A fragmentation model based on global load sharing (GLS) theory is developed to obtain stress-strain curves that describe the mechanical behavior of unidirectional composites. The model is named CNB+τ* because it is based on the Critical Number of Breaks model (CNB) and on the correction of the fiber matrix interfacial strength, τ*. Model allows both obtaining the ultimate tensile strength of CFRP and GFRP composites, and correcting the σ vs ε curve to match its peak point with the predicted strength, which is more accurate than the one obtained by previous GLS-based models. Our model is used to classify the mechanical response of the material according to the energetic contributions of two phenomena up to the failure: intact fibers (IF) and fragmentation (FM). Additionally, the influence of fiber content, Vf, on the tensile strength, σU, failure strain, εU, and total strain energy, UT, is analyzed by means of novel mechanical-performance maps obtained by the model. The maps show a dissimilar behavior of σU, εU and UT with Vf between GFRP and CFRP composites. The low influence of Vf on the percent energetic contributions of IF and FM zones, as well as the larger energetic contribution of the FM zone, are common conclusions that can be addressed for both kinds of composites.