Themost important material properties which influence the behaviour of steeltension and compression members are the modulus of elasticity, E, and the yieldstress, ay. Values for both the modulus of elasticity and the yield stress maybe obtained from tensile tests on coupon specimens or from compression tests (Parke, 1988).
Figure2.4: Stress strain curve and its fracture Theslope of the line in a region where stress is proportional to strain and isknown as the modulus of elasticity or young’s modulus. The modulus ofelasticity (E) defines the properties of a material as it undergoes stress,deforms, and then returns to its original form after the stress is removed. Itis a measure of the stiffness of a given material. To compute the modulus ofelastic, simply divide the stress by the strain in the material. This modulusis of interest when it’s far essential to compute how much a rod or wirestretches below a tensile load.Thestress applied to the material at which plastic deformation begins to arise incalled as the yield strength of a material.
In ductile materials for instance,at some point, the stress strain curve deviates from the straight linerelationship and law no longer applies as the strain increases faster than thestress. In brittle materials, little or no plastic deformation occurs and thematerial fractures close to the end of the linear elastic portion of the curve.Theultimate tensile strength (UTS) or the tensile strength, is the maximum stresslevel reached in a tensile test. The strength of a material mean its ability towithstand external forces without breaking.
In brittle materials, the UTS willbe at the end of the linear elastic portion of the stress strain curve or closeto the elastic limit. While, in ductile materials, the UTS will be well outsideof the elastic portion into the plastic portion of the stress strain curve. 2.3.1 Measuresof Ductility (Elongation and Reduction of Area) The ductility of a material IS defined as a measure ofthe extent to which a material will deform before it fail due to fracture. Moreover,the amount of ductility plays an important factor when considering formingoperations such as rolling and extrusion which provides an indication of howvisible overload damage to a component might become before the componentfractures. Besides that, ductility is also used as a quality control measure toassess the level of impurities and proper processing of a material.The common measures of ductility were the engineeringstrain at fracture (elongation) and the reduction of area at fracture.
Thesetwo properties were obtained by fixing the specimen back together afterfracture thus, measuring the change in length and cross sectional area. Thechange in axial length divided by the original length of the specimen orportion of the specimen is called elongation which expressed as a percentage.Percent elongation is defined simply as%Elongation= (Lf – Lo)/Lo / 100Where, Lo is the initial gage length and Lfis the length of the gage section at fracture.
For most materials, the amountof elastic elongation is so small that the two are equivalent. When this is notso (as with brittle metals or rubber), the results should state whether or notthe elongation includes an elastic contribution. The other common measure of ductilitywhich is percent reduction of area is defined as,%Reductionarea = (Ao _ Af)/Ao / 100Where, Ao and Af are the initialcross-sectional area and the cross-sectional area at fracture, respectively.Figure 2.
2 shows the stress strain curve comparing between brittle and ductilefracture.Figure 2.5:Stress strain curve comparing between brittle and ductile fracture. 2.
1 Structuralsteel compression memberAstructural member which carries an axial compression load is known as acompression member which are called as columns, trusses member and bracingmembers. Columns, posts or stanchions are the vertical compression members inbuildings whereas struts is a compression member called in a roof trusses.Therefore,compression test is needed in order to determine the behavior or response of amaterial or specimen while it experiences a compressive load by measuring the fundamentalvariables, such as, strain, stress, and also deformation. Furthermore, only bytesting a material in compression, then the compressive strength, yieldstrength, ultimate strength, elastic limit, and the elastic modulus among otherparameters may all be determined. Besides, with the understanding of thesedifferent parameters and the values associated with a specific material then itcan be concluded whether or not the material or the specimen is suited forspecific applications or if it will fail under the specified stresses orloading.
Eventhough steel is a very strong material and very reliable in structuralconstruction of buildings but its effectiveness, however, is only guaranteedwhen the steel is properly designed. A poor design in the structure can lead tothe variety type of failures of steel structures. The most common cause offailure is fatigues, whereas the most spectacular one is brittle fracture.
2.4.1 Bucklingof compression memberBuckling can occurwhen long slender steel members are subjected to compressive loads and suddenlyundergoes bending as shown in the Figure 2.6 (b). Consider a long slendercompression member, as an axial load, P is applied and increased slowly, itwill ultimately reach a value of the critical buckling load of the column, Pcr,which will cause buckling of the column. Buckling result in instability ofcolumns, and causes sudden failure. Buckling occurs when load, P is greaterthat the critical load, Pcr.
The most common failure modes in structural steelmember are local buckling, flexural buckling (Euler buckling) and torsionalbucklingFigure 2.6: Buckling of axiallyloaded compression members Flexural buckling(Euler Buckling) is a primary type of buckling which occurs when the membersare subjected to flexure or bending where they become unstable or when thelateral loads on the members increase beyond its limit as shown in Figure 2.6.
However, these are one of the least occurring failures in steel structure.Unlike flexuralbuckling, local buckling occurs when a part or section of the column bucklesbefore other modes of failure occurs due to its small thickness at some partsof its cross-section as shown in Figure 2.7. Local buckling depends on the slenderness(width-to-thickness ratio) of the plate element and the yield stress (fy) ofthe material.Inaddition, there is one more important failure of steel structure which known asfailure due to lateral torsional buckling as shown in Figure 2.8.This kind offailure happens when the compression flange of the steel beam is unrestrained.
The loads are present on the floor and there always in an eccentricity of theload, this eccentricity leads to a twisting moment. Therefore, as the flange ofthe steel beam is not fixed, the beam twists as well as moves laterally.