A Comparison of Methods for Forecasting Demand for Slow Moving Car Parts

This paper has a focus on non-stationary time series formed from small non-negative integer values which may contain many zeros and may be over-dispersed. It describes a study undertaken to compare various suitable adaptations of the simple exponential smoothing method of forecasting on a database of demand series for slow moving car parts. The methods considered include simple exponential smoothing with Poisson measurements, a finite sample version of simple exponential smoothing with negative binomial measurements, and the Croston method of forecasting. In the case of the Croston method, a maximum likelihood approach to estimating key quantities, such as the smoothing parameter, is proposed for the first time. The results from the study indicate that the Croston method does not forecast, on average, as well as the other two methods. It is also confirmed that a common fixed smoothing constant across all the car parts works better than maximum likelihood approaches.Count time series; forecasting; exponential smoothing; Poisson distribution; negative binomial distribution; Croston method.

Forecasting the Intermittent Demand for Slow-Moving Items

Organizations with large-scale inventory systems typically have a large proportion of items for which demand is intermittent and low volume. We examine different approaches to forecasting for such products, paying particular attention to the need for inventory planning over a multi-period lead-time when the underlying process may be non-stationary. We develop a forecasting framework based upon the zero-inflated Poisson distribution (ZIP), which enables the explicit evaluation of the multi-period lead-time demand distribution in special cases and an effective simulation scheme more generally. We also develop performance measures related to the entire predictive distribution, rather than focusing exclusively upon point predictions. The ZIP model is compared to a number of existing methods using data on the monthly demand for 1,046 automobile parts, provided by a US automobile manufacturer. We conclude that the ZIP scheme compares favorably to other approaches, including variations of Croston's method as well as providing a straightforward basis for inventory planning.Croston's method; Exponential smoothing; Intermittent demand; Inventory control; Prediction likelihood; State space models; Zero-inflated Poisson distribution

Determining the effect of strain rate on the fracture of sheet steel

A key challenge for the automotive industry is to reduce vehicle mass without compromising on crash safety. To achieve this, it is necessary to model local failure in a material rather than design to the overly conservative criteria of total elongation to failure. The current understanding of local fracture is limited to quasi-static loading or strain rates an order of magnitude too high for automotive crash applications. This thesis studies the local fracture properties of DP800 sheet steel at the macroscopic scale from strain rates of to for the first time. Geometries for three stress states, namely plane-strain, shear and uniaxial tension, were developed to determine a fracture locus for DP800 steel using optical strain measurement. These geometries were developed using Finite Element Analysis and validated experimentally for strain rate and stress state. Thermal imaging was used to determine the effect of strain rate on temperature rise and its associated effect on fracture. Fractography was used to examine the specimens’ failure modes at different strain rates. The geometries were applied to the advanced high strength steel grade DP800. Despite prior evidence from simple tensile test data, DP800 showed no significant variation in fracture strain with strain rate in all three stress states. Non-contact thermal measurements showed that the high strain rate tests ( ) were non-isothermal with temperature rises of up to being observed. As a result of this it is difficult to decouple the effect of strain rate from the effect of temperature and requires further investigation. The test geometries were also applied to the deep draw steel DX54 and the aluminium alloy AA5754 where a strain rate effect was observed. Both materials are significantly more ductile than DP800 whish exposed a limitation in the test procedures. At high fracture strains the stress state deviates from its intended value and can invalidate the test. Therefore, a method was developed for determining the validity of a test for each geometry and material from experimental data. The preliminary data from DX54 indicates significantly greater strain rate sensitivity across one order of magnitude than was observed in five orders of magnitude in DP800

Forecasting Compositional Time Series with Exponential Smoothing Methods

Compositional time series are formed from measurements of proportions that sum to one in each period of time. We might be interested in forecasting the proportion of home loans that have adjustable rates, the proportion of nonagricultural jobs in manufacturing, the proportion of a rock's geochemical composition that is a specific oxide, or the proportion of an election betting market choosing a particular candidate. A problem may involve many related time series of proportions. There could be several categories of nonagricultural jobs or several oxides in the geochemical composition of a rock that are of interest. In this paper we provide a statistical framework for forecasting these special kinds of time series. We build on the innovations state space framework underpinning the widely used methods of exponential smoothing. We couple this with a generalized logistic transformation to convert the measurements from the unit interval to the entire real line. The approach is illustrated with two applications: the proportion of new home loans in the U.S. that have adjustable rates; and four probabilities for specified candidates winning the 2008 democratic presidential nomination.compositional time series, innovations state space models, exponential smoothing, forecasting proportions

Increased IL-5 and IL-13 cytokine level in ex vivo stimulated whole blood cells from grass pollen allergic donors correlate with seasonal exposure

AbstractThere is a need for simple and physiological assays to characterize the immune status of allergic individuals. Whole blood samples from 15 adult subjects (10 with positive clinical history to grass pollen and 5 with negative clinical history) were obtained before the start (April 2010) and during the middle of the grass pollen season (June 2010). The investigators were blinded to the allergic status of the subjects. A skin prick test (SPT) to grass pollen was carried out at the end of the study. Cytokines (IL-5, IL-13, IL-10 and IFNγ) and activation of T-lymphocytes were determined after ex vivo culture of whole blood cells. IL-5, IL-10 and IL-13 cytokines were significantly elevated in allergic individuals during the middle of the season (p≤0.02) compared to the start. This assay can be a valuable tool in clinical trials especially in pediatric population where limited quantities of blood are available to study immune responses

Macro-modelling of laser micro-joints for understanding joint strength in electric vehicle battery interconnects

Laser micro-welding is increasingly being used to produce electrically conductive joints within a battery module of an automotive battery pack. To understand the joint strength of these laser welds at an early design stage, micro-joints are required to be modelled. Additionally, structural modelling of the battery module along with the electrical interconnects is important for understanding the crash safety of electric vehicles. Fusion zone based micro-modelling of laser welding is not a suitable approach for structural modelling due to the computational inefficiency and the difficulty of integrating with the module model. Instead, a macro-model which computationally efficient and easy to integrate with the structural model can be useful to replicate the behaviour of the laser weld. A macro-modelling approach was adopted in this paper to model the mechanical behaviour of laser micro-weld. The simulations were based on 5 mm diameter circular laser weld and developed from the experimental data for both the lap shear and T-peel tests. This modelling approach was extended to obtain the joint strengths for 3 mm diameter circular seams, 5 mm and 10 mm linear seams. The predicted load–displacement curves showed a close agreement with the test data. View Full-Tex

High intensity interval training (HIIT) improves resting blood pressure, metabolic (MET) capacity and heart rate reserve without compromising cardiac function in sedentary aging men

Background: This study examined a programme of pre-conditioning exercise with subsequent high intensity interval training (HIIT) on blood pressure, echocardiography, cardiac strain mechanics and maximal metabolic (MET) capacity in sedentary (SED) aging men compared with age matched masters athletes (LEX). Methods: Using a STROBE compliant observational design, 39 aging male participants (SED; n=22, aged 62.7± 5.2 yrs) (LEX; n = 17, aged= 61.1 ± 5.4 yrs) were recruited to a study that necessitated three distinct assessment phases; enrolment (Phase A), following pre-conditioning exercise in SED (Phase B), then following 6 weeks of HIIT performed once every five days by both groups before reassessment (Phase C). Hemodynamic, echocardiographic and cardiac strain mechanics were obtained at rest and maximal cardiorespiratory and chronotropic responses were obtained at each measurement phase. Results: The training intervention improved systolic, mean arterial blood pressure, rate pressure product and heart rate reserve (each P b 0.05) in SED and increased MET capacity in both SED and LEX (P b 0.01) which was amplified by HIIT. Echocardiography and cardiac strain measures were unremarkable apart from trivial increase to intra-ventricular septum diastole (IVSd) (P b 0.05) and decrease to left ventricular internal dimension diastole (LVId) (P b 0.05) in LEX following HIIT. Conclusions: A programme of preconditioning exercise with HIIT induces clinically relevant improvements in blood pressure, rate pressure product and encourages recovery of heart rate reserve in SED, while improving maximal MET capacity in both SED and LEX without inducing any pathological cardiovascular remodeling. These data add to the emerging repute of HIIT as a safe and promising exercise prescription to improve cardiovascular function and metabolic capacity in sedentary aging

Methodology for developing a macro finite element model of lithium‐ion pouch cells for predicting mechanical behaviour under multiple loading conditions

To assist in light weighting of electric vehicles by improving the volumetric and gravimetric energy density and the structural performance of the battery pack, a modelling methodology based on a macro finite element model of a pouch cell has been developed. This model treats the core cell structure as a homogeneous orthotropic honeycomb block with the pouch material being defined as an orthotropic fabric with compressive stress elimination. The model considers five compression and bending load cases simultaneously and allows a level of element discretisation that is computationally efficient and appropriate for inclusion in full vehicle and sub‐system simulations. The methodology is scalable in that it can be applied to a range of chemistries, external geometries and internal cell constructions. When considering stacks of cells, the model is predictive for both lateral compression and three‐point bend, but further work is required to improve the confined compression response