Common Failure Modes of Metal Materials And Their Judgment
The failure modes of metal materials in various engineering applications are mainly fracture, corrosion, wear and deformation.
Deformation Failure
The deformation failure at normal temperature or low temperature is mainly composed of elastic deformation failure and plastic deformation failure. The elastic deformation failure is mainly due to excessive deformation or the loss of the original designed elastic function, and the plastic failure is generally the excessive deformation. Deformation failure at high temperature includes creep failure and thermal relaxation failure.
1. Elastic Deformation Failure
In the elastic state, the solid material absorbs the loaded energy and deforms depending on the change of the atomic spacing, but because the bonding force between atoms is not exceeded, when the cutting is unloaded, all the energy is released, the deformation completely disappears, and the material is restored to its original state. To have good elasticity, we should start with increasing the elastic limit of the material and reducing the elastic modulus.
2. Plastic Deformation Failure
Plasticity means that after the stress in the material exceeds the yield limit, it can produce significant irreversible deformation without immediate destruction. This significant and irreversible deformation is called plastic deformation. The indicators that usually reflect the quality of plastic properties of materials are elongation δ and area shrinkage φ. The higher the elongation and shrinkage at break, the better the plasticity. The plastic deformation of metal can generally be regarded as the defect movement of the crystal.
3. Deformation Failure of Metal Materials Under High Temperature
Under the action of high temperature for a long time, even if the stress value of metal components is less than the yield strength, plastic deformation will slowly occur. When the deformation exceeds the specified requirements, the plastic deformation of the components will fail. The high temperature referred to at this time is higher than 0.3Tm (Tm is the melting point of the metal material expressed as absolute temperature), generally carbon steel components are above 300°C, and low-alloy strength steel components are above 400°C.
3.1 Creep Deformation Failure
The phenomenon that metal slowly produces plastic deformation under the action of constant temperature and constant load for a long time (even if the stress is less than the yield strength at this temperature) is called creep. The rupture of a material caused by creep deformation is called creep rupture. From the creep deformation and fracture mechanism, it is known that to increase the creep limit, the rate of dislocation climbing must be controlled; to increase the lasting strength, the sliding of grain boundaries and the diffusion of vacancies must be controlled.
3.2 Stress Relaxation Deformation Failure
Under the condition of constant deformation, the phenomenon that the elastic stress gradually decreases with the extension of time is called stress relaxation. The performance of metal materials against stress relaxation is called relaxation stability, which can be evaluated by the stress relaxation curve determined by the stress relaxation test. Residual stress is an index to evaluate the stress relaxation stability of metal materials. The higher the residual stress, the better the relaxation temperature.
The creep of metal is the process of plastic deformation of the component under the condition of constant stress. While the relaxation rule of metal is that under the condition of constant total deformation, the elastic deformation of the component is continuously transformed into plastic deformation, so that the stress is continuously reduced. process.
Fracture Failure
Fracture is a phenomenon in which a metal material separates into two or more parts that are not connected to each other under the action of stress.
The fracture process of metal materials generally has three stages, that is, the initiation of cracks, the metastable expansion of cracks and the unstable expansion of cracks, and finally the fracture. Metal components may initiate cracks and initiate cracks under different conditions in the material manufacturing, component forming or use stages; and are affected by different environmental factors and load-bearing states, so that the cracks propagate until they break.
After the metal component is fractured, there are two matching fracture surfaces at the fracture site, which are called fractures. The fracture and its surroundings leave information closely related to the fracture process. Through fracture analysis, the type of fracture and the mechanism of fracture process can be judged, so as to find out the cause of fracture and measures to prevent fracture.
1. Classification of Fracture Failure
According to the degree of deformation before fracture, it is divided into ductile fracture and brittle fracture.
According to the type of stress causing the fracture and the macro orientation of the section and the relative position of the stress, it can be divided into normal fracture, cut off and mixed fracture. Normal breaks may be brittle or ductile, while cuts are generally always ductile.
According to the way of crack propagation in the fracture process, it is divided into three categories: intergranular fracture, transgranular fracture and mixed crystal fracture.
According to the nature of the load and the cause of stress, it is divided into fatigue fracture and environmental fracture.
According to the microscopic fracture mechanism, it can be divided into cleavage fracture, dimple fracture, fatigue fracture, creep fracture and bond weakening fracture.
2. Ductile Fracture
Ductile fracture refers to the fracture of the container and pipeline under the action of pressure, the stress generated on the wall of the container exceeds the strength limit of the material, and a significant macroscopic plastic deformation occurs.
Ductile fracture is a slow fracture process in which plastic deformation and crack growth occur simultaneously. The crack initiation and metastable propagation have large resistance and slow speed, and the material needs to consume a considerable amount of energy during the fracture process. With the continuous increase of plastic deformation, the bearing cross-sectional area decreases. When the load on the material exceeds the strength limit σb , the crack propagation reaches the critical length and ductile fracture occurs.
There are two types of ductile fractures: one is a normal fracture with the macro-section orientation perpendicular to the maximum normal stress, also known as a plane fracture, which occurs in situations where deformation constraints are large, such as fractures under plane strain conditions. The other is the cut-off in which the orientation of the attack section is consistent with the direction of the maximum shear stress, that is, about 45° to the maximum normal stress, also known as an oblique fracture, which occurs when the slip deformation is unconstrained or less constrained. Such as fracture under plane stress conditions.
3. Brittle Fracture
Brittle fracture means that the container has no macroscopic plastic deformation when it is ruptured, and the average stress of the container wall is far from the strength limit of the material, and some are even lower than the yield limit. The occurrence conditions of brittle fracture: The equipment and the container itself have defects or sudden changes in geometry. There is a certain level of stress. The toughness of the material is very poor.
4. Fatigue Fracture
Under the action of alternating loads, although the stress level of metal components is lower than the tensile strength of metal materials, the advantage is even lower than the yield limit, but after a certain cycle period, the metal components will suddenly break, which is called Fatigue fracture, fatigue fracture is a form of brittle fracture.
Wear Failure
Wear is the phenomenon of continuous loss of materials on the surface of materials or residual deformation and fracture due to mechanical action and chemical reactions (including thermochemical, electrochemical and mechanochemical reactions). Wear is a surface phenomenon that occurs on objects whose contact surfaces must have relative motion. Abrasion necessarily produces material loss (including material transfer), and it is a progressive dynamic process with time-varying characteristics.
According to the wear mechanism, wear can be divided into adhesive wear , abrasive wear , fatigue wear , corrosive wear , erosion wear , fretting wear , and can be divided into dry wear , wet wear and fluid wear according to the environmental medium.
Corrosion Failure
Corrosion is the physical or chemical reaction between the surface of the material and the service environment, causing damage or deterioration of the material. Corrosion of components that prevent them from functioning properly is called corrosion failure.
There are many forms of corrosion, including uniform corrosion that evenly spreads over the surface of components and local corrosion that only occurs in local areas. Local corrosion is further divided into pitting corrosion, intergranular corrosion, crevice corrosion, stress corrosion cracking, corrosion fatigue, etc.
