The performance of a fender is defined by its energy absorption capacity, as well as the corresponding reaction force generated during compression. The energy absorption capacity of a fender is calculated by the area under the force-displacement curve generated during the compression test.
Fender efficiency is defined by the ratio of energy (E) and reaction force (R) and expressed by the term E/R.
If two fenders have the same energy absorption capacity, the one with the higher E/R ratio is more efficient because it absorbs the same amount of energy as the other fender, but generates a lower reaction force. An efficient fender needs lighter accessories, so fenders with higher E/R ratios are always preferred from a commercial point of view.
Trelleborg undertook a series of compression tests on foam fenders using low, standard, high and super high density foam core. The fender size selected for the testing was 1 meter diameter x 1.5 meter length.
All fenders were compressed to 70% of their diameter at a constant velocity (CV) speed of two to eight millimeters per second. The performance of each fender was studied at zero hour (1st compression) and then after one hour, 24 hours, 36 hours, 48 hours and seven days of rest period. All the tests were conducted at 23 + -5°C.
The tests showed that the first compression is not an accurate measure of true fender performance. Instead, performance after 24 hours is a more reliable reflection.
While studying closed-cell foam and its reaction under compressive load, many researchers have observed that the rate of recovery is independent to the density of the foam. Foam creeps under static loading. Therefore, the recovery after impact compression becomes a prolonged process.
Recovery occurs through the viscoelastic straightening of the cell’s buckled faces, but it is incomplete due to some plastic deformation in the structure. However, if the foam is allowed to fully recover in storage for a sufficient time, the level of stress curve will recover to that of the second compression.
The first compression of foam material provides a very high yielding load which doesn’t return to its initial compression value. This implies that some potential damage occurs during the first compression even when converted to a fender.
Trelleborg observed that a foam fender resembles the behavior of the foam material it is produced from. For a foam fender the first compression value is high, erratic and non-sustainable. Therefore, foam fenders need more than three compressions and to rest for more than 24 hours to be stabilized and produce uniform performance data. It was also observed that subsequent and frequent compressions reduce the performance of a fender. However, with sufficient resting time, fender performance improves and returns to the level of the second compression.