A preliminary investigation of adhesive discontinuity detection in glulam beams through stress wave time-of-flight (ToF) measurement

This study investigated the effectiveness of the stress wave non-destructive testing (NDT) method in detecting adhesive discontinuities in glue-laminated timber (glulam). The research measured stress wave time-of-flight (ToF) responses in southern yellow pine (SYP) lumber, loblolly pine (Pinus taeda L.) and in the corresponding glulam beams. The glulam specimens were assembled from nominal 2 × 4 lumber bonded with one-component polyurethane (PUR) adhesive. To simulate delamination, adhesive discontinuities were intentionally introduced at selected glue lines and locations, producing beams with 0, 1, 2 and 3 flaws distributed along the length and through the thickness. Beams were fabricated in two lengths—2.44 m (8 ft) and 3.66 m (12 ft)— to accommodate two discontinuity lengths, 305 mm (12 in.) and 610 mm (24 in.), respectively. Stress wave ToF measurements were conducted in three directions: longitudinal, width-wise and thickness-wise. Among these, only thickness-wise measurements across the glue lines consistently identified the location and the intensity of the introduced adhesive flaws. ToF variations in the thickness-wise direction were consistent with regions lacking adhesive, demonstrating the directional sensitivity of the ToF method in detecting glue line discontinuities irrespective of beam and discontinuity length.