The following two measurement methods are applicable to stainless steel castings, alloy steel castings, and carbon steel castings.
Endurance performance test
The endurance performance test is to determine the strength of a material that does not break at a given temperature for a given time. This strength is called endurance strength, also known as endurance fracture strength. The difference between it and the creep test is that it does not require the creep of the material to be measured. It only requires the maximum stress that can be withstood at a given temperature and after a certain time. This data is mainly used to design certain parts that operate at high temperatures and only require a service life under a given stress state without considering the deformation.
(1) Test equipment
The endurance test can be carried out using a creep tester. The difference is that the test temperature is higher, the load is larger, and the test cycle is shorter. For these reasons, it is best to use a dedicated testing machine for endurance testing.
(2) Samples, test process and results
The samples used in endurance performance tests are basically the same as those used in creep tests. The process of determining the endurance limit is also similar to that of creep tests, but it is simpler and does not require measuring its deformation over time unless there are special requirements.
Usually, isothermal endurance tests are carried out at more than 5 appropriate stress levels. It is recommended that at least 3 of the stresses have 3 data each. The stress-rupture time curve is plotted on a single logarithmic or double logarithmic coordinate using a graphical method or the least squares method, that is, the relationship between stress and rupture time at a given temperature. Of course, the relationship between stress and rupture time at different temperatures can also be plotted on the same graph for comparison. From these curves, the endurance strength under specified conditions and the required design data can be obtained by interpolation and extrapolation.
High-Temperature Creep and Endurance Performance
The phenomenon of increasing plastic deformation of a material over time under a specified temperature and constant force is called creep. This phenomenon is particularly pronounced when the temperature exceeds the recrystallization temperature of the steel. Improving the creep resistance, that is, the creep strength and endurance strength, of cast steel materials used in equipment operating under high-temperature and high-pressure conditions, such as steam turbines and boilers, is particularly important.
At a given temperature and a given tensile load, when the stress on the material is less than its yield strength, the specimen slowly elongates with loading time, which is the creep curve. In addition to the elastic deformation produced during loading, the curve can be roughly divided into three stages: initially, the creep elongation rate gradually decreases with time (stage 1 creep); then it reaches a constant value (stage 2 creep); and finally, it rapidly increases until the specimen breaks (stage 3 creep). The creep curves of ductile and brittle materials have roughly the same shape, but the third stage is shorter for brittle materials, and the total elongation at rupture is also smaller.
The creep curve has different shapes depending on the test stress and temperature. Under the same temperature but different stresses, or under the same stress but different temperatures.
Although the creep curves vary in shape under different temperatures and stresses, they can almost always be divided into the three stages described above. The main difference lies in the slope of the curve. At high temperatures or high stresses, the curve is steeper, meaning the slope is larger. Secondly, the duration of the second stage varies. Under high temperatures or high stresses, the second stage is very short, even shrinking to a point. Conversely, under lower temperatures or lower stresses, the second stage can last for a long time.
When designing mechanical parts, it is usually required that the material does not experience the third stage of creep within the design life. To achieve this, the design operating stress is very low. In this case, the first stage creep deformation accounts for a small proportion, and the second stage deformation is the main factor. Therefore, for materials used for long periods of time, the second stage creep rate is often used as an indicator of creep performance.
The creep rate refers to the relative deformation per unit time, expressed in %/h. Creep strength, also known as creep limit, is generally defined in two ways: one is the maximum stress at which the second-stage creep rate reaches a specified value within a specified time at a certain temperature; the other is the maximum stress at which the total creep elongation or creep plastic elongation reaches a specified value within a specified time at a certain temperature. Under actual stresses, the first-stage creep deformation is very small, and the difference in deformation determined by the two creep strengths is even smaller. Consequently, the difference in creep strength derived from the two definitions is also minimal. Because determining creep strength using the second-stage creep rate is more convenient and requires shorter testing times, it is widely used.
Endurance properties typically include the ultimate endurance strength, endurance elongation, reduction of area, and endurance notch sensitivity coefficient. The endurance strength is defined as the maximum stress that a specimen can withstand at a specified temperature for a specified time (e.g., 100 h) without breaking. The endurance elongation and cross-sectional shrinkage refer to the percentage of the elongation increment to the original length and the maximum reduction in cross-sectional area to the original area, respectively. The endurance notch sensitivity coefficient is the ratio of the endurance fracture time when the notch and smooth specimens are under the same stress test conditions; when the notch and smooth specimens are under the same test time conditions, it is the ratio of the test stress.
Post time: Oct-16-2025