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.