  The stopping of baryons in heavy ion collisions at beam momenta of $p_{\rm
lab} = 20-160A$ GeV is lacking a quantitative description within theoretical
calculations. Heavy ion reactions at these energies are experimentally explored
at the Super Proton Synchrotron (SPS) and the Relativistic Heavy Ion Collider
(RHIC) and will be studied at future facilities such as FAIR and NICA. Since
the net baryon density is determined by the amount of stopping, this is the
pre-requisiste for any investigation of other observables related to structures
in the QCD phase diagram such as a first-order phase transition or a critical
endpoint. In this work we employ a string model for treating hadron-hadron
interactions within a hadronic transport approach (SMASH, Simulating Many
Accelerated Strongly-interacting Hadrons). Free parameters of the string
excitation and decay are tuned to match experimental measurements in elementary
proton-proton collisions, where some mismatch in the $x_F$ distribution of
protons is still present. Afterwards, the model is applied to heavy ion
collisions, where the experimentally observed change of the shape of the proton
rapidity spectrum from a single peak structure to a double peak structure with
increasing beam energy is reproduced. Heavy ion collisions provide the
opportunity to study the formation process of string fragments in terms of
formation times and reduced interaction cross-sections for pre-formed hadrons.
A good agreement with the measured rapidity spectra of protons and pions is
achieved while insights on the fragmentation process are obtained. In the
future, the presented approach can be used to create event-by-event initial
conditions for hybrid calculations.
