Material Testing Lab SAFETY: RECORDING DATA: .Material Testing Lab ... students are required to contribute

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  • Johns Hopkins University (10/22/2014)

    Material Testing Lab

    SAFETY:

    When completing the Materials Testing Lab activities, students must wear their goggles at all times

    while in the lab. You must also wear sturdy, closed-toe shoes. It is suggested that you wear

    disposable gloves when handling the lead shot. Please wash your hands after you complete this lab.

    Failure to comply with these safety requirements may result in earning no credit for the lab activity.

    RECORDING DATA:

    Record all your data in a notebook and then transfer it to an Excel data sheet. Think about the information

    you want to record in your lab notebook. Plan ahead to decide how you will record it in a table of rows

    and columns. You will be taking a lot of data so good data management is important. Dont forget to

    measure the diameter for every piece of spaghetti you test.

    FOCUS:

    This laboratory includes three separate materials testing experiments to be done on spaghetti of 3 different

    diameters. See individual labs for respective objectives. Students will work in groups of three, and all

    students are required to contribute to the completion of the lab report (additional handout will provide

    specific details for write-up). Students are expected to put more thought into the objectives/hypotheses

    and communication of results since few instructions are given.

    BACKGROUND:

    Materials Science and Material Properties

    Materials Science involves investigating the relationships that exist between the (internal) structures and

    the properties of materials. A materials properties are quantitative traits that tell us something about the

    nature of that material, and a materials performance will be a product of its properties. These properties

    may be constant or dependent upon environmental conditions, and most important properties of materials

    may be grouped into six different categories: mechanical, electrical, thermal, magnetic, optical, and

    deteriorative.

    The mechanical properties of materials are most relevant to the design of structures. The mechanical

    properties reflect the relationship between a materials reaction (or deformation) to an applied load or

    force. Examples of mechanical properties of materials are ultimate tensile/compressive strength,

    toughness, hardness, ductility, modulus of elasticity, and stiffness.

    There are three principal ways in which a load may be applied to a structural member: tension (pulling),

    compression (pushing), and shear (frequently applied through torsion). In this Material Testing Lab,

    students will perform three tests on spaghetti to determine the modulus of elasticity (Youngs Modulus,

    E) of spaghetti, the tensile strength of spaghetti, and to prove the format of the Euler Buckling equation.

    Although useful, students will not be performing shear tests of spaghetti as part of this lab.

  • Page 2 of 12 Johns Hopkins University (10/22/2014) M. Karweit (modified)

    Terminology

    TERM DEFINITION

    Ductility a measure of the degree of plastic deformation that has been sustained at failure. A

    brittle material experiences little or no plastic deformation

    Elastic Deformation deformation in which stress and strain are proportional. This deformation is

    nonpermanent, so the specimen returns to its original shape when the applied load

    is released.

    Hardness resistance to (different types of) permanent shape change when a force is applied.

    Hookean Solid any material that displays linear-elastic elongation.

    Plastic Deformation when a material is deformed beyond the area of elastic deformation and stress is no

    longer proportional to strain, this permanent deformation occurs.

    Proportional Limit the point in material testing before which elastic deformation occurs and beyond

    which plastic deformation occurs.

    Stiffness resistance to deformation.

    Strength ability of a material to withstand an applied stress without failure.

    Tensile Strength the maximum stress sustained by a material in tension. Prior to and including the

    tensile strength, all deformation is uniform throughout the specimen tested.

    However, if this stress is maintained or surpassed, necking begins to form and all

    subsequent deformation is confined to this neck.

    Toughness resistance to fracture in a material when stressed. Toughness indicates how much

    energy a material can absorb before rupturing. Think of chipping away at ice on a

    wind shield.

    Yield Strength the stress corresponding to the intersection between the stress-strain curve and a

    line parallel to the elastic portion of the stress-strain curve at a 0.002 strain offset.

    A materials yield strength should be constant regardless of its cross-sectional area.

    Yielding plastic deformation.

    Youngs Modulus also known as the modulus of elasticity, E is the constant of proportionality as

    defined by Hookes Law. This modulus may be thought of as stiffness, or a

    materials resistance to elastic deformation (i.e., the greater the modulus, the stiffer

    the material).

    TERM FORMULA UNITS DEFINITION

    Engr. Stress =

    0 MPa the instantaneous force applied to a specimen over

    original cross-sectional area

    True Stress =

    MPa the instantaneous force applied to a specimen over the

    instantaneous cross-sectional area

    Strain =

    0 none the elongation (or change in length) of a specimen divided

    by the original length, sometimes referred to as percent

    elongation

  • Page 3 of 12 Johns Hopkins University (10/22/2014) M. Karweit (modified)

    Hookes Law = MPa strain is directly proportional to stress for Hookean materials (linear-elastic materials); this is equivalent to the

    spring equation

    Experiment 1: Tensile Test

    BACKGROUND:

    Tensile tests are one of the most common mechanical property tests. If you pull on both ends of a rod

    with an increasing amount of force (making sure to apply the force only along the length of the rod), at

    some point the rod will stretch, like silly putty, or break. The force will have exceeded the materials yield

    strength. Depending on the material, the rod may simply snap in two. It also might experience plastic

    deformation in the form of necking (a gradual reduction in cross-sectional area) before it breaks. Under

    tensile loading, brittle materials tend to fracture without deformation, while ductile materials typically

    experience plastic deformation and necking before breaking.

    The force required to pull a rod apart depends on two things: the inherent strength of the material and the

    cross-sectional area of the rod. A cotton string is easier to break than the same size steel wire. A larger

    cross-section requires a larger force to pull it apart. This makes sense because a larger cross-sectional area

    means that there are more atomic bonds to pull apart along a fracture line, which in turn necessitates a

    larger force. With a circular cross-sectional area A = R2, the force to break the rod is

    mRkP where the constant k depends on the material.

    PROCEDURE:

    You will measure the fracture strength of spaghetti by pulling on it using a

    lever arm apparatus. You will use the spaghetti epoxied to two cotter pins the

    previous day.

    1. Start with the thinnest spaghetti. Use a micrometer to measure the

    spaghetti diameter.

    2. For the lever-arm apparatus, the lower end of the spaghetti will be attached

    to a metal pin (use another cotter pin); the upper end will be attached to the

    lever arm via a chain and S-hook (see figure). Adjust the chain so that the

    lever arm is as level as possible.

    3. Fill the bucket with BBs until the spaghetti breaks. Record the mass required to break the spaghetti.

    Record whether the spaghetti beaks along its length or at the glue joint.

    4. Repeat for three samples of each spaghetti thickness. Measure the spaghetti thickness each time. You

    will have a total of 12 data points, three data points for each of the four thicknesses of spaghetti.

    DATA ANALYSIS AND INTRODUCTION TO EXCEL SPREADSHEETS

    1. Enter your data into an Excel spreadsheet, along with the proper units. It will work best if you organize your spreadsheet in the following manner:

  • Page 4 of 12 Johns Hopkins University (10/22/2014) M. Karweit (modified)

    2. Add a column C where you calculate the radius of the spaghetti in the units of meters. a. First, label the column - in cell C1 type Radius (m)

    b. Then calculate the radius for the first sample by typing this equation into cell C2 (the = at the beginning of the calculation tells Excel that you wish for it to perform a mathematical

    calculation.

    =A2/1000/2

    This will take the diameter in mm and convert it into meters (A2/1000) and then it will

    convert the diameter to a radius (1/2)

    c. To copy this equation: i. Click on the contents of Cell C2 ii. Hold down the Ctrl button and the C key at the same time (to copy) iii. Click on C3 and drag the mouse to highlight the cells where you want to copy the

    equation.

    iv. Hold down the Ctrl button and the V key at the same time (to paste)

    This will copy the equation into cells C3 and higher.

    3. Add a column D where you calculate the force required to break the spaghetti.