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FIGURE 1 Box displacement over time for a box subjected to constant compression in cyclic humidity conditions. The vertical dashed lines separate the primary, secondary, and tertiary creep regions, respectively
would yield different conclusions regarding secondary creep rate and box lifetime than creep tests performed on empty boxes. Cyclic RH creep tests can provide useful information about the performance of boxes for manufacturers. 14,21,22 In practice, the RH rarely exhibits uniform cycles especially in refrigerated conditions. 23 – 25 As boxed products move through a supply chain, they can experience periods of relatively constant RH interspersed with periods of variable RH. Thus, a secondary aim of this work was to explore how cycle-interval tests consisting of periods of constant RH in between controlled RH cycles would influence the box lifetime and secondary creep rate. This approach could allow manufacturers to simulate in a controlled testing facility the RH profiles experienced by their boxes in a supply chain and potentially economise this testing by focussing on the most harmful conditions experienced by the boxes.
box panels begin to bulge and the load is transferred to the corners of the box. 3 – 6 The cyclical nature of the displacement during secondary creep in Figure 1 is a result of hygroexpansion as the paper is the swelling and shrinking as the RH changes. 5 In the tertiary region, box failure is initiated as a result of local buckling near the corners which leads to hinge formation and ultimately catastrophic failure of the box. 3,7,8 It has been previously reported that the box compression strength is affected by humidity 2,9 – 11 ; likewise, failure rate due to creep is usually faster at higher relative humidities and previous research has reported higher secondary creep rates, shown to be closely related to shorter lifetimes 6,12 – 16 under cycling RH conditions compared with constant RH. 6,17,18 However, findings by Hussain et al. 3 challenge this notion that box performance is worse when RH is cycling as opposed to constant. They measured creep rates for a single type of box across a range of different cycling times under dif- ferent constant vertical loads. They found that boxes subject to a 20% BCT or higher at constant 90% RH failed earlier than boxes at these loads exposed to cycling conditions between 50% and 90% RH. This finding was attributed to creep time constants logarithmically shifting to shorter times as a result of the high applied load. Shorter creep time constants mean larger creep rates. High loads and high moisture contents produced large enough creep rates to quickly dissipate stress gradients leading to more creep than cyclic conditions. Compressive creep tests are conventionally done on single empty boxes, 3,18 and only recently, some researchers have put plastic balls inside the box to prevent them from inward buckling. 19 In reality, boxes are often palletised and contain product which may impart out- of-plane loading on the box panels thus promoting bucking of panels. 20 This can lower the top-to-bottom compression strength 20 and may accelerate failure due to creep. 21 Furthermore, during trans- port and storage, boxes are usually packed tightly on a pallet and only have one or two external side panels exposed to the ambient atmo- sphere. This is in contrast to having all four panels exposed in a tradi- tional creep test. The primary aim of this work was to see whether performing the compressive creep tests with boxes under conditions more aligned with those experienced in the refrigerated supply chain
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| MATERIALS AND METHODS
2.1
Materials
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For this study, single walled C-flute regular slotted containers were used. The inner and outer liners were 200 and 250 gsm Kraft liner- board made from New Zealand grown radiata pine with a 160 gsm semichem medium. The boxes were manufactured in New Zealand and obtained as flat packs with the manufacturer's flap preglued. They were stored in a controlled environment at 50% RH and 23 C. The outer dimensions of the assembled boxes were 385 248 295mm (length width height).
2.2
Box compression tests
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Box compression tests (BCTs) were conducted to determine the applied load for the creep tests. BCTs were conducted in accordance to Australian and New Zealand standard (AS/NZS 1301.800s:2006). A Wiedemann universal tester was used for this testing. Boxes were compressed at a crosshead speed of 10 mm/min until failure. The
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