503
GRAY-STUART ET AL .
FIGURE 5 Secondary creep rate from all trials, box length depicts the interquartile range and the median is the horizontal line in the box, whiskers represent the maximum and minimum ranges of the data, except the outliers (black circles) which exceed 1.5 times the interquartile range
TABLE 5
Comparison of secondary creep rate for box preparation
chain conditions where boxes contain product and do not have uni- form exposure to the ambient conditions. It is acknowledged that dynamic loading and vibration are two other phenomena which fur- ther impair box performance and are important in real world scenarios, 24 the stiffness of pallet deckboards also influence box com- pression strength. 31 However, these aspects have not been consid- ered here due to cost and complexity and because the primary aim of this work was to isolate humidity effects on box performance.
as function of trial conditions
Box preparation
Trial condition Secondary creep rate (day 1 )
9.36E 04 a
Control
Constant 90% RH
8.12E 04 a 2.29E 04 b 1.80E 05 b
Alternate
Cycling
Constant 70% RH Constant 90% RH
4
CONCLUSIONS
|
3.69E 03 a
Filled boxes
1.57E 03 b 9.72E 04 c 3.10E 05 d
Alternate
The secondary creep rate and lifetime of boxes containing product was significantly shorter than the control and foil-covered boxes. This was attributed to the internal pressure imparted on the panels by the product; this can result in out-of-plane displacement of the side panels manifesting much earlier leading to a higher creep rate and shorter lifetime. Conversely, boxes which had two panels covered with foil had lower moisture uptake and performed better than the control in cycling conditions. The resultant lower moisture content of the coated and uncoated panels of this box preparation means the box remains stronger for longer. Furthermore, the foil acts as a barrier so the panels are not subjected to as larger changes in moisture con- tent so the MC% will remain closer to the mean value. These results show that future studies should consider mimicking the actual surface available for moisture transport as it has a significant influence on the box lifetime. The lifetime of the filled boxes in the cycle-RH interval tests was shorter than those which experienced only cycling conditions, but those exposed to constant 90% RH had the shortest lifetime. The results presented here further support the findings
Cycling
Constant 70% RH Constant 90% RH
8.00E 04 a
Foil boxes
5.23E 04 a 1.31E 04 b 2.30E 05 b
Alternate
Cycling
Constant 70% RH
Note : Outliers identified using the interquartile range (IQR) criterion have been removed. Means that do not share a letter are significantly different.
lifetime and higher creep rate than the other two preparations. Fac- tors such as the internal pressure from the product 20 and the distribu- tion of moisture content of the box panels can significantly affect box performance. Including these enables the box performance to be quantified in conditions which are more representative of supply
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