Fatigue, a hot scientific research topic for centuries, can trigger sudden failure of critical structures such as aircraft and railway systems, resulting in enormous casualties as well as economic losses. The fatigue life of certain structures is intrinsically random and few monitoring techniques are capable of tracking the full life-cycle fatigue damage. In this paper, a novel in-situ wireless real-time fatigue monitoring system using a bio-inspired tree ring data tracking technique is proposed. The general framework, methodology, and verification of this intelligent system are discussed in details. The rain-flow counting (RFC) method is adopted as the core algorithm which quantifies fatigue damages, and Digital Signal Processing (DSP) is introduced as the core module for data collection and analysis. Laboratory test results based on strain gauges and polyvinylidene fluoride (PVDF) sensors have shown that the developed intelligent system can provide a reliable quick feedback and early warning of fatigue failure. With the merits of low cost, high accuracy and great reliability, the developed wireless fatigue sensing system can be further applied to mechanical engineering, civil infrastructures, transportation systems, aerospace engineering, etc.

Lab fatigue tests have been performed to verify the stability, durability, accuracy, and engineering applicability of the developed system. 6061-T6 aluminum alloy, a common material used in mechanical systems, civil infrastructure, automotive, marine structures, and aerospace structures, was selected as the test material due to its high strength, low density, good corrosion resistance, excellent cryogenic toughness, and beautiful appearance. The intrinsic characteristics of aluminum make it sensitive to stress concentration, local damage, and cracks. Table 4 shows the typical chemical elements of 6061-T6 aluminum alloys (% weight) and Figure 8 shows its S-N curve [29].