This paper investigates the dynamic fragmentation characteristics and mechanism of rock blocks with different shapes during rockfalls. Numerical simulations by 3-D discrete element method were performed for the impact of a single rock block against the ground. The block was modeled as an assembly of bonded particles and the bond strength had been calibrated by experimental data. In the analysis, the contact mode was defined by face-contact (FCM), edge-contact (ECM) and vertex-contact (VCM), while the shape was characterized by shape factor ψ. The study focuses on the characteristics of dynamic fragmentation, fragment size distribution (FSD) and the underlying link between shape factor ψ and fragmentation degree. The numerical results are as follows: (1) The dynamic fragmentation can be divided into two stages: fragmentation (including local and integral fragmentation) and ejection. (2) The damage ratio α_b of near-spherical blocks under different contact modes roughly presents the following rule: α_b(FCM) > α_b(Sphere) > α_b(ECM) > α_b(VCM). α_b under the FCM is 1.6 times that under the ECM and 1.9 times that under the VCM. (3) FSD exhibits discreteness due to the contact mode and it can be well fitted by the Weibull’s distribution function. Besides, the distribution characteristics of FSD reveals the fragmentation degree is inversely proportional to ψ under the FCM. (4) The energy dissipated by friction E_f during the fragmentation accounts for the most significant proportion, which is about 50%. E_f of the three different contact modes is shown as: E_f (FCM) < E_f (ECM) < E_f (VCM). Under the FCM, the force chains are more stable, which leads the peak impact load F is 3 ~ 4 times that under other contact modes. (5) 27 random impact tests were conducted on each block with different shapes. It was determined that the shape factor ψ is proportional to the average damage ratio M.

rockfall hazards; discrete element method; shape factor; contact mode; fragmentation