DETERMINING THE MATERIAL WITH THE BEST TENSILE STRENGTH

(Link to Data Sheet)

 

Cindy Vodraska

 

 

Main Objective:  To understand the relationship of atomic structure (bond structure) to strength and properties of materials.

 

Grade Eight:  Science Content Standards

 

Structure of Matter

 

  1. Each of the more than 100 elements of matter has distinct properties and a distinct atomic structure.  All forms of matter are composed of one or more of the elements.  As a basis for understanding this concept:
    1. Students know the structure of the atom and know it is composed of protons, neutrons, and electrons

c.       Students know atoms and molecules form solids by building up repeating patterns, such as the crystal structure of NaCl or long-chair polymers

 

Periodic Table

 

7.      The organization of the periodic table is based on the properties of the

elements and reflects the structure of atoms.  As a basis for understanding this concept:

 

c.       Students know substances can be classified by their properties including their melting temperature, density, hardness, and thermal and electrical conductivity.

 

Investigation and Experimentation

 

Scientific progress is made by asking meaningful questions and conducting careful investigations.  As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations.

 

            Students will:

 

a.       Plan and conduct a scientific investigation to test a hypothesis.

b.      Evaluate the accuracy and reproducibility of data.

c.       Distinguish between variable and controlled parameters in a test.

 

 

 

 

 

Lesson Plan:

 

  1. Instruct students on the properties of  matter (Chapter 2 in Holt Physical Science or Chapter 4 The Structure of Matter in Holt Spectrum Science – A Physical Approach)

 

  1. Instruct students on structure of atom and electron configurations.

 

  1. Discuss strength of materials and relationship to atomic structure.  Define the following terms:  compressive forces, tensile forces, shear forces, properties of materials, strength, density, elasticity, flexibility, ductility, alloys, prosthesis, and lever.

 

  1. Do exploration 5 –CD-ROM Articles Extreme Skiing – This is an interactive exploration on CD-ROM by Holt Science & Technology.  In this activity students can study questions and problems related to life processes, geology, energy and power, and chemical reactions by having access to a fully equipped virtual laboratory.  In particular, the problem is: A manufacturing company is designing an artificial leg for a ski racer who plans to compete in the next Paralympic Games.  The company’s director, Ludwig Guttman, needs help selecting the best material to use for this unusual application.  Students evaluate aluminum, carbon, steel, oak, tungsten, titanium and magnesium and measure compressive, tensile, shear and density of each substance.  From this they write a letter where they give their recommendation for the material to use and why they selected it.

 

  1. Use the computer to research topics on atomic structure, bonding and for the CD-ROM activity use www.scilinks.org to review topics.  You can log on as a teacher and then use these keywords for pertinent web sites:  Properties of substances – HK1041, Structures of Substances – HK1042, Chemical Bonding – HK1043, What Is Matter? – HSTP030, Describing Matter – HSTP035, Building a Better Body – HSTP045

 

  1.  Do a Strength of Materials lab testing aluminum, carbon lead, magnesium, wood (toothpick), copper, iron, tin – most of these were used in the CD-ROM exploration and now result in real testing and actually seeing how the materials do.  This lab would be similar to Lab 1 we did in class.

 

 

 

 

 

 

 

 

 

 

STRENGTH OF MATERIALS LAB

 

 

I.                   Background

 

Inorganic materials, such as metals, ceramics and polymers, as well as natural materials, such as bone and collagen, exhibit fundamentally different strengths.  These intrinsic differences originate in the variations in the nature of the atomic bonds between the atoms and molecules that comprise the structure of these substances.  For example, although metallic bonds are quite strong and resistant to deformation, they are relatively easy to individually break.  This ability leads directly to the ductile characteristics of most metals.  In contrast, the very strong and stiff ionic or covalent bonds that make up most ceramics, semiconductors and glasses are very resistant to any type of bond stretching or rupture, which in turns leads to their very brittle nature.

 

Here, we will focus on experimentally gaining insight into the strength and fracture characteristics of ductile and brittle materials.  We will relate these experimental observations to the intrinsic atomic bonds within these materials in order to develop an appreciation for the underlying relationships between the strengths of materials and their atomic/molecular structure.

 

II.                Objectives

1)      To establish the relationships between the properties and structure of key classes of materials (metals, ceramics/glasses, polymers, natural materials).

2)      To build and utilize a basic apparatus to experimentally measure material properties needed to verify these relationships

3)      To gain insight into basic experimental methods required in analytical science laboratory investigations

4)      To relate these findings to specific content unit, weekly and daily lesson plans as they relate to the California Science Content Curriculum Standards

5)      To explore a variety of instructional methods that may be used to teach the content lessons

 

III.             Materials & Tools:

 

2 large plastic cups (16 oz. or larger)                 Paper single-hole punch

1 small plastic cup (8 oz.)                                  Pliers

Wire                                                                 Wire clippers

Pebbles                                                            Balance

 

Specimens:  aluminum, carbon lead, magnesium, wood (toothpick), copper, iron, tin

 

 

 

IV.              Procedure:

 

Assembly of Apparatus

 

1. Punch two holes on opposite sides of the smaller cup as shown below (a).

2. Loop a ring of wire around the top of the cup, just above the holes and below the rim,

    and twist closed.  Clip the remaining end and tuck the twist under the rim for safety.

3. Connect a second U-shaped piece of wire through the loop as shown in ( c ).

4. Use this cup to collect the weight(s) added during the test.  For stronger specimens, we

    may use a larger cup.

 

 

 

 

 

 

 

 

 

 

 

 

Strength Testing of Experimental Specimens

 

1.      Place the specimen squarely across the diameter of the top of the larger cup or equivalent.  Hang the smaller cup made above from the exact center of the specimen through the U-shaped piece of wire.  If needed, gently apply a small piece of tape to hold the wire in place.  Test specimens in the order listed in materials section above.

 

2.      Gently add weights (pebbles) to the smaller cup until the specimen breaks. 

 

3.      Measure the weights needed to break the specimen, use the balance – this mass will be our measure of strength.  Record the mass required to fracture on the data table.

 

4.      Complete the data table and answer the lab questions.

 

 

 

 

 

 

This lab is adapted from Lab 1: Strength of Materials from the Amgen Summer Science Institute on Physical Science, 2003.

VI.       QUESTIONS

 

 

 

1.    Did the object bend before it broke completely?

 

 

2.    Did the object break suddenly into two pieces or did it gradually splinter apart?  Why?

 

 

3.    Did the object break at precisely the same weight?  Why or why not?

 

 

4.    Which material had the greatest tensile strength?  The weakest?  Why?  Relate your answer to atomic structure and properties.