Fatigue Crack Growth Testing

Testing to ASTM E647 & More

fatigue testing labThe fatigue crack growth test is used to characterize the rate at which a pre-existing crack will grow under time-varying crack tip stress states. Fatigue Crack Growth Testing, also known as crack propagation or da/dN testing, is performed at LTI using both the compliance and DC potential drop crack length measurement techniques. These methods are performed under either K or load control.

Advanced testing methodologies can be used to characterize threshold and full Region I, Region II (also known as the “Paris” regime) and Region III crack growth.

Routine and Specialized Testing

Laboratory Testing Inc.’s experienced fracture mechanics team offers Fatigue Crack Growth Testing at our PA (USA) lab. We can help solve your material problems and questions with routine and specialized services:

  • The test can be performed at temperatures between -150°F and 2000°F and at loads running from 40 lbf. to 100, 000 lbf.
  • Constant load, increasing and decreasing delta K, and constant Kmax control methods are offered.
  • Crack closure measurement by ACR (Adjusted Compliance Ratio), and Crack Opening Load analysis
  • Our engineers work with customers to adapt tests to help with specific engineering problems.

Advanced techniques such as crack opening load measurement, Adjusted Compliance Ratio (ACR) and constant Kmax testing are used to measure and account for crack closure. C(T) and M(T) specimens are typically used for this testing. LTI is also equipped to measure small cracks using the Kb type surface flaw specimen.

Additional information is provided in the article “Fatigue Crack Growth Testing 101” by Guy Connelly, Senior Engineer at LTI, from the June 2014 issue of Quality Magazine. Read Guy’s biography for details about his qualifications in the field of fatigue testing.

Test Method/Specification

  • ASTM E647
  • Advanced Techniques

Specimen Preparation

Test specimens can be machined in all standard geometries. Compact Tension (C(T)) and Middle Tension (M(T)) specimen configurations in a range of sizes are commonly used for fatigue crack growth testing. The M(T) specimen is primarily used for sheet materials, and the C(T) is used for a range of different material forms, including sheet, plate, bar and forgings. LTI is also capable of machining and testing surface flaw and SEN(T) specimens. Starter notches are produced using wire EDM.

About The Fatigue Crack Growth Test

The fatigue crack growth test helps evaluate the safety and reliability of materials by subjecting samples with a pre-existing crack to repeated loading and unloading. The test analyzes the crack growth rate and reports the resistance of materials to stable crack extension under this cyclic loading.

Fatigue crack growth rates from the very slow threshold regime to the fast growth rate region are typically developed using constant load or K-controlled parameters.The automated fatigue crack growth testing system at LTI uses MTS closed loop servo-hydraulic load frames controlled by the industry leading FTA ADWin based system. Both compliance and DC Potential Drop methods are available for monitoring crack length.

The result of these tests is a curve displaying the crack growth rate as the cyclic stress intensity factor (delta K) is varied. Fatigue crack growth test reports include the following:

  • Data points
  • A da/dN-dK plot
  • Threshold and/or Paris Law values, if required

LTI Capabilities

  • Control Methods – Constant Load, increasing and decreasing delta K, constant Kmax
  • Loads – load cells capable of running from 50 lbs. to 55,000 lbs.
  • Temperature – between -150° F and +2000° F
  • Test Specimens – C(T) compact tension; M(T) middle tension, Surface Flaw SEN(T)
  • Fixturing – C(T) and M(T) specimen geometries in a range of sizes

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