Split Hopkinson bar measurement using high-speed full-spectrum fiber Bragg grating interrogation.

Area of Science:

• Materials Science and Engineering
• Optical Sensing and Metrology
• Mechanical Engineering

Background:

• Accurate measurement of high strain rates and gradients is crucial for understanding material failure.
• Traditional methods often struggle with the dynamic and localized nature of these events.
• Fiber Bragg Grating (FBG) sensors offer potential for strain measurement but require advanced interrogation techniques for high-speed applications.

Purpose of the Study:

• To develop and validate a high-speed, full-spectrum measurement technique for FBG sensors.
• To analyze high strain gradients and large strains indicative of material failure under dynamic loading.
• To enable precise measurement of localized strain discontinuities without a direct line of sight.

Main Methods:

• Surface-mounting an FBG sensor onto a split-Hopkinson tensile bar specimen to achieve high strain rates (up to 500 s⁻¹).
• Interrogating the FBG using a high-speed, full-spectrum solid-state interrogator operating at a 100 kHz repetition rate.
• Analyzing captured deformed spectra for strain gradients using a combination of a default interior point algorithm and the modified transfer matrix approach.

Main Results:

• Successfully demonstrated a high-speed, full-spectrum measurement technique for FBG sensors.
• Quantified high strain gradients and localized discontinuities in materials subjected to dynamic loading.
• Validated the capability to measure these phenomena accurately up to strain rates of 500 s⁻¹.

Conclusions:

• The developed high-speed full-spectrum interrogation technique, coupled with the modified transfer matrix method, effectively measures highly localized strain gradients.
• This approach enables the analysis of material failure under extreme dynamic conditions.
• The technique provides a non-line-of-sight method for characterizing complex strain fields in materials.