## 1 Introduction  
Heavy home appliances (e.g., refrigerators, washing machines, and air conditioners) are usually cushioned by an expanded polystyrene (EPS)-based cushioning system and packaged in a shipping container (mostly corrugated fiberboard containers). These packages are unitized on a pallet in a certain quantity (palletized-unit load) and are subject to logistics operations, such as transportation, storage, and handling. They are commonly stacked in multiple stages during storage. In such a situation, where the package is subject to long-term stacking loads, the shipping container and EPS-based cushioning system in a package undergo creep deformation. The longer the package is stacked on the lower layer, the more severe the creep deformation. In a warehouse, the cumulative creep deformation of multi-stacked packages may lead to an inclination of the unitized load and even collapse. This phenomenon occurs more conspicuously by the action of the rotational moment owing to an eccentric load when the center of gravity is eccentric to one side in the package of heavy goods, such as refrigerators and washing machines. In general, the creep behavior of a viscoelastic material has been described to have three phases, as shown in Figure 1: (1) primary (transient) creep, (2) secondary (steady-state) creep, and (3) tertiary (accelerating) creep. In the primary phase, the strain increases with a decrease in the strain rate. In the second phase, the strain increases linearly with time, yielding a constant strain rate. The tertiary phase is characterized by a rapid increase in strain, rapidly leading to failure. However, the actual creep behavior of viscoelastic materials is strongly influenced by the properties of the material itself and the level of load applied, stabilizing after transient creep and leading to destruction after rapid progression through the three creep phases, or fully proceeding with the three creep phases. The creep behavior, from instantaneous elastic strain to steady-state creep after loading, can be described by the Burgers model: \(\varepsilon(t)=\sigma_{o}/E_{o}+\sigma_{o}/E_{r}\{1-exp(-t/\tau_{r})\}+(\sigma _{o}/\eta_{c})t\), where \(E_{o}\) = instantaneous elastic modulus (Pa), \(E_{r}\) = retarded elastic modulus (Pa), \(\eta_{o}\) and \(\eta_{r}\) are the viscosity coefficients of the dashpot (Pa-s), and \(\tau_{r}\) is the retardation time (=\(\eta_{r}/E_{r}\))(s). The constants of this model allow for a quantitative understanding of the characteristics of the different creep phases. In the Burgers model, the Maxwell and Kelvin elements are connected in series. The spring of the Maxwell element represents the instantaneous elastic strain, whereas the dashpot represents a non-reversible viscous flow. The Kelvin element describes the delayed elasticity [1; 2]. Corrugated fiberboard containers are mostly used as shipping containers for home appliances. In the case of a corrugated fiberboard container, the structure has a unique characteristic that renders its creep behavior as different from that of a simple viscoelastic material. The primary creep region of the corrugated fiberboard container is the deformation accommodated in the flattening process of the wing in the vertical direction to the applied load. The creep rate and quantity are large because the wing rotates and bends around the crease [3]. Part of the creep deformation generated in this phase is recoverable, occurring for a short time and not contributing to the failure of the container. The secondary creep region is dominated by the bulging of the container side and the load transfer to the container corner. The secondary creep rate is nearly constant; however, most of the deformation is inelastic and contributes to failure [4]. Local buckling at the corner of the container forms a hinge at the corner, resulting in overall buckling, eventually proceeding to tertiary creep and leading to the ultimate collapse of the container.