Tensile Testing. Objectives

Transcription

1 Laboratory 1 Tensile Testing Objectives Students are required to understand the principle of a uniaxial tensile testing and gain their practices on operating the tensile testing machine. Students are able to explain load-extension and stress-strain relationships. To evaluate the values of ultimate tensile strength, yield strength, % elongation, fracture strain and Young s Modulus of the selected metals when subjected to uniaxial tensile loading. Students can explain deformation and fracture characteristics of different materials such as aluminium, steels or brass when subjected to uniaxial tensile loading. Mechanical metallurgy laboratory

2 1. Literature Review 1.1 Stress and strain relationship Uniaxial tensile test is known as a basic engineering test to achieve ultimate strength, yield strength and ductility of interested materials. These important parameters are useful for the selection of engineering materials for any applications required. A standard specimen is prepared in a round or a square section along the gauge length as shown in figures 1 a) and b) respectively, depending on the standard used. Both ends of the specimens should have sufficient length and a surface condition such that they are firmly gripped during testing. The initial gauge length L o is standardized (in several countries) and varies with the diameter (D o ) or the cross-sectional area (A o ) of the specimen as listed in table 1. This is because if the gauge length is too long, the % elongation might be underestimated in this case. Any heat treatments should be applied on to the specimen prior to machining to produce the final specimen readily for testing. This has been done to prevent surface oxide scales that might act as stress concentration which might subsequently affect the final tensile properties due to premature failure. There might be some exceptions, for examples, surface hardening or surface coating on the materials. These processes should be employed after specimen machining in order to obtain the tensile properties results which include the actual specimen surface conditions. Figure 1: Standard tensile specimens Type specimen United State (ASTM) Great Britain Germany Sheet L / A ) ( o o Rod L / D ) ( o o Table 1: Dimensional relationships of tensile specimens used in various countries. Mechanical metallurgy laboratory

3 When a specimen is subjected to a tensile loading, the metal will undergo elastic and plastic deformation. Initially, the metal will elastically deform giving a linear relationship of load and extension. These two parameters are then used for the calculation of the engineering stress and engineering strain to give a relationship as illustrated in figure 2 using equations 1 and 2 as follow P σ = (1) A o L L f o ε = = (2) L o L L o where σ is the engineering stress ε is the engineering strain P is the external tensile load A o is the original cross-sectional area of the specimen L o is the original length of the specimen is the final length of the specimen L f 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) σ E = ε (3) If the tensile loading continues, yielding occurs at the beginning of plastic deformation. The yield stress, σ y, can be obtained by dividing the load at yielding (P y ) by the original cross-sectional area of the specimen (A o ) as shown in equation 4. P y σ y = (4) Ao Mechanical metallurgy laboratory

4 Figure 2: Stress-strain relationship under uniaxial tensile loading The yield point can be observed directly from the load-extension curve of the BCC metals such as iron and steel or in polycrystalline titanium and molybdenum, and especially low carbon steels, see figure 3 a). 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 specimen continues to extend without a significant change in the stress level. Load increment is then followed with increasing strain. This yield point phenomenon 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 as indicated in figure 3 a). 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 was 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. Mechanical metallurgy laboratory

5 Aluminium on the other hand 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 stressstrain curve. The yield strength therefore has to be calculated from the load at 0.2% strain divided by the original cross-sectional area as follows P 0.2% 0.2% y = Ao σ...(5) Note: the yield strength values can also be obtained at 0.5 and 1.0% strain. 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 as illustrated in figure 3 b). An interception between the 0.2% offset line and the stress-strain curve represents the yield strength at 0.2% offset or 0.2% strain. Figure 3: a) Comparative stress-strain relationships of low carbon steel and aluminium alloy and b) the determination of the yield strength at 0.2% offset. Mechanical metallurgy laboratory

6 Beyond yielding, continuous loading leads to an increase in the stress required to permanently deform the specimen as shown in the engineering stress-strain curve. At this stage, the specimen 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 specimen, therefore resulting in strain hardening. If the load is continuously applied, the stress-strain curve will reach the maximum point, which is the ultimate tensile strength (UTS, σ TS ). At this point, the specimen can withstand the highest stress before necking takes place. This can be observed by a local reduction in the crosssectional area of the specimen generally observed in the centre of the gauge length as illustrated in figure 4. After necking, plastic deformation is not uniform and the stress decreases accordingly until fracture. The fracture strength (σ fracture ) can be calculated from the load at fracture divided by the original cross-sectional area, A o, as expressed in equation 6. P fracture σ fracture = (6) Ao Figure 4: Necking of a tensile specimen occurring prior to fracture Tensile ductility of the specimen can be represented as % elongation or % reduction in area as expressed in the equations given below L % Elongation = 100 (7) L o Mechanical metallurgy laboratory

7 % RA = A A A A 100 = 100 A o f (8) o 0 where A f is the cross-sectional area of specimen 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 curve parallel to the slope in the linear relation. The interception of the parallel line at the x axis indicates the fracture strain of the specimen being tested. 1.2 Fracture characteristics of the tested specimens Metals with good ductility normally exhibit a so-called cup and cone fracture observed on either halves of a broken specimen as illustrated in figure 5. Necking starts when the stress-strain curve has passed the maximum point where plastic deformation is no longer uniform. Across the necking area within the specimen gauge length (normally located in the middle), microvoids are formed, enlarged and then merged to each other as the load is increased. This creates a crack having a plane perpendicular to the applied tensile stress. Just before the specimen breaks, the shear plane of approximately 45 o to the tensile axis is formed along the peripheral of the specimen. This shear plane then joins with the former crack to generate the cup and cone fracture as demonstrated in figure 5. The rough or fibrous fracture surfaces appear in grey by naked eyes. Under SEM, copious amounts of microvoids are observed as depicted in figure 6. This type of fracture surface signifies high energy absorption during the fracture process due to large amount of plastic deformation taking place, also indicating good tensile ductility. For brittle metals or metals that failed at relatively low temperatures, the fracture surfaces usually appear bright and consist of flat areas of brittle facets when examined under SEM as illustrated in figure 7. In some cases, clusters of these brittle facets are visible when the grain size of the metal is sufficiently large. The energy absorption is quite small in this case which indicates relatively low tensile ductility due to limited amount of plastic deformation. Mechanical metallurgy laboratory

8 Figure 5: Cup and cone fracture [3] Figure 6: Ductile fracture surface (Ductile metals) Figure 7: Brittle fracture surface (Brittle metals) Mechanical metallurgy laboratory

9 2. Materials and equipment 2.1 Tensile specimens 2.2 Micrometer or vernia calipers 2.3 Universal testing machine 3. Experimental procedure 3.1 The specimens provided are made of aluminium, steel and brass. Measure and record specimen dimensions (diameter and gauge length) in a table provided for the calculation of the engineering stress and engineering strain. Marking the location of the gauge length along the parallel length of each specimen for subsequent observation of necking and strain measurement. 3.2 Fit the specimen on to the universal Testing Machine (UTM) and carry on testing. Record load and extension for the construction of stress-strain curve of each tested specimen. 3.3 Calculate Young s modulus, yield strength, ultimate tensile strength, fracture strain and % elongation of each specimen and record on the provided table. 3.4 Analyze the fracture surfaces of broken specimens and sketch and describe the results 3.5 Discuss the experimental results and give conclusions. Mechanical metallurgy laboratory

10 4. Results Details Aluminium Steel Brass Diameter (mm) Width (mm) Thickness (mm) Cross-sectional area (mm 2 ) Gauge length (mm) Young s modulus (GPa) Load at yield point (N) Yield strength (MPa) Maximum load (N) Ultimate tensile strength (MPa) % Elongation Fracture strain Work hardening exponent (n) Fracture mode Fracture surfaces (Sketch) Table 2: Experimental data for tensile testing. Mechanical metallurgy laboratory

11 Engineering stress-strain curve of aluminium Describe the engineering stress-strain curve Mechanical metallurgy laboratory

12 Engineering stress-strain curve of steel Describe the engineering stress-strain curve Mechanical metallurgy laboratory

13 Engineering stress-strain curve of brass Describe the engineering stress-strain curve Mechanical metallurgy laboratory

14 5. Discussion Mechanical metallurgy laboratory

15 6. Conclusions Mechanical metallurgy laboratory

16 7. Questions 7.1 What is work hardening exponent (n)? How is this value related to the ability of metal to be mechanically formed? 7.2 If the tensile specimen is not cylindrical rod shaped but a flat rectangular plate, how do you expect necking to occur in this type of specimen? Mechanical metallurgy laboratory

17 7.3 Both yield strength and ultimate tensile strength exhibit the ability of a material to withstand a certain level of load. Which parameter do you prefer to use as a design parameter for a proper selection of materials for structural applications? Explain 8. References 8.1 Hashemi, S. Foundations of materials science and engineering, 2006, 4 th edition, McGraw- Hill, ISBN Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN W.D. Callister, Fundamental of materials science and engineering/an interactive e. text, 2001, John Willey & Sons, Inc., New York, ISBN x. Mechanical metallurgy laboratory