Required equipment

• At least two materials in strips of the same nominal size; label specimen IDs.
• One digital force gauge with an appropriate compression range and safe contact tip.
• Two equally high, fixed, rounded supports with measured and adjustable center-to-center spacing.
• Metric ruler and calipers to measure span, width and thickness.
• A fixed ruler or dial indicator measuring midpoint deflection; a fixed side camera with a metric reference scale is an acceptable observation aid.
• Safety glasses, stable work surface and teacher-approved fixture or support arrangement.

Optional: digital scale for specimen mass, camera for evidence, a gauge stand to improve axial alignment and spreadsheets for analysis.

Assign a team of 3–4

Switch roles between specimens when time and equipment permit. No one holds the test strip by hand during loading.

Before loading

1. Secure the fixture so the supports cannot slide or tip. Both supports must have the same height and a consistent contact shape.
2. Choose a common measured span L that safely fits the shortest strips, with enough overhang at both ends to stay seated.
3. Mark midpoint between the supports. Align a rounded gauge tip perpendicular to the strip over the mark.
4. Fix a ruler/indicator or camera reference so deflection is read relative to the unloaded specimen, not relative to a moving hand.
5. Check digital gauge capacity and set compression units to N if supported; otherwise convert readings before analysis.
6. Under instructor supervision, gently contact the specimen, tare force appropriately and set displacement to zero at the same reference state without appreciable preload.

Measure each individual strip

Record strip overall length, width b, and thickness t at the center (mm). Also record grain/ply/print orientation, any knots, chips or defects and measured mass if a scale is available.

Important: t is the thickness in the direction of bending. Incorrectly swapping b and t causes a large error because thickness is cubed.

Loading sequence

1. Label and inspect the strip; measure its width/thickness and any visible defects before testing.
2. Set the span: measure center-to-center support distance; keep it identical for each material.
3. Seat and tare: position the strip and gauge, apply only a tiny seating load if authorized, then establish paired zero readings (0 N, 0 mm).
4. Load slowly: increase deflection in small, teacher-specified steps. A 0.5 mm step is an illustrative example only, not a required or universally safe increment.
5. At each step: one student holds the reading steady, another measures the vertical center displacement, and the recorder logs the simultaneous force and displacement.
6. Stop at the approved limit: do not chase higher numbers after warning signs; record why the test stopped.
7. Unload gradually: measure residual (permanent) displacement once the load returns near zero, if safe.
8. Repeat with the next material, without changing the span, tip, measurement reference or loading method.

Data-quality rules

• Record actual gauge force at each observed deflection; do not assume the force equals a target.
• Record values with units: N and mm. If gauge reads lbf or kgf, convert to N before making a graph.
• Use at least three nonzero paired readings in a safe approximately linear response region per specimen if equipment resolution permits; 5 or 6 is better.
• If a specimen slips, a gauge is misaligned, or its curve becomes nonlinear, stop and annotate. Do not erase unexpected data silently.
• Repeat with another specimen of the same material if available. Repeating a load cycle on the same strip is not an independent material replicate.
• If a force gauge has a peak-hold mode, use live force readout for paired data. Peak hold can be used separately to document a maximum, but it does not pair a peak to a particular displacement automatically.

Instrument caution: Hand-held force gauges can introduce side forces and variable load rates. A fixed gauge stand and separate deflection indicator improve accuracy; without them, describe the result as an approximate comparison and report setup limitations.

Measured load versus measured displacement

Every dot is a recorded pair. Curves are not model predictions. Missing results indicate that no valid measurement was entered.

Analysis equations and limits

Use only the elastic region for the slope, with the instrument zeroed before the test. The effective E approximation assumes a slender, simply supported, rectangular linear-elastic strip. It does not predict fracture load or ultimate strength.

Wood paint stick: use actual dimensions when comparing. Carbon fiber: record fiber orientation. Polyethylene and Delrin: loading time may affect displacement. Steel: a hand-gauge test may not have enough capacity or resolution to yield an accurate slope.

Required analysis questions

1. Which safely measurable strip had the largest initial force–deflection slope k? Provide the measured value and units.
2. Which had the smaller deflection at a comparable safely applied load? If actual loads differed, use the graph trend or interpolation within your measured elastic region.
3. How did measured width and thickness compare? Explain why even small differences in t can change bending stiffness.
4. What observations indicated the strip remained elastic or began slipping, permanently deforming, cracking or otherwise behaving nonlinearly?
5. What two limitations of your equipment or method could have changed the result? Propose one improvement.
6. Given these results, which bridge-permitted material would you investigate for a lightweight bridge member, and what additional strength, buckling or joint test would be needed before choosing it?

Real-world use of the results

Use actual specimen evidence to refine the material choices from 2.11. For the bridge capstone, selecting a material still requires consideration of mass, connection quality, compression buckling, grain direction and manufacturing rules.

The bridge's final performance requirements—including 400 mm clear span, 100 N center design load, ≤10 mm deflection and ≤150 g completed mass—apply to the final bridge, not directly to these small test strips.

Core insight: You are not declaring the “strongest material” from an elastic bending experiment. You are identifying which test specimen had a larger measured flexural stiffness and what more must be tested.

Find the experimental stiffness

A sample requires 2 N at 0.5 mm, 4 N at 1 mm and 6 N at 1.5 mm. What is the force–deflection slope?

Why measure thickness?

Two nominally identical strips have the same width, but one is 2 mm thick and the other is 3 mm. Does that matter?

Gauge units

A digital gauge displays 1.0 kgf. What force should be entered in a graph labeled newtons?

Nonlinearity

At the last loading step, force drops sharply while deflection increases. Should that point be included in the initial elastic slope fit?

Repeat trials

You load the same strip twice and obtain similar slopes. Have you tested two independent pieces of material?

What can the test prove?

If plywood shows less deflection than balsa at 5 N, have you proven it will hold a larger ultimate load?

Required report evidence

• Objective, hypothesis and identification of independent, dependent and controlled variables.
• One labeled diagram or photograph showing the actual support points, gauge contact location, span and deflection reference.
• Material and specimen IDs, grain/orientation, dimensions (especially actual b and t), and force-gauge units.
• At least three nonzero paired measured force–deflection readings for each of two materials, preferably more where safe.
• A graph of force (N) versus center deflection (mm) with all measurable materials identified and units labeled.
• A clearly shown stiffness calculation (slope in N/mm) for each measurable specimen; optional effective E calculation using the measured geometry.
• Observations about return to original shape, residual set, slipping, damage or other notable behavior. Report "not observed" if no failure occurred.
• A six-material result summary and a short conclusion comparing measured performance and explaining at least two limitations plus a specific improvement.

Submission quality check

• Graphs and tables use real measurements—not assumed or fabricated data.
• Sample span, orientation, actual dimensions and measurement limitations are documented for all six materials.
• Initial slope is calculated from the linear elastic region only.
• Repeat loading of one piece is not described as independent sample replication.
• Stiffness results are not mislabeled ultimate strength or safe bridge capacity.
• Exported CSV, printed data sheets or teacher-required evidence files are included.

Due date: Check the Daily Agenda and follow your teacher's directions.