Figure 6a illustrates the effi ciency–stress plots of three kinds of honeycomb paperboards (t/l =
0.0329, 0.0263 and 0.0219) under six levels of RH. Since the effi ciency curves in the range 40% RH to 75% RH were almost superposed for each kind of honeycomb paperboard, an average curve was developed for each honeycomb paperboard. Consequently, only nine curves are shown in Figure 6a to represent the Effi ciency of honeycomb paperboards of three t/l ratios under three levels of RH (i.e. lower than 75% RH, 85% RH and 95% RH).
It can be seen from Figure 6a that the effi ciency curve goes through a maximum but for each hon-eycomb paperboard this maximum is attained at a different stress. From Equation 4, it becomes evident that the product of E and σp is equal to the energy per unit volume that is absorbed by this honeycomb when compressed to the strain εp. Therefore, iso-energetic contours corresponding to maximum effi -
ciency are superimposed on the effi ciency-stress plots as shown in Figure 6a. From these contours it becomes evident that each honeycomb paperboard can absorb a different amount of energy at its maximum effi ciency. The contours shift to the left with decreasing t/l value and increasing RH.
Ideality, as introduced by Miltz,16 was aimed to refl ect how close a real material to an ideal one
(one that transmits a constant force to the product when compressed throughout its thickness), and ideality is less than one theoretically. However, because the stress-strain curve in plateau stage of honeycomb paperboard is approximate to rectangle while a ‘steep peak’ exits at the intersection of elastic stage and yielding stage, the area of the shaded part in Figure 5 is greater than the product of σp and εp in plateau stage; therefore, the ideality will exceed one. As can be seen from Figure 6b, the ideality of three honeycomb paperboard under various humidity environments fl uctuates from 1.2 to 1.4. The ideality of honeycomb paperboard does not easily refl ect the infl uence of the initial peak stress on the cushioning and energy-absorbing properties.
Energy absorption per unit volume (W) is the best measure of the energy absorption capacity of honeycomb paperboard, which is defi ned as the area under the stress-strain curve at the onset of densifi cation strain, that is