Tensile Testing of Steel

Tensile Testing of Steel

Sample of steel is subjected to a wide variety of mechanical tests to measure their strength, elastic constants, and other material properties as well as their performance under a variety of actual use conditions and environments. Tensile test is one of them. Other tests are hardness test, impact test, fatigue test, and fracture test. These mechanical tests are used to measure how a sample of steel withstands an applied mechanical force. The results of such tests are used for two primary purposes namely (i) engineering design (e.g. failure theories based on strength, or deflections based on elastic constants and component geometry), and (ii) quality control either by the producer of steel to verify the process or by the end user to confirm the material specifications.

Uniaxial tensile test is known as a basic and universal engineering test to achieve material parameters such as ultimate tensile strength (UTS), yield strength (YS), % elongation, % area of reduction and young’s modulus. Tensile testing is done for many reasons. The results of tensile tests are used in selecting materials for engineering applications. Tensile properties are often included in material specifications to ensure quality. Tensile properties are also normally measured during development of new materials and processes, so that different materials and processes can be compared. Also, tensile properties are generally used to predict the behaviour of a material under forms of loading other than uniaxial tension.

Safely withstanding the expected maximum load without permanent deformation (or to stay within the specified deflection) is a basic requirement for a steel product. The ‘resistance’ against the load is a function of the cross-section and the mechanical properties (or in other words the ‘strength’) of the steel material. Tensile testing is done to determine the mechanical properties of the yield strength, tensile strength, and elongation.

It is known from basic principles that a tensile stress tends to pull a member apart, a compressive stress tends to crush or collapse a body, a shear stress tends to cleave a structural member, and a bending stress tends to deflect a member. The allowable torsional stress which the steel material can tolerate is measured by shear strength, and the allowable bending stress which the steel material can tolerate is based on the tensile properties. This is because bending puts the outer fibers of steel member in tension.

Elastic and plastic deformation

A straight piece of steel wire or strip, rigidly held at one end, bent by a small load to a few degrees, normally ‘spring back’ to its original shape when the load is released. By placing a double load at the end of the steel sample, the rate of deflection is then twice as high but the sample still returns to its original shape when the load is taken off. In other words, the sample is loaded within its ‘elastic’ range.

After increasing the load and the deflection to a certain limit, the sample no longer returns to its original shape upon the removal of the load. At that load, the sample remains ‘permanently’ deformed since the stresses in the steel material exceeded the yield strength limit. Similar occurrences can be observed with springs. The linear relation between load and deflection is utilized in the ‘spring balance’ scale but the load is always kept safely within the elastic range of the spring. If a spring is stretched over its elastic range (over yield limit), then it will not spring back to its original shape. This permanent deformation is known as plastic deformation.

The response of steel material response to the three major forms of stresses namely (i) tension, (ii) compression, and (iii) shear, can be measured on a universal testing machine. This machine can pull axially on a test sample (tensile load) or push on a test sample to measure response to compression loading. Shear tests are run by loading a pin in a special fixture.

Tensile test sample (Fig 1) has enlarged ends or shoulders for gripping. The ‘dog-bone’ shape ensures that the sample breaks in the centre and not in the grip area. The important part of the sample is the gauge section. The cross-sectional area of the gauge section is reduced relative to that of the remainder of the sample so that deformation and failure is localized in this region. The gauge length is the region over which measurements are made and is centered within the reduced section. The distances between the ends of the gauge section and the shoulders are to be large enough so that the larger ends do not constrain deformation within the gauge section, and the gauge length is to be great relative to its diameter. Otherwise, the stress state is more complex than simple tension. The sample size and shape is to conform to a national or international standard.

Fig 1 Typical tensile test sample

There are several ways of gripping the sample. These are (i) the end may be screwed into a threaded grip, (ii) it may be pinned, (iii) butt ends may be used, or (iv) the grip section may be held between wedges. There can be other methods also. The most important aspect in the selection of a gripping method is to ensure that the sample can be held at the maximum load without slippage or failure in the grip section. Bending is required to be minimized.

Tensile testing is normally carried out in the universal testing machines (UTM) (Fig 2). These machines test materials in tension, compression, or bending. Their primary function is to create the stress-strain diagram. Universal testing machines are either electro-mechanical or hydraulic. The principal difference is the method by which the load is applied. Electro-mechanical testing machines are based on a variable-speed electric motor, a gear reduction system, and one, two, or four screws which move the crosshead up or down. This motion loads the sample in tension or compression. Crosshead speeds can be changed by changing the speed of the motor. A micro-processor-based closed-loop servo system can be implemented to accurately control the speed of the crosshead. Hydraulic testing machines are based on either a single or dual-acting piston which moves the crosshead up or down. However, most static hydraulic testing machines have a single acting piston or ram. In a manually operated machine, the operator adjusts the orifice of a pressure-compensated needle valve to control the rate of loading. In a closed-loop hydraulic servo system, the needle valve is replaced by an electrically operated servo valve for precise control.

Fig 2 Universal testing machines

Universal testing machine applies a tensile load when one end of the test sample is attached to the movable crosshead with the other end fixed to a stationary member. The crosshead is then driven in such a manner as to pull the sample apart. Compressive loading is achieved by driving the crosshead against short stubby cylinders placed on the stationary machine plate. Attachments are used to hold various shaped specimens, but tensile sample is usually made in a ‘dog-bone’ shape. 

Stress–Strain diagram

The sample is placed in the testing machine and a force F is applied. A mechanical or electrical device – strain gauge or extensometer is used to measure the amount that the sample stretches between the gauge marks when the force is applied until the specimen fails. The stretch, both elastic (recoverable) and plastic (permanent), is converted into strain by division of the change in length (extension or elongation) by the original length. Using the original cross-sectional area of the sample, the load F is converted into stress, and an engineering stress-strain diagram is obtained.

Engineering stress, or nominal stress, S, is defined as S = F/A0 where F is the tensile force and A0 is the initial cross-sectional area of the gauge section. Engineering strain, or nominal strain, e, is defined as e = (L-L0)/L0 where L0 is the initial gauge length (original distance between the gauge marks), and L is the distance between the gauge marks after force F is applied. When force-elongation data are converted to engineering stress and strain, a stress-strain diagram that is identical in shape to the force-elongation diagram (Fig 3) can be plotted. The advantage of dealing with stress versus strain rather than load versus elongation is that the stress-strain curve is essentially independent of the dimensions of the sample.

Fig 3 Typical force-elongation diagram and stress-strain diagram

Young’s modulus

During elastic deformation, the engineering stress-strain relationship follows the Hook’s Law and the slope of the curve indicates the Young’s modulus (E). E = S/e

Young’s modulus is of importance where deflection of materials is critical for the required engineering applications. This is e.g. deflection in structural beams is considered to be crucial for the design in engineering components or structures such as bridges, buildings, ships, etc. The applications of tennis racket and golf club also require specific values of spring constants or Young’s modulus values.

Yield strength and yield point

By considering the stress-strain diagram beyond the elastic portion, if the tensile loading continues, yielding occurs at the beginning of plastic deformation. The yield stress, Ys, can be obtained by dividing the load at yielding (Fy) by the original cross-sectional area (A0) of the sample (Ys=Fy/A0).

The yield point can be observed directly from the load-elongation diagram of the BCC metals such as iron and steel especially low carbon steels. The yield point elongation phenomenon shows the upper yield point followed by a sudden reduction in the stress or load till reaching the lower yield point. At the yield point elongation, the sample continues to elongate without a significant change in the stress level. Load increment is then followed with increasing strain. This yield point occurrence is associated with a small amount of interstitial or substitutional atoms. This is for example in the case of low-carbon steels, which have small atoms of carbon and nitrogen present as impurities. When the dislocations are pinned by these solute atoms, the stress is raised in order to overcome the breakaway stress required for the pulling of dislocation line from the solute atoms. This dislocation pinning is related to the upper yield point. If the dislocation line is free from the solute atoms, the stress required to move the dislocations then suddenly drops, which is associated with the lower yield point. Furthermore, it has been found that the degree of the yield point effect is affected by the amounts of the solute atoms and is also influenced by the interaction energy between the solute atoms and the dislocations.

Material having a FCC crystal structure does not show the definite yield point in comparison to those of the BCC structure materials, but shows a smooth engineering stress-strain diagram. The yield strength therefore has to be calculated from the load at 0.2 % strain divided by the original cross-sectional area [Y(0.2 %) = F(0.2 %)/A0

The determination of the yield strength at 0.2 % offset or 0.2 % strain can be carried out by drawing a straight line parallel to the slope of the stress-strain curve in the linear section, having an intersection on the x-axis at a strain equal to 0.002. An interception between the 0.2 % offset line and the stress-strain diagram represents the yield strength at 0.2 % offset or 0.2 % strain.

The yield strength of soft materials exhibiting no linear portion to their stress-strain diagram such as soft gray cast iron can be defined as the stress at the corresponding total strain. The yield strength, which indicates the onset of plastic deformation, is considered to be vital for engineering structural or component designs where safety factors are normally used. Safety factors are based on several considerations which include (i) the accuracy of the applied loads used in the structural or components, (ii) estimation of deterioration, and (iii) the consequences of failed structures (loss of life, financial, economic loss, etc.) Generally, buildings require a safety factor of 2, which is rather low since the load calculation has been well understood. Automobiles has safety factor of 2 while pressure vessels utilize safety factors of 3 to 4.

Ultimate tensile strength

Beyond yielding, continuous loading leads to an increase in the stress required to permanently deform the sample as shown in the engineering stress-strain diagram. At this stage, the sample is strain hardened or work hardened. The degree of strain hardening depends on the nature of the deformed materials, crystal structure and chemical composition, which affects the dislocation motion. FCC structure materials having a high number of operating slip systems can easily slip and create a high density of dislocations. Tangling of these dislocations requires higher stress to uniformly and plastically deform the sample, hence resulting in strain hardening.

If the load is continuously applied, the stress-strain diagram reaches the maximum point, which is the ultimate tensile strength (UTS). At this point, the sample can withstand the highest stress before necking takes place. This can be observed by a local reduction in the cross-sectional area of the sample generally observed in the centre of the gauge length.  (UTS = Fmax/Ao)

Fracture strength

After necking, plastic deformation is not uniform and the stress decreases accordingly until fracture. The fracture strength (FS) can be calculated from the load at fracture divided by the original cross-sectional area. (FS=Ffracture/A0)

Tensile ductility

Tensile ductility of the sample is represented as % elongation or % reduction in area. % elongation = [(L-L0)/L0]*100 and % reduction are = [(A0-A)/A]*100 where A0 is the original cross-sectional area of the sample and A is the cross-sectional area at fracture.

The fracture strain of the specimen can be obtained by drawing a straight line starting at the fracture point of the stress-strain diagram parallel to the slope in the linear relation. The interception of the parallel line at the x axis indicates the fracture strain of the sample being tested.

Work hardening exponent

The material behaviour beyond the elastic region where stress-strain relationship is no longer linear (uniform plastic deformation) can be shown as a power law expression (Ts = K*e to the power n) where Ts is the true stress, e is the true strain, n is the strain-hardening exponent, and K is the strength coefficient. The strain-hardening exponent values, n, of most metals range between 0.1-0.5, which can be estimated from a slope of a log true stress-log true strain plot up to the maximum load.(log Ts =n log e + log K) or Y = m X + C

While n is the slope (m) and the K value indicates the value of the true stress at the true strain equal to unity. High value of the strain-hardening exponent indicates an ability of a metal to be readily plastically deformed under applied stresses. This is also corresponding with a large area under the stress-strain diagram up to the maximum load. This power law expression has been modified variably according to materials of interest especially for steels and stainless steels.

Modulus of Resilience

Apart from tensile parameters mentioned previously, analysis of the area under the stress-strain diagram can give informative material behaviour and properties. By considering the area under the stress-strain diagram in the elastic region, this area represents the stored elastic energy or resilience. The latter is the ability of the materials to store elastic energy which is measured as a modulus of resilience (UR).

The significance of this parameter is considered by looking at the application of mechanical springs which requires high yield stress and low Young’s modulus. For example, high carbon spring steel has the modulus of resilience of 23 kg/sq cm while that of medium carbon steel is only 2.4 kg/sq cm.

Tensile toughness

Tensile toughness (UT) can be considered as the area under the entire stress-strain diagram which indicates the ability of the material to absorb energy in the plastic region. In other words, tensile toughness is the ability of the material to withstand the external applied forces without experiencing failure. Engineering applications which requires high tensile toughness is for example gear, chains and crane hooks, etc.