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Figure 3 : Filtered ridging profile used to calculate the ridging index ( RI ).
Additional mechanical polishing is usually required to achieve the desired high-quality surface finish . In the worst cases , the amount of polishing required will be so costly that the product has to be rejected .
Ridging assessments
Traditionally , ridging is assessed by straining 20mm-wide tensile test specimens of FSS sheet to a defined elongation , typically between 7 % and 20 %. The specimens are then given a visual inspection and rated against an arbitrary scale .
While visual assessment is usually carried out by specially trained personnel , the results are often very subjective .
A more rigorous approach is to use a profilometer to assess the ridging of the specimen . Various types of profilometer are available that use either a sensitive stylus in contact with the surface or non-contact optical techniques . Usually , the arithmetic average of the roughness profile - Ra and / or the maximum height of the profile - Rt are the measure for the intensity of ridging .
The raw profile recorded by the profilometer is often relatively short , covering only a few ridges . This profile is then filtered electronically . A drawback is that this filtering uses standards that were not developed for ridging , and usually do not consider the spacing of the ridges . This means the results may not be representative . It is therefore hard to evaluate the severity of the ridging in terms of both amplitude and spacing .
Finding an improved method
The driver for Outokumpu ’ s research programme was to find a way to combine better sample geometry with a tailored filtering method . This would then provide the basis for a calculated ridging index ( RI ) that indicates both height of the ridges and their spacing . It was also desirable for the RI to correlate well with visual inspection results .
Straining a 20mm specimen by 15 % reduces its width to about 17mm . That is sufficient for a standard surface roughness measurement . However , it is not suitable for use with ridging , because the distances between ridges are typically between 1- to 3mm , so only 4 to 12 complete surface features are included .
To achieve good statistical reliability , larger sheet specimens were used ( 300mm in length and 100mm in width ). FSS sheets with thicknesses of between 0.5 and 1.5mm were strained to various elongations .
Creating the RI
Raw surface profiles are measured using two different types of profilometer : an optical 3D white light interferometer and a non-skidded bench-top 2D stylus . Filtering was applied to separate the ridging profile from the shape of the specimen , surface roughness and instrument noise . See Figure 3 .
As ridging is more detrimental when the valleys between the ridges are deeper , this was measured by the term , Rz .
It is important to know the number of ridges and valleys since the more there are , the harder the ridging is to polish away . This is measured by the peak count , Pc . A threshold is set to determine Pc , as shown by the horizontal dotted line in Figure 3 .
Rz in μm and Pc in mm-1 are multiplied to calculate RI , the dimensionless ridging index .
Putting RI to the test
The optical 3D profilometer was used to measure the surface profiles of the FSS sheet samples after elongations of 0 %, 2 %, 5 %, 10 % and 15 %. Figures 4 ( a ) and 4 ( b ) indicate that both RI and Rz have a linear relationship with elongation , while Pc is nearly constant . The highest RI resulted from the highest elongation , so all subsequent tests were carried out with an elongation of 15 %.
Profiles on the top and bottom surfaces of a sample were also measured to confirm that the choice of which side was sampled did not influence the measured RI . This is because the peaks on one side correspond to valleys on the other and vice versa .
Ensuring repeatability
To test the repeatability of the RI methodology , an FSS sheet was cut into different specimens as shown in Figure 5 ( a ). Multiple tests at 10mm spacing were carried out on one sample ( C0 ), producing an average RI of 4.4 as shown in Figure 5 ( b ).
Figure 4 : The influence of strain and straining direction on the ridging index ( RI ).
20 | sheetmetalplus . com | ISMR June 2021