ISTA Unit Load Tests

stress engineering services Virtual Simulation of ISTA Unit Load Tests ADVANCED FINITE ELEMENT ANALYSIS (FEA) TECHNIQUES ARE MAKING IT POSSIBLE TO PERFORM “VIRTUAL” COMPUTER SIMULATIONS OF STANDARD INDUSTRY UNIT LOAD TESTS. These methods allow transportation and warehouse stacking packaging materials and are happy to offer their expertise. performance of unitized loads to be studied earlier in the In situations where multiple design choices are available, it is package development process, before physical samples are often not difficult or expensive to prepare samples and run available, with commensurate reductions in risk, cost, and tests to measure the actual performance in, for example, a box speed to market. compression or drop test. More Information Means Better Decisions Package design decisions associated with unit load perform - The development of a successful packaging system requires ance have not always been so easy to assess as those for indi - information about the fragility of the product being shipped vidual cartons or boxes. Unit load tests require larger and knowledge about the hazards of the distribution environ - quantities of product and packaging to conduct and can be ment. This information must then be used to make informed more difficult to perform. Products may be packaged in unit decisions regarding cushioning materials, primary boxes or loads for extended periods of time and travel through most of packaging, and shipping boxes or cases. For high-volume the distribution system in that form. The International Safe products that are typically transported in larger quantities, Transit Association (ISTA), ASTM, ISO and other organizations unit load design adds an extra layer of packaging that must have developed tests for evaluating various attributes of unit be considered. load performance but it is still necessary to have one or more unit loads available to conduct these tests. The same is true of Each new package design project starts with considerable the “stack-and-ship” tests that are sometimes used to judge assistance in the form of an extensive body of packaging how a product will perform in the distribution environment. industry knowledge and experience to draw upon. Many sup - pliers are available who have developed a range of excellent In our packaging work at Stress Engineering Services we find that the cost and time required to evaluate changes to unit load design pres - ent an obstacle to making smart changes. Everyone wants to reduce material usage, or use different materials, to save costs. The desire to figure 1 : Finite element make more sustainable packaging choices has added a further moti - model of stacked unit loads of corrugated cases vation to continuously reevaluate packaging systems and implement changes. When considering a potential unit load design change, however, the cost and time to conduct a thorough evaluation can be daunting. All too often the result is: 1. Change is not made at all Compressive loads can occur while a unit load is being transported if 2. Change is made without enough understanding products are stacked atop one another in a trailer, rail car or inter - modal container. When this is true there can be a combination of The first outcome obviously results in never realizing the potential compression and vibratory loads present. benefits. The second runs the risk of being detrimental to the organi - zation if the new design proves to be unsuitable. Damage claims can Figure 1 shows the results of a finite element model simulating a increase and packaging costs may actually go up to implement a fix stack of two unit loads consisting of corrugated boxes. The left image if the design doesn’t work as intended. shows buckling in the box sidewalls. The color code in the right Physics-Based Approach image shows the stress in the corrugated panels with blue being low stress and red indicating high stress. To try to be more predictive about the effects of changes in unit load design, and mitigate the associated risks, we have found value in An advantage of techniques like finite element analysis is the ability applying a physics-based approach to understanding packaging per - to expose inner features of the packaging system which are not usu - formance. This has involved the use of tools ranging from hand cal - ally visible and examine their behavior. In Figure 2 the contact pres - culations based on the theories of classical mechanics to sure between product layers is shown which highlights the more computer-based simulation techniques such as finite element analy - highly-stressed load path through the case sidewalls and corners. sis. This paper discusses recent work with finite element analysis to simulate ISTA unit load tests. Most transport packaging hazards fall in the general categories of • Compression • Vibration • Impact • Shock Common industry test standards for packages reflect these hazards in the nature of the tests they prescribe. Impact, in the sense of being struck by another sliding or falling object, is not so common a hazard for unit loads as it is for small parcels or other packages that figure 2 : Detail of stress in corrugated boxes from stacking of unit loads are individually handled or sorted. The typical unit load environment is more complex than might be suggested by the simple descriptions of “compression, shock, and vibration” and this is reflected in the A further consideration for unit load compression is the time- ISTA tests for unit loads. dependence of material properties that may be significant during warehouse storage. This type of creep behavior can be an issue with Compression corrugated fiberboard boxes as well as plastic bottles and other In the most general sense, compression testing involves determining types of packaging. Over time, the progressive crushing or collapse the force required to crush a package or unit load. Alternately, unit of packages can result in warehouse unit load stacks shifting and load tests like those in ISTA Test Procedure 3E, Unitized Loads of leaning. In some situations this can reach a point where the center of Same Product, serve to verify that a unit load can support the com - gravity (CG) of the stack has shifted so far that it becomes unstable pressive loads it is expected to experience in service. and risks collapsing. ww w .stress.com 1.6 1.4 1.2 ) n 1.0 i ( E G N A H 0.8 C G C 0.6 0.4 Measured CG 0.2 Extrapolation 0.0 0 30 60 90 120 150 180 210 240 270 300 330 360 TIME (days) figure 3 : Stacked unit load CG shift with time Figure 3 shows measurements of unit load CG shift for 35 days and tions. Figure 5 shows results from a rotational edge drop of a unit extrapolates those results out to one year. With information about load. For comparison, a vertical drop is shown in Figure 6. Pure verti - the product geometry and stack height it is possible to estimate cal drops of unit loads can sometimes be difficult to perform and this whether such movement will lead to a unit load stack that is danger - simulation illustrates an advantage of analytical methods in that they ously unstable. can allow difficult tests to be safely investigated. Shock Shock can be experienced from being roughly handled with a fork truck, sharp jolts in the back of a truck, rail car coupling or other events. Clearly shock can occur in both the vertical and horizontal directions and both are addressed in ISTA 3E. FEA results from a simulation of a horizontal shock event are shown in Figure 4. ISTA 3E addresses shock with a rotational edge drop test. A vertical drop or rotational flat drop might also be appropriate in some situa - figure 5 : Stress in boxes at impact during unit load rotational edge drop test figure 6 : Stress in boxes at impact during unit load figure 4 : Stress on face of boxes at 42 in/sec horizontal impact 8” vertical drop test Vibration Proceed With Caution Vibration is, like shock, a multidimensional phenomenon; though the It is very important to point out that these models have gaps in packaging industry has focused on vertical vibration for many years. knowledge and capability that keep them from being as accurate Studies of vibration in the transportation environment have shown and true-to-life as we might want them to be. All such models the most significant vibration to be vertically oriented and this has depend heavily on having knowledge about the properties of the been the most severe loading direction for the ubiquitous corrugat - materials that they are simulating. It is not possible to model every ed fiberboard box. As packaging is reduced to save costs and use aspect of a unit load in exquisite detail. Some of the properties and less material, some of the stability provided by full corrugated cases behaviors they exhibit are simply difficult to measure and determine is being lost and unit loads are being encountered which have con - with certainty. The images and animations that can be produced are siderably reduced horizontal stability. Horizontal vibration can then terrific, but don’t let yourself be misled by the pretty pictures. play a more important role in assessing overall unit load perform - ance in a vibratory environment. Finite element modeling can be Some key limitations to current modeling technology as it applies to used to simulate vibration but it is not easy to predict damage and packaging and unit loads are in the areas of manufacturing variabili - much of the success of the method depends on the details of the ty and damage accumulation. It is easy to make perfect products specific package design. Single axis or multiple axis vibration can and packages in a computer.

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