Chapter 7

§7.3 Technical Properties of Construction Steels Tensile property  Stress-strain chart of soft steel  Strength  Plasticity Impact toughness Cold-bending property Cold working

The essential properties of steels of steel structure and reinforced concrete in civil engineering are mechanical property tensile property impact toughness fatigue strength processing property cold bending property welding property

Tensile property is the most important property of the building steels. The most magnificent property indexes, which can be showed in the stress-strain graph of soft steel, are yield stress tensile strength elongation after the steels are pulled Tensile Property

Tensile Property video

Stress-strain chart of soft steel is shown in Fig σ B C D A δ σbσb σ SU σPσP σ SL ε Flexibility Phase σ/ε=E σ p － the limit of flexibility Yield Phase B—upper yield point (yield phase) B′—lower yield point σs—yield strength Aggrandizement Phase the interior tissue alters and part of its strength restores. Shrunken Phase From the point of C, the ability of transmutation resistance reduces and the transmutation mainly occurs in some parts leading to the shrunken destroy. σ b —tensile strength 0 Fig Stress-strain Chart of Soft Steel

Definition Yield Strength Yield Strength is the capacity that steels begin to lose their ability of resisting transmutation then the ability of producing much transmutation results. Yield Strength

Significance   It is considered as the turning point from the phase of flexibility transmutation to that of plasticity.   When outside force is bigger than yield strength, the transmutation can’t be restored, and the interior stress will be distributed to the low-stress parts automatically.   To sum up, yield strength is the basis of structure design.

Formula σ s － yield strength F s － yield stress A o － cross-section of specim en

Interpretation Regarding the steels with uncertain yield strength, the stress can be referred to as σ 0.2 when 0.2% residual deformation occurs, Fig σ ε 0.2% O Fig Stress-strain Chart of Soft Steel

Definition σ b is the maximum stress in which steels can stand.Significance   When the outer force is bigger than σ ， the steels totally lose their ability of resisting transmutation and crack.   So it can’t be adopted in construction design. Tensile Strength

Yield ratio （ σ s /σ b ）  Definition   σ s /σ b is a reflection of the available ratio and security of steels.Significance   Yield ratio is important to the available ratio and security of steel(Tab.7.2.1, Fig7.2.4) Tab The relationship among yield ratio, available ratio and security of steel Low σ s /σ b High σ s /σ b Available ratioSmallGreat SecurityGreatSmall

Yield ratio （ σ s /σ b ）  Significance σbσbσbσb σ s2 σ s1 σε reasonable yield ratio ： carbon steel 0.58—0.63 soft steel and alloy structure steels 0.65—0.75 Fig Significance Fig Significance of yield ratio

Plasticity is the ability that the plasticity transmutation against the outer force is achieved without breakage. DefinitionDefinition

the percentage elongation the index and the formula d0d0 L0L0 L1L1 δn—the percentage elongation L 0 —the former distance L 1 — the distance after fracture n=5d or 10d d—diameter of steel δ 5 ＞ δ 10

Shrinking rate of a cross-section Ψ—Shrinking rate of a cross-section A0—the former cross-section A1—the cross-section after fracture the index and the formula

The index and the formula Explanation The percentage elongation is an important index of demonstrating plasticity transmutation. Steel construction works within flexibility range. plasticity However, in the area where the stress is concentrated, the stress is likely to be larger than subdued strength, and has some plasticity transmutation. So that the stress can be redistributed and the construction can escape being destroyed.