In this thesis, the impact of viscoelastic effects on the lifetime of short glass fibre reinforced (sgfr) polypropylene (PP) with 40 % glass fibre content are investigated. These sgfr plastics often have to withstand dynamic and static loads in automotive applications. To investigate the impact of the viscoelastic effects the on service life, a new test program was developed in which the test specimens are alternately loaded with cyclic and static load. A variation of the holding time of 25%, 50% and 75% and the holding stress at minimum load (UL), mean load (ML) and maximum load (OL) allows further statements about the effects. In addition to unnotched (UK) test specimens, two different notch radii were also examined. Furthermore to single-stage block tests with $R=0.1$, block tests with $R=0.5$ were also performed to investigate the influence of mean stress. The tests show a shift in the Wöhler lines, with the ML (+4.0%) and UL (+7.1%) having positive effects on the UK test specimens and the OL (-7.6%) having a negative effect. These effects become more apparent with increasing holding time. The hysteresis measurement method is used to characterize cyclic damage and creep damage. With ML and UL a relaxation of the specimen during the holding times and cyclic creep during the times of cyclic loading can be observed. Under OL the plastic creeps during the holding times and relaxes in the first cycles of the subsequent holding blocks. The mean stress influence $M$ increases by 3.1% for UK specimens at OL and decreases by 10.4% at ML compared to purely cyclic tests. The tolerable stress amplitude decreases from R=0.1 to R=0.5 when holding at OL by 39% and at ML by 37%. Notches considerably reduce the tolerable stress amplitude, at ML these have a stronger effect with -17.8% or -23% for sharply notched (SK) or very sharply notched (SSK) specimens than at OL (-13.1% or -16.5%). For pure Wöhler tests, a higher notch sensitivity can be determined for SK specimens (-18%) than for SSK (-17%) specimens. This behaviour can no longer be observed in block tests. An anisotropic structure simulation with a subsequent lifetime calculation gives a good approximation for purely cyclic tests, block tests cannot be calculated due to missing damage models yet.