The use of laminated safety glass enables residual load-bearing capacity after fracture of the glass panes. The acting load is carried by the glass fragments (adhering to the interlayer) and the interlayer itself. The interlayer locally delaminates from the glass and experiences large strains. Hence, to numerically describe the post-fracture behaviour of laminated safety glass, nonlinear (strain dependent) viscoelastic material behaviour has to be taken into account for the interlayer. The most used laminated glass interlayer is polyvinylbutyral (PVB). For this, the viscoelastic material behaviour in the linear range (Prony parameters of the generalized Maxwell model and Time-Temperature Superposition Principle TTSP) and the linearity limits, which characterize the transition from linear viscoelastic to nonlinear viscoelastic material behaviour, are well described in literature. This paper investigates the viscoelastic Consideration of the nonlinear viscoelasticity of PVB through a Time-Strain Superposition behaviour at strain magnitudes exceeding the linearity limits. For this purpose, relaxation tests in uniaxial tensile mode are carried out on the interlayer material. During the tests a high deviation from linear viscoelastic behaviour is observed. The Schapery model, which extends the linear viscoelastic constitutive law with straindependent nonlinearity factors, is often used in literature to describe nonlinear viscoelastic material behaviour. The nonlinearity factors, which include a Time-Strain Superposition Principle (TSSP), were investigated to create a mastercurve.