Lab 02: Strain Gages | Anuranan Bharadwaj

Lab Overview

Course: AE 417 – Aerospace Structures and Instrumentation Laboratory (David Sypeck / Reshma Kubsad) | October 3, 2025
Objective: Install a 120 Ω foil strain gage on an aluminum cantilever beam, measure resistance changes under incremental bending loads in tension and compression, and derive Young's modulus for both an aluminum and a Nomex/CFRP honeycomb sandwich beam.
Specimens: Al 6061-T6511 strip beam (L ≈ 299 mm, w = 25.52 mm, h = 4.82 mm); Nomex honeycomb beam with CFRP skin (L ≈ 294 mm, w = 24.41 mm, h = 10.26 mm, t_face = 0.336 mm)
Instrumentation: 120 Ω Micro-Measurements strain gages (GF = 2), digital multimeter (via banana plug), C-clamp cantilever setup, calibration masses (1, 2, 3 kg)
Installation standard: Vishay bulletins B-127-14, TT-603, TT-609

Strain gage installed on aluminum beam (Fig. 8)

Aluminum beam in tension at max mass (Fig. 11)

Honeycomb beam with weighted tension (Fig. 14)

Procedure & Methodology

Each beam's dimensions and mass were measured five times and averaged. The aluminum beam was prepared per Vishay instructions: surface degreased, sanded (320 then 400 grit), conditioned with M-Prep Conditioner A, neutralized, and dried. The strain gage was positioned using tape transfer and bonded with M-Bond 200 adhesive and catalyst. Leads were soldered to bondable terminals and connected via banana plug to the digital multimeter.

The beam was clamped to the edge of the workbench in a cantilever (“diving board”) configuration. The resting resistance was recorded as the zero reference, then 1 kg, 2 kg, and 3 kg masses were hung from the free end, with a reading taken at each load. The beam was then flipped to put the gage in compression and the mass sequence repeated. The entire procedure was repeated for the Nomex honeycomb beam using its pre-installed gage. Strain was computed as ε = (ΔR/R) / GF; stress was computed from beam bending theory (σ = My/I for the aluminum; σ = M/(h·t_f·b) for the sandwich beam); Young's modulus followed from E = σ/ε.

Installation chemicals and tools (Fig. 4a/4b)

Aluminum beam in compression (Fig. 12)

Honeycomb beam in compression (Fig. 16)

Results & Analysis

Aluminum beam resistance increased by ≈0.08 Ω in tension and decreased by ≈0.08 Ω in compression for 3 kg load, confirming symmetric bending response.
Honeycomb beam resistance changed by ≈0.10 Ω in both directions for 3 kg, reflecting higher bending stiffness due to the sandwich construction.
E_Al = 84 GPa — 17.9% error relative to the 6061-T651 handbook value of 69 GPa; attributed to minor gage misalignment and surface preparation variability.
E_Honeycomb = 74 GPa — within the expected 70–120 GPa range for woven CFRP facing skins.
Stress vs. strain curves for tension and compression were nearly coincident, validating correct gage installation and material isotropy in the elastic regime.
At the same load, the honeycomb beam showed less strain than aluminum, confirming its higher effective bending stiffness per unit cross-section owing to the sandwich core.

Strain vs Load — both beams (Fig. 17)

Stress vs Strain — aluminum (Fig. 18)

Stress vs Strain — honeycomb (Fig. 19)

MATLAB Code

Two scripts were used: one to plot strain vs. load for both beams, and one to compute and plot stress vs. strain using measured beam geometry and bending theory. Below is a representative excerpt; full scripts are Lab2_Main.m and Lab2_Main_stress_strain.m.

% Strain from resistance change and gage factor
load = [0 1 2 3];  % kg
GF   = 2;          % gage factor

al_strain_T = [0  0.000354  0.000699  0.00104];   % aluminum tension
al_strain_C = [0 -0.000350 -0.000699 -0.00105];   % aluminum compression
hc_strain_T = [0  0.000429  0.000861  0.00129];   % CFRP tension
hc_strain_C = [0 -0.000433 -0.000445 -0.00131];   % CFRP compression

% Aluminum bending stress: sigma = My/I
F    = load * 9.81;            % N
M_al = F * L_al;               % N·m
sigma_al = M_al .* y_al ./ I_al;   % Pa

% CFRP sandwich stress: sigma = M / (b * h_f * t_face)
M_cf    = F * L_cf;
sigma_cf = M_cf ./ (b_cf * h_f * t_face);  % Pa

% Young's modulus: E = sigma/epsilon  (1 kg load, tension)
E_Al  = sigma_al(2) / al_strain_T(2);  % ≈ 84 GPa
E_CFRP = sigma_cf(2) / hc_strain_T(2); % ≈ 74 GPa

Full script — Lab2_Main.m

Full script — Lab2_Main_stress_strain.m

Resistance measurement data tables