The women’s semi-final singles match of the Selkirk Red Rock Open in St. George, Utah in April 2023, featured Lea Jansen verses Salome Devidze. During this match, Devidze hit several groundstrokes for winners with an exceptional amount of velocity and topspin. Based on the lower frequency “thud” of Devidze’s paddle and the amount of velocity that she was able to put on the ball, Jansen complained to the referees that Devidze’s paddle was delaminated and illegal. Several days later, deflection testing of the Devidze paddle was performed by the PPA, and it was determined that her paddle WAS NOT delaminated and therefore legal. Readers of Pickleball Science have suggested that to address the delamination issue, pickleball paddle manufacturers are applying more robust adhesives to bond the face sheets to the honeycomb cores. Others have suggested that the delamination is due to crushed cores, and that the solution to this problem is to increase the stiffness of the materials that make up the cores. Who is right?
Key Questions
This entire series of events has uncovered several apparent contradictions and raises more questions than answers:
- Jansen claimed that she knew that Devidze’s paddle was delaminated by the sound of the paddle. Other videobloggers have confirmed that a delaminated thermoformed paddle will emit a deeper “thud” verses a higher pitched “ping”. Was Jansen right to complain about Devidze’s paddle based on its sound? Is the sound of the paddle an accurate indicator of delamination? What causes the lower frequency “thud” anyway?
- Up until thermoformed paddles came on the market, it has been accepted as “common knowledge” that delaminated conventional paddles have “dead spots” that result in softer returns. This is contradictory. Will delaminated paddles increase or decrease the amount of “pop” in the paddle? Why would thermoformed paddles behave any differently than conventional paddles?
- The Devidze paddle was tested for deflection by the PPA who found that the paddle WAS NOT delaminated. This is contradictory. Several individuals have verified that a delaminated paddle will have a characteristically deeper “thud”. If it was not delaminated, what caused the deeper “thud” of her paddle? Why were her shots so fast? Can the PPA deflection test be used to reliably detect delamination?
- Are pickleball paddle manufacturers correct in their assessment that the paddle face sheets are delaminating from the cores? Will adding more robust glue between the face sheets and core solve the delamination problem?
- If the delamination and extra pace was caused by the cores of the paddle being crushed, why wouldn’t the PPA deflection test be capable of measuring this apparent significant loss of stiffness? Can we rely on this test to provide us with an accurate and dependable indicator of delamination?
Let’s see if the science can provide us with answers to these questions.
Conventional vs Thermoformed Paddles
As we discussed in our previous article of this series, ”Thermoformed Paddle Delamination”, the folded carbon fiber edge of a thermoformed paddle increases the bending stiffness of the edge creating a “frame” that is similar to a tennis racquet. This “frame” is an important characteristic that differentiates thermoformed paddles from conventional paddles. We will further analyze why in a future article.
A cross sectional view of a thermoformed paddle is shown in Figure 1a. When the paddle strikes the ball (Figure 1b), most of the reaction force from the handle goes through the edges since they are much stiffer than the interior of the paddle face. The interior of the paddle face deforms and acts like a trampoline, which slingshots the ball back towards your opponent with higher velocity.
The foam around the edge of the honeycomb core does not add structural rigidity to the edge of the paddle, nor does it provide a load path from the stiff outer edge to the inside striking area of the paddle face. Consequently, the primary load path from the edges to the center of the paddle must be through the face sheets. This causes very high bending stresses in the face sheets in the vicinity of the edge. Furthermore, since the face sheets are folded over the edge of the paddle, they must also stretch like the springs around the periphery of a trampoline membrane (Figure 2).
Eventually, the high stresses cause cracks that weaken the face sheets near the edge, making the edge more flexible; however, the striking surface at the center of the paddle is still intact and stiff. This makes the rigid center of the paddle act like a loudspeaker cone that is suspended to the rigid frame by an elastomeric edge. This piston effect is the mechanism by which a delaminated thermoformed paddle has a lower frequency “thud” than a conventional paddle. This also explains why the trampoline effect is enhanced by delamination at the edge of a thermoformed paddle. Eventually, the cracks coalesce and propagate towards the center of the paddle until the center becomes too soft to be usable.
A cross-sectional view through the width of a conventional paddle is shown in Figure 3a. When the paddle strikes the ball (Figure 3b), the reaction force from the handle is more or less uniformly distributed, since the stiffness across the width of the paddle is fairly uniform. Here, we assume that the glued-on plastic edge guard has a very low stiffness and just goes along for the ride. As a result, the deformation across the width of the paddle is also fairly uniform.
The failure mode of a conventional paddle will therefore be different from that of a thermoformed paddle because there is no outer “frame” that transfers the reaction force from the handle to the ball. Bending stresses at the edge of a conventional paddle face will remain relatively low, and since the face sheets are not constrained by the folded edges, the membrane (stretching) stresses are also low. When delamination occurs, it will occur at the striking surface near the center of the paddle. Since delamination significantly reduces the stiffness of the face sheet and increases damping, the paddle will feel “dead” when striking the ball.
Delamination Testing
The PPA uses a deflection test to determine if a paddle is delaminated. This test involves supporting the paddle on 5” long blocks that are separated by a distance of 5.5” from the interior surfaces of the paddle. A standard weight is then applied to a 1” diameter disk placed at the center of the paddle and the deflection is measured with a dial indicator (Figure 4). Paddles that pass the deflection test will deflect no more than 0.005” under a weight of 6.6 lbs (3 kg), and 0.010” under a weight of 11.0 lbs (5 kg).
The fundamental problem with the PPA deflection test is that it cannot detect delamination of a thermoformed paddle when the delamination occurs near the edge of the paddle. The load that is applied to the center of the paddle will by-pass the delaminated edge and can only deform the center of the paddle before passing into the support blocks (Figure 5a). Such a test would be valid for detecting delamination of a conventional paddle where the delamination occurs in the center of the paddle.
A better test that would detect edge delamination of a thermoformed paddle will support the edges of the paddle to force the load path to pass through the delaminated portion of the paddle (Figure 5b). To avoid over-constraining the paddle and artificially adding stiffness to it, the supports must have spherical ends. The spherical end supports could be arranged in a three-point mounting system as shown in Figure 6. Such a test would be capable of detecting delamination at the center of the paddle as well as at the edge of the paddle and would clearly indicate loss of stiffness due to delamination.
Manufacturer's Solutions
Manufacturers of thermoformed pickleball paddles have claimed that they have “solved” the delamination problem by use of more robust adhesives between the face sheets and the honeycomb core. While this may prolong the life of the paddles, we believe that this fix may only be temporary. The fundamental problem is that there is no load path between the stiff edge of the paddle and the honeycomb core except through the face sheets. There are various methods that might be employed to address this problem, which we may explore in a future article.
Delaminated paddles have also been taken apart, where it was discovered that the cores have been crushed. In response, manufacturers have suggested that they will be using stiffer cores. We believe that as you play with a paddle that has delaminated from the edge, the cracks progress towards the middle of the paddle and cause excessive bending of the core. The core is really meant to take shear stress and not bending stress. The excessive bending of the core can cause cracks to appear in the core that propagate through the thickness of the core. The crushed cores may therefore be artifacts caused by delamination at the edges of the paddle in the first place.
Who's Right?
In our opinion, Lea Jansen was correct in complaining to the referees about Salome Devidze’s paddle based on the sound emitted from her paddle and the excessive speed and topspin she could generate.
Was Devidze’s paddle delaminated? Probably. In Devidze’s defense, cumulative damage at the edge of the paddle will come about gradually and cannot be detected if the player looks for soft spots at the center of the paddle.
Was the PPA correct in concluding that Devidze’s paddle was not delaminated? Probably not. The PPA deflection test can only detect center delamination and cannot detect edge delamination. In PPA’s defense, they are accustomed to looking for delamination of conventional paddles where the damage occurs in the center striking area. Additional work (such as the three-point mount suggested above) is needed to improve the deflection test for thermoformed paddles.
Are manufacturers correct in attempting to solve the edge delamination problem by using more robust adhesives and/or by stiffening the cores? Maybe. Interfacial delamination and core damage at the center of the paddle might be secondary effects caused by delamination at the edges of the paddle. Additional work is needed to determine a better way to transfer force from the stiff outer frame to the relatively soft center of the paddle.
