A creep-testing machine measures the alteration of a material after it has undergone stresses. Engineers use Creep machines to determine the stability and behaviour of a material when put through ordinary stresses. They determine how much strain (

load Load or LOAD may refer to: Aeronautics and transportation *Load factor (aeronautics), the ratio of the lift of an aircraft to its weight *Passenger load factor, the ratio of revenue passenger miles to available seat miles of a particular transpo ...

) an object can handle under pressure, so

engineers Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the limit ...

and researchers are able to determine what materials to use. The device generates a creep time-dependent

curve In mathematics, a curve (also called a curved line in older texts) is an object similar to a line (geometry), line, but that does not have to be Linearity, straight. Intuitively, a curve may be thought of as the trace left by a moving point (ge ...

by calculating the steady rate of creep in reference to the time it takes for the material to change.

Creep

Creep Creep, Creeps or CREEP may refer to: People * Creep, a creepy person Politics * Committee for the Re-Election of the President (CRP), mockingly abbreviated as CREEP, an fundraising organization for Richard Nixon's 1972 re-election campaign Art ...

is the tendency of a material to change form over time after facing high temperature and stress. Creep increases with temperature and it is more common when a material is exposed to high temperatures for a long time or at the

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends ...

of the material. Creep machines are used to understand the creep of materials and determine which type can do the job better, which is important when making and designing materials for everyday uses. They most commonly test the creep of

alloys An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

and

plastics Plastics are a wide range of synthetic polymers, synthetic or semi-synthetic materials that use polymers as a main ingredient. Their Plasticity (physics), plasticity makes it possible for plastics to be Injection moulding, moulded, Extrusion, e ...

for the understanding of their properties and advantages of one material's use over another.

Background

The first creep testing machines were created in 1948 in Britain to test materials for

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...

to see how they would stand in

high altitudes Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

.A. I. Smith, D. Murray, M. F. Day, "Creep Testing Equipment-Design Features and Control"; Volume 180 Pt. 3A, 1965-66, pg.303-307 The machines were first developed to further calculate and understand the steady rate of creep in materials.

Design

Researchers look to test objects with a creep machine to understand the process of

metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the sc ...

and the physical mechanical properties of a

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

, test the development of

, receive

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from the loads that are derived and to find out whether a sample or material is within the boundary of what they are testing. The basic design of a creep machine is the

furnace A furnace is a structure in which heat is produced with the help of combustion. Furnace may also refer to: Appliances Buildings * Furnace (central heating): a furnace , or a heater or boiler , used to generate heat for buildings * Boiler, used t ...

, loading device and support structure. The main type of creep testing machine is a constant load creep testing machine. The constant load creep machine consists of a loading platform, foundation, fixture devices and furnace. The fixture devices are the grips and pull rods. *Load platform or load hanger is where the object will endure

at a constant rate. *Grips hold the material in a certain position. Position is important because if the

alignment Alignment may refer to: Archaeology * Alignment (archaeology), a co-linear arrangement of features or structures with external landmarks * Stone alignment, a linear arrangement of upright, parallel megalithic standing stones Biology * Structu ...

is off, the machine will deliver inaccurate creep readings. *Dial Gauge is used to measure the strain. It is the object that captures the movement of the object in the machine. The load beam transfers the movement from the grip to the dial gauge. *Heating Chamber is what surrounds the object and maintain the temperature.

Applications

Creep machines are most commonly used in

experiments An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when ...

to determine how efficient and stable a material is. The machine is used by students and companies to create a creep curve on how much pressure and stress a material can handle. The machine is able to calculate the stress rate, time and pressure. Creep testing has three different applications in the industry: #Displacement-Limited applications : the size must be precise and there must be little errors or tendency to change. This is most commonly found in

turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating e ...

rotors in

jet engines A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, water jet, and hybrid propulsion, the term typicall ...

. # Rupture Limited applications: in this application the break cannot occur to the material but there can be various dimensions as the material goes through creep. High pressure tubes are examples of them. #Stress relaxation limited application : the tension at the beginning becomes more relaxed and the tension will continue to relax as the time goes by, such as cable wires and bolts.Dr. Sabbah Ataya, "Creep Testing Machines", 2008, http://www.slideshare.net/ea2m/creep-testing-machines-presentation

Graphing of creep

Creep is dependent on time so the curve that the machine generates is a time vs. strain graph. The

slope In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...

of a creep curve is the creep rate dε/dt The trend of the curve is an upward slope. The graphs are important to learn the trends of the alloys or materials used and by the production of the creep-time graph, it is easier to determine the better material for a specific application.

Stages of creep

There are three stages of creep: *Primary Creep: the initial creep stage where the slope is rising rapidly at first in a short amount of time. After a certain amount of time has elapsed, the slope will begin to slowly decrease from its initial rise. *Steady State Creep: the creep rate is constant so the line on the curve shows a straight line that is a steady rate. *Tertiary Creep: the last stage of creep when the object that is being subjected to pressure is going to reach its breaking point. In this stage, the object's creep continuously increases until the object breaks. The slope of this stage is very steep for most materials. By examining the three stages above, scientists are able to determine the temperature and interval in which an object will be disturbed once exposed to the load. Some materials have a very small secondary creep state and may go straight from the primary creep to the tertiary creep state. This is dependent on the properties of the material that is being tested. This is important to note because going straight to the tertiary state causes the material to break faster from its form.Experiment 4 Lab, Center for Reliability of Ceramics, Department of Mechanical Engineering, University of Houston, A

linear graph In the mathematical field of graph theory, a path graph or linear graph is a graph whose vertices can be listed in the order such that the edges are where . Equivalently, a path with at least two vertices is connected and has two terminal ...

denotes that the material under stress is gradually deforming, and this would be harder to track at what level of stress an object can handle. This would also mean that the material would not have distinct stages, which would make an object's breaking point less predictable. This is a disadvantage to scientists and engineers when trying to determine the level of creep the object can handle.Narayana, V. J. S., K. Balasubramaniam, and R. V. Prakash. "Detection And Prediction Of Creep-Damage Of Copper Using Nonlinear Acoustic Techniques." AIP Conference Proceedings 1211.1,2010, pg.1410-1417

References

{{Reflist Measuring instruments