Environment assisted cracking of high strength materials is a significant technical and safety concern for the Navy and industry. There is a current need for a simple monitoring system that can be used in the vicinity of critical high strength components to indicate the cumulative impact of conditions that can lead to hydrogen assisted cracking and premature failure.

Luna has developed a robust, low-cost monitoring system to guard against premature failure of critical high-strength components susceptible to hydrogen embrittlement or other forms of EAC.

The detection system can reduce: corrosion-related failures, unscheduled outages, and inspection and maintenance costs.  The detection system also is useful for in situ testing and monitoring of new alloys for safer introduction into specific structural applications.

Key Attributes of the EAC Monitoring System

  • Dual Measurement – Direct crack growth tensile sample combined with Ag/AgCl reference electrode to monitor local potential and enhance system confidence
  • High Sensitivity –   Micron-scale crack length resolution provides rapid crack growth rate assessment and early- warning capability
  • Low Cost, Small Size – Well suited for distributed monitoring
  •  Embedded Hardware – Battery power and onboard data storage facilitate installation and operation
  • Broad Application – Compatible with impressed current and sacrificial anode cathodic protection systems
  • Easy Interpretation – Users need little or no specialized training

The heart of the EAC sensing system is the direct crack detection element, comprised of a notched tensile specimen fabricated from the alloy of interest and preloaded to the desired stress intensity.  As a crack initiates and propagates under EAC conditions, the tensile load begins to shed.  Onboard instrumentation continuously monitors the load and translates this information into a useful crack depth assessment.

As naval technology advances, there is an increased need for high strength alloys.  However, there has been a reluctance to use these materials as a result of increased risks associated with hydrogen embrittlement (HE) under cathodic potentials.  For nickel-bearing alloys (e.g., Monel K500), hydrogen diffusion is very low, however failures of such high strength alloys have been observed in critical applications.  HE prediction using short-term tests is particularly difficult in these materials as significant variability in crack initiation can occur and long times are required to obtain crack growth measurements.

Luna’s EAC detection system reduces measurement uncertainty and time by monitoring the condition of a pre-cracked alloy sample under stress in the same environmental conditions as the structure of interest.  Crack initiation and propagation are observed by measuring the sample tensile load.  Proprietary design features enhance the crack depth sensitivity into the micron range, making this system useful for quickly identifying operating conditions conducive to EAC damage.  Ag/AgCl reference potential is monitored concurrently to increase the measurement confidence, reducing false positive indicators.

The system can be configured to operate in standalone mode (battery power, onboard data storage for periodic collection) or connected directly to the user’s existing systems to provide real-time EAC monitoring capability.  Luna has developed laboratory, shipboard, and pressure-rated EAC sensor variants.

Support and disclaimer remarks: This material is based upon work supported by the SBIR Program and the Office of Naval Research under contract N00014-07C-0731.  Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Office of Naval Research.