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ASTM D5528 | Composites | Fracture Toughness

ASTM D5528 is a testing specification that determines the opening Mode I interlaminar fracture toughness of continuous fiber-reinforced composite materials using the modified beam theory or the compliance calibration method. The sample used is a double cantilever beam (DCB) specimen, which is a rectangular, unidirectional laminated composite specimen containing a nonadhesive film on the midplane that serves as a delamination initiator.

Susceptibility to delamination is one of the major weaknesses of advanced laminated composites due to their structure. Mode I interlaminar fracture toughness measures the critical value of delamination growth as a result of an opening load. It is used to establish the design allowables used in damage analyses of composite structures. Delamination growth is characterized by strain energy release rate and the way the load is applied (opening or tensile load per Mode I). Before conducting ASTM D5528, it is important to read the entire specification in the relevant ASTM publication.

ASTM D5528 | Composites | Fracture Toughness

  1. Prepare the DCB specimens with the nonadhesive insert on the midplane of the laminate and record the width and thickness measurements at midpoint as described in the method. The standard specimen length is at least 125 mm (5.0 in), the standard width is 20 to 25 mm (0.8 to 1.0 in) and the standard laminate thickness is 3 to 5 mm (0.12 to 0.2 in).
  2. Measure the initial delamination length from the load line to the end of the insert, the standard initial delamination length is about 50 mm (2.0 in).
  3. Mark both edges of the specimen just ahead of the insert. Mark the first 5 mm (0.2 in) from the insert with thin vertical lines every 1 mm (0.04 in). Mark the remaining 20 mm (0.8 in) with thin vertical lines every 5 mm (0.2 in).
  4. Bond the load blocks or the piano hinges to the top and bottom surfaces of the end of each specimen using a room temperature cure adhesive. Make sure the load blocks or the piano hinges are capable of sustaining the maximum load anticipated for the test.
  5. If using an optical microscope or a magnifying device, mount it on one side of the specimen to observe the motion of delamination front during the test.
  6. Apply the opening load at a constant crosshead rate between 1 and 5 mm/min until an increment of delamination crack growth of 3 to 5 mm.
  7. Record the load and displacement values, continuously. Start recording the delamination length on one side of the specimen from the marks and record the data point on the load-displacement curve when visual onset of delamination is observed.
  8. Unload the specimen at a constant crosshead rate of 25 mm/min. Make sure to record the load and displacement data while unloading. Mark the position of the tip of the precrack on both edges.
  9. Reload the specimen at the same initial constant speed of 1 to 5 mm/min. Record the load and displacement data, continuously, at the initial delamination and at as many delamination length increments possible in the first 5 mm, ideally every 1 mm.
  10. Unload the specimen at a constant crosshead rate of 25 mm/min. Mark the positions of the tip of the delamination crack on both edges of the specimen.
Calculations:
  • Delamination Length: Sum of distance from the loading line to the end of the insert plus the increment growth determined from the marks.
  • Interlaminar Fracture Toughness: There are three methods to calculate the interlaminar fracture toughness: Modified Beam Theory (MBT), Compliance Calibration Method (CC) and Modified Compliance Calibration Method (MCC). Please refer to the publication to determine the recommended method for your test.

ASTM D5528 | Composites | Fracture Toughness

ASTM D5528 Recommended Equipment
Testing System
Fixturing
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