Regarding creep properties of materials, today I would like to explain it in detail in the form of an article. After discovering this characteristic of materials, we also need to pay attention when selecting materials. Especially when our products will continue to be stressed and operate at high temperatures, we need to pay attention to creep indicators of materials.
In addition, for product design, if product needs to withstand long-term fixed deformation, stress relaxation is a very important consideration. Especially for plastic parts, due to stress relaxation, plastic parts that have been subjected to long-term fixed deformation will have a reduced holding force or elasticity over time, causing product failure. For example, plastic gears, etc.

 

Phenomenon that strain of a solid material increases with time while keeping stress constant. It is different from plastic deformation, which usually occurs after stress exceeds elastic limit, while creep can also occur when stress is less than force exerted by elastic limit as long as stress acts for a long time. Many materials, such as metals, plastics, rocks, and ice, exhibit creep properties under certain conditions. Due to creep, stress state of a material at a certain instant is generally related not only to instantaneous deformation, but also to deformation process before instant. Many engineering problems involve creep. In materials that maintain constant deformation, stress will decrease with time. This phenomenon is stress relaxation, which can be understood as a generalized creep.
Simply put, it is a phenomenon in which materials slowly undergo plastic deformation under action of constant temperature and constant stress for a long time.
Occurrence of creep deformation and eventual rupture of material is called creep rupture.
A common creep phenomenon in life is that if we hang a raincoat on the wall for a long time, you will eventually find that length of the raincoat will become longer; deformation of rocks in nature, height of mountain rising at a certain rate every year, etc. are all creep change phenomenon.
In addition, we know that temperature has a great influence on mechanical properties of materials, so creep is also affected by temperature. Therefore, material will also have a corresponding creep temperature. However, under action of stress, creep can occur at any temperature. Generally, at low temperatures, creep deformation is not obvious and can be ignored. In engineering, 0.3~0.4 times melting point temperature of material is usually regarded as creep temperature of material.

Creep phenomenon of materials is mainly caused by following five aspects:
· Temperature: ≥0.3~0.4Tm (melting point temperature of material). The higher temperature, the greater creep deformation will be.
· Stress: Stress can be mechanical internal stress, but also includes external stress such as gravity, electric field, etc. The greater load, the greater creep deformation produced.
· Time factor: Creep rate has an exponential relationship with time. As time prolongs, creep phenomenon intensifies. As long as load is applied for too long, damage and cracking may occur.
· Material: Creep properties of different materials vary greatly. Even if same material is subjected to different stresses and temperatures, creep curves will be different.

 

· Creep mechanism: There are two types: diffusion and slip.

Creep can be divided into three stages, as shown in figure below.

 

I is deceleration creep stage: creep rate decreases with time. With plastic deformation of material, work hardening occurs, and deformation resistance increases, so strain change rate decreases.
II is constant-speed creep stage: creep rate remains constant, and this stage usually takes the longest time. As degree of work hardening increases, material will show softening and recovery characteristics to resist hardening and a recrystallization process will occur until the two are balanced, that is, strain rate is constant.
III is accelerated creep stage: creep rate increases with time until creep rupture occurs. As plastic deformation further expands, micropores or cracks will occur in material, necking will occur, and strain rate will increase until it breaks.

 

Figure below also shows curve of creep rate in three stages over time, for reference.

Creep limit is material's resistance to creep deformation at high temperatures.
It is divided into two situations: ① At a given temperature, maximum stress when specimen produces a specified steady-state creep rate in constant creep stage; ② Under conditions of a given temperature and time, specimen produces a specified creep rate. Maximum stress for creep strain.
Therefore, there are two ways to express creep limit (unit: MPa):
①

 

Such as

 

②

 

Such as:

 

In addition, some parts that operate at high temperatures have very little creep deformation or do not have strict requirements on deformation. They only require that parts do not break during their service life. In this case, durable strength is needed as a basis for evaluating materials and designing parts.
What is lasting strength?
It is maximum stress at which material will not creep rupture at a certain temperature and within a specified time, which is also ultimate strength limit.
Endurance strength limit represents material's fracture resistance, which is different from creep limit.
Its representation method is:

 

Such as:

 

Write at the end
Creep limit and endurance strength limit of materials are generally not directly reflected in material specifications or physical property tables. You can ask raw material supplier whether they have done creep and endurance tests. If not, you can ask supplier to provide it if you need it.