Mechanical Characterisation of Materials

Transcription

1 Department of Materials and Metallurgical Engineering Bangladesh University of Engineering and Technology, Dhaka MME298 Structure and Properties of Biomaterials Sessional 1.50 Credits 3.00 Hours/Week July 2017 Term Laboratory 1 Mechanical Characterisation of Materials 1. Objective Mechanical testing plays an important role in evaluating fundamental properties of engineering materials as well as in developing new materials and in controlling the quality of materials for use in design and construction. If a material is to be used as part of an engineering structure that will be subjected to a load and experience friction, it is important to know that the material is strong enough and rigid enough to withstand the loads and hard enough to resist wear that it will experience in service. The tensile test measures the resistance of a material to a static or slowly applied force. This laboratory experiment is designed to demonstrate the procedure used for obtaining mechanical properties as modulus of elasticity (E), yield strength (YS), ultimate tensile strength (UTS), and elongation (%E) at rupture. The hardness test, on the other hand, is designed to introduce the principles of different methods of hardness testing, emphasizing the limitations and significance of the results in each method. After completion of this experiment, students should be able to 1.1 understand the principle of a uniaxial tensile testing and gain their practices on operating the tensile testing machine to achieve the required tensile properties following a specific standard, 1.2 explain stress-strain relationships and represent them in graphical forms, 1.3 evaluate the values of ultimate tensile strength, yield strength, elongation at failure, and Young s Modulus of the selected materials when subjected to uniaxial tensile loading, 1.4 understand the principles of hardness testing and gain their practices on operating the hardness testing machine to determine hardness value of materials. 2. Materials and Equipment 2.1 Tensile Testing: Tensile specimens Universal testing machine (UTM) Micrometre or Vernia calliper Steel scale 2.2 Hardness Testing: Hardness specimens Automatic Rockwell hardness tester Different indenters MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 1 of 14

2 3. Experimental Procedure 3.1 Tensile Testing: Measure diameter of the specimen provided and record in Table 1.1. Mark the location of the gauge length along the length of each specimen Observe and record the type of tensile testing machine to be used for the experiment. Fit the specimen on to the machine. Note the mode of testing and strain rate to be used for the test. Carry on testing until fracture of the sample After the specimen is broken, join the two broken pieces of the specimen together, held them firmly, and then measure the final gauge length of the broken pieces Take the raw data from the computer and import it to Microsoft Excel or any other similar programme to plot stress-strain diagram Before plotting the chart, use the calibration chart or formula (which can be found to be attached to the machine) of the machine to calculate the actual value of the load from the observed value. 3.2 Rockwell Hardness Testing: Keep the loading and unloading lever at the unloading position Select the suitable indenter and weights according to the scale Fix the indenter in the hardness tester and switch ON the power supply of the machine. Place the specimen on testing table anvil Turn the hand wheel clockwise to raise a job until it makes contact with indenter and continue turning till the dial gauge reads between 290 and 299 and the SET indicator becomes lighted. This indicates that the minor load (usually 10 kg) is now applied to the indenter Press the button START to apply the major load (usually 60, 100 or 150 kg depending on the scale selected). The dial readings will continue changing and, after about 15 second intervals, the reading will become stationary and OK indicator will be lighted Note down the reading Turn back the hand wheel anti-clockwise and remove the job Similarly repeat the step from for different trials and for different samples Complete Data Sheet Results 4.1 Tensile Testing: Plot a stress-strain curve for each specimen Determine Young's modulus, yield strength, ultimate tensile strength, and elongation at failure. 4.2 Hardness Testing: List hardness values and determine the average hardness number of each specimen. Compare your result with those published in reference book. MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 2 of 14

3 5. Discussion 5.1 Answer the following questions: (a) Discuss the variations in nature of the stress-strain graphs of steel and aluminium. (b) What does % elongation measures of? (c) What is the area under the stress-strain curve equivalent to? What does the area under the elastic portion of the stress-strain represent? (d) 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. (e) (f) (c) Explain the reason for the pre-load applied in the procedure for the Rockwell test. What is the limitation on the thickness of specimens for a hardness test? What are the limitations for distance from specimen edge to indentation and distance between indentations? Explain why these limitations exist in both cases. Would you suggest conducting Rockwell-Type hardness tests on ceramic or polymeric materials? Why or why not? MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 3 of 14

4 Table 1.1: Data Sheet Symbol Unit Value Sample identification Average Description of material Initial diameter of sample D 0 mm Initial cross-sectional area of sample A 0 mm 2 Initial gauge length of sample L 0 mm Final diameter of sample D f mm Final gauge length of sample L f mm Young Modulus E = tan θ GPa Yield load or 0.1/0.2/0.5 offset load P y or P 0.2 kn Yield Strength or 0.1/0.2/0.5 Proof Strength σ y or σ 0.2 MPa Maximum load P max kn Ultimate Tensile Strength σ UTS MPa Elongation at Failure E % **Provide all calculations in separate pages. Table 1.2: Data Sheet for Rockwell Hardness Number Material Type of Indenter Rockwell Hardness Number Average Rockwell Hardness Number Signature with Date of the Instructor/Course Tutor MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 4 of 14

5 6. Theoretical Background 6.1 Tensile Testing Uniaxial tensile test is known as a basic and universal engineering test for determining the mechanical properties of materials, such as strength, ductility, toughness, elastic modulus, and strain hardening capability. These important parameters obtained from the standard tensile testing are useful for the selection of engineering materials for any applications required. The tensile testing is carried out by applying longitudinal or axial load at a specific extension rate to a standard tensile specimen with known dimensions (gauge length and cross-sectional area perpendicular to the load direction) till failure. The applied tensile load and extension are recorded during the test for the calculation of stress and strain. A range of universal standards provided by Professional societies such as American Society of Testing and Materials (ASTM), British standard, JIS standard and DIN standard provides testing are selected based on preferential uses. Each standard may contain a variety of test standards suitable for different materials, dimensions and fabrication history. For instance, ASTM E8: is a standard test method for tension testing of metallic materials and ASTM B557 is standard test methods of tension testing wrought and cast aluminium and magnesium alloy products. A standard specimen is prepared in a round or a square section along the gauge length as shown in Fig. 6.1 (a) and (b) respectively, depending on the standard used. Normally, the cross section is circular, but rectangular specimens are also used. This dogbone specimen configuration was chosen so that, during testing, deformation is confined to the narrow centre region (which has a uniform cross section along its length) and to reduce the likelihood of fracture at the ends of the specimen. The standard diameter of the reduced section (also known as the gauge diameter) is 12.5 mm (0.505 in.) and the standard value of gauge length is 50 mm (2.0 in.). The initial gauge length L 0 is standardized (in several countries) and varies with the diameter (D 0 ) or the cross-sectional area (A 0 ) of the specimen, because if the gauge length is too long, the % elongation might be underestimated in this case. D 0 R W L R C A L 0 B A L 0 T 12.5 mm Round Specimen 12.5 mm Plate Specimen L 0 Gauge length L 0 Gauge length D 0 Gauge diameter W Width R Radius of fillet, min. 10 T Thickness A Length of reduced section, min. 57 R Radius of fillet, min L Overall length 200 A Length of reduced section, min. 57 B Length of grip section 50 C Width of grip section, approx. 20 Figure 6.1: Standard tensile specimens. MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 5 of 14

6 6.1.1 Stress and Strain Relationship When a specimen is subjected to an external tensile loading, the metal will undergo elastic and plastic deformation. A typical deformation sequence in a tension test is shown in Fig When the load is first applied, the specimen elongates in proportion to the load, called linear elastic behaviour. If the load is removed, the specimen returns to its original length and shape, in a manner like stretching a rubber band and releasing it. These two parameters (load and extension) are then used for the calculation of the engineering stress and engineering strain to give a stress strain relationship as illustrated in Fig Figure 6.2: Stress-strain relationship under uniaxial tensile loading showing various features. The engineering stress (or, nominal stress), σ, is defined as the ratio of the applied load, P, to the original cross-sectional area, A 0, of the specimen: σ = P A 0 (6.1) The engineering strain, ε, is defined as a ratio of the change in length to its original length ε = L L 0 L 0 = ΔL L 0 (6.2) where L 0 is the original length of the specimen, and L is the instantaneous length of the specimen. The unit of the engineering stress is megapascal (MPa) according to the SI Metric Unit where 1 MPa = 10 6 N/m 2 whereas the unit of psi (pound per square inch) can also be used in the FPS system. It can be noted here that, since the stress is obtained by dividing load with a constant (i.e., the original area of the specimen) and the strain is obtained by dividing extension (or deformation) with another constant (i.e., the MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 6 of 14

7 original gauge length of the specimen), instead of stress strain diagram, the similar deformation sequence in a tensile test can also be represented in a load deformation diagram. Many materials exhibit stress-strain curves considerably different from that mentioned in Fig Low carbonsteels or mild steels, for example, show two distinct yield points (a.k.a upper yield point and lower yield point) in their stress strain diagrams, Fig In addition, the stress-strain curve for more brittle materials, such as cast iron, fully hardened high-carbon steel, or ceramics show more linearity and much less nonlinearity of the ductile materials. Little ductility is exhibited with these materials, and they fracture soon after reaching the elastic limit. Because of this property, greater care must be used in designing with brittle materials. The effects of stress concentration are more important, and there is no large amount of plastic deformation to assist in distributing the loads. Figure 6.3: Different schematic stress strain diagram of different types of materials. It can be noted from the above diagram that, the stress strain curve of normal (semi-crystalline) polymer differs dramatically from other ductile metals in that the neck does not continue shrinking until the specimen fails. Rather, the material in the neck stretches and propagates until it spans the full gage length of the specimen, a process called drawing. This leads to localised strengthening of the materials and subsequent upward trend of the stress-strain curve before failure. Young s Modulus During elastic deformation, the engineering stress-strain relationship follows the Hook's Law and the slope of the curve indicates the modulus of elasticity or the Young s modulus, E: E = σ ε (6.3) This linear relationship is known as Hooke s law. Note in Eq. (6.3) that, because engineering strain is dimensionless, E has the same units as stress. The modulus of elasticity is the slope of the elastic portion of the curve and hence indicates the stiffness of the material. The higher the E value, the higher is the load required to stretch the specimen to the same extent, and thus the stiffer is the material. Young's modulus is of importance where deflection of materials is critical for the required engineering applications. This is for examples: deflection in structural beams is crucial for the design in engineering components or structures such as bridges, building, ships, etc. The applications of tennis racket and golf club also require high values of spring constants or Young s modulus values. MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 7 of 14

8 The elongation of the specimen under tension is accompanied by lateral contraction; this effect can easily be observed by stretching a rubber band. The absolute value of the ratio of the lateral strain to the longitudinal strain is known as Poisson s ratio and is denoted by the symbol ν. For many metals and other alloys, values of Poisson s ratio range between 0.25 and Yield Strength and Proof Stress As the load is increased, the specimen begins to undergo nonlinear elastic deformation at a stress called the proportional limit. At that point, the stress and strain are no longer proportional, as they were in the linear elastic region, but when unloaded, the specimen still returns to its original shape. Permanent (plastic) deformation occurs when the yield strength of the material is reached. The yield strength, σ y, can be obtained by dividing the load at yielding (P y) by the original cross-sectional area of the specimen (A 0): σ y = P y A 0 (6.4) For soft and ductile materials such as aluminium, it may not be easy to determine the exact location on the stress strain curve at which yielding occurs, because the slope of the curve begins to decrease slowly above the proportional limit. Therefore, P y is usually defined by drawing a line with the same slope as the linear elastic curve, but that is offset by a strain of 0.002, or 0.2% elongation. The yield stress is then defined as the stress where this offset line intersects the stress strain curve and renamed as the proof stress. This simple procedure is shown in Fig The yield strength therefore must be calculated from the load at 0.2% strain divided by the original cross-sectional area as follows: σ 0.2% = P 0.2% A 0 (6.5) Offset at different values can also be made depending on specific uses: for instance; at 0.1 or 0.5% offset. The yield strength of soft materials exhibiting no linear portion to their stress-strain curve such as soft copper or grey cast iron can be defined as the stress at the corresponding total strain, for example, = Figure 6.4: Typical stress strain behaviour for a metal showing elastic and plastic deformations, the proportional limit P, and the yield strength y, as determined using the strain offset method. MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 8 of 14

9 The yield strength, which indicates the onset of plastic deformation, is vital for engineering structural or component designs where safety factors (SF) are normally used as shown in Eq. (6.6). For instance, if the allowable working strength w = 500 MPa to be employed with a safety factor of 1.8, the material with a yield strength of 900 MPa should be selected. σ w = σ 0.2% SF (6.6) Safety factors are based on several considerations; the accuracy of the applied loads used in the structural or components, estimation of deterioration, and 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 3-4. Ultimate Tensile Strength As the specimen begins to elongate under a continuously increasing load, its cross-sectional area decreases permanently and uniformly throughout its gage length. 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. As the load is increased further, the engineering stress eventually reaches a maximum and then begins to decrease (Fig. 6.2). The maximum engineering stress is called the tensile strength, or ultimate tensile strength (UTS), of the material. σ TS = P max A 0 (6.7) If the specimen is loaded beyond its ultimate tensile strength, it begins to neck, or neck down. This can be observed by a local reduction in the cross-sectional area of the specimen generally observed in the centre of the gauge length. As the test progresses, the engineering stress drops further and the specimen finally fractures at the necked region (Fig. 6.2). Tensile Ductility An important mechanical behaviour observed during a tension test is ductility the extent of plastic deformation that the material undergoes before fracture. A metal that experiences very little or no plastic deformation upon fracture is termed brittle. The tensile stress strain behaviours for both ductile and brittle metals are schematically illustrated in Figure 6.5. Figure 6.5: Schematic representations of tensile stress strain behaviour for brittle and ductile metals loaded to fracture. MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 9 of 14

10 There are two common measures of ductility. The first is the total elongation of the specimen at failure, which measures the percentage of plastic strain at fracture and is given by %EL = L f L 0 L 0 x 100 = ΔL L 0 x 100 (6.9) where L f and L 0 are measured as shown in Fig We should note here that the elongation is calculated as a percentage and it depends on the original gage length of the specimen, as a significant proportion of the plastic deformation at fracture is confined to the neck region of the specimen during necking just prior to fracture. The shorter the L 0, the greater the fraction of total elongation from the neck and, consequently, the higher the value of %EL. Therefore, L 0 should be specified when percent elongation values are cited. The second measure of ductility is the reduction of area, given by %RA = A 0 A f A 0 x 100 = ΔA L 0 x 100 (6.10) where A 0 and A f are, respectively, the original and final (fracture) cross-sectional areas of the test specimen. Note that %RA is not sensitive to gage length and is somewhat easier to obtain, only a micrometer is required. Furthermore, for a given material, the magnitudes of %EL and %RA will, in general, be different. Knowledge of the ductility of materials is important for at least two reasons. First, it indicates to a designer the degree to which a structure will deform plastically before fracture. Second, it specifies the degree of allowable deformation during fabrication operations. We sometimes refer to relatively ductile materials as being forgiving, in the sense that they may experience local deformation without fracture, should there be an error in the magnitude of the design stress calculation. Brittle materials are approximately considered to be those having a fracture strain of less than about 5%. Modulus of Resilience Resilience is the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered. The associated property is the modulus of resilience, U R, which is the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding. Figure 6.6: Schematic representation showing how modulus of resilience (corresponding to the shaded area) is determined from the tensile stress strain behaviour of a material. Computationally, the modulus of resilience for a specimen subjected to a uniaxial tension test is just the area under the engineering stress strain curve taken to yielding (Fig. 6.6), or MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 10 of 14

11 σ y U R = σ dε 0 (6.11a) Assuming a linear elastic region, we have U R = 1 2 σ y ε y = σ y 2 2E (6.12b) in which y is the strain at yielding. Thus, resilient materials are those having high yield strengths and low moduli of elasticity; such alloys are used in spring applications. The units of resilience are the product of the units from each of the two axes of the stress strain plot. For SI units, this is joules per cubic meter (J/m 3, equivalent to Pa). Tensile Toughness Tensile toughness, U T, can be considered as the area under the entire stress strain curve 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 that requires high tensile toughness is for example gear, chains and crane hooks, etc. The tensile toughness can be estimated from an expression as follows: σ f U T = σ dε (σ y + σ TS ) ε f (6.13) where TS is the ultimate tensile strength, y is the yield stress and f is the strain at fracture. 6.2 Hardness Testing Hardness is a measure of a material s resistance to localized plastic deformation (by scratching or indentation). Methods to characterize hardness can be divided into three primary categories: 1. Scratch Tests. These are the simplest form of hardness tests. In this test, various materials are rated on their ability to scratch one another. If two materials are compared, the harder one can scratch the softer one, but not vice versa. Mohs hardness test is of this type. This test is used mainly in mineralogy. 2. Dynamic Hardness or Rebound Tests. Here an object of standard mass and dimensions is bounced back from the surface of the test specimen after falling by its own weight. The hardness number is proportional to the height of rebound of the standard mass. Shore hardness is measured by this method. 3. Static Indentation Tests. Test tests are based on the relation of indentation of the specimen by a penetrator under a given load. The relationship of total test force to the area or depth of indentation provides a measure of hardness; the softer the material, the larger and deeper the indentation, and the lower the hardness index number. The Rockwell, Brinell, Knoop, Vickers, and ultrasonic hardness tests are of this type. For engineering purposes, static indentation tests are mostly used, Fig Indentation tests produce a permanent impression in the surface of the material. The force and size of the impression can be related to a quantity (hardness) which can be objectively related to the resistance of the material to permanent penetration. Because the hardness is a function of the force and size of the impression, the pressure (and hence stress) used to create the impression can be related to both the yield and ultimate strengths of materials. For materials that undergo plastic deformation primarily via slip (e.g., metals for which dislocation motion requires shear stress), it has been demonstrated that: H ~ 3 σ y (6.14) MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 11 of 14

12 Figure 6.7: General characteristics of hardness-testing methods and formulas for calculating hardness. Thus, as hardness increases so does the yield strength and the ultimate tensile strength. For this reason, specifications often require the results of hardness tests rather than tensile tests. Several different types of hardness tests have evolved over the years. These include macro hardness test such as Brinell, Vickers, and Rockwell and micro hardness tests such as Knoop and Tukon. Rockwell and Brinell testing is the most commonly applied materials test in industry due to several factors: 1. Simple to perform and does not require highly skilled operators; 2. Using different loads and indenters, hardness testing can be used for determining the hardness and approximate strength of most metals and alloys including soft bearing materials and high strength steels; 3. Hardness readings can be taken in a few seconds with minimal preparation; and 4. For Rockwell hardness testing, no optical measurements are required; all readings are direct. Factors influencing hardness include microstructure, grain size, strain hardening, etc. Measured harnesses are only relative (rather than absolute) thus care must be taken when comparing values determined by different techniques Rockwell Hardness Testing This hardness test uses a direct reading instrument based on the principle of differential depth measurement. Rockwell hardness differs from Brinell hardness testing in that the indentation size is measured by the diameter of the indentation in Brinell testing while Rockwell hardness is determined based on the inverse relationship to the difference in depths of indentation produced by minor and major load. For thin test samples or samples for which the relatively large Brinell or Rockwell indentations must be avoided, the Superficial Rockwell hardness test is often employed. Superficial Rockwell hardness testing employs lower loads to the indenter to minimize penetration. MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 12 of 14

13 The Rockwell Hardness Test consists of measuring the additional depth to which an indenter is forced by a heavy (major) load beyond the depth of a previously applied light (minor) load as illustrated in Fig Application of the minor load eliminates backlash in the load train and causes the indenter to break through slight surface roughness and to crush particles of foreign matter, thus contributing much greater accuracy in the test. The Rockwell hardness test method consists of indenting the test material with a diamond cone or hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor load F0 (Fig. 6.8A) usually 10 kg f. When equilibrium has been reached, an indicating device, which follows the movements of the indenter and so responds to changes in depth of penetration of the indenter is set to a datum position. While the preliminary minor load is still applied an additional major load is applied with resulting increase in penetration (Fig. 6.8B). When equilibrium has again been reach, the additional major load is removed but the preliminary minor load is still maintained. Removal of the additional major load allows a partial recovery, so reducing the depth of penetration (Fig. 6.8C). The permanent increase in depth of penetration, resulting from the application and removal of the additional major load is used to calculate the Rockwell hardness number. Stage A F0 applied Stage B F0 + F1 = F applied Stage C F0 applied (F1 Withdrawn) F0 = preliminary minor load in kg f F1 = additional major load in kg f F = total load in kg f e = permanent increase in depth of penetration due to major load F1 measured in units of mm E e E = a constant depending on form of indenter: 100 units for diamond indenter, 130 units for steel ball indenter Figure 6.8: Principle of Rockwell testing. The minor load is applied first, and a reference or set position is established on the measuring device or the Rockwell hardness tester. Then the major load is applied at a prescribed, controlled rate. Without moving the workpiece being tested, the major load is removed and the Rockwell hardness number is indicated on the dial gage. The entire operation takes from 5 to 10 seconds. In Rockwell testing, the minor load is 10 kg f, and the major load is 50, 90 or 140 kg f. In superficial Rockwell testing, the minor load is 3 kg f, and major loads are 12, 27 or 42 kg f. In both tests, the indenter may be either a diamond cone or steel ball, depending principally on the characteristics of the material being tested. The 120 sphero-conical diamond indenter is used mainly for testing hard materials such as hardened steels and cemented carbides. Hardened steel ball indenters with diameters 1/16, 1/8, 1/4, 1/2 in. are used for testing softer materials such as fully annealed steels, softer grades of cast irons, and a wide variety of nonferrous metals. There are 30 different Rockwell scales (regular and superficial), defined by the combination of the indenter and minor and major loads, Table 6.1. The suitable scale is determined due to the type of the material to be tested. Most of applications are covered by the Rockwell C and B scales for testing steel, brass, and other materials. No Rockwell hardness value is expressed by a number alone. A letter has been assigned to each combination of load and indenter, as indicated in Table 6.1. Each number is suffixed first by the letter H (for hardness), then the letter R (for Rockwell), and finally the letter that indicates the scale used. For example, a value of 60 in the Rockwell C scale is expressed as 60 HRC, etc. MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 13 of 14

14 Table 6.1: Rockwell hardness scales. Scale Indenter Minor Load, F0 A Diamond cone Major Load, F1 Total Load, F kgf kgf kgf B 1/16" steel ball C D Diamond cone Diamond cone E 1/8" steel ball Typical Applications Cemented carbides, thin steel and shallow case-hardened steel Copper alloys, soft steels, aluminium alloys, malleable irons Steel, hard cast irons, pearlitic malleable iron, titanium, deep case-hardened steel and other materials harder than 100 HRB Thin steel and medium case-hardened steel and pearlitic malleable iron Cast iron, aluminium and magnesium alloys, bearing metals F 1/16" steel ball Annealed copper alloys, thin soft sheet metals G 1/16" steel ball H 1/8" steel ball Aluminium, zinc, lead K 1/8" steel ball L 1/4" steel ball M 1/4" steel ball P 1/4" steel ball R 1/2" steel ball S 1/2" steel ball V 1/2" steel ball Phosphor bronze, beryllium copper, malleable irons. Upper limit is HRG 92, to avoid flattening of ball. Bearing metals, plastics and other very soft or thin materials The metal immediately surrounding the indentation from a Rockwell hardness test is cold worked thus multiple readings cannot be taken at the same point on a material s surface. If multiple tests are conducted on a single part the indentations should each be a minimum of 3 indentation diameters apart. The depth of material affected during testing is on the order of ten times the depth of the indentations. Therefore, unless the thickness of the metal being tested is at least ten times the depth of the indentation, an accurate Rockwell hardness test cannot be expected. In addition to the limitation of indentation depth for a workpiece of given thickness and hardness, there is a limiting factor on the minimum material width. If the indentation is placed too close to the edge of a workpiece, the edge will deform outward and the Rockwell hardness number will be decreased accordingly. Experience has shown that the distance from the centre of the indentation to the edge of the workpiece must be at least 3 times the diameter of the indentation to ensure an accurate test. MME298/July 17 Term/Expt. 01: Mechanical Characterisation of Materials Page 14 of 14