1. INTRODUCTION
Fiber metal laminates (FMLs) are hybrid composite structures constructed of thin sheets of metal alloys sandwiched by plies of fiber-reinforced polymeric composites. Considering the metal alloys as cover sheets. Jen et al. developed Mg/CF/PEEK nanocomposite laminates 1as well as Ti/CF/PEEK nanocomposite laminates 2 and obtained their mechanical properties at elevated temperatures. AS-4/Polyether–ether–ketone (PEEK), called APC-2, thermoplastic composites were adopted, since PEEK polymer matrix can sustain its mechanical properties at temperatures up to 143° C 3and offers excellent toughness properties. FMLs with thermoplastic-based composites offer superior resistance to aggressive environments, outstanding interlaminar fracture properties, excellent resistance to both impact and cyclic loadings, short time to cure,4,5 and potentially wide applications. FMLs also exhibit excellent fatigue resistance and durability. The reduction of the crack growth rate in FMLs is caused by the bridging effect in the fiber layers 6.
Kim et al. 7 improved the fracture toughness of carbon black and nanoclay mixed with epoxy resin and measured the fracture toughness using the single edge notched bend specimens at the room (25° C) and cryogenic (-150° C) temperatures. Jen et al. 8developed a methodology to fabricate AS-4/PEEK aromatic polymer composite and nanocomposite laminates and found the optimal content of nanoparticles (SiO2) was 1% by total weight. In fracture analysis the related famous theorems were proposed such as the maximum strain criterion by Chang 9, and energy release rate criterion by Nuismer 10. Hussain et al.11 investigated the mixed mode I and II cracking problem by using strain energy release rate concept. Ritchie et al.12 measured the effect of crack-tip shielding from the crack bridging effect for ARALL laminates. Asghar et al.13 studied the fatigue crack growth rate for CARALL, ARALL and GLARE composite laminates. Li and Johnson 14investigated the fatigue responses of hybrid titanium composite laminates. Cortes and Cantwell 15 obtained both tensile and fatigue properties of Ti/APC-2 hybrid laminates. Ilham et al. 16 presented a summary of matrix cracking in nanocomposite materials under conditions of fatigue, tensile, thermal and flexural loadings. Borowski et al. 17 reduced the interlaminar fiber-matrix cracking and delamination in CFRP laminates by adding the multi-walled carbon nanotubes to improve their fracture toughness. Mefford et al. 18 investigated the scaling of the structural strength of polymer/graphene nanocomposites. The geometrically scaled single edge notch bending specimens with varying contents of graphene were tested at fracture and studied the effect of nanomodification on the scaling.
In the consideration of crack initiation, growth rate and direction, Gross and Seelig 19 provided the methods in fracture mechanics to solve the problem of double-edge-cracked composite laminates subjected to tensile and cyclic loadings. Aliha et al.20 investigated the crack initiation behavior of different polyurethane foams under mixed modes such as I/II and I/III. The effect of specimen type, mode mixity and foam density was studied on fracture initiation angle and fracture trajectory. Wang et al.21 presented a theoretical and numerical study on the stress intensity factors for double-edge cracked steel plates strengthened with fiber reinforced polymer plates.
Guo and Wu 22 presented a theoretical model to predict the fatigue crack growth rates in fiber-reinforced metal laminates that in good agreement with experimental data. Homan 23determined the fatigue initiation in fiber metal laminates by assuming that fatigue crack initiation in FMLs is determined only by the stress cycles in the metal layers. The validation fatigue tests showed that the assumptions proved to be correct. Alderliesten 24presented a new analytical model for constant-amplitude fatigue crack propagation of through cracks in fiber metal laminates Glare. The prediction model was implemented in a numerical program and was validated with a wide range of experimental data. Spronk, Sen and Alderliesten 25 presented a methodology to predict the cycles to crack initiation in a notched FML due to cyclic loading. Gupta26 showed that the crack paths in FMLs under fatigue loading deflect because of the presence of mixed-mode loading at the crack tip. The amount of deflection depends on the mixed-mode ratio induced which, in turn depends on the Glare grade and the off-axis angle.
Herein, the Ti/APC-2 neat and nanocomposite laminates were fabricated. Then, the double-edge cracks were cut symmetrically and anti-symmetrically. From the tensile tests the load vs. displacement curves, maximum load, displacement and fracture mechanisms were received. From the base-line data of mechanical properties the cyclic tests were finished to obtain the load vs. cycles (P-N) curves, residual life and the mechanisms.