Appraisal of linear baseline-free techniques for guided wave based structural health monitoring

•Provides a comprehensive review of several baseline-free guided wave structural health monitoring (GWSHM) techniques, i.e., the time reversal (TR), the virtual time reversal (VTR), the instantaneous baseline (IB) and the reciprocity-based technique, and critical factors which could affect the damage detection and localization capabilities of the baseline-free techniques.•Proposes an integrated experimental framework for investigating the damage detection performance of baseline-free GWSHM techniques, which includes the proposal of a health index (HI) for assessing the health condition of monitored composite structures.•Experimentally investigates and compares the damage detection and localization capabilities of the VTR, IB, and the reciprocity-based technique when applied to composite structures considering various factors including material anisotropy, temperature variations, multiple damages, boundary reflections of guided waves, and structural complexities, e.g., stiffeners. Practical applications of the several baseline-free techniques to the stiffened aircraft fuselage panel are considered during the experimental investigations. This paper offers a comprehensive critical appraisal and experimental comparison of leading linear baseline-free techniques applied in guided wave-based structural health monitoring (GWSHM). The paper extensively examines the most popular linear baseline-free techniques, namely Time Reversal (TR), Virtual Time Reversal (VTR), Instantaneous Baseline (IB), and reciprocity-based methods. Detailed discussions on the principles, strengths, and limitations of each technique provide a thorough understanding of their capabilities and challenges. Critical factors affecting performance that influence the performance of baseline-free techniques in damage detection and localization is the main focus of the paper. These factors encompass varying environmental conditions such as temperature fluctuations, geometric and structural complexities, and diverse damage scenarios. The research reported conducts experimental comparisons among VTR, IB, and reciprocity-based techniques as related to the challenging case of composite materials, considering single and dual Barely Visible Damage (BVID) scenarios, temperature variations, boundary reflections, and structural complexities like stiffeners. The results demonstrate that the investigated baseline-free techniques are capable of identifying and localizing damages, albeit with differing capabilit