A Materials Testing Study: Creep Behavior and Measurement
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1 ortonceramic.com The Edward Orton Jr. Ceramic Foundation A Materials Testing Study: Creep Behavior and Measurement Understanding creep behavior provides insights for design improvements and quality assurance. A phenomenon known as creep can really wreak havoc for manufacturers of metals, refractories, ceramics, and other materials required to withstand high temperatures. As the name implies, creep is the time-dependent, permanent deformation of material that occurs at elevated temperatures (typically greater than one-half of its melting point). Creep can potentially cause unsafe conditions, reduce manufacturing efficiencies, and cost manufacturers hundreds of thousands of dollars in repair costs and lost time. Materials vulnerable to creep should undergo creep testing as part of a manufacturer’s product development and quality assurance processes. Design engineers may select a material with a high-melting temperature, high-elastic modulus and large grain size in order to resist creep. However, creep testing is necessary to understand dimensional changes over time at elevated temperature as well as differences in creep resistance due to variations in the material. This Materials Testing Study offers scientific knowledge of creep behavior and insights into design modifications for materials vulnerable to creep. The primary source of information derives from the more technically detailed article, Measuring Flexural Creep of Ceramic Investment Casting Shells, authored by Dr. Joseph Homeny, Director of Material Testing and Research at The Edward Orton Jr. Ceramic Foundation. Challenge: Material selection— will it stand the test of time and temperature? Temperature and mechanical stress can lead to creep, tempera- ture being the primary factor. A slow-failure mechanism, creep occurs in material exposed to high temperature at stresses below its elastic limit. The material deforms in the direction of the ap- plied stress. The amount of deformation (strain rate) increases as the tem- perature or stress increases. Creep can lead to catastrophic failure when the material can no longer bear the applied load. Examples of structures subject to creep failure are glass melting tanks, investment casting molds, kiln furniture, and sulfur recov- ery units (SRUs), to name a few. To avoid the consequences of creep, it is critically important ma- terial samples undergo creep testing to know the properties and advantages over other material. There are three stages of creep behavior: • Primary—rate of creep decreases • Secondary—rate of creep remains constant; this is the region of steady-state creep • Tertiary—rate of creep increases; this is the region of accelerat- ing creep, which leads to failure. Classical creep analysis usually involves the determination of the steady state creep rate as a function of stress and temperature. Creep measurement and analysis of the data leads to the calcu- lation of n, the stress power law exponent, and Q, the activation energy. These values are related to the physical processes that contribute to creep behavior. Once the creep mechanism(s) is(are) understood, the material can be engineered to be more creep resistant. CASE STUDY COMMON PURPOSES FOR CREEP TESTING R&D • New product • New raw material evaluation • New application Quality control • Internal requirement • Customer requirement Failure analysis Sales tool (proof of stability/efficiency) Typical creep curve illustrating the three stages of creep behavior DEFORMATION TIME Elastic Extension Primary Creep Secondary Creep Tertiary Creep 0 0
Transcript of A Materials Testing Study: Creep Behavior and Measurement
1ortonceramic.com The Edward Orton Jr. Ceramic Foundation
A Materials Testing Study: Creep Behavior and Measurement
Understanding creep behavior provides insights for design improvements and quality assurance. A phenomenon known as creep can really wreak havoc for manufacturers of metals, refractories, ceramics, and other materials required to withstand high temperatures. As the name implies, creep is the time-dependent, permanent deformation of material that occurs at elevated temperatures (typically greater than one-half of its melting point).
Creep can potentially cause unsafe conditions, reduce manufacturing efficiencies, and cost manufacturers hundreds of thousands of dollars in repair costs and lost time. Materials vulnerable to creep should undergo creep testing as part of a manufacturer’s product development and quality assurance processes. Design engineers may select a material with a high-melting temperature, high-elastic modulus and large grain size in order to resist creep. However, creep testing is necessary to understand dimensional changes over time at elevated temperature as well as differences in creep resistance due to variations in the material.
This Materials Testing Study offers scientific knowledge of creep behavior and insights into design modifications for materials vulnerable to creep. The primary source of information derives from the more technically detailed article, Measuring Flexural Creep of Ceramic Investment Casting Shells, authored by Dr. Joseph Homeny, Director of Material Testing and Research at The Edward Orton Jr. Ceramic Foundation.
Challenge: Material selection— will it stand the test of time and temperature? Temperature and mechanical stress can lead to creep, tempera-ture being the primary factor. A slow-failure mechanism, creep occurs in material exposed to high temperature at stresses below its elastic limit. The material deforms in the direction of the ap-plied stress.
The amount of deformation (strain rate) increases as the tem-perature or stress increases. Creep can lead to catastrophic failure when the material can no longer bear the applied load. Examples of structures subject to creep failure are glass melting tanks, investment casting molds, kiln furniture, and sulfur recov-ery units (SRUs), to name a few.
To avoid the consequences of creep, it is critically important ma-terial samples undergo creep testing to know the properties and advantages over other material.
There are three stages of creep behavior:
• Primary—rate of creep decreases
• Secondary—rate of creep remains constant; this is the region ofsteady-state creep
• Tertiary—rate of creep increases; this is the region of accelerat-ing creep, which leads to failure.
Classical creep analysis usually involves the determination of the steady state creep rate as a function of stress and temperature. Creep measurement and analysis of the data leads to the calcu-lation of n, the stress power law exponent, and Q, the activation energy. These values are related to the physical processes that contribute to creep behavior. Once the creep mechanism(s) is(are) understood, the material can be engineered to be more creep resistant.
CASE STUDY
COMMON PURPOSES FOR CREEP TESTING
R&D
• New product
• New raw material evaluation
• New application
Quality control
• Internal requirement
• Customer requirement
Failure analysis
Sales tool (proof of stability/efficiency)
Typical creep curve illustrating the three stages of creep behavior
DE
FO
RM
AT
ION
TIME
Elastic Extension
Primary Creep
Secondary Creep
Tertiary Creep
00
2ortonceramic.com The Edward Orton Jr. Ceramic Foundation
Material Testing: Measures stability under pressure A creep-testing instrument measures the deformation of a ma-terial at elevated temperatures after it is placed under different forms of stress: compressive, flexural or tensile. The instrument measures how much strain a specimen can handle under pres-sure. These measurements provide important information as to material or component properties under longer term conditions.
It is critically important to know the rate of deformation at a given load and temperature if the components are to be safely designed for high- temperature/high-strain service.
During creep testing, stress is applied to the specimen while a furnace heats it uniformly. A non-contact laser micrometer mea-sures the deformation of the specimen.
Creep-testing instruments measure time-dependent deforma-tion data which is used to calculate the steady rate of creep. As shown in the graph below, increasing either the temperature or stress results in both a greater strain to failure and a shorter time to failure. For more detailed information on creep analysis, refer to Dr. Homeny’s article, Measuring Flexural Creep of Ceramic Investment Casting Shells.
Water CooledCoupler
Creep Test Frame
Furnace
Laser DetectorLaser Source
Laser BeamSample
AI203Support
Block
AI203Support
Rods
AI203 Load Ram
AI203 Load Ram
Water CooledCoupler
Schematic diagram illustrating the setup for flexural creep measurement
Effect of temperature and stress on creep behavior
DE
FO
RM
AT
ION
TIME
Increasing stressor temperature
00
Creep testing results of these firebrick samples showed Com-position A (on left) had deformed 0.1% at 1600C while com-position B (right) deformed 5% at 1375C. Why Composition A proved more creep-resistant than B could be due to a number of different factors, including: 1) It was made from higher qual-ity raw materials (less Fe2O3, TiO2, or SiO2), 2) It contained a higher concentration of alumina 3) The composition of B was such that it formed liquid at the test temperature.
The Edward Orton Jr. Ceramic Foundation • Materials Testing and Research Center • www.ortonceramic.com
For information, email Brian Rayner at [email protected] or call (614) 818-1321.
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Key takeaways • Creep is the gradual deformation of material that occurs at a
measurable point in time and temperature.
• Materials vulnerable to creep should undergo creep testing as part of a manufacturer’s product development and quality assurance processes.
• Product engineers should know the rate of deformation at a given load and temperature if the components are to be safely designed for high- temperature/high-strain service.
• Understanding creep behavior provides insights for design improvements and quality assurance.
For inquiries about creep testing or other materials testing services, contact The Edward Orton Jr. Ceramic Foundation at (614) 818-1321. The testing services group performs physical properties tests, thermal analysis measurements, and provides consulting services on ceramics, carbon, cement, glass, refractory, plastics and numerous other materials.
