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Title: The FE simulation of the processes for the manufacture of bimetallic snap-action thermostats
Author: Moughton, Colin
ISNI:       0000 0004 2709 5310
Awarding Body: The University of Manchester
Current Institution: University of Manchester
Date of Award: 2004
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Strix Ltd is the world leader in the manufacture of thermostatic controls for water boiling applications in the domestic appliance industry. At the heart of every control are one or more snap-action bimetal thermostatic switches called `blades'. In 2003, Strix Ltd manufactured over 100 million blades and demand is increasing. In this research finite element models have been used to simulate key stages of the blade manufacturing process and the subsequent measurement of the blade switching temperatures. The purpose of this research has been to identify sensitive material and manufacturing process parameters in order to gain an understanding of the observed process variability. There are two key stages within the blade manufacturing process; a high-speed metal forming stage, and a heat treatment stage. Only the metal forming stage is modelled in this work due to the time constraints of the project, however, suitable material models are produced for both stages. The material phenomena relevant to the blade manufacturing process are, elasticity, plasticity and creep. In the first part of this work a series of experiments have been carried out to characterise the relevant material behaviour; uni-axial tensile testing and Vickers hardness testing to characterise the elastic and plastic material parameters, and uni-axial creep testing to characterise creep behaviour of the two constituent bimetal alloys. A constitutive material model is developed; linear elasticity and classical metal plasticity are used to model strain-rate and temperature dependent elasticity and plasticity. Creep deformation mechanisms in metals and techniques for modelling creep are examined and a set of creep constitutive equations selected. The experimental data is used to calibrate the material model which is then implemented within the commercial finite element code `ABAQUS'. A novel and original technique is developed to enable the calibration of cold reduction and goods-in residual stress present within bimetal strip. The boundary conditions describing the interaction of the blade-forming press tool with the blade are investigated and defined. A finite element simulation of the blade metal forming process is then developed. The residual stresses present within the blade after each metal forming operation are evaluated and their compatibility with known process sensitivities examined. A finite element simulation of the blade temperature measurement process is developed using the results of the residual stress calculations from the blade metal forming simulation as input. The snap-through switching temperatures of the simulated blade are established and the stress state of the blade is evaluated to gain an understanding of the blade operating cycle. Sensitivity studies are carried out using the finite element models above to evaluate the impact of variation of material and metal forming process parameters on blade performance. Three sensitive metal forming process parameters, and two sensitive material parameters are identified and their relative sensitivities quantified. The results from the finite element models are then compared against experimental data to establish the accuracy and validity of the approach used. The sensitive parameters identified are examined to establish their compatibility with the top five problems experienced in normal blade production. Finally, the new understanding of the sensitive material and process parameters is used to formulate recommendations to improve current and future blade manufacturing processes within Strix.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available