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A less dominant form of punch force signature shape variation was also observed in all the trials conducted. Generally, these PCs capture variation in the progression of punch force over the stroke, or the slope of the signature plateau, Figure 8.2. This as a consequence, will describe the initial level of punch force once the blank yields.

Figure 8.2: Principal Component representations for extended wear trial 1 Principal Component 3 (a) and the secondary lubricant comparison Principal Component 2 (b). These two Principal Components capture the progression of punch force over the stroke form of shape variation, despite differences in example b-value curves, both PCs describe variation in the punch force progression or slope of the signature plateau.

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The slope of punch force progression was found to be related to dynamic changes in friction over the course of the stroke. This was supported by the differences in force progression observed between fluid lubricant punch force signatures and dry/unlubricated punch force signatures. The dry contact condition was found to have approximately consistent levels of punch force after the blank material yielded. Alternatively, all fluid lubricants were found to have increasing progression over the course of the stroke that was associated with friction increase that comes with lubrication regime change.

This form of signature shape variation allowed for clear distinction between various lubricant types and BHF levels (Section 4.3.3). The results Chapter 7 suggested that late stroke friction increase as a result of lubrication mechanism changes can be detected using this PC. The inclusion of dry film lubricants in addition to the unlubricated trials supported the hypothesis that the other fluid lubricant trials were experiencing late stroke lubrication mechanism change. The clear separation in b2 values for fluid lubricant trials and the dry conditions (Figure 7.2) demonstrates the detection of this late stroke mechanism change.

The relationship between the slope of punch force progression and galling wear was also assessed. The b-values associated with the slope of punch force progression form of shape variation were observed to have a significant increase in range for the high wear trial in Section 4.3.2.

A slight trend was also observed in this form of variation in extended wear trial 2 that continued after wear severity at the start of the stroke had plateaued. A similar trend was not observed in extended trial 1 indicating that the slope of punch force progression can be influenced by some inconsistent factor of galling wear.

Channel part sampling rate proved to be an issue for assessing the relationship between galling wear and punch force progression in the extended wear trials of Chapter 6. Large variations in b-values for the slope of progression PC occurred part to part for much of the extended trials. However, this variation could not be compared to the galling wear on the part as only every 10th _{part was collected. To} address this issue, extended trial 1 was continued in Chapter 7 and all parts were collected. Visual assessment of consecutively formed parts and comparison to changes in the associated b-value provided qualitative evidence that punch force progression form of signature shape variation is correlated to the presence of

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adhesive wear damage late in the stroke. This correlation to late stroke adhesive damage was assessed quantitatively using 𝑊𝐷𝑊𝑇 measurements to track galling

severity over the course of a stroke, however, the results of this assessment were inconclusive. The 𝑊_𝐷𝑊𝑇 measurements did not demonstrate the expected

relationship in 2 of the 3 test channel part series, with the remaining parts showing a clear increase in galling severity with stroke displacement for parts with signatures that had a higher than average positive slope for punch force progression. In summary, there is qualitative evidence that the progression of punch force during the stroke is related to the dominant wear mechanism during the stroke. The different mechanisms that are observed in galling wear are associated with distinct levels of friction, and so it follows that a change will have an effect on the punch force progression. As galling wear develops, and more regions are damaged, multiple mechanisms can be present at the same time. Multiple damage complicates assessment and could have affected 𝑊𝐷𝑊𝑇 measurements taken in the quantitative

investigation (Section 7.3.4), where parts had at least 6 separate regions of damage across both sidewalls. Further work is required to clarify the quantitative link between punch force progression and galling wear, potentially by assessing parts with fewer damaged regions.

8.4Limitations and assumptions

There are a number of limitations and assumptions in this work that must be considered with the results and conclusions. Firstly, this work has been conducted on relatively small channel parts, and so the proportion of wear damage area to total contact area is relatively large. In industry much larger parts are formed, which will make the ratio of wear to contact area much smaller. At this stage the sensitivity of the punch force signature is unknown, and very small changes in the amount of wear may not be detectable.

A significant assumption made throughout this work is that the PC determined for the analysis of different punch force signature data sets represent the same general forms of signature shape variation in the cases of magnitude and slope of progression. As the mean signature for each data set is not consistent, the PCs and

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the corresponding b-values cannot be directly compared. However, in the analyses conducted the variation around the mean signatures for the dominant PCs appeared to represent the same general shapes. Hence the assumption that these different PCs represent the same general forms of signature shape variation and so are conceptually comparable.

Determining if a signature is affected by galling wear or other friction influencing factors is an aspect that has not been fully addressed, and so is a limitation of this work. Tests focusing on identifying if punch force magnitude variation is specifically due to lubrication changes or galling wear were not conducted. These factors were not assessed in the same trials in this work, with the trials focusing on varying either wear or other friction influencing factors, but not both simultaneously. It is feasible that either or both could occur in an industrial situation, requiring different remedial action, and as such, it would be beneficial to be able to distinguish between issues. With the data available it is not possible to make a direct comparison; however, a possible approach could involve combining b-values for both forms of variation in order to isolate specific issues. For example, the b2 values of the unlubricated conditions (Figure 7.2) were significantly different to the base case mill oil condition, and so in an instance where lubricant application failed it would be expected that b2 values would be very negative in addition to very positive b1 values. Based on observations from extended wear trial 1 (Figure 6.8b) the very negative b2 value would not be observed when wear is the issue, even though both situations would come with an increase in b1 values that correspond to increased signature magnitude. This, of course, has not been tested in the present work and requires further investigation to confirm. However, in an industrial situation, due to the cost of galling, any friction increases that will correspond to an increase in signature magnitude should merit inspecting the conditions of the press regardless of cause.

Another limitation relates to the DWT galling wear severity quantification methodology presented. The DWT methodology and 𝑊𝐷𝑊𝑇 values are not currently

linked to set severities of galling wear, making it difficult to infer the galling severity based off a 𝑊𝐷𝑊𝑇 value. In this work the 𝑊𝐷𝑊𝑇 have been treated as a relative

measure with reference to values acquired for known unworn parts from the beginning of trials. If the technique is not applied similarly, as a relative measure, it

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may be difficult to implement in industry or other trials immediately. Additional testing would be required to understand what values represent unacceptable levels of wear for different materials. The results of Chapter 7 also suggest that the 𝑊_𝐷𝑊𝑇

parameter does not conclusively show a distinguishable difference when measuring galling wear tracks of equal size but consisting of different wear mechanism specific damage. This may require further refinement of the 𝑊_𝐷𝑊𝑇 parameter to quantify

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CHAPTER NINE