The global supply chain has been built on the material handling capabilities provided by the use of pallets and corrugated boxes. Current pallet design methodologies frequently underestimate the load carrying capacity of pallets by assuming they will only carry uniformly distributed, flexible payloads. But, by considering the effect of various payload characteristics and their interactions during the pallet design process, the structure of pallets can be optimized. This, in turn, will reduce the material consumption required to support the pallet industry. In order to understand the mechanical interactions between stacked boxes and pallet decks, and how these interactions affect the bending moment of pallets, a finite element model was developed and validated. The model developed was two-dimensional, nonlinear and implicitly dynamic. It allowed for evaluations of the effects of different payload configurations on the pallet bending response. The model accurately predicted the deflection of the pallet segment and the movement of the packages for each scenario simulated. The second phase of the study characterized the effects, significant factors, and interactions influencing load bridging on unit loads. It provided a clear understanding of the load bridging effect and how it can be successfully included during the unit load design process. It was concluded that pallet yield strength could be increased by over 60% when accounting for the load bridging effect. To provide a more efficient and cost-effective solution, a surrogate model was developed using a Gaussian Process regression. A detailed analysis of the payloads' effects on pallet deflection was conducted. Four factors were identified as generating significant influence: the number of columns in the unit load, the height of the payload, the friction coefficient of the payload's contact with the pallet deck, and the contact friction between the packages. Additionally, it was identified that complex interactions exist between these significant factors, so they must always be considered. / Doctor of Philosophy / Pallets are a key element of an efficient global supply chain. Most products that are transported are commonly packaged in corrugated boxes and handled by stacking these boxes on pallets. Currently, pallet design methods do not take into consideration the product that is being carried, instead using generic flexible loads for the determination of the pallet's load carrying capacity. In practice, most pallets carry discrete loads, such as corrugated boxes. It has been proven that a pallet, when carrying certain types of packages, can have increased performance compared to the design's estimated load carrying capacity. This is caused by the load redistribution across the pallet deck through an effect known as load bridging. Being able to incorporate the load bridging effect on pallet performance during the design process can allow for the optimization of pallets for specific uses and the reduction in costs and in material consumption. Historically, this effect has been evaluated through physical testing, but that is a slow and cumbersome process that does not allow control of all of the variables for the development of a general model. This research study developed a computer simulation model of a simplified unit load to demonstrate and replicate the load bridging effect. Additionally, a surrogate model was developed in order to conduct a detailed analysis of the main factors and their interactions. These models provide pallet designers an efficient method to use to identify opportunities to modify the unit load's characteristics and improve pallet performance for specific conditions of use.

The optimization of pallet design in today’s competitive supply chain is imperative to reduce costs and improve sustainability. With over two billion pallets in circulation in the United States, most packaged products are handled using unit loads and the interactions between the unit load components are not being considered in the pallet design process. This study aims to investigate the effect of the interlocking of layers and the pallet stacking patterns on pallet bending. This effect is part of a greater encompassing observed behavior known as load bridging, where a redistribution of the stresses on the pallet dependent on the characteristics of the load is generated. The bending of the unit load was measured under four common support conditions, warehouse racked across the width and length, fork tine support across the width and floor stacking. Five different pallet stacking patterns were then analyzed, comparing different interlocking levels, from column stacking to fully interlocking. It was identified that interlocking the layers causes a reduction in pallet deflection of up to 53% versus column stacking, and is more significant on lower stiffness pallets. The stacking patterns and interlocking levels also presented an effect on pallet deflection, albeit only for very low stiffness pallets when supported on its weakest components. A relationship between the observed results and a ratio of load and pallet stiffness was conducted, suggesting that when the load on the pallet is not significantly high in relation to the stiffness, load bridging won’t be observed. These results provide a guideline on improving pallet design and help furthering the understanding of the load bridging effect. / Master of Science / The optimization of pallet design in today’s competitive supply chain is imperative to reduce costs and improve sustainability. With over two billion pallets in circulation in the United States, most packaged products are handled using unit loads and the interactions between the unit load components are not being considered in the pallet design process. This study aims to investigate the effect of the interlocking of layers and the pallet stacking patterns on pallet bending. This effect is part of a greater encompassing observed behavior known as load bridging, where a redistribution of the stresses on the pallet dependent on the characteristics of the load is generated. Tests were conducted to measure the pallet bending performance under common scenarios, evaluating the effect of five different pallet stacking patterns. It was identified that when the layers of a unit load are interlocked, the pallet presents lower deflection (up to 53%). A relationship between the observed results and a ratio of load and pallet stiffness was conducted, suggesting that when the load on the pallet is not significantly high in relation to the stiffness, load bridging won’t be observed. These results provide a guideline on improving pallet design and help furthering the understanding of the load bridging effect.

During unitized shipment, the components of unit loads are interacting with each other. During floor stacking of unit loads, the load on the top of the pallet causes the top deck of the pallet to bend which creates an uneven top deck surface resulting in uneven, or asymmetrical support of the corrugated boxes. This asymmetrical support could significantly affect the strength of the corrugated boxes, and it depends on the top deck stiffness of the pallet. This study is aimed at investigating how the variations of pallet top deck stiffness and the resulting asymmetric support, affects corrugated box compression strength. Pallet top deck stiffness was determined to have a significant effect on box compression strength. There was a 27-37% increase in box compression strength for boxes supported by high stiffness pallets in comparison to low stiffness pallets. The fact that boxes were weaker on low stiffness pallets could be explained by the uneven pressure distribution between the pallet deck and bottom layer of boxes. Pressure data showed that a higher percentage of total pressure was located under the box sidewalls that were supported on the outside stringers of low stiffness pallets in comparison to high stiffness pallets. This was disproportionately loading one side of the box. Utilizing the effects of pallet top deck stiffness on box compression performance, a unit load cost analysis is presented showing that a stiffer pallet can be used to carry boxes with less board material; hence, it can reduce the total unit load packaging cost. / Master of Science / Packaged products are primarily shipped as unit loads that consist of packaged products restrained to a platform, commonly a pallet. Paying particular attention to the design of the unit loads' components is necessary to safely ship products while still maintaining low packaging costs and sustainability initiatives. Stacking unit loads is a common practice to effectively use warehouse space, but warehouse stacking causes large amounts of weight for packaging to support. Pallets are not completely rigid and will deform because of this weight. The purpose of the study was to investigate the effect of pallet stiffness on the compression strength of corrugated boxes. Compression tests were completed on boxes supported by pallet designs having different deck stiffnesses. The top deck stiffness of a pallet was determined to have up to a 37% effect on the strength of corrugated boxes. Pressure data recorded between the bottom layer of boxes and the top deck of the pallet showed a larger percentage of pressure was located towards the outside edges of the unit load for boxes carried by a flexible pallet. Effectively, one side of the box was stressed more than the other causing package failure. Utilizing the effects of pallet top deck stiffness on box compression performance, a unit load cost analysis is presented showing that a stiffer pallet can be used to carry boxes with less board material; hence, it can reduce the total unit load packaging cost.

Turbine engine blades are subjected to extreme conditions characterized by significant and simultaneous excursions in both stress and temperature. These conditions promote thermo-mechanical fatigue (TMF) crack growth which can significantly reduce component design life beyond that which would be predicted from isothermal/constant load amplitude results. A thorough understanding of the thermo-mechanical fatigue crack behavior in single crystal superalloys is crucial to accurately evaluate component life to ensure reliable operations without blade fracture through the use of "retirement for cause" (RFC). This research was conducted on PWA1484, a single crystal superalloy used by Pratt & Whitney for turbine blades. Initially, an isothermal constant amplitude fatigue crack growth rate database was developed, filling a void that currently exists in published literature. Through additional experimental testing, fractography, and modeling, the effects of temperature interactions, load interactions, oxidation and secondary crystallographic orientation on the fatigue crack growth rate and the underlying mechanisms responsible were determined. As is typical in published literature, an R Ratio of 0.7 displays faster crack growth when compared to R = 0.1. The effect of temperature on crack growth rate becomes more pronounced as the crack driving force increases. In addition secondary orientation and R Ratio effects on crack growth rate were shown to increase with increasing temperature. Temperature interaction testing between 649°C and 982°C showed that for both R = 0.1 and 0.7, retardation is present at larger alternating cycle blocks and acceleration is present at smaller alternating cycle blocks. This transition from acceleration to retardation occurs between 10 and 20 alternating cycles for R = 0.1 and around 20 alternating cycles for R = 0.7. Load interaction testing showed that when the crack driving force is near KIC the overload size greatly influences whether acceleration or retardation will occur at 982°C. Semi-realistic spectrum testing demonstrated the extreme sensitivity that relative loading levels play on fatigue crack growth life while also calling into question the importance of dwell times. A crack trajectory modeling approach using blade primary and secondary orientations was used to determine whether crack propagation will occur on crystallographic planes or normal to the applied load. Crack plane determination using a scanning electron microscope enabled verification of the crack trajectory modeling approach. The isothermal constant amplitude fatigue crack growth results fills a much needed void in currently available data. While the temperature and load interaction fatigue crack growth results reveal the acceleration and retardation that is present in cracks growing in single crystal turbine blade materials under TMF conditions. This research also provides a deeper understanding of the failure and deformation mechanisms responsible for crack growth during thermo-mechanical fatigue. The crack path trajectory modeling will help enable "Retirement for Cause" to be used for critical turbine engine components, a drastic improvement over the standard "safe-life" calculations while also reducing the risk of catastrophic failure due to "chunk liberation" as a function of time. Leveraging off this work there exists the possibility of developing a "local approach" to define a crack growth forcing function in single crystal superalloys.

A study was done to determine the temperature and load interaction effects on the fatigue crack growth rate of polycrystalline superalloy IN100. Temperature interaction testing was performed by cycling between 316°C and 649°C in blocks of 1, 10 and 100 cycles. Load interaction testing in the form of single overloads was performed at 316°C and 649°C. After compiling a database of constant temperature, constant amplitude FCGR data for IN100, fatigue crack growth predictions assuming no load or temperature interactions were made. Experimental fatigue crack propagation data was then compared and contrasted with these predictions. Through the aid of scanning electron microscopy the fracture mechanisms observed during interaction testing were compared with the mechanisms present during constant temperature, constant amplitude testing. One block alternating temperature interaction testing grew significantly faster than the non-interaction prediction, while ten block alternating temperature interaction testing also grew faster but not to the same extent. One hundred block alternating testing grew slower than non-interaction predictions. It was found that as the number of alternating temperature cycles increased, changes in the gamma prime morphology (and hence deformation mode) caused changes in the environmental interactions thus demonstrating the sensitivity of the environmental interaction on the details of the deformation mode. SEM fractography was used to show that at low alternating cycles, 316°C crack growth was accelerated due to crack tip embrittlement caused by 649°C cycling. At higher alternating cycles the 316°C cycling quickly grew through the embrittled crack tip but then grew slower than expected due to the possible formation of Kear-Wilsdorf locks at 649°C. Overload interaction testing led to full crack retardation at 2.0x overloads for both 316°C and 649°C testing. 1.6x overloading at both temperatures led to retarded crack growth whereas 1.3x overloads at 649°C created accelerated crack growth and at 316°C the crack growth was retarded.