The objective of this study was to investigate the fatigue and fracture beh
avior of steel cord/rubber composites used in tire belts under constant cyc
lic strain loading. The material was a specially made tire belt layer in th
e form of rubber sheets reinforced with unidirectional cords consisting of
two pairs of twisted steel wires. Failure mechanisms, damage development, a
nd fatigue life were determined for single belt layers with different cord
orientations. Tests were conducted at cord angles of 22 degrees, 72 degrees
, and 90 degrees degrees with a cyclic strain amplitude of 8.3% at a freque
ncy of 10 Hz. Five different stages of damage development were observed: mi
crocrack initiation, microcrack multiplication, macrocrack formation, slow
macrocrack propagation, and fast macrocrack propagation leading to final fa
ilure. In the case of the 22 degrees cord specimens, where the in-plane she
ar component was dominant, damage development consisted of microcrack initi
ation at the cord/rubber interface, the formation of more microcracks and m
acrocracks, and finally the formation of a major fatal macrocrack along the
cord direction. In the case of 90 degrees cord specimens, dominated by tra
nsverse tension, initial microcracks occurred within the cord, they propaga
ted across the thickness of the specimen, and finally a major macrocrack pr
opagated across the entire width of the specimen. The final crack propagate
d in part along the cord/rubber interface and in part within the cord. In t
he case of the 72 degrees cord specimens, where both in-plane shear and tra
nsverse tension are critical, the initial microcracks occurred within the c
ord and the final macrocrack along the interface. For the same cyclic strai
n amplitude, the 90 degrees specimens had the shortest fatigue life, and th
e 72 degrees specimens had the longest. Additional tests were conducted at
different strain amplitudes. The normalized modulus decreases slowly and ne
arly linearly with normalized fatigue lifetime up to a certain value of the
latter, approximately 80% of the normalized logarithmic lifetime, and then
it drops sharply. Cyclic strain amplitude also affects the failure mechani
sms. High amplitudes produce localized damage, whereas low amplitudes produ
ce dispersed damage. A residual life model was proposed based on stiffness
degradation.