A creep testing machine is often used to test metal and non-metal materials for tensile, compression, creep, and relaxation, as well as for low cycle fatigue and creep fatigue.
What is the distinction between high temperatures creep and stress relaxation?
Creep is the process by which metals slowly change shape when they are subjected to constant temperature and load over a long period of time. When the temperature is high, the effect of creep is very noticeable. It can be broken down into high temperature compression creep, high temperature tensile creep, high temperature bending creep, and high temperature torsional creep based on how the stress is applied. Most of the time, the terms “creep limit” and “rupture strength” are used to talk about how materials change over time.
High temperature creep is when a metal’s stress stays the same even though it is exposed to both high temperature and stress at the same time, and its inelastic deformation slowly gets worse over time. Creep is caused by three things: a high temperature, stress, and time.
When there is more stress, the temperature goes up, and the longer the temperature stays high, the worse the creep. Stress relaxation is the process by which metal parts that work at high temperatures change from elastic deformation to inelastic deformation over time, even though the total deformation conditions stay the same. This causes the stress in the metal to go down slowly and stay the same.
Creep and stress relief are pretty much the same thing. They are the processes that happen when materials that can’t change shape change over time at high temperatures. Under certain conditions, stress relaxation is the change of some elastic deformation into non-elastic deformation. On the other hand, creep is the direct deformation of an inelastic material under long-term stress.
The high temperature creep testing machine is mostly used to measure the creep and durability of metals, alloys, composites, and structural parts at a certain temperature and constant tensile load. At the same time, it can be used as a machine to test how relaxed you are.
Comparison between electronic creep testing machines and mechanical creep testing machines
There are two kinds of creep testing machines: those that are mechanical and those that are electronic. Both have their own benefits. The electronic creep testing machine, on the other hand, can do more than just creep endurance tests. It can also do stress relaxation tests, creep cycle tests, and other things. It has no effect, and no test force is automatically added or taken away.
At the same time, it can control constant deformation, uniform deformation rate, and other indicators through multi-step operation. This lets the metal material be stretched at a higher temperature in a short amount of time, or the tensile test can be done at room temperature. Most of the time, an electronic creep testing machine is used for a one-quarter test, which takes a short amount of time. If the test will take a long time, the best way to do it is with a mechanical testing machine.
Electronic and mechanical creep testing machines usually have specifications of 30, 50, 100, and 200 kN. All testing machines, including creep testing machines, have a host, a high-temperature furnace, a temperature control system, a measurement and control system, accessories, and a computer system. A beam, a servo motor, a column, a photoelectric encoder, and other parts make up the host. The beam’s job is to hold things up. The top of the table and the upper beam are connected by columns, usually two. This makes the frame look like a portal.
The features of the electronic creep testing machine:
For gapless transmission, the mechanical transmission is made up of a harmonic reducer, an arc synchronous belt, and a pair of ball screws. A stable transmission and a fast speed make it possible to control the test force (or deformation). Changes in control are very small.
Because there are no gaps in the transmission, the waveform during the cycle is true and not distorted. This means that the low cycle test can be done. Because it uses a full digital measurement controller and a low-noise sensor, the measurement control is stable for a long time, and its drift is 4 code/24h or less, so it meets the requirements of the creep endurance (or stress relaxation) test.
The electronic creep testing machine can do the same things as the mechanical testing machine, plus the following:
- At the beginning of the creep test, the test force is applied without stopping, and the output elongation curve is drawn automatically to show the elastic deformation and the first plastic deformation.
- It can do stress relaxation tests, automatically draw stress time or relaxation rate time curves, and measure stress relaxation performance parameters like stress relaxation rate and relaxation stress.
- It has a gapless collet and other parts that let it test metals for axial equal amplitude low cycle fatigue. On the strain failure cycle number curve and the stress-strain curve, it can get information about the fatigue strength index, the fatigue ductility index, the fatigue strength coefficient, the fatigue ductility coefficient, the hardening index, and the cyclic strength coefficient.
Things to know before using a high temperature creep testing machine
Even though the high temperature creep testing machine has made our lives easier, we need to pay attention to the following things when we use it:
Pull rod system
The pull rod system is mostly used to make changes to the structure of the equipment. The pull rod system is in charge of making sure the equipment is balanced. If there is a problem with the pull rod system, it will make it hard to measure the equipment normally and you won’t be able to use it. So, before using, it’s important to make sure the pull rod system is working right. Once a problem with the pull rod system is found, it needs to be fixed.
Temperature control system
The temperature control system mostly makes sure that the equipment works in a reasonable way so that it can work normally. Temperature is one of the things that can affect how the testing machine works, as it is just one of many things that can affect how it works. So, when we use a temperature control system, we should pay attention to how we use it to make sure the data we get is correct and not affected by other things.
When used for a long time, the indicator light above the host base will show that pulling the sample to break it will cause the lever to become very out of balance. At the same time, the micro switch on the end of the lever is opened, which lets electrical appliances be used in strange ways. Right now, the problem needs to be fixed as soon as possible. But the indicator light shows something different, and the crack happens in a different way. If the broken sample does not affect other parts, the light will show “red.” If it does affect other parts, the light will show “green.” Check and deal with the equipment no matter what color you see.
These are just some of the things we need to keep in mind as we work and make sure that the data we collect is accurate, like:
Place the testing machine correctly.
When the creep testing machine is being used, it needs to be set up in the right way. The testing machine can’t be tested until it’s in place. If the testing machine is not put in a stable place, it will not only change how materials are measured, but it will also do a lot of damage to the machine itself. So, be careful about where you put the testing machine so that it doesn’t change the final results.
Check the testing machine before use.
Before using the creep testing machine, it should be checked. When the testing machine is not in use, some parts may not be in the right place, so all parts should be in the right place to keep the measurement results from being affected. If the measurement pointer is not in the right place, it won’t be measured at the beginning. This is especially true if the measurement pointer is not in the right place.
Materials are placed correctly.
When testing materials, the materials should be placed well, not at will, so that the equipment cannot measure the material results. When placing materials, it is recommended to conduct multiple inspections to avoid incorrect placement.
Close after using
After using the creep testing machine, make sure to turn it off and set all the parts back to zero at the same time. This will make sure there are no problems the next time it is used. Here’s a reminder that the testing machine should be cleaned after use so that it doesn’t hurt the equipment.
How to reduce the error rate of the creep testing machine?
Factors causing errors
For example, if the installation of the main engine is not sufficiently horizontal, it will lead to a certain friction between the working piston and the working oil cylinder wall, which will cause errors, usually positive errors. Then, with the increase in load, the errors will gradually decrease.
The Dynamometer section
If the dynamometer is not installed horizontally, there will be friction between the pendulum shaft bearings, which will cause a negative difference.
Of course, these two errors will have a great impact on small load measurement. If it is a large load measurement, the impact will gradually become smaller.
What are the representations of the creep test limits?
The creep limit is the maximum stress that can be applied to a sample without causing it to deform or move too quickly at the temperature and time it is supposed to be. This stress is called the creep limit.
It can be expressed in two ways:
- Expressed as, where t is the test temperature (°C), t is the test time (hours), and % is the specified creep deformation. For example, 150 MPa, which means that the creep limit of a material is 150 MPa when the temperature is 600 °C, the test time is 100,000 hours, and the total creep deformation is 1%.
- It is represented by symbols, where t represents the test temperature (°C) and V represents the creep speed of the second stage of creep (%/hour). For example, 100 MPa means that the creep limit of a material is 100 MPa when the temperature is 700 °C and the creep rate is (1/105%) %/hour.
The tensile creep test method is to: under a certain constant temperature, put a group of samples under different constant stresses to test, get a series of creep curves, and then draw the relationship curve between creep speed and stress at this temperature on double logarithmic coordinate paper, so as to calculate the creep limit under the specified creep speed.
The electronic creep testing machine can do stress relaxation tests, low-cycle creep cycle tests, and creep endurance tests. It can automatically put the test force on or take it off without making a sound. It has several steps that let you control the constant test force, the uniform test force, the constant deformation, and the uniform deformation rate. At the same time, it can quickly test the tensile strength of metals at high temperatures or at room temperature.