Metal Material Mechanical Property Codes and Their Meanings
I. Strength
1. Tensile strength, code: σb, meaning: when a material sample is subjected to tension, the maximum stress it endures before it breaks.
2. Compressive strength, code: σbc, meaning: when a material sample is subjected to pressure, the maximum stress it endures before it is crushed. Large stress
3. Bending strength, code: σbb, meaning: the maximum stress that the material sample bears before being subjected to bending force
4. Shear strength, code: τ, meaning: the maximum shear stress that the material sample bears before shearing when it is subjected to shear force
5. Torsional strength, code: τb, meaning: when the material sample is subjected to torsion force, the maximum shear stress it endures before breaking
6. Yield strength
During the tensile process of a material sample, the phenomenon that the load does not or begins to increase but the deformation continues is called yielding, and the minimum stress at yielding is called the yield point or yield limit. When a metal material yields, the stress point at which plastic deformation occurs without force during the test is called the yield strength. A distinction should be made between upper yield strength and lower yield strength. The stress before the specimen yields and the force decreases for the first time is the upper yield strength. During yielding, the lowest stress excluding initial transient effects is called the lower yield strength.
For some metal materials whose yield phenomenon is not obvious, it is difficult to determine the yield strength. To facilitate measurement, the stress when the permanent deformation is equal to 0.2% of the original length of the sample is usually called the yield strength or conditional yield strength. Rp0.2 represents the stress when the non-proportional elongation is 0.2%
(1) Yield point, code: σs,
(2) Yield strength, code: σ0.2
(3) Upper yield strength, code: ReH
(4) Lower yield strength, code: ReH
(5) Specified non-proportional extension strength, code: Rp
7. Elastic limit, code: σe, proportional limit: code: σp
The maximum stress at which a material can elastically deform is called the elastic limit.
The true elastic limit is difficult to determine. The standard stipulates that the stress value when the residual elongation is 0.01% represents the maximum stress at which the material stress and strain are proportional to the elastic deformation stage, which is called the proportional limit.
The two values of σp and σe are very close, and the prescribed σp is often used instead of σe.
8. Elastic modulus and shear modulus
The elastic modulus and shear modulus are the proportional constants when stress and strain are proportional to each other within the proportional limit. They are indicators of material stiffness.
E = σ / ε, (ε is the longitudinal linear strain of the specimen.)
G = τ / γ, (γ is the shear strain of the sample.)
II. Poisson's ratio
Poisson's ratio μ: the ratio of the transverse linear strain to the longitudinal linear strain of the sample within the elastic range
μ = |ε' / ε|, where ε' = -με (ε' represents the transverse linear strain of the specimen)
III. Fatigue limit, code σ-1, σ-1n
The maximum cyclic stress at which a metal material can undergo infinite stress cycles without fracture under alternating loads is called the fatigue limit. National standards stipulate that for steel materials, the stress cycle should be times, and for non-ferrous metal materials, stress cycles should be times or more. σ-1 represents the symmetrical bending fatigue limit of the smooth specimen; σ-1n represents the symmetrical bending fatigue limit of the notched specimen.
IV. Creep strength and lasting strength
1. Creep strength code: σ Temperature / (Strain/Time)
Metal materials are subjected to stress above a certain temperature. Even if the stress is less than the yield strength, the specimen will undergo plastic deformation over time. This phenomenon is called creep. The stress that causes a certain amount of creep deformation in the sample at a given temperature and within a specified use time is called creep strength. For example, σ500/(1/100000) =100Mpa, it means that the creep strength of a material at a temperature of 500°C with a post-strain of 1% is 100MPa. Creep strength is a performance indicator of a material's resistance to plastic deformation under high temperature load.
2. Durable strength code: σb Temperature/Time
The stress when a metal material breaks after a specified period of time under high temperature conditions is called the enduring strength. The lasting strength usually refers to the breaking strength of the sample after being exposed to certain temperature conditions. Indicates the stress with a duration of 100h when the test temperature is 700°C.
V. Elongation and Shrinkage
1. Elongation
(1) Code: A(δ), A11.3(δ), δ10
(2) Meaning:
a. δ is the percentage of the ratio of the gauge length to the original gauge length after the material sample is broken.
b. δ5 is the elongation when the gauge length of the sample is equal to 5 times the diameter.
c. δ10 is the elongation when the gauge length of the sample is equal to 10 times the diameter.
For proportional specimens, A11.3 represents the elongation after fracture with an original gauge length (L0) of 11.3 (S0 is the original cross-sectional area parallel to the length). For non-proportional specimens, the symbol A should be accompanied by the following footnote indicating the original gauge length used, expressed in mm. For example, A80mm indicates the elongation after break when the original gauge length (L0) is 80mm.
2. Area shrinkage
(1) Code: Z(ψ)
(2) Meaning:
The area shrinkage rate is the percentage of the reduction in the cross-sectional area at the fracture point of the material sample after it is broken and the original cross-sectional area. Shrinkage and elongation are both indicators of material plasticity.
VI. Impact toughness and impact absorption energy
1. Impact toughness
(1)Code: akU or akV
(2) Unit: J/cm²
2. Impact absorption energy
(1)Code name: AkU or AkV
(2) Unit: J
On a pendulum-type primary testing machine, the ratio of the work Ak consumed when punching a standard specimen of a certain size and shape to the fracture cross-sectional area is called the impact toughness ak.
According to national standards, aku is the impact toughness of the Charpy U-notch sample, and Aku is the impact absorbed energy (J) consumed when the Charpy U-notch sample is punched;
αkv is the impact toughness of the Charpy V-notch sample, Akv is the impact absorption energy (J) consumed when the Charpy V-notch sample is punched.
Since the value of a not only depends on the material itself, but also changes with changes in sample size, shape and test temperature, the value of a is only a relative indicator. At present, many countries in the world directly use the impact absorption energy Ak as an indicator of impact toughness. Our country will gradually replace ak with Ak.
VII. Hardness
1. Brinell hardness
(1) Code: HB (HBS or HBW), unit: kgf/mm²
(2) Hardness refers to the ability of a metal to resist hard objects being pressed into its surface. Use a small hardened steel ball or a cemented carbide ball to press into the metal surface. After a certain period of time, the deformation is stable and then unloaded. The load on the steel ball is divided by the indentation area. The resulting quotient is the Brinell hardness value of the metal. When the stool degree is less than or equal to 450HBS, use a steel ball to measure it. When the hardness is less than or equal to 650HBW (see B/T231.1), use cemented carbide balls to measure
When the test force unit is N, the Brinell hardness value is
In the formula.
F--Load on the cemented carbide ball, N
D--diameter of cemented carbide ball, mm
d--average diameter of indentation, mm
If the test force unit is kgf, the coefficient 0.102 in the formula should be 1
2. Rockwell hardness
In the formula:
K-constant, the K value of HRC and HRA is 100, and the K value of HRB is 130
-Indentation depth, mm
0.002--The depth of indentation represented by each small grid on the dial of the testing machine (each small grid represents one degree of Rockwell hardness), mm
(1) Rockwell hardness level C, code: HRC
Meaning: Use a 1471N load to press a conical diamond indenter with an apex angle of 120° into the metal surface, and use the depth of the indentation to calculate the hardness, which is the HRC hardness of the metal. HRC is used to measure metal materials with a hardness of 230~700HB. It is mainly used to measure hard metal materials such as quenched steel and quenched and tempered steel (see GB/T230, the same below)
(2) Rockwell hardness level A, code: HRA
Meaning: refers to the hardness measured using a 588.4N load and a conical diamond indenter with a vertex angle of 120°. It is generally used to measure metal materials with very high hardness or hard and thin materials, such as carbides, cemented carbide or surface quenching layers. HRA is used to measure metal materials with a hardness greater than 700HB.
(3) Rockwell hardness level B, code: HRB
Meaning: Refers to the hardness measured using a 980.7N load and a hardened steel ball with a diameter of 1.5875mm (i.e. 1/16in). Mainly used to measure soft metal materials with hardness of 60~230HB, such as mild steel, annealed steel, normalized steel, copper, aluminum and other non-ferrous metals
3. Surface Rockwell hardness
(1) Code: HRN, HRT
(2) Meaning
The test principle is the same as Rockwell hardness, but the difference is that the test load is lighter. The indenter of HRN is a diamond cone with an apex angle of 120°, and the indenter of HRT is a hardened steel ball with a diameter of 1.5875mm. The loads of both are 15kgf, 30kgf and 45kgf. The two labels are HRN15, HRN30, HRN45 and HRT15, HRT30, HRT45 respectively. Surface Rockwell hardness is only applicable to the hardness of the surface layer of steel surface treated with carburizing, nitriding, etc., as well as the hardness measurement of thinner and smaller specimens, and the numerical value is relatively accurate (see GB/T1818)
In the formula, t--the difference between the indentation depth of the main load and the initial load, mm
4. Vickers hardness
(1)Code: HV
(2) Meaning: Use a load of 49.03~980.7N (divided into 6 levels) to press a diamond square pyramid indenter with a vertex angle of 136° into the surface of the metal. After a certain holding time, unload it. The quotient obtained by dividing the load by the indentation surface area is the Vickers hardness value. HV is only suitable for measuring the hardness of thin metal materials, thin metal coatings or surface layers after chemical heat treatment (such as chromium plating, carburizing, nitriding, carbonitriding layers, etc.) (see GB/T4340.1)
In the formula: P--Load on the pressure head, N
d--Indentation diagonal length, mm
5. Shore hardness
(1)Code: HS
(2) Meaning: A punch of a certain weight is dropped from a certain height on the surface of the test sample, and the hardness is expressed by the rebound height of the punch. It is suitable for measuring some precision measuring tools with smooth surfaces or large parts that are difficult to move (see GB/T4341-2001)
HS=Kh/H0
K-Shore hardness coefficient
h-The height of the diamond punch from the surface to be measured
h0-height of the punch rebounding from the surface being measured
