  We investigate the cyclic mechanical behavior in uniaxial tension of three
different commercial thermoplastic polyurethane elastomers (TPU) often
considered as a sustainable replacement for common filled elastomers. All TPU
have similar hard segment contents and linear moduli but sensibly different
large strain properties as shown by X-Ray analysis. Despite these differences,
we found a stiffening effect after conditioning in step cyclic loading which
greatly differs from the common softening (also referred as Mullins effect)
observed in chemically crosslinked filled rubbers. We propose that this
self-reinforcement is related to the fragmentation of hard domains, naturally
present in TPU, in smaller but more numerous sub-units that may act as new
physical crosslinking points. The proposed stiffening mechanism is not
dissimilar to the strain-induced crystallization observed in stretched natural
rubber, but it presents a persistent nature. In particular, it may cause a
local reinforcement where an inhomogeneous strain field is present, as is the
case of a crack propagating in cyclic fatigue, providing a potential
explanation for the well-known toughness and wear resistance of TPU
