Observation of fracture behavior of 3-D printed specimens under rolling contact fatigue in water

Polymer bearing was widely used in the corrosive conditions because of its high corrosion durability. The polymer bearing had been formed using molding and machining until the new 3-D printing method was developed. In this study, we performed the rolling contact fatigue tests of the 3-D printed specimens in water and observed the fracture behaviour of the specimens. We found that the surface cracks are related to both the rolling direction and the lamination directions.


Introduction
When engineers use bearings, they require various properties from the materials such as high hardness, low friction coefficient and formability.Bearing steel is popular because of its high hardness.However, it cannot be used in oil-less conditions; for example, in dry conditions which result in direct contact between the materials, in solutions which include some corrosive materials.In these special conditions we use the polymer bearings.
Some of the good properties of polymer are selflubrication ability; light weight; and high corrosion resistance.In particular, corrosion resistance is important because wet and corrosive lubricating conditions often occur in industrial situations.Therefore researchers have studied polymer bearings in wet conditions [1][2][3].
Two major methods of making polymer bearing's formation are the injection molding and machining.Recently, a new 3-D printing method was developed and it had become popular because we can easily make complex and seamless parts using the method.Along with the expansion of the use of 3-D printing, researchers had focused on the effect of the laminated layers joins on the mechanical properties of the 3-D printed parts [4][5].
In this study, we performed rolling contact fatigue tests on the 3-D printed specimens in water and observed these fracture behaviour of the specimens 2 Experimental procedure

Specimens
The specimens were printed by 3-D printer (Projet 3510 HD plus, 3-D systems, Inc.).Figure 1  The tests were done in water conditions.The specimens set-up was similar to our previous work [6].The rotation speed (rpm) was 1200 rpm and the total number of rotations was 2.88×10 5 cycles.This number of rotations simulated one day's working time (8 hours at 600 rpm).After testing the specimens were washed in ethanol and measured the weights.We observed the specimens using Laser Confocal Microscope (LCM, VK9710, Keyence).

Specimen surface observation
We observed the specimen surfaces using a LCM. Figure 4 shows the donut-shaped specimen's surface whose lamination direction was perpendicular to the rolling direction.Figure 4(a) shows the surface before the test.We observed the lamination gaps, and the roughness of the surface was about Ra = 4.6 m (the measurement direction parallel to rolling direction).Figure 4  Figure 5 shows the donut-shaped specimen's surface whose lamination direction was the same as the rolling direction.Figure 5(a) shows the surface before the test.The roughness of the surface was about Ra = 4.0 m (the measurement direction parallel to rolling direction).Figure 6 shows the donut-shaped specimen's surface whose lamination direction was 45 degrees to the rolling direction.Figure 6

Fig. 2 Fig. 3
Fig. 2 RCF test set up image (b) shows the donut-shaped specimen's surface after the 500 N thrust load test.The circular cracks and the deep peeling can be seen.The circular cracks are evenly distributed at increments between 50 and 100 m.

Figure 4 (
c) shows a 3-D image of Fig. 4(b).The peeling depth was over 350 m after the 500 N thrust load test.

Figure 5 (
b) shows the donut-shaped specimen's surface after the 500 N thrust load test.We observed a deep hole which was the result of flacking.We also observed two types of cracks, surface circular cracks which are perpendicular to the rolling direction, and cracks which are parallel to the rolling direction at the bottom of the flaking failure.Figure 5(c) shows a 3-D image of Fig. 5(b) and the two types of cracks can be clearly observed.
(a) shows the surface before the test.The roughness of the surface was about Ra = 4.7 m (the measurement direction parallel to rolling direction and rolling direction opposite to that in other pictures).Figure 6(b) shows the donut-shaped specimen's surface after the 500 N thrust load test.Some of the cracks were in the same direction as the lamination direction.Figure 6(c) shows a 3-D image of Fig. 6(b) with cracks in various directions.