Comparison of normal and oblique incidence ultrasonic measurements for the detection of environmental degradation of adhesive joints

Abstract

SummaryNormal-and oblique-incidence ultrasonic scans have been conducted on two-and three-layer adhesive-joint specimens exposed to water at 50 o C for periods of up to 18 months. The joints consisted of aluminium alloy adherend(s) which was subjected to one of four different surface pretreatments prior to being coated (for the two-layer specimens) of bonded (for the three-layer specimens) with an epoxy polymer. Overall, the oblique-incidence technique detected the same types of defect as the normal incidence scans, except when the resolution was too poor to detect the smallest defects. No defect was detected using oblique-incidence scans that could not be detected using normal-incidence scans. In the two-layer specimens, two main types of defects were detected: edge disbonds and micro-defects. Micro-defects were detected in regions remote from the edges and these small-scale, isolated defects appeared took several forms. In the case of the two-layer specimens, the detection of micro-defects was clearly a strong indicator that the interphase of the specimen had been attacked and weakened by the ingressing water molecules and, indeed, the number of such defects correlated to the loss of interfacial toughness. In the case of the three-layer specimens, the extent of edge disbonding was much lower than compared to the corresponding twolayer specimens. Also, no micro-defects were detected ultrasonically in any of the three-layer specimens. However, examination of some of the failure surfaces from the three-layer specimens suggested that some micro-defects may have been present but that they were too small to be detected at the spatial resolution of the ultrasonic scans. Therefore, for the three-layer joint specimens, there were no indicators from the ultrasonic scans which could reveal whether the joint had suffered attack and weakening of the interphase regions by the ingressing moisture. Finally, for all the tests conducted it was very noteworthy that there was no evidence of a gradual change in either the normal-or oblique-incidence reflection coefficient moving into an apparently wellbonded region from either the edge disbonds or the micro-defects, when present.

Nondestructive Evaluation of Strain-Induced Phase Transformation and Damage Accumulation in Austenitic Stainless Steel Subjected to Cyclic Loading

“…A schematic diagram of the experimental setup used to measure the acoustic nonlinearity of an oblique incidence measurement technique is shown in Figure 1. The oblique incidence technique has a practical utility for the structural facilities of mechanical plants due to how easily accessible the structure surfaces under investigation are [21]. The amplified high-voltage signal passes through a 6-dB attenuator (RA-31) to decrease the driving voltage to less than 60 Vpp and 50 Ω termination, which impedes a transient electric current from the mismatch of electrical impedance between the amplifier and the piezoelectric transducer.…”

Cluster-enhanced sparse approximation of overlapping ultrasonic echoes

“…once the signals indexes Ω k , which belong to the kth cluster, have been obtained by (2), they are removed from Γ (i.e., Γ = Γ − Ω k ). We continue to identify new clusters by repeating the process described above, until all signals are associated to one of the clusters (i.e., Γ = Ø).…”

“…2) The ε parameter in (2). This parameter should account for the discrepancy between the approximation Dβ to the noiseless ultrasonic signal and for the amount of noise present in the measured signal.…”

Pulse-echo NDT of adhesively bonded joints in automotive assemblies

“…Many publications have been devoted to the problem of ultrasonic wave propagation in layered media which include metaladhesive-metal structures [8][9][10][15][16][17][18]. Nevertheless, it is perhaps helpful to consider a simple model ( Fig.…”

“…To visualize defects in adhesive joints with high spatial resolution, high frequency focused systems have been developed using a pulse-echo technique [6,8]. Here the variation of the reflection coefficient at the first metal/adhesive interface (closest to the transducer) that results from disbonds is strong enough to obtain sufficient contrast in resulting acoustical images.…”