01.29 · Short fibers: alignment and stiffness

This exercise examines how reinforcement geometry and orientation affect the effective elastic response of a short-fibre material. The aim is to connect the model inputs to the resulting directional properties, while keeping elastic load transfer separate from a prediction of fibre breakage or composite strength.

Open this exercise in Workbench

Prepare the baseline

Before running, note fibre length, diameter, fractions and orientation settings. Change the quantity requested in the task while retaining the other settings, then record all directional moduli rather than only the largest one. Where two geometries have the same aspect ratio, compare their results explicitly; this helps identify which geometric information the selected idealization actually uses.

Worked procedure

1. Begin with planar-random short fibers: Fa = 0 and Fp = 1. Review the representative carbon/epoxy properties, 30% fiber volume and zero voids. Fiber length is 0.35 mm and diameter is 7 µm, giving aspect ratio 50. These fixed teaching inputs isolate orientation rather than changes in composition.

2. Run the Micro stiffness calculation and record E1, E2 and G12. Create separate copies for Fa = 0.25, 0.50, 0.75 and 1.00, retaining Fp = 1 and all other inputs. Fa is the fraction in the preferred direction; the remainder is planar-random, not a specified angular spread.

3. Compare the preferred-direction stiffness with transverse and shear stiffness. Explain why better alignment for one loading direction is not necessarily better for another. Check that the Cox length-transfer efficiency remains unchanged while the orientation average changes.

4. Save the elastic material snapshots for the companion laminate exercise. Keep strength allowables unset: this exercise predicts stiffness only and does not assess failure, a safe load or progressive damage.

Review checkpoints

Planar-random fibers give E1 = E2.

With the supplied inputs, E1 rises from approximately 24.11 to 61.38 GPa; E2 falls from approximately 24.11 to 5.45 GPa.

Cox transfer efficiency stays approximately 0.854 because fiber geometry and composition are fixed.

Model limits

Illustrative elastic screening, not measured material data or strength prediction. Prescribed axial/planar orientation mixture with stiffness-domain averaging, perfect interface and monodisperse fibers. Transverse and shear closure is approximate. Zero strength inputs mean failure is not assessed.

Interpret the comparison

Use the model’s reported transfer and efficiency outputs to explain the stiffness comparison. Treat any displayed fibre geometry as an illustration unless its dimensions are explicitly part of the calculation. If you save the result as a material, inspect which fields were predicted and which remain blank; an elastic snapshot is not a complete set of qualified material properties.

How information passes between models

Materials → Micro: Constituent stiffness, strength, density and thermal / moisture properties.

Models → Micro: Applied model assignment: Cox shear-lag elastic. Model parameters and formulation are used by Micro.

Further reading and evidence

• Create and connect a lamina
• Edit material properties and units
• Run and review a model
• Review effective properties
• Connected inputs and result freshness
• Related case study: NASA lamina analysis

Review the recorded validation scope. Retain the original inputs and solver notices with the results. Representative teaching data are not design allowables.