Characteristics and causes of internal cracks

Introduction

Internal cracks are important physical defects in metal materials. It can cause metal materials to lose their mechanical properties, thus losing their large performance and service life. At present, the identification of internal cracks mostly relies on conventional non-destructive testing methods and other related test methods, but these methods have certain limitations, such as the time-consuming process and the high cost.

In order to reduce the impact of internal cracks, it is necessary to understand the characteristics and causes of internal cracks. The characteristics and causes of the internal crack problem can be attributed to the mechanical properties of the material, the process parameters of the production, the structure and properties of the microstructure, and the erosive environment. This paper will discuss the characteristics and causes of internal cracks.

Characteristics of Internal Cracks

Internal crack refers to longitudinal or transverse crack on the internal surface of the material. The most common type of internal crack is usually manifested in two forms: sub-surface crack and inter-granular crack. Sub-surface cracks are also known as surface-initiated cracks and are caused by impurities in the material or an inhomogeneous structure. They often occur at the surface of the material and can penetrate into the interior. Inter-granular cracks are cracks between grains, which are formed during the deformation or welding of the material.

In addition, the internal crack of the material also has other characteristics. For example, the depth of the internal crack is closely related to its magnitude and its formation mechanism. The depth of surface-initiated cracks is usually shallow, and it is deep for inter-granular cracks. In addition, both can also cause brittle fracture of materials and cause damage to the inner surface of the material.

Causes of Internal Cracks

In general, there are three main causes of internal cracks in materials.

The first is due to the mechanical properties of the material. Materials with weak strength and poor plasticity often have internal cracks after long-term loading or fatigue. For example, if the material is subjected to a large impact force or a large cyclic load, the material structure will have a large internal stress or strain, thus forming an internal crack.

The second is due to the process parameters of production. Volatile process parameters during production will cause the material structure to become inhomogeneous, thus making the material prone to forming internal cracks. For example, in the welding process, if the welding current is too large, the welding joint will generate a high local internal stress, which will cause the weld to crack and the material to form an internal crack.

The third is due to the structure and properties of the microstructure of the material. The microstructure of the material is the main factor in determining the properties of the material, and it is also one of the main causes of internal cracks. For example, materials with an inhomogeneous microstructure, such as large and uneven grain size, low carbon content, and dense small particles, will easily form internal cracks during the production process.

Conclusion

In conclusion, internal cracks are caused by certain mechanical properties of the material, processing parameters of production, and structural and property characteristics of the microstructure. It is important to understand the characteristics and causes of internal cracks in order to reduce the impact on the material’s performance and service life.

Internal cracks are cracks which occur inside a material, rather than on the surface. They can be identified by visual inspection or by using nondestructive testing such as X-ray or ultrasound. In some cases, internal cracks can weaken the material, reducing its strength and making it more prone to failure.

Common causes of internal cracks include stress corrosion cracking, hydrogen embrittlement, residual stress, thermal cycling and fatigue. Stress corrosion cracking occurs when a material is exposed to a combination of stress and a corrosive environment. Hydrogen embrittlement occurs when hydrogen atoms accumulate within the material, weakening it. Residual stress can be present due to welding or other fabrication processes. Thermal cycling occurs when the material is exposed to repeated cycles of heating and cooling. Fatigue occurs when a material is subjected to repeatedly cycling forces, leading to cracks forming due to material fatigue and eventual failure.

It is important to identify, and address, the underlying cause of the internal crack in order to prevent further damage or failure. This can involve changing material properties, improving fabrication processes or changing the environment or application to reduce stress and corrosion.