Strength of notched and un-notched thermoplastic composite laminate in biaxial tension and compression

Composite laminates are being increasingly used in a wide variety of industrial applications, but there are difficulties in applying these materials in ways that exploit their full potential, in particular under multi-axial loading. The objective of the present study is to determine by experiments the biaxial failure data for composite laminates produced by Fokker Aerostructures based on the thermoplastic UD carbon reinforced material AS4D/PEKK-FC. A test machine and accompanying cruciform specimens for in-plane biaxial failure tests have been developed. A coupon-level biaxial test program covering various biaxial load combinations in tension-tension, tension-compression and compression-compression has been successfully executed and biaxial failure values for the thermoplastic laminate have been determined. Besides, the experimental biaxial test program, also finite element models and analyses have been used to predict the global outcomes of the biaxial tests and to interpret the test results. Both plain (un-notched) and open-hole (notched) specimens of the thermoplastic laminate have been tested. The biaxial failure data have been collected and further processed in biaxial failure criteria. From the experiments, the failure strains, stresses and loads are determined and a failure envelope is created for both plain and open-hole specimens. Good agreement is found between the theoretically predicted envelopes and the test data. From the findings for biaxial failure criteria from this study, it is expected that structural weight saving can be achieved in the design of multi-axially loaded composite parts as compared to the design with the previous uni-axially based failure criteria.