Lab 03: Aluminum and Composite Tensile Testing

Lab Overview

Course: AE 417 – Aerospace Structures and Instrumentation Laboratory (David Sypeck / Reshma Kubsad) | October 24, 2025
Objective: Perform uniaxial tensile testing of three aerospace materials to fracture, extract mechanical properties from stress-strain curves, and validate CFRP modulus using the rule of mixtures and optical metallography.
Specimens: Al 2024-T351 (d̄ = 8.58 mm); GFRP polyester matrix rod (d = 9.53 mm, L = 295 mm, m = 39.4 g); CFRP rod (d̄ = 9.38 mm, L = 320 mm, m = 31.78 g)
Instrumentation: Tinius Olsen 150ST with LVDT extensometer, Motic metallurgical microscope, grinding/polishing machine, digital calipers, mass scale
Test speeds: 5 mm/min (metal), 1 mm/min (composites)

CFRP, GFRP, and Al-2024 specimens (Fig. 1)

Specimens after fracture (Fig. 2)

Tinius Olsen 150ST testing machine (Fig. 3)

Procedure & Methodology

Each specimen was lightly sanded, cleaned with acetone, and fitted with 6061-T6 aluminum gripping tubes bonded with cyanoacrylate adhesive. The LVDT extensometer was attached to the gauge section using hot glue at the knife edges. The cross-sectional area (mean of 5 diameter measurements) was entered into the Tinius Olsen Horizon software before each test. Aluminum was tested in metal tensile mode; GFRP and CFRP in composite tensile mode.

Young's modulus was extracted by averaging σ/ε at five points within the elastic region. Ultimate stress was taken as the maximum stress value. Yield stress for the aluminum was determined via the 0.2% offset method. For CFRP, a polished cross-section was prepared by hand-sanding through 240–600 grit SiC and final polishing with 1 μm alumina slurry. Microscopy images were used to count fibers and measure average fiber radius; the rule of mixtures (E_c = E_m V_m + E_f V_f) was then applied to estimate Young's modulus independently.

CFRP polishing and microscopy setup (Fig. 5/6)

CFRP fiber cross-section — microscopy (Fig. 9)

CFRP fiber distribution detail (Fig. 11)

Results & Analysis

Al 2024-T351: E = 693 MPa, σ_u = 497 MPa, σ_y = 361 MPa. Percent difference vs. handbook (E = 731 MPa, σ_u = 469 MPa, σ_y = 324 MPa): 1.3%, 1.4%, 2.7% — all within acceptable range.
GFRP: E = 373.9 MPa, σ_u = 469 MPa. Percent difference vs. handbook (E = 276 MPa): 0.4%.
CFRP: E = 1118.6 MPa (stress-strain method), σ_u = 1030 MPa. The stress-strain E shows ≈49% error vs. published; CFRP requires the rule of mixtures for accurate modulus extraction.
CFRP E via rule of mixtures = 126,984 MPa (V_f ≈ 55%, E_f = 228 GPa, E_m = 3.3 GPa) — 6% difference from published value.
Strength-to-weight ratios: Al 2024 = 178,777; GFRP = 250,482; CFRP = 717,297 MPa·mm³/g. CFRP outperforms by ≈4× over aluminum.
Al 2024 exhibited ductile yielding and necking before fracture; both composites fractured in a brittle manner without yielding.

Stress-strain — Al 2024 (Fig. 14)

Stress-strain — GFRP (Fig. 15)

Stress-strain — CFRP (Fig. 16)

MATLAB Code

Stress and strain were read directly from the Tinius Olsen CSV output. Young's modulus was extracted by averaging σ/ε at five elastic-region data points per specimen. The rule of mixtures was applied using fiber volume fraction measured from microscopy images. Below is a representative excerpt; the full script is the Lab 3 appendix code.

% Load Tinius Olsen CSV data
Al_data  = readtable('Al_2024_metal_sample_tensile_test.csv');
Al_stress = table2array(Al_data(:,3));   % MPa
Al_strain = table2array(Al_data(:,4));   % %

% Young's modulus: average of 5 elastic-region points
E_Al_average = mean([Al_stress(10)/Al_strain(10), ...
                     Al_stress(25)/Al_strain(25), ...
                     Al_stress(50)/Al_strain(50), ...
                     Al_stress(86)/Al_strain(86), ...
                     Al_stress(90)/Al_strain(90)]);  % ≈ 693 MPa

% Rule of mixtures for CFRP Young's modulus
E_m = 3300;   E_f = 228000;  % MPa
Vf  = mean([Vf1, Vf2, Vf3, Vf4, Vf5]);   % fiber volume fraction from microscopy
Vm  = 1 - Vf;
E_c = E_m*Vm + E_f*Vf;                    % ≈ 126,984 MPa

% Strength-to-weight
density_Al   = 2.78/1000;           % g/mm³
sigma_u_Al   = max(Al_stress);      % MPa
specific_str = sigma_u_Al / density_Al;   % MPa·mm³/g

Full MATLAB script — Lab 3 appendix

Tensile test CSV data files

Fiber radius measurements from microscopy