Scope:

ASTM D5528 test is used for determining opening Mode I interlaminar fracture toughness of continuous fiber-reinforced composites. This is accompanied by using modified beam theory or compliance calibration method. The test is used to measure the critical value of delamination growth due to opening load. The delamination growth is denoted by strain energy release rate and also by how the load is applied. It is also used for establishing an allowable design that is useful for analyzing the damage of composite structures. 

Test Procedure:

For the ASTM D5528 test, the sample used is a double cantilever beam (DCB) which is a rectangular, unidirectional laminated composite with a non-adhesive film on the mid-plane used as a delamination initiator. The DCB test samples are prepared with the non-adhesive insert on the midplane of the laminate. The width and thickness measurements are noted as specified by the test. The initial delamination length is measured from the load line to the end of the insert. The required delamination length is about 50 mm (2.0 inches). 

Both edges of the sample are marked just ahead of the insert. Using a room temperature cure adhesive, the load blocks are bonded to the top and bottom surface of the end of each sample. It is ensured that the load blocks can sustain the maximum expected load.  Then, the optical microscope or a magnifying device is mounted on one side of the sample to observe the motion of the delamination front in the ASTM D5528 test. The opening load is applied at a constant crosshead rate within 1 and 5 mm/min unless an increase in delamination growth of 3 to 5 mm appears. The load and displacement values are noted regularly. The delamination length is recorded on the side of the sample from the marks and the data points are also recorded on the load-displacement curve if the visual onset of delamination is observed. The sample is unloaded at a fixed crosshead rate of 25 mm/min. Also, the load and displacement date is noted while unloading. The position of the tip of the pre-crack is marked on both edges. Then, the sample is reloaded at the same initial fixed speed of 1 to 5 mm/min. Again, the load and displacement data are recorded continuously.

Sample size:

The typical sample length for ASTM D5528 is at least 127 mm (5.0 inches), the typical laminate thickness is 3 to 5 mm (0.12 to 0.2 inches), and the standard width is 20 to 25 mm (0.8 to 1.0 inches).

Data:

Interlaminar Fracture Toughness

Delamination Length

Conclusion:

Hence, ASTM D5528 is a standard test method for mode I interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites.