12.16 SURFACE FATIGUE FAILURE MODELS—DYNAMIC CONTACT

There is still some disagreement among experts as to the actual mechanism of failure that results in pitting and spalling of surfaces. The possibility of having a maximum shear stress at a subsurface location (in pure rolling) has led some to conclude that pits begin at or near that location. Others have concluded that pitting begins at the surface. It is possible that both mechanisms are at work in these cases, since failure initiation usually begins at an imperfection, which may be on or below the surface. Figure 12-24 shows both surface and subsurface cracks in a case-hardened steel roll subjected to heavy rolling loads.[18]

An extensive experimental study of pitting under rolling contact was done by Way[19] in 1935. Over 80 tests were made with contacting, pure rolling, parallel rollers of different materials, lubricants, and loads, run for up to 18 million cycles, though most samples failed between 0.5 E 6 and 1.5 E 6 cycles. The samples were monitored for the appearance of minute surface cracks, which inevitably presaged a pitting failure within less than about 100 000 additional cycles in the presence of a lubricant.

Harder and smoother surfaces better resisted pitting failure. Highly polished samples did not fail in over 12 E 6 cycles. Nitrided rolls with very hard cases on a soft core

FIGURE 12-24

Photomicrograph (100x) of surface and subsurface cracks in a carburized and hardened roll (HRC 52-58) subjected to a heavy rolling load (Source: J. D. Graham, Pitting of Gear Teeth, in C. Lipson, Handbook of Mechanical Wear, U. Mich. Press, 1961, p.137.)

were longer-lived than other materials tested. No pitting occurred on any samples in the absence of a lubricant even though dry-running produced surface cracks. The cracked parts would continue to run dry with no failure for as many as 5 E 6 cycles until some lubricant was added. Then the surface cracks would rapidly enlarge and turn to pits of a characteristic arrowhead shape within 100 000 additional cycles.

The suggested explanation for the deleterious effect of the lubricant was that once suitably oriented surface cracks form, they are pumped full of oil on approaching the rollnip, and then are pressed closed within the roll-nip, pressurizing the fluid trapped in the crack. The fluid pressure creates tensile stress at the crack tip, causing rapid crack growth and subsequent break-out of a pit. Higher-viscosity lubricants did not eliminate metal-to-metal contact but did delay the pitting failure, indicating that the fluid must be able to readily enter the crack to do the damage.

Way reached a number of conclusions regarding how to design rollers to delay surface fatigue failure.[19]

1 Use no oil (though he was quick to point out that this is not a practical solution, as it promotes other types of wear as discussed in previous sections).

2 Increase the viscosity of the lubricant.

3 Polish the surfaces (though this is expensive to do).

4 Increase the surface hardness (preferably on a softer, tough core).

No conclusions were drawn with respect to the reasons for the initiation of the initial cracks on the surface. Though, with pure rolling, the shear stresses are not maximal at the surface, they are nonzero there at some locations (see Figures 12-12, p. 354 and 12-17, p. 361).

TABLE 12-3 Modes of Surface Failure and Their Causes

Source: W. E. Littmann and R. L. Widner, Propagation of Contact Fatigue from Surface and Subsurface Origins , J. Basic Eng. Trans . ASME , vol. 88, pp. 624–636, 1966.

Littman and Widner[20] performed an extensive analytical and experimental study on contact fatigue in 1966 and describe five different modes of failure in rolling contact. These are listed in Table 12-3 along with some factors that promote their occurrence. Some of these modes address the crack initiation issue and others the crack propagation issue. We will briefly discuss each in the order listed.

INCLUSION ORIGIN This describes a mechanism for crack initiation. It is assumed that the crack originates in a shear-stress field at a subsurface or surface location containing a small inclusion of “foreign” matter. The most commonly identified inclusions are oxides of the material that formed during processing and were captured within it. These are typically hard and irregular in shape and create stress concentration. Several  researchers[21],[22],[23] have published photomicrographs of (or otherwise identified) subsurface cracks starting at oxide inclusions. “These oxide inclusions are often present as stringers or elongated aggregates of particles . . ., which provide a much greater chance for a point of high stress concentration to be in an unfavorable position with respect to the applied stress.”[24] The propagation of a crack from the inclusion may remain subsurface, or break out to the surface. In the latter case it provides a site for hydraulic pressure propagation, as described above. In either case, it ultimately results in pitting or spalling.