Technical Tips

November 2012

Titanium Driver Face Fatigue - Fact or Fiction?

Tom Wishon - Golf Technology

Recently a golfer contacted us to ask if it was possible for the face of a titanium driver to fatigue over time and experience a drop in the COR of the face. In addition, we have also heard the stories of long drive competitors “wearing out the face” of their drivers. Since we felt this was an interesting question related to club head performance, we wanted to use this topic in an article in this month’s TWGT ETECHreport newsletter.

Metal fatigue is a weakened condition in which repeated stress causes the strength of a metal part to drop below the normal designed stress threshold of the part. Micro-fractures begin to form from a high level of repeated stress which upon further stress becomes larger and more populated until the yield strength drops well below the level of the stress being induced on the part.

It is highly unlikely a driver could ever be hit so many times that the titanium face could develop metal fatigue. The face is simply not flexed in and out far enough to create such a condition. In addition, few drivers are ever hit more than a few thousand times in their lifetime, with a high percentage of those impacts occurring off the center of the face. It is true that much higher club head speeds will push the impact stress higher on the face, but again, the face is simply not flexed in and out dimensionally to a point that fatigue could occur.

From previous Finite Element Analysis projects commissioned by TWGT, at a club head speed of 110mph, the face of a typical 0.830 COR titanium driver flexes inward approximately 1/16” for an on-center hit. For club head speeds below this level, the face flexes less and for impact speeds above 110mph, it flexes inward a little more. It is possible that the COR and resulting smash factor could drop from other factors that can change in a driver head over repeated use which are not related to any form of metal fatigue.

Chief among the factors which can drop the COR and ball speed of a driver from repeated hits to falsely give the impression that metal fatigue has set in is a permanent change of the horizontal bulge and vertical roll radii on the face. If the clubhead speed is high enough, if the face thickness is less than its designed specification, and if the head is subjected to enough impacts, the bulge and roll radii can begin to flatten after repeated hits.

One of the regular and routine tests performed on any new driver head design before production is approved is an air cannon durability test. When TWGT designs a new titanium driver, or any thin face, high COR clubhead model, final clubhead prototypes with the face thickness, bulge and roll verified to be on spec are subjected to air cannon durability tests. One of the tests typically involves firing 5,000 shots at 120mph at the center of the face. After each 500 shots, the bulge and roll radii are checked to determine if they have changed. If the face radii flatten by up to 1 inch after 5,000 hits, the head is considered to pass. However, if the face radii flatten by 1.5” or more, the head is deemed to have failed. If that happens, an analysis of the face is done to determine the cause of the failure from which adjustments of the face thickness or the heat treatment of the face material may need to be done before new prototypes are made and re tested.

Face flattening can happen with production driver heads made after all durability testing is performed if a club head happens to be on the minus side of the +/- tolerances for factors such as the face thickness or the bulge/roll radii, and such heads randomly end up being played by a golfer with a very high club head speed. The very best face thickness tolerance that can be had in the club head production industry is +/- 0.1mm (+/- 0.004”). Only the very best of the best of the club head foundries can manufacture club heads with a face thickness tolerance of that precision. Many foundries deliver a face thickness tolerance in the area of +/-0.2mm.

If a driver head is designed such that its threshold for bulge/roll flattening is 120mph, were a golfer with a 125mph club head speed happen to use a driver which comes out of production 0.1mm thinner than the face thickness design specification, within 200 hits the face radii on that head would start to flatten. Upon repeated center face hits it would continue to flatten out until very likely at around 400-500 hits, the head would no longer perform properly.

Driver and wood faces will continue to perform perfectly well when the face radius is only slightly flattened from repeated hitting. If the face radii change substantially, the face will no longer perform properly – the ball speed will drop, distance will drop and the feeling of impact becomes more “dead” – all factors which can make a golfer believe the fatigue has set into the face.

Excessive flattening of the bulge or roll is the number one reason long drive competitors experience a drop in the performance of a driver head. While some drivers built for these players with club head speeds as high as 150mph are made with a thicker face to begin with, some of the long drive players do not want the face thickness of their drivers increased too much because they know this drops the COR of the head and can rob them of getting the most distance possible in a competition. In those cases the competitors use a driver with a normal production face thickness and simply live with the fact their incredibly high impact speeds will most definitely flatten out the face and require replacing the head on a frequent basis.

The other way a titanium driver can give the golfer the impression the face has fatigued is when micro-fractures begin to form on the welding lines that secure the face to the body of the head which are not yet visible on the outside of the edges of the face to the eye. This can happen for several reasons – a welding defect or repeated high speed impacts near the welding lines on the face are the two predominant reasons for the beginning of a welding line failure which can affect the face performance enough to drop the COR and distance of the shot.

So the bottom line is metal fatigue in the face of a driver is highly unlikely, but most definitely changes in the bulge, roll and welding lines can happen occasionally from very high speed impacts which can reduce the performance of the driver.

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