1. Introduction
The global courier, express and parcel market has grown steadily over the past decade and the net income of these services doubled in this time [
1]. In this service, the first and last step of distribution occurs by small vehicles, when the operators collect or deliver the parcels. This paper focuses on this part of the transportation. All over the world, the volume of travel for parcels has exponentially increased, so better testing is needed. Companies like Amazon (
www.amazon.com, accessed on 15 January 2021) and carriers like UPS (
www.ups.com, accessed on 15 January 2021) are now the global leaders in providing online sales for customers and providing delivery of parcels. Amazon this year started to deliver packages, on the way to become the largest parcel delivery company.
This mode of express delivery brought new circumstances during transportation. The huge amount of goods causes a very intensive flow in the distribution channel by collecting and delivering single and various products day by day. This often means that the operators try to use the most of the vehicle capacity; thereby many identical or different kinds of parcels are stacked on top of each other. This situation is complicated further by the fact that the fixation accessories such as fixing straps or bands are virtually non-existent in this transportation mode. This is a common practice in Europe, but on some continents, this is different because the operators use vehicles equipped with shelves.
Figure 1a,b shows some examples for parcel shipments during daily courier service.
The shipments for courier parcel packages usually consist of small size, lightweight and time sensitive products. The vibration environment of these stacked shipments produces unique physical events due to free movement spaces around them [
2]. Since they are not restrained as unitized loads with stretch wrap or banding. In the vertical direction these parcels practically can move freely. This physical condition is very important to analyze in order for packaging engineers to design suitable protective packaging systems.
The vibration environment during distribution can be based on factors such as vehicle body structure, type of vehicle suspension and tires, road roughness, vehicle speed and actual payload, respectively [
3]. Previous studies have measured and analyzed vibration levels for parcel delivery vehicles and CEP (Courier Express Parcel) modes of transportation [
4,
5,
6,
7,
8,
9]. Factors regarding the effect of payloads, suspension system and road condition to vibration levels during express in delivery vehicles have also been previously studied [
4,
9,
10]. However, all this previous research measured and analyzed the vibration levels on the floor of the vehicle and none of them took into consideration that most of these parcels are transported in “unrestrained stacked configuration” and without shelving due to the intensive flow of goods and relatively limited capacity of vehicles. Furthermore, most of the previous studies did not investigate the differences and similarities between the laboratory simulation and field transportation. The vibration levels that is experienced in stacked loads (in layers) is different from the direct input citation that can be observed on the vehicle floor. Furthermore, these parcels do not act as a unitized pallet as they are not fin contact or restrained to each other.
Another aspect is that the results of study are also important, namely the generally widely used protocols for parcel shipment simulation that do not deal with a stacked unit. The only one, which separately deals with small parcel delivery shipments, is the ASTM D7386 [
11]. However, this protocol also does not expose specimens for stacking in order to test the upper layers if it is made from small packages. The only way to try to simulate small parcel delivery is a so-called vibration testing under dynamic load, which means that there is a top load bag apparatus for the test specimen that tries to simulate an additional load above the packages.
The main goal of this paper was to measure the vibration levels in layers of stacked packages in parcel delivery shipments and to compare the results to the laboratory experience and show the differences. The new results can be useful for packaging engineers to make better and precise pre-shipment testing. Furthermore, the results support the use of the vibration technique in the simulation of parcel delivery goods in a stacked way. It is important to note that existing ASTM method are not an under-test causing damage, but an over test.
4. Conclusions
In express stacked parcels delivery without vertical fixation the higher parcel position has a tendency to have higher acceleration peaks and RMS acceleration. Among various road conditions the urban road produced the highest acceleration events. This was 4.63 g for this study in the 3rd level of a stacked unit. It is clear based on the data that all accelerations above 1 g cause parcels to be air-lifted.
This increasing phenomenon can also be observed at the vibration intensity, but the field vibration resulted in much lower vibration intensity than the laboratory test (1–200 Hz) frequency range), even in the top row of the unit where it was observed that the field vibration intensity was the highest. The vibration intensity in the field was higher in any unit level than the vibration intensity on the floor due to the out-of-phase motion during distribution. This not true for the laboratory simulation where the PD levels were always higher on the vibration table in regard to the in-phase motion of the unit.
The PD levels of the layers in the stacked unit in the field, in the frequency range of 7–50 Hz, practically reached or exceeded the levels of the ASTM D7386 test spectrum, so here the artificial amplification of the test spectrum cannot be interpreted. In the 1–6 Hz lower frequency range and over 50 Hz the response vibration of the unit rows was significantly lower than the test spectrum. Therefore, the time-compression method of standard protocol can be determined even for the top level of a stacked unit without fixation.
The authors further suggest for the applied distribution practice that it is advisable to place packages on the top row of the set that are less sensitive to vibration on the one hand, or to use damping materials that have more favorable damping properties between 7–50 Hz. Furthermore, in laboratory tests, the time compression methods used by the standards should only be carefully considered in the frequency range of 7–50 Hz to reproduce random vibrations generated for upper level of stacked packages. Further consideration should be given to securing the stacked sensitive goods to avoid amplification of vibration intensity at the top of the stacked unit.