Stress-Strain Curve in Material Science
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Questions and Answers

What typically characterizes brittle polymers in a stress-strain curve?

  • They fail in the elastic region. (correct)
  • They have a distinct yielding point.
  • They undergo significant plastic deformation before breaking.
  • They exhibit necking before fracture.

    • In the stress-strain curve for ductile materials, what follows the linear elastic deformation?

  • Necking forms.
  • Homogeneous plastic deformation. (correct)
  • Cold drawing begins.
  • Fracture occurs.

    • Which stage occurs immediately after necking forms in a stress-strain curve?

  • Necking grows unstably. (correct)
  • Cold drawing.
  • The entire sample is drawn.
  • Linear elastic deformation.

    • What is represented by the symbol σ in the context of tensile tests?

    <p>Force per unit area.</p> Signup and view all the answers

    During what stage of the stress-strain curve does cold drawing occur?

    <p>After the neck stabilizes.</p> Signup and view all the answers

    What is the definition of strain in a tensile test?

    <p>The ratio of the change in length to the original length.</p> Signup and view all the answers

    At which point in the stress-strain curve does fracture typically occur?

    <p>At the end of the plastic deformation phase.</p> Signup and view all the answers

    What distinguishes highly elastic polymers like rubbers in stress-strain analysis?

    <p>They can return to their original shape after deformation.</p> Signup and view all the answers

    What does tensile modulus indicate about a material?

    <p>The relative stiffness of a material</p> Signup and view all the answers

    Which types of plastic data are commonly compared?

    <p>Tensile strength and elongation</p> Signup and view all the answers

    What is a significant limitation of the tensile tests for plastic materials?

    <p>Data is unreliable for different testing speeds</p> Signup and view all the answers

    What shape are tensile test pieces most commonly made in?

    <p>Dumb-bell</p> Signup and view all the answers

    What does ASTM D412 and ISO 37 specify?

    <p>Methods for determining tensile properties of rubber</p> Signup and view all the answers

    What is the primary purpose of conducting tensile tests?

    <p>To determine the force necessary to break a specimen</p> Signup and view all the answers

    Which aspect is NOT commonly discussed regarding rubber tensile tests?

    <p>Material hardness</p> Signup and view all the answers

    How does environmental sensitivity affect tensile property data for plastics?

    <p>It limits the application in varying conditions.</p> Signup and view all the answers

    What does the cold draw process primarily affect in thermoplastics?

    <p>Carbon chain alignment</p> Signup and view all the answers

    How is the toughness of a thermoplastic quantified?

    <p>By the area under the stress-strain curve</p> Signup and view all the answers

    Which factor has the least influence on tensile strength values?

    <p>Ambient light conditions</p> Signup and view all the answers

    What does an increase in strain rate affect in terms of material properties?

    <p>Increased tensile strength</p> Signup and view all the answers

    Which of the following is true regarding molecular orientation's impact on tensile strength?

    <p>Higher values result when loads are applied parallel to orientation</p> Signup and view all the answers

    What is indicated by the tensile modulus in thermoplastics?

    <p>Mechanical stiffness</p> Signup and view all the answers

    Which sampling condition is likely to produce lower tensile strength values?

    <p>Machined specimens</p> Signup and view all the answers

    What relationship exists between strain rate and elongation in tensile tests?

    <p>Inversely proportional</p> Signup and view all the answers

    What is the recommended standard lab temperature for conducting tensile tests on plastics?

    <p>23±2°C</p> Signup and view all the answers

    Which measuring device is primarily used to measure the width and thickness of specimens before testing?

    <p>Thickness gauge</p> Signup and view all the answers

    What is the proper testing speed for a rubber specimen in tensile tests?

    <p>500 mm/min</p> Signup and view all the answers

    During tensile testing, what happens to the load value as the specimen elongates?

    <p>It increases as resistance of the specimen increases.</p> Signup and view all the answers

    What type of machine is the Instron universal tester primarily used for?

    <p>Tensile testing of materials</p> Signup and view all the answers

    According to standard procedure for tensile tests, what should be done to the grips before starting the test?

    <p>Tighten them evenly and firmly.</p> Signup and view all the answers

    What does the load cell detect during the tensile testing process?

    <p>Resistance of the specimen as it elongates</p> Signup and view all the answers

    What characteristic is shown in the elastic region of a generalized tensile stress-strain curve for polymeric materials?

    <p>Stress is proportional to strain.</p> Signup and view all the answers

    What is the primary purpose of measuring the sample dimensions in the waist region?

    <p>To verify material properties before testing</p> Signup and view all the answers

    What component is typically used to measure resistance to deformation during tensile testing?

    <p>Load cell</p> Signup and view all the answers

    Which shape is commonly used for tensile specimens to ensure breaking points occur in the specified region?

    <p>Dumbbell / dog-bone</p> Signup and view all the answers

    What is the standard test method for tensile testing materials as mentioned?

    <p>ASTM D638</p> Signup and view all the answers

    Which equipment is typically used to monitor extension during a tensile test?

    <p>Extensometer</p> Signup and view all the answers

    What type of specimens can be prepared from materials before tensile testing?

    <p>Injection molded or compression molded specimens</p> Signup and view all the answers

    At what point during the tensile test is the load/extension curve usually generated?

    <p>Automatically by the machine during the test</p> Signup and view all the answers

    Which of the following accurately describes the gauge length in a tensile specimen?

    <p>The portion of the narrowed section of the sample</p> Signup and view all the answers

    What is the internal diameter of the normal size ring test piece?

    <p>44.6 mm</p> Signup and view all the answers

    Which formula is used to calculate the tensile strength for the ring test piece?

    <p>$ rac{F}{2A}$</p> Signup and view all the answers

    What is a disadvantage of using a dumb-bell test piece?

    <p>Difficult to grip</p> Signup and view all the answers

    What is the thickness specification for the normal size ring test piece?

    <p>4 ± 0.2 mm</p> Signup and view all the answers

    Which of the following is a major advantage of using a ring test piece?

    <p>No gripping problems</p> Signup and view all the answers

    How is the % elongation at break calculated for the ring test piece?

    <p>$ rac{(I - I0)}{I0} imes 100$</p> Signup and view all the answers

    What does the modulus formula for the ring test piece have in common with the tensile strength formula?

    <p>Both are $F/2A$</p> Signup and view all the answers

    What is a common issue when using a dumb-bell test piece as highlighted in the disadvantages?

    <p>Difficulty in gripping the test piece</p> Signup and view all the answers

    Study Notes

    Polymer Physical Testing & Analysis

    • The course is PST472
    • Tensile testing (plastic) is the topic
    • The presenter is Nor Mazlina Abdul Wahab (UITMPS)

    Learning Outcomes

    • Describe tensile testing of polymers.
    • Sketch and label typical stress-strain curves for tensile testing.
    • Describe tensile testing procedures.
    • Define related terms in tensile testing of polymers.
    • Sketch and explain the typical shape of the stress-strain behaviour of different polymer properties.
    • Calculate tensile strength, % elongation, and Young's Modulus of polymeric materials.

    Content

    • Introduction
    • Tensile specimen
    • Testing apparatus (testing machine)
    • Conditioning
    • Test procedure
    • Typical stress strain curve
    • Related terms and symbols for tensile tests
    • Typical shape of stress-strain behaviour
    • Factors affecting test results

    Introduction

    • Tensile testing is the most common short-term mechanical test.
    • It's relatively easy to perform and produces reproducible results.
    • Results include tensile strength, elongation, and modulus.
    • Machines typically measure the resistance to deformation and extension rate.

    Introduction (cont.)

    • Tensile tests measure the force needed to break a specimen and how much it stretches.
    • A stress-strain diagram is used to find tensile modulus.
    • Data is useful for material specification, component design, and quality control.

    Introduction (cont)

    • Sample dimensions (gauge section) are measured first using a micrometer or dial gauge.
    • The sample is firmly gripped in the machine's jaws.
    • An extensometer (optional) may be clipped to the sample.
    • One jaw is moved away from the other at a set speed.
    • Resistance to deformation is measured by a load cell.
    • A load/extension curve is automatically generated.
    • A standard test method (ASTM D 638) is used.

    Tensile Specimen

    • Sample preparation: injection-molded, compression-molded, machined from sheets/plates/slabs.
    • Dumbbell/dog-bone shape is common to avoid break points in grips.
    • Various tensile specimen types are shown in the slides. (Examples: ASTM D638 Type I tensile bar, ASTM D882 for thin sheet/film).

    Testing Apparatus/Testing Machine

    • The machines are usually Instron universal testers..
    • Grips hold the specimen firmly.
    • A force measurement system is used.
    • The rate of the test is constant.

    Testing Apparatus / Testing Machine (cont.)

    • Proper specimen alignment in the grips is crucial for reliable results.
    • Incorrect alignment can lead to inaccurate measurements.

    Conditioning

    • Standard conditioning procedures (e.g., ASTM D 618 Procedure A) are used.
    • The lab temperature (23 ± 2°C) and relative humidity (50 ± 5%) are controlled.
    • Rapid temperature changes can affect tensile properties.

    Test Procedure (ASTM D638)

    • Measure specimen width and thickness using a gauge.
    • Position the specimen vertically in the testing machine grips (Instron).
    • Securely tighten the grips to prevent slippage.

    Test Procedure (ASTM D638) (cont.)

    • Set the testing speed (e.g., 50mm/min for plastics, 500mm/min for rubber).
    • Record the load-extension values as the specimen is stretched (usually automatically).

    Typical Stress-Strain Curve

    • A generalized curve for polymeric materials is shown.
    • Different types of polymers (brittle, ductile, highly elastic) have different stress-strain curves.
    • Key points: elastic region, yield point, tensile strength, break/failure.

    Typical Stress-Strain Curve (cont.)

    • Various curve shapes for different types of polymers are displayed.
    • Examples: brittle, ductile, and highly elastic polymer curves.

    Typical Stress-Strain Curve (cont.)

    • Ductile material curves show uniform elastic deformation, followed by uniform plastic deformation and finally necking before fracture.
    • Semicrystalline polymer stress-strain curves also show stages of linear, elastic, and different types of plastic deformation.

    Typical Stress-Strain Curve (cont.)

    • LDPE (Low Density Polyethylene) plastic stress-strain curve is characterized by various stages (linear region, bending, various stages of plastic deformation, and eventual breaking).

    Terms and Symbols (Tensile Test)

    • Stress: force per unit area
    • Strain: change in length/original length
    • Elastic region: material returns to initial shape, and stress is linearly related to strain. Elastic modulus = slope
    • Yield point: start of irreversible deformation
    • Tensile strength: maximum stress at break
    • Toughness: energy absorbed by sample until it breaks (area under the curve).

    Terms and Symbols (Tensile Test) (cont.)

    • Elongation: increase in length
    • Modulus of elasticity: Stress/Strain
    • Necking: the localized decrease in cross-sectional area
    • Cold draw: the neck increases in size
    • Stiffness: resistance to deformation

    Factors Affecting Test Results

    • Specimen preparation and size
    • Strain rate (speed of testing, often a crucial factor)
    • Temperature (significant effect; higher strength and modulus at lower temperatures for many plastics)
    • Molecular orientation
    • Gate location(important, especially with composites and certain special products.)

    Typical Tensile Strength, Elongation and Tensile Modulus of Polymers

    • A table is presented with values for different polymers.
    • These data points are useful for comparing properties across different types of plastics.

    Exercise

    • Data for tensile test calculations is provided.
    • Calculations involving tensile strength and elongation are required from the students.

    Exercise (cont)

    • Definitions for tensile stress, tensile strain, cold draw, and gauge length are required.

    Conclusion

    • Tensile testing is an important method for characterizing the ability of materials to withstand pulling forces.
    • Results are helpful in material selection, design considerations, and quality control for plastics.
    • Important properties assessed include tensile strength, elongation, and modulus.
    • Factors include the speed of testing, temperature, and sample preparation.

    Conclusion (cont.)

    • Results for plastics often vary based on the speed of testing and environmental conditions.
    • The choice of test type and specimen size could influence the outcome.

    Polymer Physical Testing & Analysis (Rubber/Elastomer)

    • Specific notes on tensile testing, including typical stress-strain curves, test procedures, specimen types (ring, dumbbell), and significance of the testing method, are shown
    • Information on the importance of the test is provided for different rubber types/formulations

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    Tensile Test (Plastic) - PST472 PDF

    Description

    This quiz tests your understanding of the stress-strain curve characteristics in material science, focusing on brittle and ductile polymers. You will explore concepts such as necking, tensile test symbols, and the cold drawing process. Perfect your knowledge on material deformation behavior!

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