At 11:43 PM 7/26/03 -0700, Alan Brooks wrote:In principle, I agree with Dave T. A plug of epoxy on the inside of the shaft will cause a stress concentration. That said, however, I find it unlikely that effect is significant. The modulus of elasticity of the epoxy is so much lower than that of the steel hosel and the graphite reinforced composite shaft that it's effect is going to be negligible.
Offhand, I'd have thought so too. An article in Golfsmith "Clubmaker" a few years ago claimed otherwise. They said that their observations of broken shafts suggested that an epoxy plug at or above the tip (1) increased the chance of breakage substantially, and (2) when breakage occurred, it was always at the top of the plug.
Interesting. I'd be interested to know what they mean by "substantial". Clearly, if you take a design and load it close to failure then add a stress concentration it's going to break, and at the stress concentration. It seems unlikely to me that if I fill the tip of my Graphalloy shaft with epoxy to just above the hosel I'm suddenly going to break it. If I swung at 150-mph, then maybe. I think that I would have to work to get enough excess epoxy in the joint that it would fill the inside of the shaft to above the hosel. If I have a shaft and club head that has a, say, 1% failure probability at 150-mph and I plug the shaft tip with epoxy does the failure probability go up to 2%? 10%? 50%? I think probably more on the order of 2%. The other way of looking at it is that the 1% failure probability for the plugged tip goes down to 145-mph. For the average golfer with swing speeds below 100-mph I think this is a non-issure.
As I said in my other post, I think this breakage is either due to the shaft tip reinforcing never being designed for this club head speed, or something in the assembly process that is reducing the amount of reinforcing above the hosel. And, again, if the shaft is breaking off straight across (as opposed to a spiral fracture)
Interesting point about the spiral fracture. If y'all don't know what Alan is talking about, try this experiment. Take two sticks of round chalk:
* With one stick, bend it till it breaks. It will be a jagged but mostly straight-across break.
* With the other stick, TWIST it till it breaks. Twist it about its own axis. The break line will spiral around the stick at a 45-degree angle.
Anyway, if it were a metal shaft (or other anisotropic material) and if the break were not right at the hosel, I'd certainly agree. I'm not as sure in this case because:
* The material definitely has strength-aligned directions. This might affect the spiral. But more important...
* There is a definite discontinuity at the top of the hosel. A long, well-graded cone filled with epoxy will minimize its effect. But if the coning is too short or filled with air, there will be a decided stress concentration there. I could be convinced that a torsional stress combined with a concentration point at the top of the hosel could result in a straight-across break rather than a spiral.
Shaft failure due to torsional loading is a shear failure, and it occurs in the maximum shear plan. For a homogenous isotropic material in simple torsion this is at 45-degrees because that is the direction of the principle shear stress. For a non-isotropic material, such as a composite golf shaft, the direction of principle shear is different but the failure will still be in that plane and you will see a spiral failure plane.
A golf shaft is loaded in anything but simple torsion. There is also bending due to centrifugal force on the club head and the reaction of the club head to impact with the ball (which can also produce a torsional load), and tensile loads due to the centrifugal forces. Any of these can initiate a failure in one mode and it can then propagate in another mode. A straight across break indicates that the primary failure is bending or tensile, although it could initiate from torsional stresses. It would sure be fun to get ahold of one of the main line shaft designers and see what they have learned over the years.
That doesn't mean that that the impact stress that causes the break is torsional. It certainly could be the clubhead bending backward due to the impact. But it could also be torsional, as originally suggested.
it is a consequence of bending or tension in the shaft and if it is consistently breaking just above the hosel, it's most likely bending from the centrifugal toe down forces that is initiating the failure, with tension finally separating the head and shaft tip from the rest of the shaft.
I have a lot of trouble with this. The reason is that the forces due to impact with the ball are much greater than the forces due to the swing alone (toe down due to centrifugal force. I've never EVER heard of a shaft that broke at the hosel during a practice swing that didn't hit anything. It always involved contact, either with a ball, the ground, or something else (a tree?). If it were the centrifugal toe-down forces, then a practice swing is as likely as a real swing to cause the problem.
Good point, although my practice swings never seem to get above about 85-mph and I get around 100-mph with a ball. Go figure. How about a combination of the toe down bending and the bending associated with an off-center hit? I still maintain that if the break is straight across it is a tensile/bending failure, not a torsional one (primarily)
Cheers! DaveT
Regards,
Alan
