Determination of structure fatigue properties

A test system comprising:

• a load frame operational to:
• apply repeatedly an adjustable load to a composite structure over a plurality of cycles, wherein the composite structure has a crack with a crack size;
• and
• measure a displacement of the composite structure in each of the plurality of cycles;
• and
• a computer coupled to the load frame and operational to:
• store the crack size at a start of the plurality of cycles;
• update the crack size in response to the displacement in each of the plurality of cycles;
• update the adjustable load in each of the plurality of cycles, wherein the adjustable load as updated maintains a strain energy release rate approximately constant based on the crack size, wherein the update of the adjustable load comprises lower the adjustable load as the crack size increases;
• calculate a growth rate of a crack propagation in the composite structure based on the displacement in each of the plurality of cycles;
• and
• store the growth rate in a file, wherein the file is suitable for analysis in a virtual crack closure technique.

A method for determining interlaminar fatigue properties in a composite structure, the method comprising:

• mounting the composite structure in a load frame, wherein the composite structure has a crack with a crack size;
• initializing an adjustable load exerted on the composite structure by the load frame based on the crack size and a strain energy release rate;
• applying the adjustable load to the composite structure in a current cycle of a plurality of cycles;
• measuring a displacement of the composite structure in the current cycle;
• updating the crack size used in a next cycle of the plurality of cycles in response to the displacement as measured in the current cycle and the adjustable load used in the current cycle;
• updating the adjustable load used in the next cycle, wherein the adjustable load in the next cycle maintains the strain energy release rate approximately constant based on the crack size used in the next cycle, wherein the updating of the adjustable load comprises lowering the adjustable load as the crack size increases;
• applying a plurality of different values of the adjustable load over the plurality of cycles;
• calculating a growth rate of a crack propagation in the composite structure based on the displacement in each of the plurality of cycles;
• and
• storing the growth rate in a file, wherein the file is suitable for analysis in a virtual crack closure technique.