# Analysis of Acoustic Emissions for Determination of the Mechanical Effects of Scratch Tests
## 1 Introduction  
Sheet metal forming (SMF) is a common manufacturing technique used to make components such as automotive body panels and aircraft skins. It is widely utilized in industry for high-volume production. SMF is a fast, cheap, near net shape (limited scrappage) process which produces high-quality parts when galling does not occur. The galling damage mechanism is of high industrial relevance, particularly for the SMF industry, where restrictions on lubricants and the use of high-strength sheet materials may lead to galling [1]. Galling wear of forming tools is a well-known problem in SMF and can result in a combination of adhesive and abrasive wear [2]. During the galling wear process, material is transferred from the sheet to the tool, thus damaging the tool and future parts [3]. The definition according to ASTM G40-17 is \"a form of surface damage arising between sliding solids, distinguished by macroscopic, usually localized, roughening, and the creation of protrusions above the original surface; it is characterized by plastic flow and may involve material transfer\" [4]. The most important factor affecting the quality of the final productsin SMF processes is wear control in the contact area between the sheet material and the tool surface [5]. Galling failure accounts for up to 71% of the cost for die maintenance [6]. It is widely reported that galling exhibits three distinct regimes described by an increasing coefficient of friction [7; 8; 9]. Initially, contact is between a clean tool and the sheet surface. However, continued sliding leads to an intermediate stage through local adhesive wear, and this gives rise to individual lumps of adhered sheet material on the tool surface. At this point, the (local) contact becomes a sheet-to-sheet material contact. Further sliding in this first regime grows the adhered lumps to a final stage of microscopic scratching. In the second regime, adhesive and abrasive wear mechanisms interact, and growth of lumps occurs primarily where sheet-to-sheet contact prevails. In the third regime, the entire contact area becomes covered by a layer of sheet material, which leads to severe adhesive wear of the entire sheet track [10]. It has been established by multiple other works that these three regimes are detectable by acoustic emissions (AE) under many different experimental regimes [11; 12]. AE is showing promise as a method for online tool condition monitoring by measuring elastic waveforms in the forming tools during manufacture. AE is the release of transient elastic energy accumulated in a material during deformation processes. [PERSON] et al. identified that the main drawback of AE as a method of measuring friction processes or initial cracking in a forming process is that several phenomena can appear as the same signal when analyzed in the time-frequency domain [13]. [PERSON] et al. showed that defective products could be detected by AE during manufacturing by analyzing simple signal parameters; however, the superposition of a signal from a defective component and one from tool wear could not be distinguished when analyzing the AE in terms of amplitude and energy [14]. [PERSON] et al. did not investigate if time-frequency techniques could separate this super-positioning. [PERSON] and [PERSON] found that a strong relationship between the AE root mean square (RMS) and the coefficient of friction (COF) exists under steady state using cross-correlation analysis (based on instantaneous features in both AE RMS and COF) [15]. A key conclusion of [PERSON] and [PERSON] is that the AE energy rise due to failure is a characteristic of the ball/disk material combination. [PERSON] et al. also proposed the use of mean frequency to infer the progression of galling wear and showed that decreasing mean frequency was indicative of increasing wear [16]. [PERSON] et al. also showed that materials are separable during wear by both counts per mm and AE pulse energy [17]. [PERSON] et al. studied the AE of galling from a metal-forming point of view by using the bending under tension test.