Papers by Author: Michael A. Sek

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Authors: Michael A. Sek, Vincent Rouillard
Abstract: This paper presents some of the latest results of a research project aimed at using composite corrugated paperboard structures for protection of products against mechanical shocks and vibration during transportation and handling. Specifically, the behaviour of multi-layered corrugated paperboard (MCPB) under shock loading is investigated. Conventionally, packaging cushion design requires the determination of the maximum expected shock levels or equivalent drop which are usually determined from statistical analysis of original field measurements. With this approach, it is generally acknowledged that the cushioning element is engineered to provide adequate protection for statistically likely events but not for extreme events with low statistical likelihood. It is reluctantly accepted that, should it occur, the latter will result in damage to the product. MCPB can be formed with a broad range of compressive characteristics and with various proportions of elastic and plastic behaviour. The objective of this experimental investigation was to determine the optimum elastic/plastic proportion to extend the protective range to include large shock levels. The experimental results obtained include the effects of compression history on the stress-strain properties of MCPB as well as the behaviour of the material in both virgin and pre-compressed form under impulsive loads. The mechanism of deformation of the corrugations (flutes) was studied using high-speed video equipment. The complex acceleration signals produced during deformation of the composite corrugated paperboard cushions under shock loading were analysed by means of the shock response spectrum. Experiments have shown that inserting a sacrificial crumple element of virgin corrugated paperboard at the optimum contact area ratio dramatically lowers the overall level of the resulting shock response spectrum. This has the effect of increasing the allowable drop height for a limited number of extreme events. The main conclusion of the research is that MCPB in both virgin and pre-compressed forms can be combined to provide significantly enhanced protection to products against mechanical hazards during distribution.
Authors: Michael A. Sek, Vincent Rouillard, Anthony Parker
Abstract: There are two main characteristics of cushioning materials that are required to design a robust cushioning system for the protection of critical element against enviro-mechanical hazards: the attenuation of shocks as a function of the static load and the vibration transmissibility. The effect of a shock on a hypothetical critical element is normally evaluated by the Shock Response Spectrum (SRS) whereas the Frequency Response Function (FRF) performs similar function in relation to vibrations. This paper is concerned with the latter. Cushioning materials are generally nonlinear and, together with the interacting mass, form a nonlinear dynamic system. This paper shows how the Reverse Multiple Input-Single-Output (R-MISO) method can be used to describe the nonlinear characteristics of cushioning systems by generating a series of FRF terms. However, this creates ambiguity in relation to the effect of transmitted vibration on the critical element. This paper proposes to resolve this, by analogy to the SRS, through a numerical calculation of the Vibration Response Spectrum (VRS) for a hypothetical critical element, using as the excitation of the critical element either experimental cushion response data or data synthesised via R-MISO FRFs. Values of the VRS are defined as the ratio of acceleration rms of the critical element to the rms of the cushion excitation, although other descriptors of critical element's exertion can also be considered. The VRS can be considered as the true transmissibility. It is shown that the R-MISO method is superior over the Single Input-Single-Output (SISO) method in determining the transmissibility of polystyrene cushions. Since the cushion system is nonlinear, the excitation of the linear critical element will in general be non-Gaussian, although this paper has shown that it is near- Gaussian in the vicinity of cushion resonance. A chosen hypothetical critical element can be linear or, if its characteristics is known in advance, nonlinear. Results presented in this paper demonstrate how the R-MISO and the VRS can be used to determine the dynamic characteristics of EPS as a nonlinear cushioning material.
Authors: M.A. Garcia-Romeu-Martinez, Michael A. Sek, Vincent Rouillard, V.A. Cloquell-Ballester
Abstract: During distribution, consignments undergo numerous handling processes both mechanized and manual. These operations are known to produce drops and impacts of varying severity which have a potential to cause damage to the product. These shocks are the main parameters required for the optimum design of protective packaging systems. The severity of the shocks is often described in terms of their effective (free-fall equivalent) drop height (EDH) and impact orientation, in order to facilitate the laboratory testing conducted on a free-fall apparatus. The preferred approach is to survey the shocks with self-contained tri-axial shock recorders and process the data in such a way that statistical distributions of expected drop heights and orientations are obtained. On the other hand the Real Drop Height (RDH) method, based on the measurement of free fall time, is also used, mainly to discriminate between free-fall events and more commonly occurring complex causes of shocks, primarily for the quality control of distribution environment. The focus of the paper is on the EDH method and on the use of characteristic parameters of the tri-axial acceleration shock pulse to determine the EDH. An accurate estimate of the coefficient of restitution between the instrumented test package and the impact surface must be known and this poses a problem as it cannot always be established for every event during distribution. Consequently, the adopted approach is to calibrate an instrumented test package and obtain an estimate of the coefficient of restitution between the package and a test impact surface which is generally assumed to be hard relative to the cushioned package. The paper addresses the pitfalls and investigates various algorithms of determining the EDH from recorded shock data. It presents an analysis of the influence and errors associated with various methods used to estimate velocity change from characteristic parameters of a shock pulse such as the pulse width, the peak acceleration and its temporal location. The effects of analyzing the orthogonal acceleration vectors separately, as opposed to the resultant vector, are discussed. The results of a number of free-fall experiments, undertaken in controlled conditions, are used to validate and calibrate the proposed method for determining the EDH for free-fall drops on hard surfaces.
Authors: Michael A. Sek
Abstract: This paper describes the development of a software simulation tool of discrete elements which has been developed for the purpose of investigating the dynamic response of multilayer sandwich structures that incorporate highly nonlinear crumple (buckling) elements. These structures are to be optimised as cushions in order to minimise the transmission of shocks when exposed to transient excitation, such as in a free fall. Presented results are for multilayer corrugated paperboard. A single layer was modelled as a nonlinear 2-DOF system with an additional elastoplastic element to reflect contact conditions. Numerical models of the platen and the exciter with either acceleration or displacement control were developed and applied to perform numerical compression tests of the sandwich layer at various strain rates to validate the model of a single layer. Sandwich structures were then numerically assembled and subjected to simulated impacts. The model predicted inter- and intralaminar forces, displacements, velocities and accelerations. The shock attenuation characteristics were obtained and presented as the time-acceleration-static stress maps. A postprocessor was developed to produce animations to reveal complex dynamic interactions within modelled sandwich structures.
Authors: M.A. Garcia-Romeu-Martinez, Vincent Rouillard, Michael A. Sek, V.A. Cloquell-Ballester
Abstract: During the transportation phase of the distribution cycle, packaging systems are subjected to random dynamic compressive loads that arise from vibrations generated by the vehicle. The level and severity of these dynamic compressive loads are generally a function of the vibration levels, the stack configuration and stack weight. The container’s ability to withstand these compressive loads for sufficiently long periods depends on the material’s characteristics as well as the container design. The research presented herein tests the hypothesis that cumulative damage in the material under random dynamic compression will result in a reduction in the overall stiffness as well as an increase in the overall damping of the element. These are expected to be manifested, respectively, as a shift in the fundamental resonant frequency as well as an increase in the bandwidth of the frequency response function of the material at resonance when configured as a single degree of freedom system. The paper presents the results of preliminary experiments in which a number of corrugated paperboard samples were subjected to dynamic compressive loads by means of broadband random base excitation with a vibration table coupled with a guided dead-weight arrangement. The level of cumulative damage in the sample was continuously evaluated by monitoring the stiffness and overall damping of the sample which were extracted from the Frequency Response Function (FRF) of the system. This was obtained from continuous acceleration measurements of the vibration table and the guided dead weight.
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