Figure 4.3 shows a typical stress-strain curve for each classification of structural steels defined in Art. 4.40.4. The diagram illustrates the higher-strength levels achieved with heat treatment and addition of alloys.
Tensile Properties of Structural Steels
The curves in Fig. 4.3 were derived from tensile tests. The yield points, strengths, and modulus of elasticity obtained from compression tests would be about the same.
The initial portion of the curves in Fig. 4.3 is shown to a magnified scale in Fig. 4.4. It indicates that there is an initial elastic range for the structural steels in which there is no permanent deformation on removal of the load. The modulus of elasticity E, which is given by the slope of the curves, is nearly a constant 29,000 ksi for all the steels. For carbon and high-strength, low-alloy steels, the inelastic range, where strains exceed those in the elastic range, consists of two parts: Initially, a plastic range occurs in which the steels yield; that is, strain increases with no increase in stress. Then follows a strain-hardening range in which increase in strain is accompanied by a significant increase in stress.
FIGURE 4.3 Typical stress-strain curves for structural steels.
FIGURE 4.4 Magnification of the initial portions of the stress-strain curves for structural steels.
The curves in Fig. 4.4 also show an upper and lower yield point for the carbon and high-strength, low-alloy steels. The upper yield point is the one specified in standard specifications for the steels. In contrast, the curves do not indicate a yield point for the heat-treated steels. For these steels, ASTM 370, ‘‘Mechanical Testing of Steel Products,’’ recognizes two ways of indicating the stress at which there is a significant deviation from the proportionality of stress to strain. One way, applicable to steels with a specified yield point of 80 ksi or less, is to define the yield point as the stress at which a test specimen reaches a 0.5% extension under load (0.5% EUL). The second way is to define the yield strength as the stress at which a test specimen reaches a strain (offset) 0.2% greater than that for elastic behaviour. Yield point and yield strength are often referred to as yield stress.
Ductility is measured in tension tests by percent elongation over a given gage length—usually 2 or 8 in—or percent reduction of cross-sectional area. Ductility is an important property because it permits redistribution of stresses in continuous members and at points of high local stresses.
Poisson’s ratio, the ratio of transverse to axial strain, also is measured in tension tests. It may be taken as 0.30 in the elastic range and 0.50 in the plastic range for structural steels.
Cold working of structural steels, that is, forming plates or structural shapes into other shapes at room temperature, changes several properties of the steels. The resulting strains are in the strain-hardening range. Yield strength increases but ductility decreases. (Some steels are cold rolled to obtain higher strengths.) If a steel element is strained into the strain-hardening range, then unloaded and allowed to age at room or moderately elevated temperatures (a process called strain aging), yield and tensile strengths are increased, whereas ductility is decreased. Heat treatment can be used to modify the effects of cold working and strain aging.
Residual stresses remain in structural elements after they are rolled or fabricated. They also result from uneven cooling after rolling. In a welded member, tensile residual stresses develop near the weld and compressive stresses elsewhere. Plates with rolled edges have compressive residual stresses at the edges, whereas flame-cut edges have tensile residual stresses. When loads are applied to such members, some yielding may take place where the residual stresses occur. Because of the ductility of steel, however, the effect on tensile strength is not significant but the buckling strength of columns may be lowered.
Strain rate also changes the tensile properties of structural steels. In the ordinary tensile test, load is applied slowly. The resulting data are appropriate for design of structures for static loads. For design for rapid application of loads, such as impact loads, data from rapid tension tests are needed. Such tests indicate that yield and tensile strengths increase but ductility and the ratio of tensile strength to yield strength decrease.
High temperatures too affect properties of structural steels. As temperatures increase, the stress-strain curve typically becomes more rounded and tensile and yield strengths, under the action of strain aging, decrease. Poisson’s ratio is not significantly affected but the modulus of elasticity decreases. Ductility is lowered until a minimum value is reached. Then, it rises with increase in temperature and becomes larger than the ductility at room temperature.
Low temperatures in combination with tensile stress and especially with geometric discontinuities, such as notches, bolt holes, and welds, may cause a brittle failure. This is a failure that occurs by cleavage, with little indication of plastic deformation. A ductile failure, in contrast, occurs mainly by shear, usually preceded by large plastic deformation. One of the most commonly used tests for rating steels on their resistance to brittle fracture is the Charpy V-notch test. It evaluates notch toughness at specific temperatures.
Toughness is defined as the capacity of a steel to absorb energy; the greater the capacity, the greater the toughness. Determined by the area under the stress-strain curve, toughness depends on both strength and ductility of the metal. Notch toughness is the toughness in the region of notches or other stress concentrations. A quantitative measure of notch toughness is fracture toughness, which is determined by fracture mechanics from relationships between stress and flaw size.
Shear Properties of Structural Steels
The shear modulus of elasticity G is the ratio of shear stress to shear strain during initial elastic behaviour. It can be computed from Eq. (5.25) from values of modulus of elasticity and Poisson’s ratio developed in tension stress-strain tests. Thus G for structural steels is generally taken as 11,000 ksi.
The shear strength, or shear stress at failure in pure shear, ranges from 0.67Ft to 0.75Ft for structural steels, where Ft is the tensile strength. The yield strength in shear is about 0.57Ft .
Creep and Relaxation
Creep, a gradual change in strain under constant stress, is usually not significant for structural steel framing in buildings, except in fires. Creep usually occurs under high temperatures or relatively high stresses, or both.
Relaxation, a gradual decrease in load or stress under a constant strain, is a significant concern in the application of steel tendons to prestressing. With steel wire or strand, relaxation can occur at room temperature. To reduce relaxation substantially, stabilized, or low-relaxation, strand may be used. This is produced by pretensioning strain at a temperature of about 600F. A permanent elongation of about 1% remains and yield strength increases to about 5% over stress-relieved (heat-treated but not tensioned) strain.
Hardness of Structural Steels
Hardness is used in production of steels to estimate tensile strength and to check the uniformity of tensile strength in various products. Hardness is determined as a number related to resistance to indentation. Any of several tests may be used, the resulting hardness numbers being dependent on the type of penetrator and load. These should be indicated when a hardness number is given. Commonly used hardness tests are the Brinell, Rockwell, Knoop, and Vickers. ASTM A370, ‘‘Mechanical Testing of Steel Products,’’ contains tables that relate hardness numbers from the different tests to each other and to the corresponding approximate tensile strength.
Fatigue of Structural Steels
Under cyclic loading, especially when stress reversal occurs, a structural member may eventually fail because cracks form and propagate. Known as a fatigue failure, this can take place at stress levels well below the yield stress. Fatigue resistance may be determined by a rotating-beam test, flexure test, or axial-load test. In these tests, specimens are subjected to stresses that vary, usually in a constant stress range between maximum and minimum stresses until failure occurs. Results of the tests are plotted on an S-N diagram, where S is the maximum stress (fatigue strength) and N is the number of cycles to failure (fatigue life). Such diagrams indicate that the failure strength of a structural steel decreases with increase in the number of cycles until a minimum value is reached, the fatigue limit. Presumably, if the maximum stress does not exceed the fatigue limit, an unlimited number of cycles of that ratio of maximum to minimum stress can be applied without failure. With tension considered positive and compression, negative, tests also show that as the ratio of maximum to minimum stress is increased, fatigue strength is lowered significantly.
Since the tests are made on polished specimens and steel received from mills has a rough surface, fatigue data for design should be obtained from tests made on as-received material.
Tests further indicate that steels with about the same tensile strength have about the same fatigue strength. Hence the S-N diagram obtained for one steel may be used for other steels with about the same tensile strength.