Determination of Tensile Properties of Materials

Attachments
I- Purpose/Objective:

The objectives of this laboratory are: 

  1. To provide a basic understanding of the mechanical tests required to determine the monotonic tensile properties of structural metal alloys and polymeric foams
  2. To examine the strain localization associated with tensile failure in ductile alloys
  3. To investigate the mechanisms of ductile fracture by SEM fractography and to reinforce the development of alternative sensor schemes to track deformation using Labview capture.


Three materials in sheet form will be examined:

  • 360 Brass
  • 6061 Aluminum alloy (heat treatment unknown)
  • 1018 Plain Carbon Steel (processing unknown)
  • Separately a polystyrene foam (density TBD, processing unknown) will also be characterized using a self-loading system.

II - Experimental Procedure:

A. Specimen preparation and tensile testing

Rectangular metallic tensile bars are prepared in advance and supplied to each laboratory group. Determine the dimensions of the test bars and insure that they comply with the ASTM testing procedure for metallic materials.  You will use the MTS Insight screw driven mechanical testing frame and perform tests at strain rates corresponding to.

Perform tensile tests according to published laboratory practice and record the force/displacement data required for constructing stress/strain curves for each specimen. Calibrate the mechanical testing frame and extensometer following the procedures outlined in supplemental instructions. Place specimen in the grips and attach the extensometer provided to the specimen.  Note the gage length of the extensometer.  Place marks at a specified distance apart on the gage section using a small punch or scribe.  Measure the spacing with a micrometer.

Place shield in front of the mechanical testing frame before beginning each test and transfer data to student file spaces after the tests are complete.

Make all measurements on fractured specimens that are necessary to determine tensile ductility, taking care to isolate individual fracture surfaces, please do not touch them with your hands, or rub them together, and carefully store them for later analysis. Following the completion of these experiments, please send to your GSI a table of your mechanical testing results for brass and aluminum for wider submission to the CTools web site so that you can do a larger statistical analysis using the results from the rest of the groups and to compare with the literature.  You can determine whether results you collected in a set of three are within statistical boundaries of the composite results collected by the rest of the class.

SOP: MTS Insight 10



 

 

B.  Bend and crack propagation testing of foams

Each lab group will be supplied with very large samples of polystyrene foam.  Since this is such a compliant material compared with the more standard, metallic samples you have also received, it is considered important to recognize that the mechanical behavior of these foams could be evaluated in a different, more demonstrative way.

Evaluate the force-deflection curve of polystyrene foam specimen in 3-point bending using sawhorses as fixture supports and loading the center point of the foam.  Mark the specimens at the point of contact on the saw horses and track whether these move as a function of loading.  Measure the deflection as a function of center point load, and generate a force-deflection curve to determine the appropriate elastic response, and from your knowledge of beam bending and moments of inertia, calculate the bending modulus of the foam.

Each group will load in bending a different piece of foam using a different length dimension between the supports nominally at either 5 or 7 feet apart.  The dimensions of the foam will be needed and an appropriate density sample is also required.

 

 

 

For the crack propagation experiments, you will evaluate notched specimens.  Armed with a drill and/or box cutters, each group will create a notched specimen that can be loaded in a crack opening mode to failure in this instance. Half of the groups will create a round defect (using a coring drill bit) and the other half can use the box cutters to create a sharper notch.  Measure the dimensions of the defects you install in your foam specimens.  You might be able to load and measure crack propagation like a type of fracture toughness measurement.  Recognize that this may not be a true measure of plane-strain fracture toughness based on the strain release characteristics of the foam. Create a force-crack extension curve as best as you can determine as well as a critical failure load required for critical crack propagation through your specimen.  You will need to consider the best approaches to load this specimen in the most controlled mode.

For the crack propagation experiments, you will evaluate notched specimens.  Armed with a drill and/or box cutters, each group will create a notched specimen that can be loaded in a crack opening mode to failure in this instance. Half of the groups will create a round defect (using a coring drill bit) and the other half can use the box cutters to create a sharper notch.  Measure the dimensions of the defects you install in your foam specimens.  You might be able to load and measure crack propagation like a type of fracture toughness measurement.  Recognize that this may not be a true measure of plane-strain fracture toughness based on the strain release characteristics of the foam. Create a force-crack extension curve as best as you can determine as well as a critical failure load required for critical crack propagation through your specimen.  You will need to consider the best approaches to load this specimen in the most controlled mode.

Send to your GSI your analytical results indicating the type of defect, size, and the force-resulting deflection curve for posting on Ctools.

 

 

 

 

 


C:  Cantilever bend testing of aluminum using Labview

Samples of 360 brass will also be received bonded with a strain gauge which can be subsequently loaded in a cantilever bending mode.  The strain gauge will be appropriately calibrated and then loadings will occur through an attached loading reservoir on the end of the specimen.  Knowing the mass attached to the cantilevered end of the specimen, you can use the strain gauge input to determine the strain on the outside surface of the specimen.  These will

image16Figure 2: Strain Gauge Wheatstone Bridge Circuit.

 

 

Apparatus

Cantilever Beam apparatus, labview strain gauge system including 1 kg set of 100g masses (individually marked with accurate masses).

Preliminary Experiment

  1. With the cantilever beam unloaded, measure and note the resistance of one of the gauges using a Digital Multimeter (in resistance mode). What is the change in gauge resistance when a 1 kg load is suspended from the beam? 

Setting up the Wheatstone Bridge Circuit

  1. Assemble the Wheatstone Bridge Circuit with labview inputs.
  2. Connect the high sensitivity voltmeter to measure output voltage.
  3. Using labview interface, construct program to measure relative resistance change occurring with each incremental loading of specimen.  

Experiment

  1. Measure the bridge output voltage with beam loadings from 0.00 to X 00 kg in x/5 kg steps.
  2. Plot a graph of output voltage (in Volts) on the vertical axis versus beam loading (in kg) on the horizontal axis.
  3. Hooke's Law states that for elastic behaviour that the strain is proportional to the load applied, does your graph verify Hooke's Law?
  4. If the design of the apparatus is correct, the output voltage of the strain gauge circuit should be image16. Use this formula to calculate the output voltage you expect for one of the loadings you have measured.
  5. Calculate the maximum experimental error in the expected value of Vo using the formula image16.
  6. Does the output voltage you expect agree with output voltage you have measured within their respective experimental errors? What does this tell you? 

    Symbol

    Description

    Value

    Units

    K

    Gauge Factor

    2.10 ± 0.02

    dimensionless

    m

    suspended mass

    ± 0.01 per mass

    grams (g)

    g

    acceleration due to gravity

    9.816 ± 0.001

    (ms-2)

    D

    load to gauge distance

    0.504 ± 0.003

    (m)

    VEX

    bridge excitation voltage

    estimated error

    volts (V)

    E

    Young's modulus

    195 ± 5

    (GPa)

    w

    width (horizontal) of cantilever

    2.53 ± 0.01

    (cm)

    t

    thickness (vertical) of cantilever

    3.55 ± 0.05

    (mm)

 

 

D. Scanning electron microscopy (SEM)

Cut a section of one half of each fractured metallic samples (approximately 15 mm from the fracture) using the cutoff saw. Clean the specimen to degrease and remove debris. Mount the specimen on an aluminum stub with carbon paint and dry for use in the SEM. Acquirepictures of each fracture surface in sufficient detail to describe the fracture processes in your report.

 

At least one sample will have been prepared for your examination by optical microscopy.  Examine the specimen(s) and capture representative microscopy images for detailed analysis.

 

If you can resolve anything from the fracture surfaces of the failed foam specimens, report on that as you see fit.

SOP: FEI (Philips) XL30 Teaching SEM with BSE mode and EDAX analysis

 


III - Theory/Background Information:

B.  Bend and crack propagation testing of foams

image1

Stress between load and support pointsimage1

 

 

Stress at center of constant cross sectionimage3

 

 

Deflection between load and support points image3

 

 

 

 

 

Maximum deflection at load  image3

 

 

 

Where:

E =

Modulus of Elasticity

psi

(N/m2)

I =

Moment of Inertia

in4

(m4)

W =

Load

lbs

(N)

s =

Stress at the cross-section being evaluated

Lbs/in2

(N/m2)

y =

Deflection

inches

(m)

x =

Some distance as indicated

inches

(m)

l =

Some distance as indicated

inches

(m)

Z =

section modulus of the cross-section of the beam

 

 

Z =

I ÷ distance from neutral axis or plane to extreme fiber (edge)

 

 

c =

Distance from extreme fiber edge to neutral plane

inches

(m)

 

 

 

 

BRASS 360
Copper Alloy Number: 36000
Typical Analysis in Percent:
Cu 60.0 - 63.0
Pb 2.5 - 3.7
Fe .35
Zn bal.
Typical Physical Properties:

Melting Point (Liquidus): 899 o C
Melting Point (Solidus): 888 oC
Density (20 oC): .307 lb/in3
Coefficient of Linear Thermal Expansion (20 oC - 300 oC): 11.4 x 10-5/oF
Thermal Conductivity (20 oC): 67 BTU/ft2/ft/hr/oF
Electrical Conductivity, Annealed (20 oC): 26% IACS
Specific Heat (20 oC): .09 BTU/lb/oF

Typical Mechanical Properties:
Yield Strength
Size Tensile (.5% Ext Elongation
Section Strength under Load) in 2 in.
Form (in.)           Temper (ksi)             (ksi) (%)
Rod 1.0              Soft Anneal 49.0 18.0 53
.250                    Half Hard (25%) 68.0 52.0 18
1.0                      Half Hard (20%) 58.0 45.0 25
2.0                      Half Hard (15%) 55.0 44.0 32

 

C:  Cantilever bend testing of aluminum using Labview 

Strain Gauges and the Wheatstone Bridge

The electrical resistance (R) of a metal wire is given by  (where r is the resistivity), so the resistance is proportional to the length (L) and inversely proportional to the area (A). As the wire stretches it becomes longer and thinner (because the volume of the wire stays approximately the same). Hence the resistance of the wire is increased by stretching. When a wire is compressed it becomes shorter and fatter, this reduces the resistance. The material structure changes slightly during stretching and compression, this produce small resistivity changes.

Strain Gauges are thin wires that can be glued to a metal structure. When the structure flexes under a load the resistance of the strain gauges changes and this can be used to measure the strain in the structure. In this way, the strain in a structure (e.g. an oil rig or an aircraft wing) can be measured to verify the design calculations.

 

 

Theory

The change in resistance DR in a strain gauge of resistance R is very nearly proportional to the applied strain. Hence: 
image8

 

 

image9Figure :  Loaded Cantilever Beam.

 

 

 

 

The gauges are a distance D from the load (see figure), a load of mass m and weight mg is suspended from the cantilever beam (g is the acceleration due to gravity). The beam has thickness t and width w and is made from stainless steel with a Young's Modulus E.  image9

 

 

The calculated strain due to the suspended mass is: image9

 

 

Therefore the relative resistance change of the strain gauge is given by: image9

 

 

The resistance changes in the strain gauges are very small, therefore the gauges are connected in a Wheatstone Bridge Circuit (see figure ). The gauge on top of the beam is in tension, the gauge underneath the beam is in compression, hence strain causes equal and opposite resistance changes in the gauges. By using two gauges the effects of temperature variations on the gauge resistances are cancelled out.

The left hand end of the bridge circuit is at zero volts (see figure 1), the circuit is powered by the bridge excitation voltage VEX applied to the right hand end of the bridge (see figure 1).

If the strain increases the resistance of Gauge One from R to R + DR then the resistance of Gauge Two is decreased from R to R - DR. Hence the voltage VG (see figure 2) is given by:

image9

 

 

 

 

 

 

 

To balance the Wheatstone Bridge the Zero Adjust resistor is adjusted to produce a voltage of image9. Therefore the output voltage Vo of the Wheatstone bridge is given by:

image9

 

Substituting image16then:

image16


IV - Theory/Background References:

  1. W. D. Callister Jr., Materials Science and Engineering: An Introduction, Fifth Ed., Chapter 17, “Composite Materials”, John Wiley & Sons, Inc., New York, 2000.
  2. ASTM E8-03: Standard Test Methods for Tension Testing of Metallic Materials, ASTM International, 100 Barr Harbor Drive, PO Box C700 West Conshohocken, PA.
  3. Metals Handbooks, ASM International, Metals Park, OH.  Available in MSE Laboratory.

V- Activity Schedule:

Week one

For the metal specimens, perform tensile tests on three alloys provided, analyze results and plot stress vs. strain data for each alloy. At least three measurements of each sample type is sufficient.  Complete the analysis of tensile data to determine modulus of elasticity, E, yield stress, ultimate tensile stress, elongation at fracture and tensile ductility.

Week two:

For the foam, focus on the crack extension-load diagram and an identified critical load at failure using a notched specimen. Perform a cursory determination of the foam density by cutting a specific shape with defined dimensions and determine the mass.   Prepare fractures samples for observation by scanning electron microscopy and perform fractography on each material. Share appropriate data identifying whether your group’s defect shape and size for the foam, foam density measurement, etc.

Week three:

For the labview work, construct an appropriate program to measure the strain with each successive loading…Once this is constructed, you can do the same thing with either the brass or the aluminum.  Mainly focus on the elastic region of these specimens, this will save them for the other groups. It would also be good for you to make a sample with a strain gauge so that you have the experience of bonding these devices together.

 

Date/Time        
Group 1
Group 2
Group 3
Group 4
Day 1
first half

1:50-3:40


Activity #3

Indentation
Toughness

Activity #4

Residual Stress

Activity #1

Flexural Testing
Day 1
second half

3:40-5:30

Activity #3

Indentation 
Toughness

Activity #4

Residual Stress

Activity #1

Flexural Testing

Activity #2

Fractography
Day 2
first half

1:50-3:40

Activity #4

Residual Stress

Activity #1

Flexural Testing

Activity #2

Fractography

Activity #3

Indentation 
Toughness
Day 2
second half

3:40-5:30

Activity #1

Flexural Testing

Activity #2

Fractography

Activity #3

Indentation 
Toughness

Activity #4

Residual Stres
Day 3
first half

1:50-3:40

Activity #2

Fractography
Finish Tasks Finish Tasks Finish Tasks
Day 3
second half

3:40-5:30

Finish Tasks Finish Tasks Finish Tasks
Finish Tasks

VI -Format and Important Questions for Lab Report:

A. Introduction

In this report you are required to write a more extensive background section that: (1) describes the characteristics and mechanisms of ductile fracture in structural alloys and how microstructure influences strength and ductility, (2) how tensile and bend testing can be used to qualify materials and in design of components and structures and (3) how accumulated damage affects the load capacity of samples with specific types of defects.


B. Experimental Procedure

Metals: Provide a more extensive materials section which gives the likely phases, the alloy microstructure, common strengthening mechanisms and common uses for each class of alloy.

Foam: Provide a schematic of the loading schemes, the defects used, and corresponding force-deflection curves of both the notched sample and the bend sample.

Labview: Provide code and details on the cantilever bending results comparing them with the results from other work to resolve the true bending modulus from the measurement of the strains, and the determinations of the force-deflection curve in bending.

Determine from a summary analysis from all groups what the impact of defect size is on reductions in strength.  Can a comparison be made with other strength measurements of other foams of similar density?

Testing: Provide a detailed description of the tensile test procedures and the conformance of lack of conformance with ASTM standards. Provide a complete (but concise) description of the characterization of microstructure and fractography.


C. Results

Present standard stress-strain curves for the data that your group acquired.  In a table show the required data and the data for steel (this will be made available). Perform a standard statistical analysis of (1) yield stress, (2) ultimate tensile stress, (3) elongation after fracture. Present fractographs of each material that allow you to describe the type of fracture and any important differences between materials.


D. Discussion

Explain the important feature of your results, both in terms of practical issues with tensile testing and in terms of fundamental deformation mechanisms (for example, describe how your fractography results reinforce what you know about the type of fracture that occurs).

Explain conceptually whether the sharp flat defect or the round defect in the foam should have more effect on reducing crack propagation in the foam.  Does the trend in the data analysis support conceptually what is intuitive easy to understand.

Explain why the plane strain fracture toughness K1c might not be truly being measured for these foam specimens.  What constraints are placed on a normal fracture toughness specimen that is different with the foam samples?  How does value compare with fracture toughness values for other polymers and does it make sense?

Relate the bending modulus to comparison moduli measurements for bulk polystyrene and other polystyrene foam measurements you have found in the literature or on the web.