cis-1,4-Polyisoprene (IR) elastomers harden via strain-induced crystallization (SIC) when the imposed stretch (λ) exceeds the onset value of SIC (λ*). We investigate the Mode-I fast crack growth in the IR sheets as a function of λ in a pure shear geometry. The steady-state crack velocity (V) increases with increasing λ, and V exceeds the shear wave speed of sound at λ > λs. Further stretch beyond λ* (>λs) causes SIC-driven hardening, resulting in a pronounced increase in V. The characteristics of the crack-tip strain field are also significantly influenced by the SIC-driven hardening: The crack-tip opening displacement increases with increasing λ at λ < λ* but exhibits an abrupt reduction beyond λ*. The crack-tip singularity and the area of strain increment caused by the crack growth change discontinuously around λ*. The abrupt variations in these crack-tip characteristics result from the considerable differences in the mechanical properties prior to the crack growth between the entirely amorphous state at λ < λ* and the partially crystallized state at λ > λ*.

Simultaneous measurements of a stress-strain curve with two-dimensional small-angle X-ray scattering (2d-SAXS) patterns were conducted at room temperature to reveal the relationship between the formation of the kink structure and necking of the specimen comprising hard and soft lamellar microdomains under the uniaxial stretching. As a result, the 2d-SAXS pattern typical of the chevron structure was observed without changing throughout the first stage of the stretching (up to the strain of 2.1) where the stress was kept constant and the specimen underwent the necking. Note that necking appeared around the strain of 0.04 ~ 0.05 and it was propagating throughout the specimen with an increase of strain up to 2.1, which is the final point of the first stage of the stretching where the stress turned into increasing linearly with strain. Upon releasing the load applied to the stretched specimen, the chevron structure was transformed into biaxially-oriented lamellar structure, suggesting that the chevron structure is only stably existing under the stretched state. To rigorously understand the relationship between the necking region and the formation of the chevron structure, we conducted 2d-SAXS measurements using the microbeam of 2-µm diameter under the stretched state of the specimen with the strain of 0.5. As a result, it was found that the lamellar structure was not suffered from necking in the band region of 160 µm width near the boundary between the necking and unstretched regions though the lamellar structure was slightly deformed. This result is significant because it was believed that the chevron structure was formed throughout the necking region. Furthermore, the lamellar structure was drastically changed into the chevron structure as a function of the distance from the boundary in the narrow necking region within 160~240 µm from the boundary.