| name | structural-testing |
| description | Structural testing — proof load, static load test, load application methods, strain gauge rosettes, load measurement, test-analysis correlation, qualification testing, scale models. |
| metadata | {"priority":6,"promptSignals":{"phrases":["structural test","proof load","load test","strain gauge","rosette","qualification test","load application","bridge test"],"minScore":4}} |
Structural Testing — Complete Skill
Proof Load Testing
Purpose and Level
Demonstrate structure safely carries design load without yielding/failure
Proof load level: 1.1-1.5× design load (depends on code and consequence)
ASME VIII pressure vessels: 1.3× MAWP (hydrostatic proof test)
AISC/ASCE qualification: load × appropriate factor (varies by structure type)
Crane/hoist: 1.25× rated load (ASME B30 series)
Aerospace: 1.5× design limit load (FAR Part 25 for aircraft)
Criteria
No permanent deformation (< 0.2% permanent strain at critical sections)
No visible cracking (VT + PT/MT after test)
Load-deflection linearity maintained during proof test
Return to zero: structure returns to original position after unloading
Static Load Testing
Load Application
Dead weight: direct mass stacking; accurate, slow, labor intensive
Hydraulic actuator: fast, precise, programmable; requires reaction frame
Pneumatic: pressure vessels, inflatable structures
Water filling: tanks, vessels, floating structures
Turnbuckle/come-along: in-plane tension loads
Reaction frame: transmits load from actuator to test structure without damaging surroundings
Load path: actuator → spreader beam → test structure → support structure → reaction frame → floor/wall
Spreader beams: distribute point load to multiple application points
3-point loading: creates moment M = P×a at loaded section
4-point loading: constant moment between two load points (no shear in constant-M zone)
Instrumentation
Strain gauges: bondable foil gauges (ASTM E251)
Gauge length: 1-10 mm; long gauge for averaging, short for high-gradient
Full bridge: 4 gauges, self-compensating for temperature
Half bridge: 2 gauges (active + dummy), moderate temperature compensation
Quarter bridge (1 gauge): simplest; needs Wheatstone bridge completion with dummy
Load cells: compression/tension column, proving ring, load pin
Accuracy: ±0.1-0.5% full scale (class A precision load cells)
Calibration: ASTM E74, traceable to NIST
LVDTs / string pots / laser displacement: deflection measurement
String pots: large displacements (up to 2m), ±0.1% FS accuracy
Optical: Photogrammetry (3D point tracking), DIC (full-field strain)
Strain Gauge Rosettes
Principal Strain Calculation (45° Rectangular Rosette)
ε_0°, ε_45°, ε_90° measured
Principal strains: ε₁,₂ = (ε_0° + ε_90°)/2 ± √[(ε_0° - ε_90°)²/4 + (ε_45° - (ε_0°+ε_90°)/2)²]
Shear strain: γ_max = ε₁ - ε₂
Angle: tan(2θ_p) = (2ε_45° - ε_0° - ε_90°)/(ε_0° - ε_90°)
Principal Stress from Strains (Plane Stress)
σ₁ = E/(1-ν²) × (ε₁ + ν×ε₂)
σ₂ = E/(1-ν²) × (ε₂ + ν×ε₁)
Von Mises: σ_VM = √(σ₁² - σ₁σ₂ + σ₂²)
Delta Rosette (60° spacing, 3 gauges at 0°, 60°, 120°)
More complex formulas; less common than 45° rosette
Gauge Installation (ASTM E251 / HBM guidelines)
- Surface prep: grind, abrade (silicon carbide), degrease (acetone)
- Apply adhesive (cyanoacrylate for short-term, M-Bond 200 for long-term)
- Cure: 24hr room temp or 2hr at 120°C
- Solder lead wires; strain relief
- Waterproofing (for outdoor/cyclic): polyurethane or silicone coating
Temperature compensation: use dummy gauge on unstressed same-material piece (quarter bridge) or use self-temperature-compensating gauge (matched alloy)
Test-Analysis Correlation (Test vs. FEA)
Comparison Methods
Strain correlation: compare ε_measured vs. ε_FEA at gauge locations
Target: < 10% difference for design loads
Larger difference → model error in stiffness, material, BCs, or gauge placement
Deflection correlation: measured vs. FEA deflection profile
Load-deflection linearity: test shows deviation from linear → yielding, geometric NL, slack
Common Causes of Discrepancy
FEA too stiff: over-constrained BCs, rigid connection assumptions
FEA too flexible: missing stiffeners in model, wrong section properties
Strain mismatch near joints: stress concentration modeling (fillet radius, weld toe)
Temperature effects: thermal strains contaminating mechanical strains
Model Updating from Test
Use test data to update FEA model — see modal-analysis skill for MAC/COMAC
Qualification Testing
Component Qualification
Demonstrate part meets all requirements without failure
Pass/fail criteria per specification
Test conditions: worst-case load, temperature, environment simultaneously if possible
Test levels (MIL-STD-810, DO-160):
Development test: exploratory, find margins, typically 3× design level or until failure
Qualification test: demonstrate margin above max design (1.5-2×), pass/no-fail
Acceptance test: production screening (1.0-1.1× design load)
Scale Model Testing
Dimensional analysis: Π groups must match between model and prototype
Key dimensionless groups: Re (fluid), Fr (gravity), St (vibration), σ/E (stress)
Similitude: if geometry scaled 1:10, loads scale differently depending on test type
Structural scaling:
Same material: σ_model = σ_prototype if loads scale as L²
Length scale = 1:10; area scale = 1:100; load scale = 1:100 for same stress
Wind tunnel scaling:
Match Re = ρVL/μ (aerodynamics) AND Fr = V²/(gL) (gravity) simultaneously — often impossible
Compromise: match dominant dimensionless group for phenomenon studied
Load Measurement and Control
Open-Loop (Force-Controlled)
Apply known displacement of actuator → measure force output
Good for: rigid structures where stiffness doesn't change
Closed-Loop Force Control
Feedback: load cell signal → compare to setpoint → adjust actuator
Better for: specimens with changing stiffness (yielding), soft specimens
Stroke Control (Displacement Control)
Feedback: LVDT or actuator position → control loop
Essential for: post-peak softening, snap-through, compression buckling tests
Prevents uncontrolled load drop after peak
Load Frame Compliance Correction
Frame and fixtures add compliance → actual specimen displacement ≠ actuator displacement
Measure frame stiffness k_frame separately → subtract from total
δ_specimen = δ_total - F/k_frame
Standards Summary
| Test | Standard | Level |
|---|
| Pressure vessel proof | ASME VIII UG-99 | 1.3× MAWP |
| Crane/hoist proof | ASME B30 | 1.25× rated |
| Bridge load test | AASHTO | 1.0× (diagnostic) |
| Structural component | AISC / ASTM | Per design code |
| Aircraft structure | FAR 25.305 | 1.5× DLL (ultimate) |
| Environmental test | MIL-STD-810H | Per category |
Output
Provide: load application method, instrumentation plan (gauge locations, type, bridge config), proof load level [kN], measured vs. FEA strain comparison table, acceptance criteria, permanent set limit [mm].
