved; and the possibility of getting a hole with poor quality is eliminated. This leads to an economy in the drilling process. By contrast, direct methods involve measurement of tool wear using optical techniques. The principle disadvantage of the direct method is that the tool can be inspected only after the machining process. In indirect methods, process variables such as changes in hole quality, cutting force, vibrations, and spindle current or power can be monitored using acoustic emission signals with the help of different sensorssuch as thoselisted below:

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  Accelerometers for measuring vibration

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  Acoustic emission sensors for measuring acoustic signals during metal cutting

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  Dynamometers for measurement of cutting force and torque

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  Sensors to measure current or power of feed drives and main spindle

Among these, acoustic emission sensors are widely used for monitoring metal-cutting applications (Arul, 2007; Byrne, 1995). The principle advantages of these sensors are that they are not affected by machine noise and that they are sensitive to the changes taking place in the material during the metal-cutting process. These are transient elastic waves generated by the rapid release of energy from a localized source within a material when subjected to a state of stress. This energy release is associated with the abrupt redistribution of internal stresses; and as a result of this, a stress wave propagates through the material. The above definition indicates that processes that are capable of changing the internal structure of a material—processes such as dislocation motion, directional diffusion, creep, and grain boundary sliding which results in plastic deformation and fracture—are sources of Acoustic Emissions (AE). In metal cutting, the sources of AE signals are

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  Plastic deformation in the primary and secondary shear zones

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  Chip friction

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  Chip collision and breakage

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  Tool wear and breakage