# Subsurface Fracture Mapping in Adhesive Interfaces Using Terahertz Spectroscopy

## TL;DR

This paper introduces a new method using terahertz spectroscopy to map hidden cracks in adhesive materials, improving the accuracy of fracture toughness measurements.

## Contribution

The novel use of terahertz time-domain spectroscopy to directly map subsurface crack geometry in adhesive interfaces.

## Key findings

- THz-TDS resolves crack openings down to 100 μm and reveals non-uniform crack propagation.
- Surface-based measurements can misestimate fracture toughness depending on location.
- THz-derived fracture toughness agrees with J-integral estimates from digital image correlation.

## Abstract

Adhesive fracture in layered structures is governed by subsurface crack evolution that cannot be accessed using surface-based diagnostics. Methods such as digital image correlation and optical spectroscopy measure surface deformation but implicitly assume a straight and uniform crack front, an assumption that becomes invalid for interfacial fracture with wide crack openings and asymmetric propagation. In this work, terahertz time-domain spectroscopy (THz-TDS) is combined with double-cantilever beam testing to directly map subsurface crack-front geometry in opaque adhesive joints. A strontium titanate-doped epoxy is used to enhance dielectric contrast. Multilayer refractive index extraction, pulse deconvolution, and diffusion-based image enhancement are employed to separate overlapping terahertz echoes and reconstruct two-dimensional delay maps of interfacial separation. The measured crack geometry is coupled with load–displacement data and augmented beam theory to compute spatially averaged stresses and energy release rates. The measurements resolve crack openings down to approximately 100 μm and reveal pronounced width-wise non-uniform crack advance and crack-front curvature during stable growth. These observations demonstrate that surface-based crack-length measurements can either underpredict or overpredict fracture toughness depending on the measurement location. Fracture toughness values derived from width-averaged subsurface crack fronts agree with J-integral estimates obtained from surface digital image correlation. Signal-to-noise limitations near the crack tip define the primary resolution limit. The results establish THz-TDS as a quantitative tool for subsurface fracture mechanics and provide a framework for physically representative toughness measurements in layered and bonded structures.