Metallic material properties
Many mechanical parts and engineering components work under alternating loads. Under the action of alternating load, although the stress level is lower than the yield limit of the material, after a long time of repeated stress cycle, sudden brittle fracture will occur, which is called the fatigue of metal materials. The characteristics of fatigue fracture of metal materials are:
1) Load stress is alternating;
2) The loading time is long;
3) The fracture is instantaneous;
4) Both plastic and brittle materials are brittle in the fatigue fracture zone. Therefore, fatigue fracture is the most common and dangerous fracture form in engineering.
Fatigue phenomena of metal materials can be divided into the following types according to different conditions:
1) High-cycle fatigue: refers to the fatigue with more than 100000 stress cycles under the condition of low stress (the working stress is lower than the yield limit of the material, or even lower than the elastic limit). It is the most common kind of fatigue failure. High cycle fatigue is commonly referred to as fatigue.
2) Low cycle fatigue: refers to the fatigue under high stress (working stress is close to the yield limit of the material) or high strain condition, and the number of stress cycles is less than 10000~100000. Because the alternating plastic strain plays a major role in this fatigue failure, it is also called plastic fatigue or strain fatigue.
3) Thermal fatigue: refers to the fatigue damage caused by the repeated action of thermal stress caused by temperature change.
4) Corrosion fatigue: refers to the fatigue damage of machine parts under the joint action of alternating load and corrosive medium (such as acid, alkali, sea water, active gas, etc.).
5) Contact fatigue: this refers to the contact surface of machine parts, under the repeated action of contact stress, pitting spalling or surface crushing spalling, resulting in failure of parts.
Hardness is the ability of a material to resist hard objects pressing into its surface. It is one of the important performance indexes of metal materials. General hardness is taller, wear resistance had jumped over. Commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.
Brinell hardness (HB) : with a certain load (generally 3000kg), the hardened steel ball of a certain size (generally 10mm in diameter) is pressed into the surface of the material and kept for a period of time. After the load is removed, the ratio of the load and the indentation area is the Brinell hardness value (HB), the unit is kg force /mm2 (N/mm2).
Rockwell hardness (HR) : when HB>450 or the sample is too small, the Brinell hardness test can not be used, but rockwell hardness measurement. It is to use a diamond cone with a top Angle of 120° or a steel ball with a diameter of 1.59 or 3.18mm to press into the surface of the measured material under a certain load, and the hardness of the material can be calculated by the depth of the indentation. According to the hardness of the test material, different indenter and total test pressure can be used to form several different Rockwell hardness scales, each scale is indicated by a letter after the Rockwell hardness symbol HR. Commonly used Rockwell hardness scales are A,B,C three (HRA, HRB, HRC). The C scale is the most widely used.
HRA: Hardness obtained by using a 60kg drill cone press for extremely hard materials (such as hard alloys).
HRB: the hardness is obtained by using 100kg load and 1.58mm diameter hardened steel ball, used for low hardness materials (such as annealed steel, cast iron, etc.).
HRC: hardness obtained by 150kg load and drill cone press for very hard materials such as hardened steel.
Vickers hardness (HV) : With a load of less than 120kg and a tip Angle of 136°, a square cone diamond indenter is pressed into the surface of the material, and the surface area of the material indentation pit is divided by the load value, which is vickers hardness value (HV). Hardness test is the most simple and feasible test method in mechanical property test. In order to replace some mechanical properties tests with hardness tests, a more accurate conversion relationship between hardness and strength is needed in production. It has been proved by practice that there are approximately corresponding relations between hardness values and strength values of metal materials. Because the hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, the higher the hardness value.