• 2023-11-13
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Die material fatigue fracture

The concept of die fatigue and fracture
1. Fatigue: The performance change of mold metal materials under the repeated action of stress or strain is called fatigue.

2. Fatigue fracture: When the mold material is subjected to alternating cyclic stress or strain, the local structural changes and the continuous development of internal defects cause the mechanical properties of the material to decline, and eventually lead to the complete fracture of the product or material, this process is called fatigue fracture, which can also be referred to as metal fatigue. The stress causing fatigue fracture is generally very low, and the occurrence of fatigue fracture is often characterized by suddenness, high locality and sensitivity to various defects.

Classification of die fatigue fracture
1. High cycle fatigue and low cycle fatigue

If the stress level acting on the mold parts or components is low, the fatigue with more than 100,000 cycles of destruction is called high-cycle fatigue. For example, spring, drive shaft, fasteners and other products are generally seen in high cycle fatigue.

The stress level of the mold parts is high, the number of cycles of destruction is low, and the fatigue is generally less than 10,000 times, which is called low-cycle fatigue. For example, the fatigue damage of pressure vessels and steam turbine die parts belongs to low-cycle fatigue.

2. Stress and strain analysis

Strain fatigue - high stress, low cycle times, called low cycle fatigue;

Stress fatigue - low stress, high cycle times, called high cycle fatigue.

Mold composite fatigue, but in practice, it is often difficult to distinguish between stress and strain types, in general, two types of both, so called composite fatigue.

3. According to the mold load type classification

Bending fatigue, torsion fatigue, tension fatigue, contact fatigue, vibration fatigue, fretting fatigue.

3. Characteristics of mold fatigue fracture

Macro: crack source → expansion zone → transient fault zone.

Crack source: An area with grooves, defects, or stress concentration on the surface is a prerequisite for a crack source.

Fatigue expansion area: the section of the mold is relatively flat, and the fatigue expansion is perpendicular to the direction of stress, resulting in an obvious fatigue arc, also known as the beach or shell line.

Transient fracture zone: The mold fatigue crack rapidly expands to the instant fracture area, the fracture has traces of metal slip, and some products have radioactive stripes and shear lip zone in the transient fracture zone.

Micro: The typical feature of mold fatigue fracture is fatigue striation.

Cleavage and quasi-cleavage phenomena (crystallographic names, small planes appearing in microscopic images), as well as microscopic regional features such as dimple, also appear in some microscopic die samples.

4 Characteristics of fatigue fracture
(1) There is no obvious macroscopic plastic deformation when the mold breaks, and there is no obvious omen before the fracture, which is often caused by sudden occurrence, so that the damage or fracture of mechanical mold parts is very serious.

(2) The stress causing fatigue fracture is very low, often lower than the stress load of the yield strength under static load.

(3) After fatigue failure, the components of the three regions where the crack occurs, expands and finally breaks can generally be clearly displayed at the fracture.