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A Creep Testing Machine is one that accurately measures the creep of a material under constant load and at elevated temperatures until final rupture. Basically, it demonstrates the effect of Temperature on Creep. Creep is defined as the time dependant deformation of a material under constant load at constant elevated temperature. The resulting strain is a function of applied stress, temperature and time. Creep is increasingly important in industry in many different applications ranging from turbine rotors, high pressure steam tubes, suspended cables, tightened bolts where materials are subjected to extremely high temperatures and stresses which may cause them to change size, shape and lead to rupture.
So the main objective of a creep test is to measure how a given material will perform under constant loads and elevated temperatures to make sure they are ready for use in industry under such conditions. In a creep test, a tensile specimen is subjected to a constant load inside a furnace set to a specified temperature maintained at a constant high temperature. The material will go through 3 phases of creep; primary, secondary (steady state, lengthiest stage) and tertiary until it ruptures. The test may run on for days until eventually the specimen fails and the creep properties are recorded.
Applications in industry
Creep testing is increasingly important in large areas of industry. There are three types of high temperature industry applications.
Displacement limited applications: where precise dimensions must be maintained, examples of which are in turbine rotors/aircraft turbine blades.
Rupture limited applications: where fracture must be avoided, such as in steam tubes.
Stress-relaxation limited applications: where initial tension relaxes with time such as in suspended cables and tightened bolts.
Aims and Objectives:
To fully design a Creep Testing Machine within a hard budget in the given time frame – by the end of the semester in week 13.
The reason behind the project is to provide AUT Engineering School with its first working Creep Machine. It will provide an opportunity for materials to be tested for Creep under extremely high temperatures for research or educational purposes after the project is completed. The very high temperatures will allow for testing stronger more complex materials such as alloys with higher melting points.
A fully detailed, clear and FEA tested 3D CAD drawing will be produced, effectively demonstrating the complete final design of the Creep Testing Machine in all of its dimensions and absolute properties.
The machine will be designed to be cheap, practical, robust, reliable, easy to use, relatively lightweight, safe (to the touch), long lasting and professional looking.
The project meets the academic requirements of my qualification as it will require a great deal of knowledge I have obtained from papers such as: material science, manufacturing technology, CAD/solidworks engineering design, quantitative techniques, thermodynamics, solid mechanics and heat transfer papers.
Potential Industry organisations involved:
high temperature material suppliers
high temperature measuring/control device distributers
electronic and electrical control distributers
insulation material suppliers
Resources (likely required):
Solidworks/CAD design programs, matlab/computer programming programs, Microsoft excel, electrical/electronic equipment.
Workshop machines: milling, lathe, drilling, soldering, welding machines and more.
Plan/chart will be subject to refinement throughout the duration of the project.
This project is supposed to be carried on through until the end of the 4th year industrial project. From now until the end of the semester we will be designing the Creep Testing Machine completely, putting the designs through numerous tests and immense scrutiny until it is certain that the design will be achievable and the project a total success.
Our supervisor, Tim, informed us with the estimated budget for this project of around $5,000NZ. This is a hard budget and we are not to exceed it under any circumstances. Therefore an important goal of this project is to stay under budget (by a decent sized margin if possible) and design a relatively cheap Creep Testing Machine that can be delivered realistically for the 4th year industrial project (where the machine should be produced).
The Creep machine consists of several main components that fit into three categories; Heat, mechanics and control. The components are as follows: Frame, Furnace, Control, Electronics + Data acquisition, Strain measurement device (extra), Emergency shut off, Temperature measurement device, Grip system, Timer, On/Off switch and Loading mechanism.
The furnace should be designed to reach exceedingly high temperatures of up to 800C so that it may be suitable for creep testing on a wide range of specimen materials (high T alloys etc) for research and educational purposes at the AUT Engineering department.
During our first group meeting after the first formal meeting with our supervisor, we (the project team) assigned several components to each of us to carry out detailed research and gather our findings before the next meeting with Tim. The larger, more complex components namely the furnace and loading mechanism were both shared between me and Ramez, Steven and John-Paul respectively. The full details are illustrated in my logbook.
So the components assigned to me for research were the Furnace, strain measuring device and the frame. I began dissecting the furnace into its individual components and features. A standard muffle furnace consists of insulation, the body, heating element, temperature measurement/controller and door with locking mechanism, compartments for other components (load train, measuring apparatus), mounting kit and air vents.
After I completed some general research I developed a good and original idea of what our Creep testing machine will consist of:
Furnace insulation will either consist of two different materials namely refractory firebrick and refractory ceramic fibre blanket or solely just ceramic fibre blanket/wool. These are highly heat resistance materials (one that has especially low thermal conductivity value – k).
Fire brick; would be the first line of insulation and the main barrier to heat loss from the furnace chamber with a k value of ~ 0.21 (@800C).
Ceramic fibre; would be the surrounding/main layer of insulation and will be put around the fire brick and on the inner door surface. It has a k value of ~ 0.22 (@800C). There are several types of suitable fibre and one will be chosen based on its cost and thermal properties.
The reason that firebrick is being considered as a layer of insulation is because of the ease of fitting it with heating elements. However they are more bulky and considerably heavier than ceramic blankets.
The insulation choices will be theoretically tested for suitability using standard thermal resistance equations:
Qtransfer = âˆ†T/Rth
Rth = (1/hiA)+(L/kA)+ (1/hoA)
Where; L is the minimum thickness of insulation, k is its thermal conductivity, h is the convection heat transfer coefficient and Rth is the total heat resistance.
Choice and list of possible heating elements + why chosen. Take into accnt start up hting time
The minimum insulation thickness required is found as follows;
Power in: 1.6kW heating element
âˆ†T = TMAX – TAMB
Insulation, Ceramic fibre: k = 0.22W/m2K
hi = 10W/mK, ho = 40W/mK
Rth = (1/hiA)+(L/kA)+ (1/hoA)
Rth = (1/10*0.35*0.15)+(L/0.22*0.35*0.15)+ (1/40*0.35*0.15)
L = 21.87mm
Round dog bone: Length, L = 127mm
Flat dog bone: Length, L = 101mm
Gripping mechanism: Length, L ~ 40mm
Only round dog bone specimens will tested – ASTM creep/fatigue specimens. Full dimensions below: ((+ gripping mechanism size))
The bottom pull rod will be fixed in place but the top pull rod will be adjustable such that the specimen can be placed into the gripping mechanisms. Therefore this will be considered when estimating the height of the furnace and its chamber. Preferably, the chamber should be small in size such that it can be heated up quicker and use up less energy thereby increasing the efficiency of the furnace.
Chamber width, WC = 150mm
Chamber Length, LC = 150mm
Chamber Height, HC = 350mm
As the minimum insulation thickness ranges from 21 to27mm (depending on the heating element’s power rating) therefore, it will be necessary to have two layers of 25mm thick insulation around the chamber. So the outer body dimensions will be about:
Width, W = 250mm
Length, L = 200mm
Height, H = 450mm
The main body will almost certainly be made from mild steel due to its relatively low cost and robust, tough nature. The heating element will have to be either kanthal A1 or Nichrome most probably in wire form so that it may be easily routed into specially made grooves in the firebrick. With temperature ranges up to 1800C and relatively low cost kanthal A1 may be more suitable in this case. The time for the furnace to reach its required temperature will also be taken into account and the choice of heating element (power rating, shape and material) will be based on the time it takes to heat the chamber to a stable working temperature and the thickness of insulation required to work at such energy inputs. Also whether or not the heating element can be fitted into the insulation material will be a factor.
Temperature measurement and control device will most probably be a standard high temperature thermocouple. There are many different types, shapes and sizes but most are relatively cheap and affordable regarding the project budget. The thermocouple will give the give the reading and control the temperature in the chamber (keeping it at a constant working temperature) by clicking the relay on and off when necessary.
The furnace body will have to be made from a hard, tough and relatively thick material. This is why I think that mild steel sheets should be purchased and formed to the desired shape. Processes such as bending and cutting can be undertaken at the mechanical engineering workshop at AUT.
Mild steel plates/sheets prices per quantity + list of possible materials, choose most suitable.
An idea that I have deemed suitable for the furnace is that a simple solid, robust steel filing cabinet could be converted into a furnace body. Simple tools and workshop resources available at AUT can be used to machine the necessary features to make it work as a muffle furnace. This could potentially save a great deal of investment that could be used in other areas where it is needed more (concerning the project).
The strain measuring device will have to be one that works efficiently under the high temperatures experienced with muffle furnaces. I have narrowed it down to either a high temperature extensometer or an LVDT. The most suitable device is the high temperature extensometer as they are specifically suited to such elevated temperatures and give an extremely accurate strain/displacement measurement beyond ASTM standards. They can also be attached easily to standard creep testing furnaces. However, a strain measuring device is an optional extra as the specimen displacement can be accurately measured after the specimen ruptures and simple calculations can be used to determine the strain experienced. A strain measuring device would be for convenience purposes only.
List of strain measuring devices, filter to high temp use devices, then the only suitable model + prices and deem if actually suitable. Speak with Wassim. Extensometer or LVDT.
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