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RYERSON UNIVERSITY DEPARTMENT OF MECHANICAL AND INDUSTRIAL ENGINEERING BIOMATERIALS LABORATORY BME 423 MANUAL Name of Student: _____________________________________________ Instructor: _____________________________________________ NO REFUNDS NO EXCHANGES 1 Table of Contents Experiment Title Page General Instructions ii 1 Construction of Atomic Models of Metallic Materials 1 2 Identification of Materials by X-Ray Diffraction 3 3 Corrosion of Metals 10 4 Impact and Hardness Testing 15 5 Tensile Properties of Polymeric Materials 19 2 BIOMATERIALS LABORATORY – GENERAL INSTRUCTION Laboratory Hours and Attendance Most of the experimental work will be completed within the assigned hours. Most of the experiments will be done in a group mode and attendance of all members in a group is compulsory. Attendance will be taken at 12 minutes after the hour. In the case of late arrival, it is the responsibility of the student to have the attendance record revised. Laboratory Manual The students must read the laboratory manual and gain an understanding of the procedure prior to coming to the laboratory. Laboratory Procedure 1. When an experiment is carried out by a group, all members are expected to participate in the actual laboratory work. A neat, concise record of observations is required for the report which is to be submitted following completion of the experiment. The instructor will advise on the form and evaluation procedure for reports due. 2. Every group cleans all equipment and the area employed for the particular experiment prior to the end of the period. 3. At all times exercise maximum care in order to avoid damage to the equipment or injury to yourself or others. All of you must pay utmost attention to this point all the time. 4. Before leaving the laboratory, submit worksheet to the instructor for observation. Laboratory Reports 1. Reports will be submitted as instructed. 2. Reports should be submitted in a suitable folder with the name of the experiment, date of the experiment performed, submission date, author of the report, and the group members. 3. The submitted report should contain the following: Title, date, and objectives of the experiment, Experimental material and equipment employed, Procedure - a concise summary of the key procedural points is required, not an entire description, Observations and results - use graphs and tables suitably arranged, and include sample calculations where possible. Always include the unit of measurements. Discussion - detailed discussion of experimental results, relationship to theory where applicable, answers to questions posed, statements of how well the objectives have been achieved, and conclusions. 4. Marking. Evaluation is partly based on 1aboratory performance, neatness of report, academic value of the discussion (clarity, organization, English usage, headings, tables, figures, table and figure captions), correct answers to the questions posed for each experiment, and conclusions. 3 EXPERIMENT #1 CONSTRUCTION OF ATOMIC MODELS OF METALLIC MATERIALS Introduction Most materials in the solid state consist of atoms arranged in relatively fixed positions with respect to each other. These solids are known as crystalline solids and as such they exhibit properties distinct from so-called amorphous solids in which atomic arrangement is irregular and behavior more resembles that of viscous liquids. All metals and alloys are crystalline solids and interatomic forces between atoms produce a regular three-dimensional array or pattern. This regularity is sometimes, although infrequently, observed in the external symmetry of a crystalline material when conditions are favourable to the growth of certain geometric faces. In most cases, however, these conditions are absent and crystals have irregular external surfaces although the internal arrangement is perfectly geometric. Many of the physical properties associated with crystalline solids can be interpreted on the basis of the stacking arrangement of the atoms in a given solid. Although many different crystal structures are possible the symmetry of any crystal can be described as one of only fourteen possible space arrangements or space lattices. Fortunately, most of the common metals such as iron, copper, aluminum and titanium exhibit relatively simple geometric arrangements of high degree of symmetry and can be described as a cubic or hexagonal atomic array. The cubic structure is characterized by a space lattice in which all edges of a unit cell (the smallest array of atoms which completely describes the crystal symmetry) are of equal length and mutually perpendicular. The hexagonal system is characterized by a space lattice in which three axes are of equal length, coplanar and 120º apart and a fourth axis of unequal length and perpendicular to the three coplanar axes. It is instructive to construct models of these systems as an aid in visualizing the possible stacking arrangements of atoms in metals and to show how knowledge of atomic arrangement can explain many of the observed physical properties of metals and alloys. In this experiment, construction will be limited to the face-centered cubic, body-centered cubic and hexagonal close-packed structures. Procedure Obtain from instructor around 50 – 60 cork balls (representing spheres of atoms) and the peg boards for model construction. Consult reference texts for stacking arrangements of the facecentered cubic, body-centered cubic and hexagonal close-packed structures. A proper understanding of the positions of the atoms in each of the three structures is essential before 4 subsequent calculations on the cell geometry can be made. All questions from the report should be attempted and understood using the models during this laboratory period. Questions 1. Given that silver has a face-centered cubic structure and an atomic radius of 1.44 Å, determine the density of silver. Show your detailed calculations. Compare this value with the density published in reference texts. 2. For each of the above crystal structures, which is the plane of closest packed atoms? 3. Explain clearly the differences in stacking arrangements of the three crystal models constructed. How can the face-centered cubic and hexagonal close-packed structure be converted from one to the other? 4. Determine, in terms of the radius of the atom, the distance between adjacent closest packed planes in each of the above structures. 5. What differences in the mechanical properties (e.g. strength and ductility) of metals result from the differences in crystal structure? Use as examples, iron, aluminum, copper, zinc and titanium. 5 EXPERIMENT #2 IDENTIFICATION OF MATERIALS BY X-RAY DIFFRACTION Introduction Solid materials may generally be categorized as crystalline or amorphous substances. The distinguishing characteristics of crystalline solids are that the particles (ions, atoms or molecules), which comprise the solid, are arranged in some definite three-dimensional geometric pattern. Despite the existence of so many crystalline solids it is known that the geometric configuration of each is described by one of only 14 possible geometric patterns. Most metals and alloys and many inorganic compounds exhibit relatively simple geometric configurations. The following diagrams illustrate the unit cells and the positions of the particles in each of three common structures. Figure 2. 1 Principal metal crystal structure unit cells: (a) body-centered cubic, (b) facecentered cubic, (c) hexagonal close-packed. [1] For further explanations of these crystal structures consult your textbook. Although atoms cannot be seen even with the most powerful microscope the determination of the geometric structure and identification of substances is possible by x-ray diffraction. X-rays are diffracted by a crystal in a similar way that ordinary light is spread out by a diffraction grating. Crystals, being made up of layers of particles a few angstroms apart, act as a diffraction grating for x-rays, which have wavelengths on the order of 0.1 angstroms (Å). (1Å = 10-8 cm). 6 Figure 2. 2 The reflection of an x-ray beam by the (hkl) planes of a crystal. (a) No reflective beam is produced at an arbitrary angle of incidence. (b) At the Bragg angle, eflected rays are in phase and , the r reinforce one another. (c) Similar to (b) except that the wave representation has been omitted. [2] 7 The basis for all X-ray diffraction analysis is the Bragg Law nλ = 2d sin θ where θ is the angle at which diffraction occurs; d, the distance between successive planes of atoms; λ, the wavelength of monochromatic x-rays and n, an integer representing the diffraction order. The distances between planes of atoms (the d values) are characteristic of the material and thus it is possible to identify a substance by the spacings between the planes of atoms. These distances may be determined by several methods one of which is the Debye-Scherrer powder method. In this technique a finely ground sample is rotated while bombarded with x-rays of suitable wavelength. The result of diffraction is a plot of diffracted beam intensity as a function of rotation angle, 2θ, as seen in Figure 2.3. Figure 2. 3 Diffraction pattern of a crystalline metal. [1] If a circular strip of film is used, any cone of reflected radiation, representing diffraction of xrays from a given set of crystal planes (say the (111) planes), produces two arcs on the photographic film. Similarly, other sets of planes will diffract the incident beam and produce like arcs on the photographic film. Thus, a series of arcs will be produced and the number and distance between them is a characteristic of one specific material. Hence by measuring the distance between these arcs, it becomes possible from Bragg’s law to determine the interatomic spacing between the planes “d”. Since they are peculiar to the given material only, it becomes possible to identify the material. To facilitate determination of the “d” spacings, the Debye-Scherrer method utilizes a camera of specific dimensions, usually 57.3 mm diameter. Thus, consider a plane, which diffracts the incident beam at an angle θ, as illustrated in Figure 2.4. 8 Figure 2. 4. Schematic diagram of the Debye-Scherrer camera showing front reflection. The cone of reflection produced subtends an angle of 4θ for direct reflection and 2π−4θ for back reflection. Hence by geometry, S/R(radians) = 4θ for direct reflection and S/R(radians) = 2π−4θ for back reflection; or (S/R) x 57.3 = 4θ°for direct reflection and (S/R) x 57.3 = 360 – 4θ° for back reflection. Since the diameter of the camera is exactly 57.3mm then, S, in mm = 2θ°for direct reflection and S, in mm = 180 – 2θ° for back reflection. Thus when the film strip is unrolled and laid out flat the distance between corresponding diffraction lines can be readily measured and determined. Application of Bragg’s Law then permits determination of the characteristic “d” spacings. Identification of a material is made by comparing the “d” values of the three most intense lines on the photographic film with a reference index (see next page). The intensities of the lines may in fact be measured instrumentally but relative intensities determined by eye on the assumption the darkest line in the film has a value of 100 is generally sufficient for identification. 9 X-Ray Powder Diffraction Data The following data, from the ASTM index, are for a selection of substances. “d spacings” are listed for the three most intense powder lines, in order of relative intensity, followed by the corresponding relative intensities. Note: if other lines appear approximately equal to the highest intensity, check each one of them in the first column of “d spacings”. Experimental variables cause some deviation from the values given, Check within a range of about 0.05Å on each side of your results, to find the substance, which gives closest agreement with your experimental values. d Spacing (angstroms) Relative intensities Substances 4.88 3.58 3.40 100 60 60 As2O5 4.38 3.40 2.88 100 90 65 V2O5 4.15 2.53 1.64 100 80 60 SiO2 3.85 3.21 3.44 100 60 40 S 3.75 2.29 1.96 100 83 53 α - Sn 3.72 3.68 2.15 100 53 28 BaCO3 3.59 3.50 2.49 100 43 32 PbCO3 3.58 3.89 2.78 100 73 56 PbCl2 3.54 3.45 2.05 100 70 50 SrCO3 3.53 2.50 4.08 100 70 42 KI 3.51 1.89 2.38 100 33 22 TiO2 3.50 2.80 1.86 100 100 100 PbO2 3.44 3.10 2.12 100 97 80 BaSO4 3.44 2.61 2.28 100 55 43 PbI2 3.40 4.20 2.86 100 75 25 CrO3 3.40 1.98 3.27 100 65 52 CaCO3 3.35 2.64 1.77 100 81 63 SnO2 3.31 3.13 2.93 100 86 84 α - ZnS 3.29 2.33 1.48 100 57 17 KBr 3.28 2.27 2.39 100 41 40 Bi 3.24 3.74 2.29 100 82 63 NaI 3.23 2.68 1.67 100 90 80 Bi2O3 3.22 1.97 2.79 100 42 40 Sb2O3 3.20 6.39 2.54 100 63 38 As2O3 3.18 1.95 1.66 100 100 100 Pb2O3 3.15 2.22 1.82 100 59 23 KCl 3.14 2.42 1.64 100 80 80 MnO2 3.14 1.92 1.64 100 60 35 Si 3.12 1.91 1.63 100 51 30 β - ZnS 3.11 2.25 1.37 100 70 67 Sb 3.07 2.95 2.74 100 31 28 PbO 10 3.02 2.98 4.14 100 55 20 Mg 2 P 3 O 7 3.01 3.78 2.07 100 53 35 Se 2.99 3.45 2.11 100 64 63 NaBr 2.97 3.43 2.10 100 84 57 PbS 2.92 2.79 2.02 100 90 74 β - Sn 2.86 2.48 1.49 100 50 32 Pb 2.83 3.42 3.24 100 40 40 SnS 2.82 1.99 1.63 100 55 15 NaCl 2.81 1.90 3.05 100 75 67 CuS 2.78 5.90 2.14 100 50 50 SnS 2 2.77 3.52 1.88 100 26 26 As 2.77 1.96 3.20 100 50 49 AgCl 2.74 2.10 1.70 100 43 34 MgCO 3 2.67 2.48 1.67 100 96 90 Cr 2 O 3 2.53 1.48 2.97 100 70 60 Fe 3 O 4 2.52 2.32 2.53 100 96 49 CuO 2.52 1.48 2.95 100 53 34 Fe 2 O 3 2.48 2.82 2.60 100 71 56 ZnO 2.47 2.14 1.51 100 37 27 Cu 2 O 2.45 2.61 2.78 100 41 35 Mg 2.36 2.04 1.23 100 52 36 Au 2.36 2.04 1.23 100 40 26 Ag 2.35 2.81 2.58 100 65 32 Cd 2.34 2.02 1.22 100 47 24 Al 2.32 1.64 1.34 100 60 17 NaF 2.15 2.49 1.52 100 80 60 FeO 2.11 1.49 0.94 100 52 17 MgO 2.10 2.00 1.24 100 66 30 Mn 2.09 2.98 2.67 10 0 90 90 Fe 2.09 2.55 1.60 100 92 81 Al 2 O 3 2.09 2.47 2.31 100 53 40 Zn 2.09 2.41 1.48 100 91 57 NiO 2.09 1.81 1.28 100 46 20 Cu 2.03 1.76 1.25 100 42 21 Ni 2.03 1.17 1.43 100 30 19 Fe 1.96 2.78 3.20 100 35 20 Cu 2 S 1.93 3.15 1.65 100 94 35 CaF 2 1.91 1.25 1.15 100 80 80 Co 1.63 2.71 2.42 100 84 66 FeS 2 1.54 1.81 1.17 100 80 70 Sb 2 O 5 11 Procedure Obtain from instructor, two previously developed x-ray films. Place the film on the reader such that the front reflection lines (generally darker and more distinct than the somewhat fuzzy back reflection lines) are on the left side. Centre the film such that the measuring cross-hairs intersect the arcs at their widest points. Determine the reading for the exact center of the left hole (2θ = 0) by measuring the average distance between all corresponding arcs which are on either side of the hole. Now measure successively the lengths, in millimeters, of all arcs between the two punched holes. Before removing the film from the reader, recheck your readings and the assign arbitrarily an intensity value of 100 to the darkest arc on the film. Then on a comparative basis, assign a number to the next darkest arc (e.g. 60) and so on until all the arcs have been assigned a relative intensity number. On the basis of Bragg’s Law, assuming first order diffraction, determine the characteristic “d” values for all lines on the film. (Assume Cu radiation of wavelength λ= 1.542 Å). Then select the “d” values corresponding to the three strongest intensity lines and from the previous table attempt to identify the unknown sample. Report 1. Tabulate all readings. 2. Show sample calculations for evaluation of characteristic “d” spacings. 3. Tabulate “d” values determined and corresponding relative intensities. Identify the three strongest lines. 4. Show clearly how you identified the unknown sample. 5. An x-ray diffraction analysis of a crystal is made with a molybdenum target (λ=0.711 Å). If one of the interplanar spacings is 1.82 Å, what is the angle of diffraction for this particular set of planes (assuming first order diffraction)? 6. Sodium metal crystallizes in a body-centered cubic arrangement where the length of one edge of the unit cell is 4.24 Å. What is the closest distance between centers of adjacent sodium atoms? 7. An x-ray difffractometer for an element which has either the BCC or the FCC crystal structure shows diffraction peaks at the following 2θ angles: 40, 58, 73, 86.8, 100.4 and 114.7. The wavelength of the incoming x-ray was from a Cu target. (a) Determine the cubic structure of the element. (b) Determine the lattice constant of the element. (c) Identify the element. References [1] Smith, “Principals of Materials Science and Engineering”, 2nd ed., McGraw-Hill, 1990. [2] Guy and Hren, “Elements of Physical Metallurgy”, 3rd ed., Addison-Wesley, 1974, p201. 12 EXPERIMENT #3 CORROSION OF METALS Introduction Corrosion generally refers to the dissolution of a metal in contact with a suitable medium. Ordinary rust, for example, is a typical end product of the corrosion of iron in aqueous solutions. Metallic corrosion is electrochemical in nature and may be viewed as a reaction of a metal with its environment such that the metal returns to an ionic state. For instance, in the case of zinc, corrosion may be expressed by the reaction: Zn → Zn++ + 2e− where Zn++ represents zinc ions in solution and e- an electron. This reaction involving a metal going into solution is referred to as an anodic reaction. However, for corrosion of dissolution to progress the electrons must be removed, otherwise a charge builds up and corrosion stops. Thus, if electrons can flow to a point where they are used up, the reaction continues. The absorption of electrons is considered a cathodic reaction. Several cathode reactions are possible. For example, hydrogen ions in solution (always present in water or acids) may combine with electrons to form hydrogen gas: 2H+ + 2e− → 2H → H2 (gas) Similarly, a metal ion in solution may combine with electrons to form a metallic element. For example, in the presence of copper ions: Cu++ + 2e−→ Cu (metal) Thus for corrosion to continue there must be anode and cathode reactions. The anodic reaction involves the metal going into solution and the release of electrons. Thus the metal in this region is negative. The cathodic reaction involves electron absorption by ions to form a gas or a metal. This region is positive because positively charged ions accumulate to receive electrons. The driving force for corrosion depends on the metal itself and the medium or electrolyte. Each metal has a different driving force for a solution which can be measured as a voltage. Since a voltage cannot be measured on a single electrode by itself, the electromotive force (emf) is a comparative value based on some arbitrary standard such as hydrogen. On this basis it is then possible to measure the driving voltage or tendency to corrode of a given corrosion couple in a given electrolyte. This value is influenced not only by the electrolyte but the concentration of the electrolyte as well. Thus a measurement of the potential or emf of a corrosion cell indicates the anode and the cathode and the tendency to corrode. The higher the potential the greater is this tendency. 13 There are many factors which may alter the corrosion rate of a metal, some of which are polarization effects caused by the evolution of hydrogen at the cathode, the electrolyte composition, stress effects, the metal with which it is coupled, relative size of the electrodes and the presence of passive or active films. These factors will be investigated in the following experiment. Procedure (a) Difference in Potentials Using a set-up similar to that illustrated in Figure 3.1, measure the difference in potentials in millivolts between: i) copper and steel ii) copper and zinc iii) copper and aluminum iv) copper and magnesium v) copper and stainless steel From your readings, list the metals in the order of most anodic to least anodic. Which metal would be most likely to corrode in the presence of copper? Which metal is least likely to corrode in the presence of copper? Figure 3. 1 Experimental set-up for the corrosion experiment. 14 (b) Influence of Medium (Electrolyte) Add 5 ml 1% NaOH to the previous solution used in part (a). Measure again the potentials in millivolts as in section (a). Discuss any significant changes with respect to the findings in section (a). (c) The Galvanic Cell With the copper and zinc plates still in the solution, connect the copper plate to the positive (red) lead of the ammeter. Set the ammeter on the 0-0.2 mA range. As soon as you clamp the negative lead on the zinc plate observe the reading on the ammeter. Record the initial current and the current after 30 seconds. Wipe the copper plate off with a piece of paper towel and measure the current again. On the basis of your results: i) Which metal is corroding and which metal is not? (Note that the more negative metal is called the anode and the relatively positive one the cathode. In the zinc-copper galvanic cell the zinc is the anode and the copper is the cathode.) ii) Does the current increase or decrease with time? This effect is known as polarization. iii) Does polarization increase or decrease the tendency for corrosion? iv) Why did we use a salt solution in this part of the experiment? Could we have also used distilled water or a sugar solution? (d) Stress-Corrosion Take 1 ordinary nail and 1 nail that has been stress-relief annealed. Clean both by placing them in a test tube of 10% hydrochloric acid for 2 minutes. Rinse with water. Immerse each nail in a separate test tube and fill the tubes with acidified 10% calcium chloride solution. Place the test tubes in a rack and do not touch them while proceeding with (e) and (f). Observe after 5 or 10 minutes, which nail shows corrosive activity and which one seems passive. Try to explain the difference based on your knowledge of the differences between the two nails. (e) Corrosion of Stainless Steel The resistance to corrosion of stainless steel is due to a thin air-tight film of chromium oxide. However, if this film is destroyed the electromotive potential is about the same as that of iron and its corrosive resistance is correspondingly the same. The presence of an 15 adherent film indicates a passive condition whereas destruction of the film leads to an active state. Clean an ordinary steel plate for about 2 minutes in 10% HCl and rinse in water. Passify 2 pieces of stainless steel in prepared dilute nitric acid for about two minutes, then rinse with water. Fill a 250 ml beaker with about 200 ml 5% H2SO4. Clamp one stainless steel plate and the ordinary steel plate as seen in Figure 3.1 and connect the voltmeter leads. Use the 0 - 2 volt range. Lower the assembly into the beaker and observe and record the potential. Now bring the two electrodes into contact. During contact and upon separation of the electrodes, record the potential. Repeat the procedure several times recording the potentials upon contact and upon separation. Replace the ordinary steel electrode with the second stainless steel strip (passivated) and observe and record the potential. (If negative, reverse the leads and record the potential.) Explain the changes in potential for each case. Explain the practical significance of these effects, e.g. the corrosion resistance of stainless steels under different conditions. (f) Effect of Anode and Cathode Size Using a set up similar to Figure 3.1, place approximately equal size strips of copper and steel in a 250 cc beaker. Fill the beaker to approximately 3/4 full with 5% NaCl solution. Also ask the instructor to set up the oxygen bubbling system for the cell. Connect the milliammeter to the cell as in section (c) and record the value observed. Remove the steel plate and replace it with a thin ordinary steel nail. Measure and record the current observed. Now measure and record the current observed for the steel plate and a thin copper wire. On the basis of your results, explain the effect of the relative anode and cathode size on the tendency for corrosion. (g) Measurement of the Rate of Corrosion Using a triple beam balance, weigh and record to the nearest 0.1 g an ordinary steel nail. Using the solution in (g) set up a cell with the copper plate and measure the current every minute for about 10 minutes. Calculate the average current reading based on these 10 readings. 16 The corrosion rate can be determined on the basis of Faraday’s law. Using this law the weight of metal (w) dissolving in grams can be determined from the expression: w = kIt where k = atomic weight of metal number of electrons transferred x 96 500 I = current (amperes) t = time (seconds) Assuming that the atomic weight of the nail (mostly iron) is 56 and that two electrons are transferred (Fe → Fe ++ + 2e−), calculate from the current observed the time (in days) required to completely dissolve the nail. Questions 1. A child is receiving braces for the first time. The Orthodontist places a Ti-Ni wire in the groove of a stainless steel bracket. What corrosion problems might he encounter? 2. Fixation plates occasionally corrode at the countersunk holes used for mounting. What is this type of corrosion called and describe the mechanism? 3. Proteins can bind to metal ions and transport them away from the implant. How does this affect the free energy at the metal surface? 4. A surgeon is having difficulty trying to get a Steinmann pin to fit in a treatment for a tibial fracture. In a last ditch effort to finish the operation, he bends the pin to make it go in. What corrosion problems might this cause? References [1] Corrosion in Action, The International Nickel Company of Canada, Limited [2] Munro, L. A., Chemistry in Engineering, Prentice-Hall Publ. 17 EXPERIMENT #4 CHARPY IMPACT AND HARDNESS TESTING Introduction Two common tests employed in industry to determine certain mechanical properties of materials are the hardness and the Charpy impact test. Hardness Testing One of the simplest tests which gives a measure of strength related to a material’s resistance to localized plastic deformation is the hardness test where hardness may be defined as resistance to penetration or to abrasion. In general, increased hardness indicates increased strength but lower toughness and ductility. Thus hardness testing is often used as a simple quality control evaluation procedure. There are several types of hardness testers, the Shore scleroscope, the Brinell hardness tester, the Vickers hardness tester and the Rockwell hardness tester. They differ principally in the amount of load and the type of penetrator used in testing. For example, the Shore scleroscope utilizes a diamond tipped dart dropped from a standard height. The Vickers machine utilizes weights from 1 kg to 120 kg and a square based diamond pyramid penetrator. The Shore scleroscope is of low accuracy and is utilized mainly because of its portability. The Vickers unit is more of a research tool. In practice, routine hardness evaluations are generally restricted to the Brinell and Rockwell testers, because of their simplicity and ease of measurement. The Brinell tester uses a standard load (500 or 3000 kg) and a tungsten carbide ball of fixed diameter. The diameter of penetration is measured with a low power microscope and the hardness is calculated or read off a chart for the loading employed. As a general rule of thumb, the tensile strength in lb/in2 of ferrous materials is considered to be approximately 500 BHN where a 3000 kg loading is employed. The Rockwell hardness tester measures the difference in depth of penetration between a minor and a major load. The advantage of this machine is its relative simple operation and the range of scales (hardness values) available by combining different loadings (60, 100 and 150 kg) with different penetrators (1/16 , 1/8 steel possible to estimate the tensile strength in lb/in2 based on the hardness values. Charpy Test This test is used to determine several properties; the effect of sudden loading (impact), the effect of triaxial stresses as simulated by the presence of a notch of certain geometry (notch sensitivity) and the effect of temperature on the toughness of a given material. In the test a hammer is raised to a certain height and is caused to strike a sample mounted horizontally with the notch positioned away from the anvil face. The height to which the anvil rises after impacting the 18 sample is a measure of its toughness under the conditions of the experiment. This is automatically recorded by the machine and is measured in foot pounds (ft-lb). A high reading represents considerable energy absorbed on impact and therefore high toughness. A brittle material, however, would fracture readily with little energy absorption and a low reading would result. Certain steels exhibit a drastic reduction in toughness under certain conditions. This tendency of steel to fail in a brittle manner is increased by stress concentration, speed of load application and a decrease in temperature. The effect of temperature on the ductile-brittle properties of a steel may be conveniently measured by the Charpy test since the stress concentration (notch effect) and the speed of load application may be held constant. A plot of the energy absorbed as determined by the Charpy test versus temperature for a given steel may show a temperature at which the impact values drop sharply. This temperature is known as the ductile-to-brittle transition temperature. It is a function of the composition of the steel as well as the heat treatment it undergoes. (a) (b) Figure 4.1 Typical results of the effect of temperature on the energy absorbed upon impact of Charpy V-notch samples; (a) of different types of materials, (b) illustrating the influence of carbon content on the behavior for steel. Procedure Charpy Impact Test 1. Obtain five Charpy steel samples from the instructor. One sample will be tested at room temperature. The remaining four samples are to be tested at 100°C, 0°C, -20°C and -40°C temperatures. 2. Place one sample in the -20°C deep freeze cabinet and another in the -40°C deep freeze cabinet, one in the ice-water bath (0°C) for about 1 hour and another in a beaker of boiling water (100°C) for about 10 minutes. 3. While waiting for the samples to reach the appropriate temperatures, measure the Rockwell C hardness (HRC) of the room temperature sample. Take care to restrict 19 hardness indentation marks to the extremities of the test sample and not close to the notch. 4. Begin Charpy testing with the room temperature sample. CAUTION: SEE INSTRUCTOR BEFORE USING THE CHARPY TESTER. THERE IS RISK OF SERIOUS INJURY IF PROPER PRECAUTIONS ARE NOT TAKEN. For each test, set the indicator of the Charpy Tester to the far left (240 ft-lb). Raise the hammer and ensure that it is securely held in the raised position. Place the specimen in the proper horizontal position. Be sure that no obstacles are in the path of the swinging hammer. Release the hammer and record the amount of energy absorbed on impact. Retain the broken samples for fracture examination. Remember that these tests are temperature dependent so that the low temperature samples must be removed with tongs and positioned in the testing unit as fast as possible so that the effect of temperature can be evaluated. It is suggested the placement of a sample in the machine with the tongs be practiced prior to testing the actual samples. Record the impact reading and corresponding temperatures for each sample tested. Hardness Tests Ask the instructor to demonstrate the use of the Rockwell and Brinell hardness testers. For the Rockwell tests, several scales corresponding to differences in loading and penetrator can be employed. Rockwell ‘C’ scale (HRC) is generally used for steels which have been hardened and tempered. Rockwell ‘E’ scale (HRE) may be used for soft steels and non-ferrous metals and alloys. See the instructor for the range of samples to be tested using the Rockwell and Brinell testers. HARDNESS READINGS MUST BE TAKEN ON ONLY ONE SIDE OF THE TEST BLOCK. For the Rockwell tests, at least two readings, which are within 2 hardness units, must be taken for each sample. For the Brinell tests, one impression per sample is required but two readings on each impression should be made to determine the average diameter of the impression. Report 1. Tabulate all of the results for the Charpy and Hardness tests. 2. Construct a graph of the energy absorbed (ft-lb) versus temperature and estimate the transition temperature for the steel supplied. 3. Comment on the relative ductility of the impact test pieces based on the fracture at the notch as a function of temperature. 4. Estimate from the hardness data the tensile strength in lb/in2 of all the samples tested. 20 Use either the chart in the laboratory or the appropriate formula. 5. Comment on the relative strengths of the materials tested based on the hardness tests. 6. Discuss the significance of the above tests in biomedical engineering applications. References Avner, S.H., Introduction to Physical Metallurgy, McGraw-Hill Publ. Smith, C.O., The Science of Engineering Materials, 3rd edition, Prentice-Hall, New Jersey, 1986. 21 EXPERIMENT #5 TENSILE PROPERTIES OF POLYMERIC MATERIALS Introduction Biomaterials in practice are subjected to various forces which if large enough could cause catastrophic failure in service. Once the failure of an implant occurs in the human body, it is a serious clinical problem since inevitable revision surgery is more difficult and has a lower success rate. It causes additional damage to the surrounding tissues and significantly increases health care costs, not to mention extra pain. Consequently, some measurements of the forces which materials can withstand without failure are necessary prior to the use in a given application. Two parameters which materials are subjected to are (1) stress defined as the force per unit area, generally expressed in MPa or ksi and (2) strain defined as the elongation per unit length of material expressed in mm/mm or in/in. In most materials there exists a condition in which the stress is directly proportional to the strain, i.e., the elongation is directly proportional to the load applied. Under these conditions, the removal of the applied load results in a return of the material to its original dimension. Beyond a given loading, however, the elastic property is no longer observed and increased loading results in either permanent elongation and deformation of the material or sudden fracture. The minimum stress at which permanent deformation occurs is known as the elastic limit or proportional limit. Design of a structure is often based on this elastic property and for convenience relative stiffness of materials can be evaluated by comparison of the ratio of stress/strain for each. This ratio is known as Young’s modulus or the modulus of elasticity and its value is readily determined experimentally by the stress-strain diagram of the material. In certain ductile materials, with continued loading beyond the elastic limit, a stress is reached at which the material deforms without increasing the load. This value of stress is known as the yield point. In many materials, however, this point is not well defined and the yield strength is taken as the stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain. The yield strength is generally determined from the stressstrain diagram by drawing a line parallel to the linear portion of the curve at a value of normally 0.2% strain. Where this line intersects the stress-strain curve is considered the yield point and the corresponding yield strength is readily determined from the graph. With continued loading of the test sample both stress and strain increase but eventually a point is reached where the stress is a maximum. This maximum stress based on the original crosssectional area is known as the ultimate tensile strength, or sometimes just the tensile strength. A ductile material will continue to stretch beyond this point but a brittle material would break when stressed to this limit. In a ductile material, beyond the maximum stress “necking” occurs and the load decreases as the cross-sectional area decreases. Elongation at this stage is rapid and the point of failure is soon reached. The stress based on the original cross-sectional area at the point of failure is known as the breaking strength or fracture strength. 22 Mechanical properties of polymeric materials are defined by similar parameters employed for metals, e.g., modulus of elasticity, hardness, tensile strength, impact strength and fatigue strength. Unlike metallic materials, however, the mechanical properties of polymeric materials are very much dependant on the rate of deformation, testing temperature in relation to the transition temperature, and polymer structure. Thus some modifications in testing techniques and specimen configuration are required as compared to metallic materials. In this experiment stress-strain behavior of some common polymeric biomaterials will be considered. Procedure Obtain from the instructor a few prepared tensile specimens of three different polymeric materials. Accurately measure the width and thickness in the narrow parallel section of each specimen. Also, using a felt tip marker, draw the initial gauge marks on each sample. The mode of testing of these materials to obtain a force-elongation diagram will be demonstrated. From these data, a stress-strain diagram for each material can be derived and its mechanical properties evaluated, e.g., modulus of elasticity, tensile strength, etc. Table 5. 1 Measurements of test coupons. Sample ID Initial dimensions Dimensions after fracture Width, mm Thickness, mm Gauge length, mm Width, mm Thickness, mm Gauge length, mm Using the raw data supplied from the computer, create force-elongation curves for each polymer up to fracture or until maximum elongation is achieved. A USB flash drive will be required to obtain the data only after a proper anti-virus scan. Report 1. From the raw data files, derive and tabulate stress-strain data for each material. Plot on a single graph the stress-strain curves for each material and determine the modulus of elasticity, tensile strength and percent elongation. 2. Compare the values obtained for mechanical properties to the published data in reference texts and account for differences in the values observed. 3. Compare the properties of each material tested with regard to stiffness, strength and ductility, and analyze the potential sources of error. 23