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GRAY-STUART ET AL .
faster rate. The overall strength and response to loading of these panels could be altered by this imbalance in moisture content between the inner and outer liners and result in the panels bowing inwards. This inward deflection would not occur under supply chain condi- tions as the product in the box would provide support and prevent this from happening. This is shown in Figure 4C,D which shows the uncovered and foil covered panels of the same box which failed in trial (B). The mean box lifetime for the filled boxes was around half that of the control boxes in trials (B) and (C). The boxes with foil coated panels had the longest lifetime and lowest creep rate across all trials. The mean and median box lifetimes were similar with the largest dif- ference being 2.4 days for the control boxes in trial B. If the number of box failures is low the box lifetime in is not a useful metric. However, the box failure rate provides a good indica- tion of relative box performance across different preparations and test conditions. The failure rate of boxes which experienced 12-h cycling was lower than those at constant 90% RH (Table 3), this finding is in line with Hussain et al. 3 but in contrast to several previous studies It is noteworthy that no foil boxes under cycling conditions failed while a significant number of foil boxes failed when exposed to constant 90% RH and the cycle/interval conditions. These results support the notion that exposure to constant high RH% can be worse for box perfor- mance than cycling conditions with a load greater than 20% BCT. Furthermore, this shows that if one wants to mimic the conditions a box will experience in the supply chain, periods of constant high RH should not be omitted and may and increase creep rate more so than continuous cycling. Further research in this area will undoubtedly be of some benefit to the packaging industry.
flaps. If there is a product occupying most of the internal space, the rate of mass transfer from the inner liner to the air space and box flaps will be reduced. This will result in the inner liner of the panels having a higher moisture content as there is less moisture transfer to the air space and box flaps than in an empty box. As can be seen Table 2, some preparations affect the moisture content under some test conditions. The moisture content and its var- iation in fibreboard during cycling for different box preparations is something which has not previously been investigated. The response of moisture content in relation to changes in RH is further compli- cated by the adsorption and desorption phenomena which is also worth pursuing in future studies. Given the correlation between box performance and moisture content such information could help understand vulnerability of boxes to creep.
3.3
Box failure and lifetime
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Boxes were examined at the end of the trials to assess the failure they experienced. Failure was observed in all trials and preparations except for constant RH 70% and the foil covered boxes with the 12-h cycling time where there were no failures; 100% failure was observed for the control and filled boxes in trial B. The type of fail- ure exhibited by the control and filled boxes was as previously described 26,34 where buckling occurs in the corners which results in hinge formation leading to out-of-plane displacement on the larger side panels (Figure 4A,B). An interesting observation was that boxes with the foil coating behaved differently to the control and filled boxes where the panels with foil buckled inwards. Having foil on the outer liner likely results in the inner liner gaining moisture at a
FIGURE 4 Manifestations of box failure as a result of creep deformation observed in this study: (A) and (B) show all panels of a control box in which it can be seen that creases and hinges form at the corners of the box which leads to buckling and global failure; (C) and (D) show the panels of a foil box, where similar creases and hinge formation is observed while the foil coated panels moved inward, this was observed for all foil boxes which failed
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