What you need to know
- 4.9Core Practical 8: determine the tensile strengthThe pulling force a material can withstand before it breaks, usually given per unit cross-sectional area. of plant fibres.
- 4.4Relate results to celluloseA structural polysaccharide of β-glucose. Straight chains are linked by hydrogen bonds into microfibrils that strengthen plant cell walls. microfibrilA bundle of parallel cellulose chains held together by many hydrogen bonds, giving the cell wall high tensile strength. and secondary thickening.
By the end, you should be able to…
- Calculate tensile strength as force per unit cross-sectional area.
- Identify independent, dependent and control variableA variable kept the same throughout an investigation so that it does not affect the result..
- Evaluate hanging-mass methods, including safety and uncertainty.
Build your understanding
What you are measuring
tensile strengthThe pulling force a material can withstand before it breaks, usually given per unit cross-sectional area. is the force required to break a fibre divided by its cross-sectional area. Use consistent units, for example newtons and mm². Measure diameter in more than one place with a micrometer (or a calibrated eyepiece graticuleA scale in the microscope eyepiece. It has no units until it is calibrated with a stage micrometer.), take a mean, and treat the fibre as a circle only if that assumption is stated: area = πr². A millimetre ruler is too coarse: a ±0.5 mm reading on a 0.5 mm fibre is a 100% uncertainty.
The independent variable is often fibre type, soaking time, or length. The dependent variable is breaking force, later converted to strength. Controls include how the fibre is clamped, the rate masses are added, fibre moisture and the gauge length between clamps.
A workable school method
Follow the approved local protocol. A typical hanging-mass method clamps a fibre securely, adds masses gently until it breaks, and records the largest mass supported. Force in newtons is mass in kilograms × 9.81 N kg⁻¹, or use g = 10 N kg⁻¹ if the question says so. Add masses, do not drop them.
Soaking, retting or using wet fibres can change results because water affects hydrogen bonding and flexibility. The direction depends on the fibre: some, such as flax, are often reported to be stronger wet, while others change little or weaken. If soaking is not the independent variable, keep moisture the same. Repeat with new fibres from the same batch; a fibre that has already been loaded is no longer the original sample.
Risk and evaluation
Falling masses can injure feet and crush fingers. Keep feet clear, add masses close to the floor or into a sand tray, and do not stand under the load. Wear eye protection if fibres or clamps can snap. Report damaged stands.
Uncertainty comes from diameter measurement (a squared radius amplifies error), fibres that slip rather than break, knots that act as weak points, and masses that are too coarse. A fibre that unravels has not given a clean tensile break. Compare mean strengths with the range or a simple spread, and do not claim a difference smaller than the variation between repeats.
The plant fibre tensile strength lab
Measure a strand with a micrometer, load it gently until it snaps, then calculate force per unit area and compare fibres. The fibres, diameters and breaking masses are fictional.
Simulation only. Wear eye protection, put a sand tray under the masses and keep feet clear. A teacher sets the masses, stand and fibres you may use.
Method 7 steps
- Put on eye protection. Set up a clamp stand on the bench, with a sand tray (or soft landing) under where the masses will hang. Keep feet clear.
- Measure the diameter of a fresh strand at several points along its length with a micrometer, and calculate the mean.
- Clamp the top of the strand securely without a knot, and attach a mass hanger at a fixed gauge length below the clamp.
- Lower slotted masses onto the hanger one at a time, gently, waiting a few seconds after each. Use smaller masses as the strand nears breaking.
- Record the largest total mass the strand supported, including the hanger, before it broke.
- Calculate force (F = mg, stating g), cross-sectional area (A = πr², assuming a circular section) and tensile strength = F ÷ A.
- Repeat with at least three fresh strands of each fibre type, then compare means and the spread of repeats.
Aligned with the order of the specification practical. Simulation only: a teacher sets the real volumes, concentrations and risk controls; this is not a laboratory protocol.
Variables
- Independent
- Fibre type (or, as an extension, gauge length or soaking).
- Dependent
- Breaking force, converted to tensile strength (N mm⁻²).
- Controlled
- Gauge length between clamp and hanger
- Moisture: all dry, or all soaked for the same time
- How the strand is clamped (no knots, no slipping)
- Rate and size of mass additions (gentle, same steps)
- Strands from the same batch, a fresh one each time
Safety
- Wear eye protection: a strand can whip back when it snaps.
- Place a sand tray or soft landing under the masses, keep feet clear and never stand under the load.
- Clamp the stand to the bench or counterweight it so it cannot topple. Report a damaged stand.
Follow your school’s risk assessment; these notes are not a substitute for it.
Interpreting
- Strength = force ÷ area. Compare strengths, not the masses held: a thicker strand can hold more without being stronger.
- 1 N mm⁻² = 1 MPa = 10⁶ Pa. Quote the value of g you used.
- Strands break at their weakest point, so repeats vary. Longer strands contain more weak points and tend to be weaker.
- Only claim a difference if it is larger than the spread of repeats. Soaking can raise or lower strength depending on the fibre, so test it rather than assume.
Guided method · step 1 of 10
- In 0 · Safety, just below these steps, press Put on eye protection.
- Press Place sand tray to put the tray under the hanger.
A strand under tension can whip back when it snaps, and the masses fall. Eye protection guards against flying fragments; the sand tray catches the masses so they cannot land on feet or bounce.
0 · Safety
1 · Set up
Slotted masses
Start with safety, then set up and clamp a fresh strand.
2 · Micrometer
Resolution 0.01 mm. A millimetre ruler cannot resolve a 0.5 mm strand usefully.
3 · Record the result
When the strand fails, choose the mass to record here. Masses include the 50 g hanger.
4 · Calculate tensile strength
Record a valid strand to calculate its strength: F = mg, A = πr² (assuming a circular cross-section), strength = F ÷ A.
5 · Compare fibres
Test 3 valid strands of two different fibres (same gauge length and moisture) to compare them.
Evaluate the claim
School strands are bundles of many fibre cells, so their strengths are far lower than published values for single fibres.
Make your reasoning visible
Divide by area
A thick fibre can bear more force without being stronger. Always calculate force/area.
State g
If you convert mass to force, write the value of g you used.
Avoid this misconception
The claim: The fibre that holds the most masses is the strongest.
The correction: Only if cross-sectional areas are equal. Strength is force per unit area.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
A fibre of diameter 0.50 mm breaks at a mass of 0.80 kg. Calculate tensile strength using g = 10 N kg⁻¹.
Show suggested answer
Force = 8.0 N. Radius = 0.25 mm, area = π × 0.25² ≈ 0.196 mm². Strength ≈ 8.0 / 0.196 ≈ 41 N mm⁻².
Practice question
Retrieval and explanation · suggested answer
Why measure diameter at several points?
Show suggested answer
Fibres are not perfectly uniform. A mean diameter reduces the effect of a local thin or thick region on the area used in the calculation, though the break still occurs at a weak point.
Practice question
Retrieval and explanation · suggested answer
Give two reasons two hemp fibres from the same stem can give different strengths.
Show suggested answer
Different diameters or secondary thickening; damage, knots or slipping in the clamp; different moisture; measurement uncertainty in force or diameter.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Fibre P breaks at 10 N with diameter 0.40 mm. Fibre Q breaks at 12 N with diameter 0.50 mm. Which has the higher tensile strength? Show the comparison.
Show a hint
Do not rank by force.
Explore the reasoning
Area P ≈ π × 0.20² = 0.126 mm², strength ≈ 80 N mm⁻². Area Q ≈ π × 0.25² = 0.196 mm², strength ≈ 61 N mm⁻². P is stronger even though Q held a larger force.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare sclerenchyma fibres with xylem vessels as materials humans exploit.
Cell walls, xylem and fibres
Practice question
Retrieval and explanation · suggested answer
Compare sclerenchyma fibres with xylem vessels as materials humans exploit.
Lesson: 4A 5: Using Plant Starch and Fibres
Try this from memory first.
Beyond the classroomWhy fibre direction mattersResearch · plant materials · Enrichment, not required recall
Plant-material reviews describe celluloseA structural polysaccharide of β-glucose. Straight chains are linked by hydrogen bonds into microfibrils that strengthen plant cell walls. microfibrilA bundle of parallel cellulose chains held together by many hydrogen bonds, giving the cell wall high tensile strength. as stiff cables in a softer matrix. Aligning those cables with the load is why a fibre is strong in tension along its length and weaker if you try to tear it across. School hanging masses are a simple version of that mechanical question.
Back to the spec: 4.4 and 4.9: microfibril arrangement explains the property you measure as tensile strength.
Gibson, hierarchical structure and mechanics of plant materials (PMC)Question
If you tested the same fibre across its width, would you expect the same strength?
Explore a possible response
No. Microfibrils are aligned along the fibre, so tensile strength is anisotropic. A cross-wise test loads hydrogen bonds and matrix differently.