Issue link: https://cp.revolio.com/i/586959
Such an approach serves to fine tune the applied safety factor "F" based on knowledge of a how a box will be unitized, and expected variables of time, temperature, and humidity. The end result should be a BCT specification is better "tuned" to a box and its intended supply chain, and to that end, such specifications are largely effective. However, the Fibre Box Handbook method admittedly does not address the effects of Cyclic Humidity, except to acknowledge that "cyclical RH variation further increases compressive loss." (Fibre Box Association; 2005) It also does not address interactions of the variables, nor does it account for differences in how individual boxes (due to design, component paper, and/or production variation) respond to the variables. So, while useful in setting BCT specifications which are largely effective, this approach does not assure that a BCT specification will provide the desired long term compression performance. This discussion of safety factors is not intended to slam, or belittle, usage of safety factors, nor various approaches to setting them. Indeed, usage of safety factors in setting BCT specifications is a practical approach to dealing with an incredibly complex set of variables which determine box compression strength, which are then dynamically affected by environmental factors during shipping, storage, and handling. At least 17 elastic constants (not including corrugated board and box production variables) have been identified for corrugated fiberboard boxes – and that does not even include inelastic constants! A unified mathematical model utilizing all known constants is a daunting – and as yet uncompleted – task. Approximations of a BCT mathematical model have nonetheless been developed, the most well-known being the classic McKee equations (which provide BCT prediction error of 11%), although an important recent update has been proposed by Urbanik and Frank in 2005 (which reduces prediction error to 8.5%). To truly appreciate the difficulties involved in mathematically predicting BCT, the Urbanik/Frank paper on this topic is highly recommended. (Urbanik & Frank; 2005) When faced with the difficulty of adequately mathematically modeling BCT at a point in time, it is not surprising that no mathematical model has yet been developed which robustly models reduction in BCT due to multiple environmental factors, although single variable approximations have been reported. For example, Burgess, Singh, and Srinangyam provide a method for using a short term creep test to predict collapse time for boxes under constant load, in long term storage conditions (Burgess, Singh, Srinangyam; 2005). Koning and Stern showed that under static temperature and humidity conditions, a short term dead load test could estimate long term performance based on secondary creep and load duration (Koning, Stern; 1977). In both of these cases, however, life estimates were made with static assumptions of load, temperature, and humidity. So, based on current knowledge, we cannot calculate the expected life of a box, except perhaps for those cases where we are able to very carefully control environmental conditions in the supply chain. Having said that – we can still understand the involved physical mechanisms, use this knowledge to propose ways to slow the rate of BCT loss, and use accelerated testing (cyclic humidity) to prove the proposals. Why Compression Strength Degrades Compression strength of a corrugated box is the result of the properties of the component papers, and the geometry of corrugated board, which combines the long known load bearing strength of the arch (or triangle) and the column. In simplistic terms, boxes fail in compression due to two factors – material and geometry failures. Vertical panels bulge, and linerboards experience localized buckling, until a crease finally forms which indicates a breakdown in the board geometry (Kellicut, Landt; 1951). Why do component papers lose their strength, especially compression? In most simplistic terms, moisture absorption causes hygroexpansion, which induces strain, both within component papers, and within the corrugated geometric structure. Hygroexpansion strain correlates to creep, and box life is inversely proportional to creep rate (Leake, Wojcik; 1993). 19 ista views • October 2015 • www.ista.org > MORE ON PAGE 20 Figure 1: Typical Compression Failure