◂ the notebook

phycists.desmondbrown.me

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No. 07December 2025.
Materials lab, with the Strain tool

Where linear elasticity ends: stress and strain from test data

A stress–strain curve starts straight because of entry 01: near equilibrium every bond is a spring. The slope of that first part is the modulus. Strain, the tool I wrote for the materials lab, fits it by least squares below 0.25 % strain, after dropping the first 0.05 %.

051015200.00.10.20.3maximum, 18.4 MPafailureσ, MPaε, mm/mm
051015200.00.10.20.3maximum, 18.4 MPafailureσ, MPaε, mm/mm
A 5 % sisal-fibre specimen from the composite batch, plotted from the testing machine’s export by Strain’s own method. The shaded sliver on the left is the fit window; the steep line is the fitted modulus. Computed on this page.

what I don’t understand yet

Where the straight part really ends. The fit window is a rule of thumb; the proportional limit should come from the data itself, and I haven’t found a method I trust yet.