The effects of fibre concentration on the mechanical response of welded glass-fibre-reinforced polypropylene (GF-PP) were studied in-depth. Experimental observations during tensile tests of unwelded and welded GF-PP have shown a weld ratio reduction – ratio between the strength of the welded material and that of the bulk material – as high as 60%. For all conditions studied, no significant change in the polymer matrix was observed. Increasing the fibre content on the welded material was additionally associated with a decrease in the stress at break and the strain at the maximum stress, respectively 68 and 84% for 50 wt% GF-PP. The DIC technique was used to retrieve the local response in the welded zone, showing local strain as high as 19.5 times the macroscopic strain. Using X-ray microtomography, the strain magnification could be explained by the significant increase of the fibre density at the welded zone. It was also shown that at least 2/3 of the fibres were orientated within the weld plane limiting the transverse strain, favouring void nucleation and embrittlement of the material. As a consequence, the rupture can be explained by the amplification of the strain linked with the fibre concentration and orientation of the welded material.