Lab 03: Aluminum and Composite Tensile Testing

Experiment Overview

Choosing between aluminum alloys and fiber-reinforced composites is one of the most consequential decisions in aerospace structural design. Aluminum is reliable, well-understood, and easy to repair; composites offer far superior strength and stiffness per unit weight but require different design methods. This lab pulled three materials to fracture — Al-2024-T351, glass-fiber reinforced polymer (GFRP), and carbon-fiber reinforced polymer (CFRP) — and built the stress-strain curves that quantify those trade-offs in direct, measurable terms.

Perform uniaxial tensile tests on Al-2024-T351, GFRP, and CFRP rods to fracture using a Tinius Olsen LVDT extensometer
Extract Young’s modulus E, ultimate stress σ_u, and yield stress σ_y (aluminum only) from stress-strain curves
Validate CFRP modulus independently using the rule of mixtures applied to optical fiber volume fraction measurements
Compare strength-to-weight ratios across all three materials to quantify the weight efficiency trade-off

Figure 2: Specimens after tensile testing

Equipment & Tools

2024-T351 aluminum, GFRP (polyester matrix), and CFRP tensile specimens with 6061-T6 aluminum gripping tubes
Tinius Olsen 150ST Electromechanical Universal Testing Machine with MTS 647 hydraulic wedge grips
Tinius Olsen LVDT Extensometer
Mitutoyo digital calipers and precision mass scale
Hand sander (240, 320, 400, 600 grit SiC) and hot glue gun
Buehler grinding and polishing machine (320 grit SiC; 1 μm alpha alumina slurry)
Motic metallurgical microscope with digital camera and imaging software

Approach

Each rod was sanded, cleaned with acetone, and fitted with 6061-T6 aluminum gripping tubes bonded with cyanoacrylate adhesive. An LVDT extensometer measured elongation at the gauge section. Young’s modulus was computed as the average of σ/ε at five points in the elastic region. Ultimate stress was the maximum recorded value; yield stress for the aluminum was found by the 0.2% offset method.

For CFRP, the standard stress-strain method severely underestimated E (by ~49%). An independent estimate was obtained using the rule of mixtures E_c = E_mV_m + E_fV_f, where the fiber volume fraction V_f was measured by counting and sizing fibers in polished cross-section microscopy images. Strength-to-weight ratios were computed by dividing ultimate stress by the measured density of each rod.

Figure 3: Zoomed picture number 1 of CFRP fibers

Figure 4: Zoomed picture number 2 of CFRP fibers

Figure 5: Zoomed picture number 3 of CFRP fibers

Figure 6: Zoomed picture number 4 of CFRP fibers

Figure 7: Zoomed picture number 5 of CFRP fibers

Key Results

Al-2024-T351: E = 693 MPa, σ_u = 497 MPa, σ_y = 361 MPa. Percent differences vs. handbook: 1.3%, 1.4%, and 2.7% — excellent agreement, confirming reliable test execution for ductile metals.
GFRP: E = 373.9 MPa, σ_u = 469 MPa. Percent error vs. handbook E: 0.4% — the polyester matrix composite behaved predictably.
CFRP stress-strain E = 1,118.6 MPa — showed ~49% error vs. handbook. Rule-of-mixtures E = 126,984 MPa (V_f ≈ 55%) agreed within 6%, confirming that the stress-strain method cannot accurately capture CFRP modulus and that metallography-based analysis is required.
Strength-to-weight ratios: Al-2024 = 178,777; GFRP = 250,482; CFRP = 717,297 MPa·mm³/g — CFRP outperforms aluminum by ~4× on this metric, directly explaining its dominance in modern aircraft primary structure.

Figure 8: Stress-strain graph for aluminum 2024 with yield and ultimate stress

MATLAB Code

Stress and strain were read from Tinius Olsen CSV output. Young’s modulus was averaged from five elastic-region data points, and rule-of-mixtures E was computed from fiber volume fraction measured in microscopy images.

% Young's modulus: average of 5 elastic-region points
E_Al = 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
E_m = 3300;  E_f = 228000;            % MPa
Vf  = mean([Vf1, Vf2, Vf3, Vf4, Vf5]);  % fiber volume fraction from microscopy
E_c = E_m*(1-Vf) + E_f*Vf;           % ≈ 126,984 MPa

% Strength-to-weight: sigma_u / density
specific_strength = max(Al_stress) / (2.78e-3);  % MPa·mm³/g

Experimental equipment used - (1) Mass Scale, (2) Acetone, (3) Cyanoacrylate Adhesive, (4) MBond 200 Adhesive — Figure 11: Experimental equipment used - (1) Mass Scale, (2) Acetone, (3) Cyanoacrylate Adhesive, (4) Small CFRP polishing sample, (5) 240 grit SiC sandpaper, (6) Calipers

Figure 13: Grinding and Polishing Machine

Valuable Takeaways

No single test method works for all materials. The stress-strain approach accurately characterized aluminum and GFRP but failed for CFRP, where the rule of mixtures plus microstructural analysis was required — a lesson in choosing the right tool for the material at hand.
CFRP’s specific strength advantage is enormous. At 717,297 MPa·mm³/g versus 178,777 for aluminum, CFRP can carry the same structural load at roughly a quarter of the weight — the driving force behind the Boeing 787 and Airbus A350 being >50% composite by weight.
Brittle vs. ductile failure modes tell different stories. Al-2024 yielded, necked, and fractured gradually; GFRP and CFRP fractured suddenly with no warning. This distinction drives damage tolerance requirements and inspection intervals for aerospace structures.
Gained experience with the complete tensile test workflow: specimen preparation, extensometer attachment, universal testing machine operation, fracture characterization, and cross-validation using metallography.