Virginia Tech: Researchers Study Modeling Compressive Stress Distributions Across Pallet Deck Surfaces
About $10 billion is lost annually to product damage that occurs largely within the distribution cycle of the supply chain.
By Yoo, Han, White, Rupert
Date Posted: 3/1/2009
All activities within a supply chain involve the transfer of products from suppliers to customers. In the marketplace, products are manufactured and distributed, and bought and sold according to the principles of supply and demand.
Unfortunately, in the U.S., about $10 billion is lost annually to product damage that occurs largely within the distribution cycle of the supply chain. Products that become damaged during distribution and storage significantly increase overall supply chain costs.
A unit load is defined as a single item, a number of items or bulk material arranged and restrained so that the load can be stored, picked up, and moved between two locations as a single mass. The three components of a unit load are: the packaging, the pallet, and any potential material handling equipment.
These components physically and mechanically interact during handling, storage, and transportation. It is important for pallet manufacturers to understand that by giving more attention to the design of unit loads, they can ultimately reduce the likelihood of preventable and costly damages from occurring. Unit loads that are designed based on the characteristics of the product and packaging, the pallet being used, as well as the characteristics of the materials handling environment will provide the most efficient distribution of consumer and industrial products.
In terms of the interactions between the components within a unit load, particularly between wooden pallets and corrugated fiberboard packaging, previous research has shown that dynamic stresses such as shock and vibration significantly influence interactions between components in unit loads. However, studies evaluating structural interactions between shapes and forms of packaging and pallets in a unit load system are complicated and not well understood. Furthermore, transportation, storage and distribution systems have been changing quickly over time.
Static stresses, especially static compressive stresses caused by the mechanical behavior of the interface between pallets and packaging during long-term warehouse storage, has not been well-documented by experimental research. Nevertheless, it is widely observed that non-uniformly distributed compressive stresses imposed by packaging at the interface of pallets and packaging can cause significant economic losses and unsafe working condition in a supply chain storage and distribution environment.
When deck boards deflect under load, non-uniform stress distributions occur across the surface and create stress concentrations around the areas where the deck boards are supported (stringers or blocks, depending on pallet style). Figure 1 illustrates this concept. As this occurs, stress levels experienced by the packaging materials located above the area of greatest deflection — in the middle of the span — relax, and shift the stress caused by the weight of the load outward becoming concentrated toward the deck board connections. Packaging materials, and the products they are designed to protect, can become damaged at these locations due to this unforeseen phenomenon.
In this example, designers can either bulk up the packaging materials to withstand these stress concentrations or stiffen the deck boards (by making them thicker or reducing the span between stringers). In most cases, it is more cost effective to simply stiffen the pallet deck surface.
A better understanding of the interactions between the primary components in a unit load design will have a significant impact on reducing economic losses caused by inefficiencies in the use of pallets and packaging. Moreover, it will also help improve workplace safety.
Research at Virginia Tech
Researchers at Virginia Tech’s Center for Unit Load Design are attempting to quantitatively analyze and model compressive static stress distributions across pallet deck surfaces as they support both flexible and rigid packaging types using simulated warehouse storage systems (racking and stacking). By understanding and being able to predict these stress distributions, designers can incorporate strategies to minimize both packaging costs and damages caused by stress concentrations resulting from uneven distributions.
For the purposes of this research, and to better isolate stress distributions, pallet sections were constructed using only one top deck board and one bottom deck board connected with three stringer segments. The thickness of the deck boards varied to evaluate the deck board stiffness effect on compression stress. Also, three different densities of polyolefin foams (2, 4, and 6 lb/ft3) were used to simulate a variety of flexible and rigid type packaging materials with a range of stiffness properties. A layer of single wall C-flute corrugated fiberboard was used as a sensing medium and to also simulate the bottom of a corrugated box.
To detect compressive static stresses at the interface between the polyolefin foams and pallet deck boards, pressure sensitive films were applied to the entire deck board surface. When placed between any two objects that contact, the film uses variable color intensities to reveal pressure distributions. The darker the film image, the higher the pressure. The pressure sensitivity of the film can vary from 2 to 40,000 psi. Since actual stresses that occur in the warehouse (when racking and stacking palletized loads) range from approximately from 2 to 6psi, this study used a micro sensitivity pressure film with a detecting range from 2 to 20psi. An image analysis computer software program was developed to quantitatively characterize stress distributions imprinted on the pressure sensitive film.
For pallet storage rack testing, a 280-pound compression load was applied to pallet sections with both ¾-inch deck thickness and ½-inch deck thickness. A 700-pound compression load was applied to similar pallet sections set up for simulated floor stack storage testing. Figure 2 shows an example of a typical pallet section and test set-up used in this research.
Using the results obtained from these tests, finite element analysis (FEA) models were developed to predict pallet deck board deflections depending on deck board and packaging characteristics. The finite element method (FEM) is one of the most popular numerical analysis techniques used for finite element analysis (FEA). FEA is a computer simulation technique used mostly by engineers, scientists, and mathematicians. FEM was developed to solve complex structural analysis and elasticity problems. The basic principle of FEM is that a complex structural model can be cut into smaller components called “elements.” To solve a problem in FEM, the elements must be reconnected by nodes at selected points. The behavior of the entire structure can be determined by the individual behavior of the elements.
In this study the predicted FEA models of the deck board deflections were validated through comparison with experimentally measured deflections in the simulated warehouse storage systems.
In the final models, the stress distributions on the pallet decks resulting from the three different packaging foams, for both rack and floor stack storage simulations, were non-uniform. The changes in the degree of stress concentrations and maximum stress levels along the deck boards varied, depending on the stiffness of the foams and deck boards and the support conditions used. The tests indicated that the 2pcf and 4pcf foams represented non-rigid sack products and the 6pcf foam represented more rigid packaging and contents.
Because the time to conduct these tests was relatively short in duration, the results focus only on the compressive stresses at the deck board/packaging interface caused by initial loading. The compressive stresses may change over time, and this will be investigated in future research.
The measure used to describe the level of stress concentration across a specific deck board surface, based on its stiffness characteristics, is called the Stress Intensity Factor (SIT). The SIT is defined as the ratio of initial maximum compressive stress to the applied stress. Using this measure, designers can calculate the appropriate design criteria for packaging by simply multiplying the SIT by the weight of the load to determine the compressive strength required by the packaging to prevent failure.
The results also showed that the stress distributions and concentrations (SIT) were more affected by the stiffness of the foam (package and product stiffness) than the stiffness of the pallet deck boards. In other words, the stiffer foam resulted in a greater change in stress levels along the deck board under the compression load.
Quantitatively, the results showed that initial maximum compressive stresses were 89% to 414% higher than the applied stress around the outer stringers during simulated rack storage and 31% to 311% higher than the applied stress around all three stringers in simulated floor stack storage. However, the compressive stresses are greater over the center stringer.
Clearly these results will be very useful in understanding the interactions that occur between pallets and packaging. Moreover, they have tremendous potential to substantially reduce product damage and improve the safety of the workplace during the long-term storage of the unit loads. The predicted FEA models will allow the industry to better optimize pallets, packaging, and unit load designs.
For more information, contact Peter Hamner at the Virginia Tech Center for Unit Load Design via e-mail at firstname.lastname@example.org.
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