The Correlation of Non-Destructive Measurements and Toughness Changes in Adhesive Joints during Environmental Attack

Abstract

Normal-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 adherends which were subjected to one of four different surface pretreatments prior to being coated (for the two-layer specimens) or bonded (for the three-layer specimens) with an epoxy polymer. The four commonly used pretreatments which were investigated were a grit-blast, a chromic-acid etch (CAE), a chromic-acid anodise (CAA) and a phosphoric-acid anodise (PAA). Techniques have been developed to measure the fracture toughness, G c , of the specimens before and after water exposure such that fracture toughness maps could be ascertained, where the measured values of G c may be assigned to spatially discrete positions within the specimens. The relative performance of the different pretreatments used in the present work followed that expected from the literature: the chromic-acid anodised (CAA) surface pretreatment giving the most durable two-and three-layer specimens and the grit-blasting pretreatment giving the least durable. In the two-layer specimens, the ultrasonic inspections detected two main types of defects: corrosion-driven edge-disbonds and micro-defects. The edge-disbonding mechanism usually started at an unsealed flush edge, and was initiated by a region of corrosion which developed on the edge of the specimen and which undercut the epoxy layer. Edge-disbonds were easily and accurately detected ultrasonically. Micro-defects were detected in regions remote from the edges and these small-scale, isolated defects took several forms. In the case of the three-layer joint specimens, only edge-disbonds could be detected ultrasonically. Nevertheless, in some of the three-layer joints which were attacked and weakened by ingressing water, the failure surfaces suggested that micro-defects were present. For both the two-and the three-layer specimens, the results from the ultrasonic scans have been correlated with the values of the fracture toughness of the specimens, before and after water exposure. Whilst the ultrasonic scans detected the presence of micro-defects in the two-layer specimens, which appeared to correlate with the extent of interphase toughness loss upon water exposure, the scans clearly failed to detect any changes in the interphase regions which would indicate the general loss of interphase toughness seen with the three-layer grit-blast and PAA specimens.

Assessment of Adhesive Bond Strength Using the Ultrasonic Technique

“…Poor adhesion is a reduction in the strength of the bond between the adhesive and the adherend. This is extremely difficult to detect using conventional ultrasonic techniques [21]. Imperfections along the bond line can significantly degrade the performance of the adhesive bond joint.…”

Non-linear methods based on ultrasonic waves to analyse disbonds in single lap joints

“…However, conventional ultrasonic techniques are not so effective at detecting poor adhesion between the adhesive and the substrate as the thickness of this thin interfacial layer could be orders of magnitude less than the wave length used. 6 Numerous scientific literature sources evaluated in Ehrhart et al., 7 clearly confirm that adhesive bond quality is a critical parameter that cannot yet be assessed by any NDT technique. Several conventional methods are suitable and have been improved for better detection capability but, to this day, no method can reliably and with a good reproducibility detect any weak adhesive bond, either on metal or composite substrates.…”

“…However, conventional ultrasonic techniques are not so effective at detecting poor adhesion between the adhesive and the substrate as the thickness of this thin interfacial layer could be orders of magnitude less than the wave length used [6]. Numerous scientific literature sources evaluated in [7] clearly confirm that adhesive bond quality is a critical parameter that cannot yet be assessed by any NDT technique.…”

Simple model to estimate adhesion of structural bonding during humid ageing

“…This reduction in strength can be associated with the presence of water near, or at, the interface between substrate(s) and adhesive, leading to (at least apparently) adhesive separation, or with physico-chemical degradation of one of the components, often, though not always, the adhesive, thus causing cohesive failure [2][3][4][5]. Both aspects are clearly of considerable importance in the context of long-term durability, although problems in the interfacial region, or interphase, tend to dominate the literature.…”