Shear break-up processes of polydisperse fractal clusters are investigated by the ultrasound scattering technique. Within the framework of fractal aggregation and the hybrid approach model for polydisperse correlated scatterers, the concept of variance in the local filler concentration is used to derive a new expression for the scattering cross-section for polydisperse fractal aggregates in the Rayleigh scattering regime. Considering the scaling laws for the shear-induced disruption of the clusters, the shear stress dependence of the ultrasound scattered intensity for polydisperse fractal aggregates is also derived. The fractal scattering regime is further discussed for both monodisperse and polydisperse clusters of size larger than the wavelength. In-line ultrasonic measurements for the shear disruption processes of silica fume fillers compounded with polypropylene during extrusion are investigated. A critical disaggregation shear stress is determined and is found to decrease with the filler surface treatment concentration. This stress is representative of the particle adhesiveness and aggregate dispersion in the matrix. This is confirmed by the improvement in impact resistance tests. On the basis of the scaling laws and the self-consistent-field approximation usually used in the microrheological models, the shear-thinning behaviour of silica fume clusters is successfully simulated.