For materials that do not work at room temperature, the stress-strain relationship cannot be simply used to evaluate the mechanical properties, but two factors need to be added, temperature and time, and the high and low temperature properties of the material need to be considered, and the durable strength is one of the indicators that represent the high temperature performance of metal materials.
Long-lasting strength
The endurance strength of a metal material is its ability to resist fracture under the action of high temperature and long-term load, and its endurance strength can be judged by testing the endurance strength limit of the metal material, that is, the maximum stress of the metal material under constant temperature and constant tensile load to reach the specified duration without breaking. Especially for those parts that do not consider the amount of deformation during high-temperature operation, but only consider their service life under a given stress, it is very important to use scientific methods to test their lasting strength. 
For high-pressure boilers, steam turbines, aero engines, diesel engines, high-temperature and high-pressure pipelines and other equipment, when they work, some parts of the metal materials are in a high-temperature load-holding state for a long time, which will not only lead to the occurrence of plastic deformation of the material, if the lasting strength is not up to standard, when the design life is not reached, the metal material will be broken, which will affect the ability of the product or device to work normally. Imagine what would happen if the design life of a certain type of aircraft jet engine was 1,500 hours, and when it suddenly broke after 1,000 hours of operation.
Influencing factors
A variety of factors will have a direct impact on the lasting strength of metal materials, such as material quality, design scheme, manufacturing process, working environment, etc., for example, the corrosive effect of environmental medium on materials will intensify with the increase of temperature, thereby affecting the mechanical properties of materials;Selecting the right manufacturing process based on the characteristics of the material can ensure the quality and performance of the metal material, thereby improving the long-lasting strength of the metal material. 
Among the many influencing factors, temperature is one of the key factors, and in a high-temperature environment, metals and alloys will undergo complex changes such as diffusion, recovery and recrystallization, resulting in changes in the structure of the materials. The "high" or "low" temperature is relative to the melting point of the metal, so the high temperature of all kinds of metal materials cannot be unified into a certain temperature value. Time is another key factor of the lasting strength of metal materials, in order to accurately measure the lasting strength of the material, usually according to the actual use conditions and design life of the test, for the design life of the longer parts, because the long-term test has certain difficulties and practical challenges, so generally make the test data of greater stress and short breaking time, and use the extrapolation method to find the lasting strength of the material.
Test detection
One of the main steps in testing the enduring strength of metal materials is the preparation of the specimen. When selecting the sample blank, the representative part in the metallurgical structure and mechanical properties should be selected, and there should be enough machining allowance, and the structure and properties of the tested metal should not be changed in the process of sample preparation, and it should be inspected according to the requirements after the sample preparation is completed. In order to ensure the smooth operation of the test, the test should be carried out in a laboratory with a clean environment, away from the seismic source, and free of corrosive gases, and when a specimen breaks, the adjacent testing machine or specimen should not be subjected to impact forces. The testing machines, fixtures, heating devices, temperature measuring devices and timing devices used should meet the relevant requirements and be verified or calibrated as needed. 
Before stretching, the cross-sectional area and original length of the specimen should be measured according to the regulations, and the thermocouples should be arranged. The temperature overshoot during the heating process of the sample should not exceed the corresponding specified value, and the heat preservation should be started after the temperature is stable, and the time and temperature of heating, heat preservation, and loading and unloading should be recorded as required. If an unexpected situation occurs during the test and the test is stopped, the load on the sample should be removed, and the original load should be kept warm for 30 to 60min after reaching the test temperature before reapplying the original load again. Since the durable strength test of metal materials is to be carried out until the specimen breaks, this test can effectively reflect the strength and plasticity of metal materials when they are used for a long time at high temperature until they break, and provide a scientific basis for material research and development and project design, which has very important practical significance in the development of many industries.