Adhesive-adherend Interface Research Articles

Ultrasonic Study of Environmental Damage Initiation and Evolution in Adhesive Joints

This article reports on an experimental study of environmental degradation of adhesive joints by an ultrasonic angle-beam technique. The technique is based on measurements of the frequency response of the reflection of obliquely incident ultrasonic signals from a joint bondline. Ultrasonic measurements were performed using a special ultrasonic goniometer with only one ultrasonic transducer. By this method, the degradation of single lap adhesive joints was studied as a function of exposure in NaCl solutions at 68°C under static tensile load. It was found that joint degradation is accompanied by a shift of the ultrasonic reflection spectrum minimum to a lower frequency. Two stages of adhesive joint environmental degradation can be distinguished: a) a relatively slow adhesive joint degradation dominated by adhesive creep, and b) delamination along the adhesive/adherend interface, leading to failure. Several degradation mechanisms are found in the first stage to affect the position of the spectral minimum. The first is adhesive creep caused by normal-to-bond-plane stress concentration at the joint overlap edges. This mechanism is found to have the dominant effect on the ultrasonic signature. Second, changes of the effective density and elastic moduli of the adhesive layer also affect the spectrum of the reflected signal. The third mechanism is the degradation of the adhesive-adherend interface. In the second stage of the joint degradation process, delamination along the adhesive/adherend interface occurs and is followed by joint failure. While the time span of the first stage changes significantly from joint to joint, the time span of the second stage (failure by delamination) in our conditions is about 30–40 h. The delamination results in a significant additional spectral minimum shift to a lower frequency that can be used as an indicator of failure initiation.

The characterization of oxide layers in adhesive joints using ultrasonic reflection measurements

The use of ultrasonic reflection coefficient measurements at both normal and oblique incidence for the detection and characterization of oxide layers at the adhesive-adherend interface in adhesive joints has been investigated. Tests have been carried out on aluminium alloy-oxide-epoxy specimens with a minimal (< 0.1 $\mu $m) thickness oxide layer, 43 and 15 $\mu $m thick sulphuric acid anodized oxide oxide layers and an industry standard, 3.5 $\mu $m thick chromic acid anodized oxide layer. A superpure aluminium-90 $\mu $m thick oxalic acid anodized layer-water system has also been investigated. The results indicate that the cell wall density of the sulphuric acid anodized oxide layer is only 1990 kg m$^{-3}$, compared to 2850 kg m$^{-3}$ for the oxalic acid anodized layer. The cell wall density and Young's modulus are obtained from normal incidence measurements. Predictions of the oblique incidence reflection coefficients using the effective elastic constants of the oxide layer obtained from a standard porosity model assuming these cell wall properties and the porosity measured from TEM images showed good agreement with the measurements made on both the sulphuric acid anodized and oxalic acid anodized specimens, only small adjustments to the properties being required to obtain a very good fit. The frequency range over which it is necessary to take measurements increases as the thickness of the oxide layer decreases. Maximum frequencies of 15 MHz for the oblique incidence measurements and 70 MHz for the normal incidence measurement were adequate to obtain values of $\rho $, C$\_{11}$ and C$\_{33}$ for a 43 $\mu $m thick sulphuric acid anodized layer to an estimated accuracy of $\pm $10% and to set a lower bound on C$_{55}$. In order for a 3.5$\mu $m thick oxide layer with similar properties to be detectable at normal incidence, the frequency would have to be increased to over 200 MHz, while the frequency of the oblique incidence measurements would have to be raised to about 25 MHz if a 5% change could reliably be detected, or 50 MHz if a 10% change is necessary in order to ensure detection.

Ultrasonic feature-based classification of the interfacial condition in composite adhesive joints: Research in Nondestructive evaluation, Vol. 2, No. 4, pp. 207–224 (1990)

The sensitivity of ultrasonic features to the condition of the adherend-adhesive interface in composite joints was studied. The inspected specimens represent different bonding characteristics due to the variety of surface pretreatments to the composite adherends. Each signal obtained from ultrasonic inspection of a specimen was characterized by a feature vector, consisting of features derived from both the time and frequency domain representations of the signal. Some of the features were found to be significantly sensitive to the interfacial characteristics of the joints. The sensitivity was verified by means of conventional statistical tests. The sensitivity of the features enabled the application of basic pattern recognition techniques for the classification of the joints according to the surface preparation of the adherends.

Evaluation of interfacial properties in adhesive joints of aluminum alloys using angle-beam ultrasonic spectroscopy

This paper reports an ultrasonic angle-beam technique for the evaluation of interfacial properties of adhesive joints. The technique is based on measurement of the frequency response of obliquely incident ultrasonic signals from an adherend-adhesive interface. A theoretical model was developed for analysis of the interaction between the obliquely incident ultrasonic waves and multilayered adhesive joints. A special ultrasonic goniometer using only one ultrasonic transducer was built to measure the reflected signals. Samples with controlled interfacial properties were prepared for experimental determination of these properties. The experimental results were in good agreement with those predicted from the model. It was found that some of the minima loci in the reflected frequency spectra are independent of either the thickness or the elastic properties of the interfacial layer. This enables us to develop a simple and stable reconstruction procedure to determine the thickness and interfacial properties from ...

Ultrasonic detection of kissing bonds at adhesive interfaces

Intimate mechanical contact between the adhesive and adherend counterparts can easily occur without an actual bond. Such apparently flawless, but severely defective 'kissing' bonds are especially dangerous because they often remain hidden from most types of conventional ultrasonic inspection. A special high-frequency, high-inspection angle ultrasonic technique is suggested to improve the detectability of such kissing bonds. It is shown to be essential to interrogate the adhesive-adherend interface from the adherend side without sending the ultrasound through the inherently inhomogeneous and highly attenuating adhesive layer.

Ultrasonic feature-based classification of the interfacial condition in composite adhesive joints

The sensitivity of ultrasonic features to the condition of the adherend-adhesive interface in composite joints was studied. The inspected specimens represent different bonding characteristics due to the variety of surface pretreatments to the composite adherends. Each signal obtained from ultrasonic inspection of a specimen was characterized by a feature vector, consisting of features derived from both the time and frequency domain representations of the signal. Some of the features were found to be significantly sensitive to the interfacial characteristics of the joints. The sensitivity was verified by means of conventional statistical tests. The sensitivity of the features enabled the application of basic pattern recognition techniques for the classification of the joints according to the surface preparation of the adherends.

Interfacial chemistry of corrosion-induced bond degradation for epoxy/dicyandiamide adhesive bonded to cold-rolled and galvanized steels

The interfacial chemistry of a conventional epoxy/dicyandiamide adhesive formulation applied to cold-rolled and galvanized steels has been studied by X-ray photoelectron spectroscopy (XPS). In both cases, the interfacial corrosion process appears to be dominated by attack on the metal substrate, and is reminiscent of crevice corrosion phenomena. On cold-rolled steel, analysis of the interfacial bond failure surfaces suggests that the contamination layer initially present on the substrate is not disturbed during bond formation. Bond failure appears to occur between the substrate and the contamination layer; the contamination layer remains largely intact on the interfacial surface of the adhesive after bond failure. On galvanized steel, the locus of failure appears to be at the adhesive-adherend interface, within a layer of zinc corrosion products. The contamination layer appears to have been partially displaced by the adhesive during bonding.

Ultrasonic Feature-Based Classification of the Interfacial Condition in Composite Adhesive Joints

Abstract The sensitivity of ultrasonic features to the condition of the adherend-adhesive interface in composite joints was studied. The inspected specimens represent different bonding characteristics due to the variety of surface pretreatments to the composite adherends. Each signal obtained from ultrasonic inspection of a specimen was characterized by a feature vector, consisting of features derived from both the time and frequency domain representations of the signal. Some of the features were found to be significantly sensitive to the interfacial characteristics of the joints. The sensitivity was verified by means of conventional statistical tests. The sensitivity of the features enabled the application of basic pattern recognition techniques for the classification of the joints according to the surface preparation of the adherends.

Surface Energetics and Adhesion

Abstract The relationships between surface energetics and adhesion are critically reviewed. New data that confirm such relationships, for peel tests as well as lap shear tests, are presented. The effect of hydrothermal aging of aluminum surfaces on surface energetics can be used to predict degradation in bond strength. The mechanism of failure for elastic adhesives (such as Scotch ® tape) in peel tests may be essentially the same as for more brittle adhesives (such as epoxies) in lap shear tests. This mechanism may involve brittle fracture that forms a critical flaw at the adherend-adhesive interface (on a microscopic level), followed by crack propagation which then may include considerable elastic and plastic deformation. The locus of propagation (fractography) is generally not (but may be) relevant to the problem of how to remedy mechanical weakness in an adhesive joint, since the local region of critical flaw formation rather than the general surface area determines the joint strength.

The Strength of Adhesive Joints

Abstract The main rules pertaining to the strength of adhesive joints are: (1) This strength is a mechanical (or rheological) property. The local stress which causes the extension of a pre-existing crack can be determined only if the stress pattern in the whole adhint is known and the intensification of stress at flaws is taken into account. (2) The rupture occurs in a material, not between two materials. Consequently, the molecular forces across the adhesive-adherend interface are irrelevant, and the “adhesion tension” does not determine the adhint strength.