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

Abstract

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 compressionÀcompression 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.

Advances in Cruciform Biaxial Testing of Fibre-Reinforced Polymers

“…As a summary, the equipment with the best acceptance in the scientific community follows this same scheme of two orthogonal pairs of axially aligned actuators with independent load cells, whether they are electromechanical or hydraulic. Examples of similar facilities are found in [16,17,[41][42][43][44][45][46][47]. Hannon and Tiernan [48] reviewed the in-plane biaxial systems applied to the characterisation of sheet metal because biaxial states can be present in sheet metal forming processes and the interest of the biaxial response due to the anisotropy resulting in the material after manufacturing.…”

“…However, there is a consensus in the research field to prefer the use of full-field techniques, emphasising the case of DIC, which has been applied, to date, in numerous biaxial tests with cruciform specimens [44,46,60,61,65,67,69,71,78,81,[83][84][85][86][87][88]. In addition to the detailed visualisation of the strain contours in different directions at every point on the surface, this technique provides numerous advantages.…”

“…In this way, the material to be tested must only be machined in its outer contour, and the tabs are machined in their central area, leaving the gauge zone accessible for measuring. This option is firstly described in the works by Kumazawa and Huang et al [42,81,120], and similarly reproduced in subsequent works by other authors [44,83,85,87,104]. Most of these works obtain the specimen by adhering the machined tabs on the contoured specimen, but Escárpita et al [104] performed a modification in the manufacturing methodology.…”

Permanent Deformation and Stiffness Degradation of Open Hole Glass/PA6 UD Thermoplastic Composite in Tension and Compression

“…This is not surprising, considering that 1) these properties are matrix-dominated and 2) the stiffness of the TP matrices are higher. A numerical and experimental study on the notched and unnotched hole biaxial tension and compression strength of UD carbon AS4/PEKK-FC are presented by Vankan et al [ 11 ]. Under both tension and compression, a quasi-brittle response in terms of stress–strain is shown.…”

“…The literature shows that most authors focus on retrieving the notched strength under static or fatigue conditions. Except for [ 11 ], the strain field (and its evolution) around the hole are not studied. Next to that, glass and carbon fibers are frequently studied as reinforcement fibers.…”