Aircraft Structural Integrity

Many aircraft fleets around the world are being operated beyond their intended service life span and in environments more severe than their airframes were designed to accommodate — conditions that place increased demands on structural reliability requirements for aging military and commercial aircraft.

Structural Design and Analysis

Southwest Research Institute (SwRI) performs design and analysis evaluations, including investigations often associated with aircraft structural modification and life extension programs, for a variety of aircraft systems. The Institute works with aircraft manufacturers and modification facilities to provide specialized engineering expertise in design, analysis, and testing. Services include:

• Computer-aided design
• Production and installation drawings
• Finite element stress analysis
• Dynamic and aeroelastic analysis
• Fracture mechanics/crack growth analysis
  • Stress intensity factors
  • Crack growth models
  • Cycle-by-cycle crack growth analysis
  • Residual strength

SwRI uses a wide variety of computer-aided design and finite element programs for aircraft structural design and analysis.

Material Property Evaluation

Extensive laboratory facilities are maintained for measurement of mechanical and physical properties for all types of structural materials. SwRI capabilities include:

• Static strength
• Fatigue crack growth rate
• Fracture toughness
• Chemistry
• Coupon spectrum testing

Tests are often performed under controlled environmental conditions of temperature, humidity, vacuum, high pressure, and immersion in chemically controlled media. Specialized testing includes programmed spectrum loading, coupled thermal and mechanical loading, variable strain rates, and multi-axial stress conditions. Other physical measurements are made as prescribed by ASTM standards. Specialized test equipment can be designed and built to meet unique test requirements.

Fatigue testing under spectrum loading is conducted on actual and simulated structures. The Institute develops specimens and spectra for computer controlled tests.

Component and Full-Scale Testing

Verification testing is often required as part of aircraft design and modification programs. SwRI has extensive laboratory facilities for performing static, dynamic, and fatigue tests on components, sub-assemblies, and full-scale test articles. Typical tests include:

• Component and full-scale static strength to limit and ultimate load
• Full-scale durability and damage tolerance
• Bird strikes and debris impingement
• Structural flight demonstration
• Aircraft model and aeroelastic flutter

Tests are usually specialized, and SwRI routinely designs and fabricates all mechanical, hydraulic, and electrical components of test systems. Extensive instrumentation is provided for load and deflection measurements, strain gaging, crack growth monitoring, and data telemetry.

The Institute's engineering and technical staff develops test plans tailored to client needs and provides test setups and instrumentation, test monitoring, data reduction and analysis, and post-test documentation.

A modified T-37B jet trainer is subjected to full-scale structural testing in an SwRI aircraft service life extension program.

Aircraft Structural Life Management

Aircraft structural life management requires the integration of design and analysis, materials behavior, structural testing, and knowledge of aircraft usage. SwRI has experience in usage monitoring of military and commercial aircraft to provide data for structural life assessments. These flight recording programs define aircraft maneuver and stress spectra for fatigue analysis and fracture mechanics-based damage tolerance assessments of critical structures.

Damage tolerance methodology is used to establish initial and recurring inspection requirements for aircraft flight-critical structures.

Typical Projects

• Structural design, analysis, and modification
• Parts drawings for fabrication and installation
• External loads development
• Stress analysis
• Aircraft usage monitoring
• Maneuver and stress spectra development
• Vibration and static/dynamic fatigue testing
• Component and full-scale damage tolerance testing
• Damage tolerance and economic life assessment
• Structural and metallurgical failure analysis
• Nondestructive inspection development and application
• Structural condition inspection
• Life extension for aircraft
• Advanced aerospace materials studies, development, and application

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