Polymer Film Tensile Test Discussion Examples | How to Interpret Stress, Strain, and Elastic Modulus

What Is a Tensile Test of a Polymer Film?

A tensile test of a polymer film is an experiment in which a film cut into a specified shape is pulled using a tensile-testing machine and its deformation and fracture behavior are measured.
During the test, the force applied to the sample and the elongation are recorded, and a stress-strain curve is created.
From this curve, the hardness, strength, extensibility, toughness, and other properties of the material can be evaluated.

Polymer materials often undergo much larger deformation than metals or ceramics and are easily affected by temperature, humidity, tensile speed, molecular orientation, crystallinity, film thickness, and additives.
Therefore, in discussing a tensile test, it is important to consider not only the numerical values but also the condition of the sample and the measurement conditions.

Main Items to Include in the Results

In the results of a tensile test, organize the specimen width, thickness, initial gauge length, tensile speed, maximum load, stress, strain, elastic modulus, tensile strength, elongation at break, yield stress, fracture position, and other information.
Because the calculated stress and strain depend on the specimen dimensions, it is important to measure the width, thickness, and gauge length accurately.

Main Items to Include in the Results

• Sample name
• Type of polymer
• Specimen shape
• Specimen width
• Film thickness
• Initial gauge length
• Tensile speed
• Measurement temperature
• Measurement humidity
• Maximum load
• Stress
• Strain
• Elastic modulus
• Yield stress
• Tensile strength
• Elongation at break
• Fracture position
• Fracture appearance
• Number of repeated measurements
• Average value and variation
• Sources of error and points for improvement

Reference Experimental Values and Analysis Examples for Tensile Testing of Polymer Films

Here, reference experimental values are used to organize the method for calculating stress, strain, elastic modulus, tensile strength, and elongation at break from a tensile test of a polymer film and for interpreting the stress-strain curve.

In a tensile test, the load applied to the specimen and the elongation of the specimen are measured.
By converting load into stress and elongation into strain, specimens with different dimensions can be compared.
The shape of the stress-strain curve reflects the stiffness, strength, extensibility, and fracture behavior of the material.

Reference Experimental Conditions

Calculation of Cross-Sectional Area

The cross-sectional area of the film specimen is calculated from its width and thickness.

Cross-sectional area A = Width × Thickness

If the width is 10.0 mm and the thickness is 0.050 mm,

A = 10.0 × 0.050 = 0.500 mm²

Calculation of Stress

Stress is calculated by dividing the load by the initial cross-sectional area of the specimen.

Stress σ = Load F ÷ Cross-sectional area A

If the load is 6.0 N and the cross-sectional area is 0.500 mm²,

Stress = 6.0 ÷ 0.500 = 12.0 MPa

Calculation of Strain

Strain is calculated by dividing the elongation by the initial gauge length.

Strain ε = Elongation ΔL ÷ Initial gauge length L₀

If the initial gauge length is 50.0 mm and the elongation is 1.0 mm,

Strain = 1.0 ÷ 50.0 = 0.020

Expressed as a percentage, this is 2.0%.

Reference Stress-Strain Data

Example Calculation of Elastic Modulus

The elastic modulus is determined from the slope of the initial linear portion of the stress-strain curve.

Elastic modulus E = Change in stress ÷ Change in strain

If the stress increases from 6.0 MPa to 12.0 MPa as the strain increases from 0.010 to 0.020,

E = (12.0 − 6.0) ÷ (0.020 − 0.010)

E = 6.0 ÷ 0.010 = 600 MPa

Therefore, the elastic modulus of this film is 600 MPa.

Comparison of Tensile Properties of Different Films

Polyethylene film has a low elastic modulus and large elongation at break, indicating that it is a relatively soft and highly extensible material.
PET film has a high elastic modulus and tensile strength but a smaller elongation at break.

Calculation of Elongation at Break

Elongation at break is calculated by dividing the elongation at fracture by the initial gauge length.

Elongation at break (%) = Elongation at fracture ÷ Initial gauge length × 100

If the initial gauge length is 50.0 mm and the elongation at fracture is 125.0 mm, the elongation at break is 125.0 ÷ 50.0 × 100 = 250%.

Tensile strength is calculated by dividing the maximum load obtained during the test by the initial cross-sectional area of the specimen.
If the maximum load is 16.5 N and the cross-sectional area is 0.500 mm², the tensile strength is 16.5 ÷ 0.500 = 33.0 MPa.

Effect of Measurement Error in Film Thickness

Because stress is calculated by dividing the load by the cross-sectional area, measurement error in film thickness directly affects the stress value.

In thin films, even a small measurement error in thickness can greatly change the stress value, so it is important to measure the thickness at multiple points and use the average value.

Differences Depending on Specimen Direction

During film formation and stretching, polymer chains may become oriented, causing the mechanical properties to vary depending on the tensile direction.

If the elastic modulus and tensile strength are higher in the MD direction, the molecular chains are considered to be oriented in the film-forming direction, making the film stronger in the tensile direction.

Changes in Mechanical Properties With Draw Ratio

When a polymer film is stretched, the molecular chains become oriented, and the tensile strength and elastic modulus may increase.

As the draw ratio increases, the elastic modulus and tensile strength increase, while the elongation at break decreases.
This is considered to be because the molecular chains become oriented in the tensile direction and become more resistant to deformation.

Effect of Tensile Speed

Because polymer materials exhibit viscoelasticity, the measured mechanical properties may change depending on the tensile speed.

Changes in Tensile Properties With Temperature

Polymer materials are readily affected by temperature, and as the temperature increases, they may become softer and easier to stretch.

As the temperature increases, the molecular chains become more mobile, and the elastic modulus and tensile strength tend to decrease while the elongation at break increases.

Examples of Fracture Appearance

Example of Repeated Measurements and Variation

Tensile properties of films may vary depending on thickness variation, specimen width, cutting direction, and the way the specimen is fixed in the grips.

Specimen 4 fractured near the grip and showed lower tensile strength and elongation at break.
It is necessary to discuss whether such data should be treated in the same way as data from normal fracture.

Example of How to Write the Results

In the tensile test of polyethylene film, stress and strain were approximately proportional in the initial region.
When the elastic modulus was determined in the strain range from 0.010 to 0.020, E = 600 MPa was obtained.
After that, yielding was observed near a strain of 0.05, followed by large plastic deformation and elongation.

The maximum load was 16.5 N, and dividing this by the specimen cross-sectional area of 0.500 mm² gave a tensile strength of 33.0 MPa.
In addition, the elongation at fracture was 125.0 mm, corresponding to an elongation at break of 250% relative to the initial gauge length of 50.0 mm.
From this result, the sample is considered to be a relatively soft and highly ductile film that can undergo large elongation before fracture.

Compared with PET film, polyethylene film had a lower elastic modulus and tensile strength and a larger elongation at break.
This is considered to be because PET has a rigid molecular structure and high orientation, whereas the molecular chains in polyethylene are relatively mobile and can undergo plastic deformation more easily.

Points for Connecting the Results to the Discussion

In a tensile test of a polymer film, it is important to explain not only the magnitude of the numerical values but also the relationship between the shape of the stress-strain curve and the material structure.

• Are stress and strain correctly calculated from load and elongation?
• Is the cross-sectional area calculated from the width and thickness and used for the stress calculation?
• Can the elastic modulus be determined from the slope of the initial linear region?
• Can the tensile strength be calculated from the maximum load?
• Can the elongation at break be calculated from the elongation at fracture?
• Can elastic modulus, tensile strength, and elongation at break be related to the hardness, strength, and extensibility of the material?
• Can it be discussed that stretching and molecular-chain orientation increase strength and elastic modulus while reducing elongation at break?
• Can changes in viscoelastic behavior of polymers with tensile speed and temperature be explained?
• Have sources of error such as thickness measurement error, specimen cutting direction, and fracture near the grips been considered?

Example Discussion

In this experiment, a tensile test of a polymer film was performed, and the elastic modulus, tensile strength, and elongation at break were determined from the stress-strain curve.
In polyethylene film, stress was approximately proportional to strain in the initial low-strain region, so this range was regarded as the elastic-deformation region and the elastic modulus was determined from its slope.
In the strain range from 0.010 to 0.020, the stress increased from 6.0 MPa to 12.0 MPa, giving an elastic modulus of 600 MPa.

In the stress-strain curve, the increase in stress became less steep near a strain of 0.05, indicating behavior corresponding to yielding.
The sample then underwent plastic deformation while stretching greatly and eventually fractured.
Dividing the maximum load of 16.5 N by the initial cross-sectional area of 0.500 mm² gave a tensile strength of 33.0 MPa.
In addition, because the elongation at fracture was 125.0 mm, the elongation at break was calculated to be 250%.
From these results, polyethylene film is considered to be a relatively soft and highly ductile material.

When samples with different draw ratios were compared, the elastic modulus and tensile strength increased as the draw ratio increased, while the elongation at break decreased.
This is considered to be because stretching oriented the molecular chains in the tensile direction, making the material more resistant to deformation under external force.
On the other hand, because the molecular chains were already oriented, there was less room for further elongation, resulting in lower elongation at break.

Regarding the effect of tensile speed, the elastic modulus and tensile strength tended to increase and the elongation at break tended to decrease as the speed increased.
Because polymer materials exhibit viscoelasticity, slow deformation allows molecular chains to relax and the material to stretch greatly.
In contrast, when the material is pulled rapidly, molecular-chain motion cannot easily follow the deformation, making the material more likely to behave in a hard and brittle manner.

Possible sources of error include measurement error in film thickness, variation in specimen width, cutting direction, slipping in the grips, and stress concentration at the grips.
Because stress is calculated by dividing load by cross-sectional area, underestimating the thickness causes the calculated stress to become larger, while overestimating the thickness causes it to become smaller.
In addition, a specimen that fractured near the grip may not represent the intrinsic strength of the material, so the fracture position must be checked and the treatment of the data considered when necessary.

Summary

In tensile testing of polymer films, stress and strain are calculated from load and elongation, and the elastic modulus, yield stress, tensile strength, and elongation at break are evaluated from the stress-strain curve.

This reference example used polyethylene film, polypropylene film, PET film, and stretched film to examine the effects of elastic modulus, tensile strength, elongation at break, film thickness, test direction, draw ratio, tensile speed, temperature, and fracture mode.
In a report, it is useful to discuss the numerical results in relation to the molecular structure, orientation, viscoelasticity, and test conditions of the material.

What Is Stress?

Stress is the force applied per unit cross-sectional area of a specimen.
In a tensile test, it is calculated by dividing the load applied to the sample by the cross-sectional area of the specimen.
Units include Pa, MPa, and N/mm².

Stress σ = Load F ÷ Cross-sectional area A

For a film, the cross-sectional area is calculated as the product of the width and film thickness.
Therefore, measurement errors in film thickness and width directly affect the calculated stress.
Even under the same load, a specimen with a smaller cross-sectional area has a larger stress.

What Is Strain?

Strain is a value representing how much a specimen has elongated relative to its original length.
In a tensile test, it is calculated by dividing the elongation by the initial length.
Strain is dimensionless but may also be expressed as a percentage.

Strain ε = Elongation ΔL ÷ Initial length L₀

A larger strain indicates that the material underwent greater deformation.
In polymer films, flexible materials may stretch to a large strain without breaking.
However, because the value changes depending on the gauge length and the method used to read the elongation, it is important to clearly state the measurement conditions.

How to Read a Stress-Strain Curve

A stress-strain curve is a graph with strain on the horizontal axis and stress on the vertical axis.
The initial linear portion corresponds to elastic deformation, and the elastic modulus is determined from its slope.
After that, depending on the material, yielding, plastic deformation, strain hardening, and fracture may be observed.

A hard and brittle material fractures at a small strain, and the curve ends abruptly.
A flexible and tough material may show a large strain before fracture and may have a large area under the curve.
By examining the shape of the curve, it is possible to discuss not only strength but also the deformation behavior of the material.

What Is Elastic Modulus?

Elastic modulus is a value representing the resistance of a material to deformation.
It is determined as the slope of the initial linear portion of the stress-strain curve.
A material with a high elastic modulus requires a large stress to produce the same strain and can be regarded as a hard material.
A material with a low elastic modulus deforms greatly under a small stress and can be regarded as a soft material.

Elastic modulus E = Stress σ ÷ Strain ε

The elastic modulus of a polymer film is affected by molecular structure, crystallinity, crosslinking, molecular orientation, plasticizers, water content, and temperature.
When elastic moduli are compared, it is important to determine the slope over the same strain range.

What Is Tensile Strength?

Tensile strength is the maximum stress that a sample withstands during a tensile test.
The highest stress value in the stress-strain curve is commonly treated as the tensile strength.
A material with high tensile strength can be regarded as strong against tensile loading.

However, a material with high tensile strength is not necessarily highly extensible.
Some materials can withstand high stress but fracture quickly, while others do not withstand as high a stress but can stretch greatly.
Therefore, tensile strength is discussed together with elongation at break and elastic modulus.

What Is Elongation at Break?

Elongation at break indicates how much a specimen stretches before it breaks.
It is often expressed as the strain at fracture in percentage form.
A material with a large elongation at break can undergo large deformation before fracture and is therefore considered to have high ductility or flexibility.

Elongation at break is affected by molecular-chain mobility, crystallinity, crosslink density, plasticizers, film thickness, defects, and tensile speed.
If the film contains small scratches or notches, fracture may begin at those points and the elongation at break may become smaller.

What Is the Yield Point?

The yield point is the point near the boundary at which a material changes from elastic deformation to plastic deformation.
Once the yield point is exceeded, the deformation no longer returns completely to its original state even if the load is removed.
Some polymer materials show a clear yield point, while others deform gradually and do not show an obvious yield point.

Yielding is related to molecular-chain slipping, orientation, deformation of crystalline regions, and flow of amorphous regions.
If the stress decreases once after yielding or the sample stretches at approximately constant stress, necking or plastic deformation may be progressing.

Difference Between Elastic Deformation and Plastic Deformation

Elastic deformation is deformation that returns to the original shape when the force is removed.
In the initial linear portion of the stress-strain curve, elastic deformation mainly occurs.
In contrast, plastic deformation is deformation that does not return to the original shape even after the force is removed.
After the yield point is exceeded, molecular-chain slipping and orientation progress and permanent deformation remains.

Viscoelasticity is also important in polymer materials.
They are not completely elastic or plastic but may show time-dependent deformation.
Therefore, the measurement results may change depending on tensile speed and holding time.

Discussion of the Area Under the Curve and Toughness

The area under a stress-strain curve corresponds to the energy absorbed by the material before fracture.
A material with a large area under the curve can absorb a large amount of energy before fracture and is therefore considered to have high toughness.
Materials that are both strong and highly extensible tend to have a large area under the curve.

On the other hand, even if a material has a high elastic modulus or strength, the area under the curve is small when its elongation at break is small.
Such a material may be hard but brittle.
In material evaluation, strength, elongation, and toughness must be considered separately.

Why Polymer Films Stretch

Polymer films stretch because molecular chains become oriented along the tensile direction, entanglements loosen, and molecular chains in amorphous regions move.
Flexible polymers have high molecular-chain mobility and may therefore undergo large deformation.
If plasticizers or water are present, intermolecular interactions may weaken and the material may become easier to stretch.

On the other hand, when crystallinity is high, crosslink density is high, or the molecular chains are rigid, molecular-chain motion becomes difficult and the material becomes less extensible.
The extensibility of polymer films is strongly related to their molecular and internal structures.

Causes of a Hard Film

Causes of a polymer film becoming hard include rigid molecular chains, high crystallinity, strong intermolecular interactions, high crosslink density, a small amount of plasticizer, and advanced drying.
Hard films tend to have high elastic moduli but may have lower elongation at break.

For example, in polymers with strong interactions such as hydrogen bonding or ionic bonding, the molecular chains may become less mobile and the material may become harder.
When there are many crystalline regions, the molecular chains are also regularly fixed, increasing rigidity.

Causes of a Soft Film

Causes of a film becoming soft include highly flexible molecular chains, low crystallinity, the presence of plasticizers or residual solvent, high water content, and low crosslink density.
Soft films may have a low elastic modulus and a large elongation at break.

However, being soft is not necessarily always desirable.
If a film is too soft, it may have low tensile strength, stretch excessively during measurement, or deform at the grips.
The balance between flexibility and strength is important depending on the intended application of the material.

Effect of Crystallinity

The crystallinity of polymer films greatly affects their tensile properties.
When crystallinity is high, the molecular chains are regularly arranged, and strength and elastic modulus may increase.
On the other hand, a large amount of crystalline regions restricts molecular-chain motion and may reduce elongation at break.

However, even highly crystalline materials may have high toughness depending on the structure of the crystalline and amorphous regions and the degree of molecular orientation.
Crystallinity cannot be determined from tensile-test results alone, but its influence can be discussed from trends in elastic modulus and elongation.

Effect of Molecular Orientation

In polymer films, molecular chains may become oriented in a particular direction during film formation or stretching.
If the molecular chains are aligned in the tensile direction, the strength and elastic modulus may become higher in that direction.
On the other hand, different properties may be observed when the film is pulled perpendicular to the orientation direction.

The measured values may change if the film-forming direction, specimen cutting direction, and tensile direction are different.
It is important to consider direction dependence in film materials.

Effect of Crosslinking

Crosslinking refers to polymer chains being connected to one another by chemical bonds or strong interactions.
When the crosslink density is high, molecular-chain motion is restricted, and the elastic modulus and shape retention may increase.
On the other hand, if there are too many crosslinks, the material may fracture before undergoing large elongation and the elongation at break may decrease.

When crosslinking is low, flexibility tends to increase, but strength and durability may become insufficient.
The mechanical properties of a film are determined by the balance between crosslink density and molecular-chain mobility.

Effects of Plasticizers and Residual Solvent

Plasticizers and residual solvents may enter between polymer chains and weaken interactions between the chains.
As a result, the film may become softer, the elastic modulus may decrease, and the elongation at break may increase.
However, if too much plasticizer or solvent is present, the strength may decrease or the film may become sticky.

In insufficiently dried films, the solvent may evaporate over time and change the mechanical properties.
It is important to standardize the drying conditions and storage conditions before tensile testing.

Effect of Film Thickness

Film thickness greatly affects both stress calculation and fracture behavior.
Because stress is calculated by dividing load by cross-sectional area, overestimating film thickness makes the calculated stress smaller, while underestimating film thickness makes it larger.
Therefore, error in thickness measurement directly affects tensile strength and elastic modulus.

In a film with nonuniform thickness, stress may concentrate in thinner portions and fracture may begin there.
If there is thickness variation, the variation in elongation at break and tensile strength becomes larger.

Effect of Specimen Shape

In tensile testing, specimen width, length, cutting shape, and the condition of the edges affect the results.
If the width is not constant, the cross-sectional area changes depending on the position, producing error in the stress calculation.
If scratches or notches are introduced at the edges during cutting, stress may concentrate there and cause early fracture.

When a dumbbell-shaped specimen is used, the shape is designed so that fracture occurs in the central portion.
For strip-shaped specimens, care must be taken to prevent fracture at the ends or grips.
Recording the fracture position makes it possible to judge the validity of the measurement.

Discussion When Fracture Occurs at the Grips

In a tensile test, it is desirable for the specimen to fracture in the central region.
If fracture occurs at the grips, the sample may not have been pulled normally and may have been affected by compression from the grips, slipping, stress concentration, or edge damage.
In this case, the measured value may not correctly represent the intrinsic tensile strength of the material.

To prevent grip fracture, the specimen should be fixed correctly, the grip pressure should be adjusted appropriately, anti-slip measures should be used, and the specimen shape should be properly prepared.
In a report, the fracture position should always be checked and recorded as either central fracture or grip fracture.

Effect of Tensile Speed

Because polymer materials exhibit viscoelasticity, the measurement results change depending on tensile speed.
At high tensile speeds, the molecular chains have less time to follow the deformation, and the material may appear harder and more brittle.
As a result, the elastic modulus and strength may increase while the elongation at break decreases.

At low tensile speeds, the molecular chains have time to rearrange and may become more extensible.
However, the effects of creep and relaxation may also become larger.
When measurement results are compared, it is important to standardize the tensile speed.

Effects of Temperature and Humidity

The mechanical properties of polymer films are affected by temperature and humidity.
As the temperature increases, molecular-chain mobility increases and the material tends to become softer.
Near or above Tg, the elastic modulus may decrease greatly.

The effect of humidity is also important.
In hygroscopic polymers, water may act like a plasticizer, softening the material and increasing the elongation at break.
On the other hand, moisture may cause swelling or structural changes and reduce strength.

Discussion of Fracture Position and Fracture Surface

Observation of the fracture position and fracture surface provides clues for considering the reliability of the tensile test and the fracture mechanism.
If fracture occurs in the central portion of the specimen, it is easier to judge that the tensile test was relatively valid.
On the other hand, if fracture occurs at an edge or near the grips, the effects of stress concentration or improper fixing must be considered.

If the fracture surface is sharp and straight, relatively brittle fracture may have occurred.
If the sample stretches greatly before fracture and whitening or necking is observed, plastic deformation or molecular orientation may have progressed.

Discussion of Necking

Necking is a phenomenon in which part of a specimen becomes locally thinner during tensile deformation.
After yielding, deformation concentrates in a specific region and that region is stretched.
In polymer films, molecular chains may become oriented in the tensile direction during necking, producing local structural changes.

Materials that undergo necking may show a decrease in stress after yielding or a region of approximately constant stress in the stress-strain curve.
The presence or absence of necking provides a clue for considering ductility and the ability of molecular chains to rearrange.

Discussion When Whitening Occurs

A film may turn white during tensile deformation.
Possible causes include formation of microscopic voids or crazes, changes in crystalline and amorphous structures, and light scattering caused by molecular orientation.
Whitening is an observation indicating that microscopic structural changes are occurring inside the material.

If whitening occurs before fracture, stress may have concentrated in that region, causing microscopic defects to grow and eventually leading to fracture.
For transparent films, recording whether whitening occurs is useful for the discussion.

Causes of Variation in Measured Values

Causes of variation in tensile-test values include uneven film thickness, differences in specimen width, scratches introduced during cutting, slipping at the grips, differences in fracture position, differences in tensile speed, drying condition of the sample, temperature and humidity, and defects inside the film.
Because polymer films are thin and flexible, even small defects can greatly affect the results.

To reduce variation, it is important to measure multiple specimens and show the average value and standard deviation.
The fracture position and appearance of each specimen should also be checked to determine whether abnormal data should be excluded.

Causes of Stress Being Lower Than Expected

Causes of stress being lower than expected include overestimating the film thickness, defects in the sample, insufficient drying that softened the sample, slipping at the grips, and fixing the specimen at an angle.
In particular, measurement error in film thickness directly affects the stress calculation.

If the sample slips at the grips, the elongation recorded by the instrument no longer reflects only the elongation of the specimen, and the stress-strain curve may become unnatural.
It is important to check the fixing condition.

Causes of Elastic Modulus Being Higher Than Expected

Causes of an excessively high elastic modulus include underestimating the film thickness, the sample becoming hard because of drying, high molecular orientation or crystallinity, and a high tensile speed.
In addition, insufficient correction of the initial strain may cause the slope of the initial linear region to appear larger than it actually is.

If slack at the beginning of the test or play in the grips is not properly corrected, an error occurs in the strain range used to calculate the elastic modulus.
Because elastic modulus is determined from the slope of the initial region, the range of the graph used for the calculation is important.

Causes of a Small Elongation at Break

Causes of a small elongation at break include scratches in the specimen, uneven film thickness, high crystallinity, rigid molecular chains, high crosslink density, excessive drying, high tensile speed, and stress concentration at the grips.
Elongation at break is strongly affected not only by the flexibility of the material but also by defects in the specimen.

If the elongation at break is extremely small, it is necessary to consider separately whether the material itself is brittle or whether there was a problem with specimen preparation or fixing.
Checking the fracture position is useful.

When Tensile-Test Results Can Be Considered Good

Tensile-test results can be considered good when the specimen fractures in the central region rather than at the grips, the stress-strain curve is smooth, and the results of multiple specimens do not vary greatly.
In addition, if the film thickness and width are measured appropriately and the initial linear region used to determine the elastic modulus is clear, the results are more likely to be reliable.

The meaning of a good result in material evaluation depends on the intended purpose.
For applications such as packaging films, where flexibility is required, elongation at break and toughness are important, while for structural materials requiring rigidity, elastic modulus and tensile strength are important.

Example Discussion When the Experiment Did Not Go Well

When a tensile test does not go well, possible causes are considered from results such as fracture at the grips, slipping of the specimen, an unnatural stress-strain curve, large variation in measured values, extremely low elongation at break, or an abnormally high or low elastic modulus.
Organizing the possible causes into specimen preparation, film-thickness measurement, fixing method, tensile speed, drying condition, and the presence or absence of defects makes the discussion easier.

How to Write Points for Improvement

In a discussion of tensile testing, including not only sources of error but also points for improvement makes the report easier to organize.
Improvements can be divided into specimen preparation, film-thickness measurement, test conditions, and data analysis.

Improvements to Specimen Preparation

• Cut the specimens to a constant width
• Avoid creating scratches or notches at the edges
• Standardize the specimen cutting direction
• Use areas with little variation in film thickness
• Check the appearance before testing

Improvements to Film-Thickness and Dimensional Measurements

• Measure the width and thickness at multiple locations
• Check not only the average value but also the variation
• Use a thickness gauge or micrometer appropriately
• Do not compress the sample excessively
• Record the measurement positions

Improvements to Test Operation

• Fix the specimen straight
• Prevent slipping at the grips
• Use appropriate grip pressure
• Keep the tensile speed constant
• Standardize temperature and humidity
• Perform multiple measurements and calculate the average value

Improvements to Analysis

• Standardize the strain range used to determine the elastic modulus
• Distinguish data from specimens that fractured at the grips
• Check the stress-strain curve for abnormalities
• Show the average value and standard deviation
• Record the fracture position and appearance

Difference Between a Superficial Discussion and a Good Discussion

In a tensile test of a polymer film, writing only that “it stretched,” “it broke,” or “the strength was high” results in a superficial discussion.
Relating stress, strain, elastic modulus, elongation at break, molecular structure, film thickness, and test conditions produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of a tensile test of a polymer film.
Adjust the necessary parts according to your own experimental results.

• Stress is the load applied to the sample divided by the cross-sectional area.
• Strain indicates how much the sample has stretched relative to its original length.
• Elastic modulus is determined from the slope of the initial linear portion of the stress-strain curve.
• A material with a high elastic modulus is resistant to deformation and is considered to be hard.
• A material with a large elongation at break can undergo large deformation before fracture and is therefore considered to have high flexibility or ductility.
• Tensile strength indicates the maximum stress that the sample withstood during the test.
• After the yield point, plastic deformation involving molecular-chain slipping and orientation is considered to progress.
• Error in film-thickness measurement directly affects the calculation of stress and tensile strength.
• If fracture occurs at the grips, the intrinsic strength of the material may not have been evaluated correctly.
• Because polymer materials exhibit viscoelasticity, measured values change with tensile speed, temperature, and humidity.

Points to Check When Discussing a Tensile Test

Checking the following points before writing the report makes the discussion easier to write.

• Are the definitions of stress and strain stated?
• Is elastic modulus explained as the slope of the initial linear portion?
• Are tensile strength and elongation at break distinguished?
• Has the shape of the stress-strain curve been discussed?
• Has the presence or absence of a yield point and plastic deformation been checked?
• Has the fracture position been recorded?
• Have measurement errors in film thickness and width been considered?
• Have scratches at the specimen edges and cutting accuracy been discussed?
• Have the effects of molecular orientation, crystallinity, crosslinking, and plasticizers been considered?
• Have the effects of tensile speed, temperature, and humidity been considered?
• Has the variation among multiple specimens been checked?
• Do the points for improvement correspond to the sources of error?

Summary

In a tensile test of a polymer film, the specimen is pulled while load and elongation are measured, and a stress-strain curve is created.
Stress is the load divided by the cross-sectional area, while strain is the elongation relative to the original length.
The elastic modulus is determined from the initial linear portion of the stress-strain curve, the tensile strength from the maximum stress, and the elongation at break from the strain at fracture.

A film with a high elastic modulus is hard and resistant to deformation, while a film with a large elongation at break is flexible and capable of large deformation.
However, tensile strength, elastic modulus, and elongation at break are affected by molecular structure, crystallinity, molecular orientation, crosslinking, plasticizers, residual solvent, film thickness, specimen shape, tensile speed, temperature, and humidity.
Therefore, it is necessary to discuss not only the numerical values but also the sample condition and measurement conditions.

In a report, rather than simply writing that “the film was strong” or “it stretched,” organize the meanings of stress, strain, and elastic modulus and explain the shape of the stress-strain curve, fracture position, thickness variation, specimen damage, and the relationship with molecular-chain orientation and crystallinity.
In tensile testing of polymer films, it is important to distinguish between the intrinsic properties of the material and the effects of the measurement procedure.