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Racquets going soft

Q: Do racquets go soft with age, and if so, how do you measure it?

A: All racquets do “go soft” with age, partially due to temperature cycling and partially due to impact stress. With wood racquets, the typical failure when subjected to temperature extremes is either to warp (in the case of heat) or crack/crush (in the case of cold). The typical failure from impact is breakage.

According to materials engineer Tom Kosinski, the carbon fiber and graphite in modern racquets are subject to the same stresses as wood frames, but they usually fail differently. When heated, the filler material (resin in most cases) will tend to expand slightly, and will become more pliable. The overall strength and modulus of the carbon fiber are such that you do not really notice the expansion, and the greater the ratio of pure fiber to resin, the less expansion there will be.

Given enough temperature extremes and enough temperature cycles over time, however, even the best modern materials will begin to develop microscopic cracks. It is common in composites for temperature cycling to cause the materials to fail on a microscopic level, developing small cracks in the material matrix. Over time, these cracks grow and cluster, and the material starts to “soften.” This is the mechanism that people experience when their carbon-fiber frames have seen a lot of use, and the frame doesn’t seem to have the punch anymore. The frame will not look any different, but the material has lost the characteristics it had when new.

In the cold, especially with frames strung at very high tensions, the frames tend to crack and snap. While the materials are best under compression, microscopic cracking will still occur, and the colder it gets the more brittle the material becomes. Typical failures in extreme cold include the development of microscopic cracks and the enlargement of other defects in the material matrix. Every frame has some defects in it after the molding process. Most defects are small, or not in an area where they can be a problem. In cold temperatures the material will grow those defects quicker and tend to fail more catastrophically. Most frames that have developed problems due to cold will tend to crack on the inside of the frame hoop, because the material in this area is under tension, and carbon and ceramics are not as strong under tension.

Modern composite racquets are much more resilient to impact-related damage than wooden racquets, but repeated impacts over time do promote the propagation of microscopic cracks, just as thermal cycling does. Also, the stress of re-stringing can speed the breakdown of the composite matrix.

Even so, softening happens very slowly over time. Most players won’t be able to feel it happening because they will adjust to the softening as it happens. The only way to measure the amount of “softening” is to use diagnostic equipment such as the Babolat Racquet Diagnostic Center (RDC) or RA Test Machine. Most players cannot afford devices such as these, so those who want to track this softening will need to find a local shop or club that has one. In order to measure the change in flex of a racquet, you have measure the flex when the racquet is new, using a calibrated device, so you can continue to measure the flex over time and compare current readings to the initial readings.

From the January 2007 issue
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