Ultrasonic Characterization of Interfaces in Composite Bonds

Abstract

ABSTRACT. The inverse determination of imperfect interfaces from reflection spectra of normal and oblique incident ultrasonic waves in adhesive bonds of multidirectional composites is investigated. The oblique measurements are complicated by the highly dispersed nature of oblique wave spectra at frequencies above 3MHz. Different strategies for bond property reconstruction, including a modulation method, are discussed. The relation of measured interfacial spring density to the physico-chemical model of a composite interface described by polymer molecular bonds to emulate loss of molecular strength on an adhesive composite interface is discussed. This potentially relates the interfacial (adhesion) strength (number of bonds at the adhesive substrate interface) to the spring constant (stiffness) area density (flux), which is an ultrasonically measurable parameter.

Hydrogen bonds, interfacial stiffness moduli, and the interlaminar shear strength of carbon fiber-epoxy matrix composites

“…16,19 In addition, the model allows the calculation of normal and transverse fiber-matrix interface stiffness moduli (also called the spring constant flux) K N and K T , respectively, that not only are related directly to the adhesive strength but also can be measured nondestructively using ABUS. [11][12][13][14] The theoretical calculations and experimental measurements of K T for moderate strength epoxies agree to within roughly 10-20 percent over the entire range of disbonds from nearly fully bonded to fully disbonded specimens, corresponding to a range of K T and of absolute bond strength of more than three orders of magnitude. [11][12][13][14][15] The very good agreement between the H-bond model and the experimental measurements for epoxy-bonded monolithic aluminum plates establishes the scientific underpinning for the quantitative nondestructive evaluation of the adhesive strength of such materials using ABUS.…”

“…[11][12][13][14] The theoretical calculations and experimental measurements of K T for moderate strength epoxies agree to within roughly 10-20 percent over the entire range of disbonds from nearly fully bonded to fully disbonded specimens, corresponding to a range of K T and of absolute bond strength of more than three orders of magnitude. [11][12][13][14][15] The very good agreement between the H-bond model and the experimental measurements for epoxy-bonded monolithic aluminum plates establishes the scientific underpinning for the quantitative nondestructive evaluation of the adhesive strength of such materials using ABUS. We here extend the model, and potential application of ABUS or similar ultrasonic technique, to enable a nondestructive assessment of the contribution to the ILSS of carbon fiber-epoxy composites from chemical bonding at the fiber-matrix interfaces of the composite.…”

“…More significantly for nondestructive evaluation, the constant value of K N /|τ| suggests that it may be possible to quantify the H-bond contribution τ to the ILSS directly from ABUS [11][12][13][14] or other ultrasonic measurement of the fiber-matrix interfacial stiffness modulus K N . Since ultrasonic John H. Cantrell AIP Advances 5, 037125 (2015) displacement amplitudes are typically of the order of fractions of nanometers (i.e., smaller than atomic diameters), ultrasonic measurements are expected to be quite sensitive to the short-range chemical bonding but much less sensitive to friction or mechanical interlocking, which become dominant for much larger displacements.…”

“…We seek to establish a theoretical underpinning for the development of a non-destructive method of assessing chemical bond strength based on previous findings using angle beam ultrasonic spectroscopy (ABUS). [11][12][13][14][15] It has been previously shown 15 bonding of the hydroxyl groups formed on the oxidized aluminum (Al 2 O 3 ) surface with epoxy adhesives. H-bonds are the primary source of adhesive strength for epoxies bonded to smooth aluminum surfaces.…”

Bondline Integrity Monitoring of Adhesively Bonded Structures via an Electromechanical Impedance Based Approach

“…Many factors may lead to kissing bonds, including surface contaminates, adhesive chemistry, inappropriate curing stress, residual stress, moisture ingress, etc. As a result, kissing bonds can appear in a local fashion and the only way to detect them is to measure the local adhesive during fabrication and in service to track its degradation [5,[9][10][11][12][13][14][15][16][17]. Kissing bonds are the most critical and challenging defect to be detected in bonded joints and significantly influence the confidence of bondline integrity after the joints are placed in service.…”