This paper focuses on the blade peak stress prediction from limited strain gage/tip-timing measurements when multiple vibratory modes are present and in the absence of a validated finite element model. The current protocol assumes that only one mode is present and only provides upper and lower bound estimates of the blade peak stress when multiple modes are important. Two candidate approaches are proposed here to resolve this situation; they are based on test measurements and data provided by the manufacturer/basic blade geometry information, e.g. aspect ratio, taper, thickness, twist, stagger, referred to as "blade descriptors". The first approach, referred to as the finite element model reconstruction approach, focuses on reconstructing a most likely finite element model of the blade given all available blade information. In the second approach, referred to as the knowledge-based approach, a database of finite element models corresponding to a broad set of blade descriptors is initially established then queried for the application considered. The formulation of these approaches and their successful preliminary validation is presented in details.