Throughout the world of high-performance athletic equipment, there exists a puzzling dichotomy: some rackets, sticks, bats, paddles, etc. are prone to frequent breakage, while others are not. Badminton rackets, golf clubs and other flimsy “Impact Extensions” (what I will call the family of hitting instruments from now on) almost never break in every-day use while sturdier hockey sticks, baseball bats and other items break quite often. Numerous physical and dynamic circumstances create this dichotomy, so I want to provide a high-level look at why this difference manifests.
Let’s start with one of the easy ones: the non-malfunctioning golf club. Simply put, golf clubs should not break unless you use a driver for 100,000 clean swings, or you dig a club into the ground constantly and create internal failures. As a general rule, golf club heads will never break. They are made of lightweight metal that should never break from contact with a ball or soft ground. The shafts of irons (whether graphite, carbon fiber, or metal) should rarely break as well. The majority of flexion that a club shaft experiences actually happens before contact with the ball and is proportional to the length of the shaft and weight of the club head. Irons and shorter clubs are sturdy in these regards— when the club contacts the ball, it presents a mechanically sturdy form (unless you swing as hard as Happy Gilmore).
Although the static golf ball does transfer a decently sharp impact on the shaft of the club, the amount of flexion that heavier-headed clubs (drivers, woods) undergo before and after ball-contact would contribute more to any breakage than impact forces. However, carbon fiber compositions and geometries of the shafts are strong enough to handle this non-contact flexion (see hockey sticks for the polar opposite of this). Since the impact with the ball or any other surface does not add additional moment while the shaft bows, clubs do not easily break. To see what I am talking about with pre- and post-impact shaft flexion, check out Tiger Woods’ drive in slow-motion:
Similar to golf clubs, rackets used in badminton, squash, tennis and other racket games rarely break, but for different reasons. While modern rackets do whip less than golf clubs, their saving grace is the energy dissipation provided by the strings of their racket face. For every respective racket impacting a birdie, tennis ball or squash ball, the strings of that racket do an incredible job dissipating the energy across a wide surface area. Pretty much any structural damage occurring to the actual carbon fiber structure of the racket would be due to an angry throw or slam into the ground. If anything in a “stringed” racket fails, it will 99.9% of the time be the strings during normal use, but this is also very rare as they normally come loose before snapping. (Fun fact: Vibrational nodes determine the “sweet-spot” of stringed rackets!)
In stark contrast to the flimsy-yet-unbreaking golf and badminton impact extensions, sturdy hockey sticks break almost daily… but why? Well incredibly forceful contact with the ground, or ice in this case, contributes to most stick breakages. Disregarding the relatively large mass of the puck, the amount of vertical force that a hockey player applies to the ice in order to bend their stick and create a whipping motion that propels the puck forward (seen also in golf clubs and also rowing oars) is daunting. I highly recommend you check out this video on the subject of slapshot physics.
To compensate for the ridiculous forces and deformations that a hockey stick experiences, manufacturers have made their sticks… well flimsier I guess. Certainly, players prefer the ultra-light composite constructions available today, but their endurance (especially for money-tight players) can be disappointing. Though the graphite-carbon fiber (sometimes Titanium!) blends are certainly stronger than old wooden sticks, it astonishes me that no major changes to hockey stick geometries or materials have been attempted commercially. While Reebok did make an attempt during my youth, the addition of strain-rate sensitive materials or more advantageous internal geometries would easily make up for weight with extended durability, tunability and performance.
Even more so than hockey stick failure, the subject of baseball-bat breakage can be a scary one. Oftentimes, baseball bats break in shocking ways. They splinter spectacularly into a wooden shower, or break in half and go flying into the field or stands. Interestingly, the failure mechanisms leading to either type of failure are fairly unique to the types of wood used to make the bats. However, the reason that any bat can break at all is mainly due in part to two common factors: the high momentum of the baseball when the bat meets it, and the slow growth of imperceptible cracks within the bat.
In relation to golf balls or birdies, baseballs are very heavy (about 4x or 50x heavier respectively). This, combined with the fact that both golf balls and birdies have zero or very low velocity upon contact, demonstrates the potency of any impact between a 100 mph fastball and a piece of wood. Additionally, many imperceptible, often internal, cracks can appear within a baseball bat that contribute a catastrophic failure down the line. These cracks can grow along the grain of a bat, out of sight, preparing for an explosion upon impact with a ball. Failure due to these internal cracks is also much more guaranteed when they form within or near the handle of the bat— the weakest and most breakable point on any bat.
As I mentioned though, the wild differences between the different failure mechanisms in bats are largely in part due to the type of wood that compose their form. In the old days, weighty, sturdy Hickory bats were popular, and in general, heavier wooden bats are more durable. More recently, there has been a transition from heavy Hickory to lighter Ash and Maple. Ironically, these similarly-weighted materials have very different fast failure patterns. Differences in both the grains of the woods and the channels that transport water through the living trees create wildly different fracture patterns. While Ash bats explode into smaller shard, Maple bats typically break into two large pieces, which can become dangerous projectiles and often fly into the field or stands.
I hope that I have inspired you to take a deeper look into the incredible mechanics that describe the failure mechanisms of impact extensions. While the dynamic movements of badminton rackets and golf clubs may never produce a spectacular failure, expect to see dozens of broken bats and hockey sticks in your lifetime. The in-depth mechanics grounding this brief overview are pretty incredible and worth a look. And of course, we must never forget that even tennis rackets are prone to structural failure in the right hands (strong language in video):