Measurement Indicators of Yield Strength

 

Yield Point (σs)
The yield point is the most direct measurement indicator of the yield strength of steel. It refers to the stress value corresponding to the point where the yield plateau begins to appear on the stress - strain curve during the tensile process of steel. For example, for Q235 steel, its yield point is usually around 235 MPa. In building structure design, designers will select steel with an appropriate yield point according to the load magnitude borne by the structure and in line with the requirements of design codes. For instance, when designing a general small - scale residential structure, the yield point of Q235 steel can meet the requirements of most structural members (such as beams, columns, etc.) to bear the roof and floor loads as well as their own weights.

 

Yield Strength Design Value (fy)
The yield strength design value is the yield strength value used for structural design calculations, which is obtained after considering the material partial safety factor of steel. It is calculated by dividing the yield point by a material partial safety factor greater than 1 (generally determined according to the type of steel and design codes). For example, in steel structure design, for Q345 steel, its yield strength design value may be lower than its yield point. This is to take into account unfavorable factors such as the discreteness of steel quality and construction errors, so as to ensure the safety of the structure within the designed service life. Designers will use the yield strength design value as a judgment basis when performing strength checking calculations of structural members (such as calculating the flexural strength of beams and the compressive strength of columns).

 

 

 

Measurement Indicators of Toughness

 

Impact Toughness (Ak)
Impact toughness is an important indicator for measuring the toughness of steel through impact tests. During the test, a standard specimen with a notch is impacted on a specified testing machine, and the impact toughness is represented by the energy absorbed per unit area when the specimen is broken. For example, in the Charpy V - notch impact test, the obtained impact toughness value can intuitively reflect the ability of steel to resist damage when subjected to impact loads. For building structure steels used in cold regions, since low temperatures may cause a decrease in the toughness of steel, it is necessary to ensure that its impact toughness in low - temperature environments meets the design requirements. For example, the impact toughness index of Q345D steel at low temperatures is clearly specified to ensure the safety of the structure under cold climate conditions.

 

 

 

 

 

 

NV E420 extra high Strength Chemical Composition
Grade	The Element Max (%)
	C	Si	Mn	P	S	Al	N
NV E420	0.20	0.10-0.55	1.7	0.030	0.030	0.015-0.08	0.020
	Nb	V	Ti	Cu	B	Ni	Mo
	0.02-0.05	0.04-0.10	0.02		0.005

 

 

 

 

 

Fracture Toughness (KIC)
Fracture toughness is an indicator for measuring the ability of steel to resist crack propagation. In building structures, due to various reasons (such as welding defects, fatigue loads, etc.), tiny cracks may be generated. Fracture toughness can be used to evaluate the ability of steel to resist the rapid propagation of cracks until fracture when there are cracks. For example, for some key structural members that bear high stress levels (such as the main steel girders of large bridges and the key supporting members of high - rise buildings), the magnitude of fracture toughness is directly related to the safety of the structure. Through fracture mechanics theories and corresponding test methods, the fracture toughness value of steel can be determined, and in the design process, requirements for the fracture toughness of steel will be put forward according to the importance of the structure and the possible occurrence of cracks.

 

Strain Hardening Index (n)
The strain hardening index is an indicator that reflects the strain hardening characteristics of steel during the plastic deformation process. When steel is subjected to external forces such as tensile forces, the strain hardening phenomenon will occur after yielding, that is, as the strain increases, the stress also increases. The higher the strain hardening index, the stronger the ability of steel to resist deformation during the plastic deformation process, and the relatively better the toughness. For example, in some structures that need to bear repeated deformations (such as energy - dissipating members in seismic structures), steel with a high strain hardening 指数 can effectively absorb and dissipate energy during multiple deformation processes, thereby improving the seismic performance of the structure. In materials science research and building structure design, the strain hardening index of steel can be determined through methods such as tensile tests and used as one of the reference bases for evaluating the toughness of steel.

 

 

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