How to Analyse Wear Geometrically in 4 Simple Steps

10 March 2021  |  Form, Surface, Defect
4 Simple Steps

We often get asked about how best to analyse wear with a view to measuring wear depth and wear volume once the wear scar has been measured with a CMM.

We are explaining the steps here at the example of an artificial hip joint, but the same principle is transferrable to other wear scars.

Failed resurfacing device

Failed resurfacing device (cup angle: 50°, revised after 3.5 years)

Prerequisites

The wear scar and the surrounding area have been measured. In this particular case, the hip head has been measured on an OrthoLux 4 AB with 901,800 data points.

 

Step 1: Identify the wear patch.

This is usually achieved by fitting the nominal shape to the surface. Initially this includes all data points:

Raw data set after initial sphere fitRaw data set after initial sphere fit

From this initial fit, it is possible to separate the original surface, which is later used as the datum for wear measurements from the wear scar.

 

Step 2: Exclude the wear scar.

The next step is to exclude measurement points on the wear scar. This important step allows establishing a clear datum surface for further calculations.

Wear scar excludedWear scar excluded

 

Step 3: Refit to datum surface.

Now that the original, unworn surface has been established as a datum, the sphere fit can be repeated.

Sphere fit to datum surfaceSphere fit to datum surface

Steps 2 and 3 might have to be iterated a few times until all of the wear scar has been isolated and removed.

 

Step 4: Include all data points and characterise wear.

Sphere fit to datum surface-1Sphere fit to datum surface

In this case, the wear scar is 53.7 µm deep and the wear volume is 18.6 mm ³.

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