Ultrasonic Inspection of Thick GFRP-Steel Hybrid Structures

Abstract

Thick hybrid-composite structures combining steel and glass-fiber-reinforced polymer (GFRP) are used in some offshore applications. Reliable inspection methods are essential to detect potential defects in these structures, such as debonding. This thesis investigates the applicability of ultrasonic guided and bulk waves for detecting such defects in representative steel-GFRP samples.

Guided waves were first evaluated on a constant-thickness sample by comparing pristine and debonded conditions. However, due to the large thickness and narrow geometry, multiple wave modes and side reflections prevented clear identification of defect-sensitive wave packets, limiting the suitability of guided waves.

The focus then shifted to bulk waves, tested in an oblique incidence pitch-catch configuration. Two metrics, signal envelope energy and instantaneous phase of the reflected longitudinal wave, were assessed for their ability to indicate debonding. In constant-thickness samples, both metrics reliably detected defects. Application to a tapered sample showed that local thickness variations affect wave propagation and metrics, complicating direct comparison across locations.
Nevertheless, instantaneous phase analysis successfully distinguished a 50 mm debond from surrounding regions, indicating potential for localized defect detection if more measurements are taken to ensure the nominal value can be predicted accurately across the sample.

This study demonstrates the challenges of applying guided waves in thick hybrid structures, proposes an effective bulk-wave strategy for debond detection in constant thickness, and highlights the conditions under which this approach can be extended to more complex geometries.