Michael J. Rich

Electrochemical Sensors for Nondestructive Evaluation of Adhesive Bonds

Abstract An in-situ corrosion sensor based on electrochemical impedance spectroscopy (EIS) has been used to detect moisture ingress into aluminum-aluminum and aluminum-composite adhesive bonds. Both wedge tests and tensile button tests (aluminum-aluminum bonds only) were performed. Upon moisture absorption, the impedance spectra change shape with the low-frequency region becoming resistive. The low-frequency impedance decreases by several orders of magnitude, depending on the adhesive and the experimental conditions. For bonds with stable interfaces, such as phosphoric acid anodized (PAA) aluminum, the absorbed moisture causes an initial weakening of the adhesive resulting in reduced strength or small crack propagation. A substantial incubation time prior to substrate hydration and bond degradation allows warning of potential joint deterioration and enables condition-based maintenance. For bonds with smooth interfaces with little or no physical bonding (mechanical interlocking), crack propagation can proceed interfacially with minimal moisture absorption. A comparison of the incubation times for Forest Products Laboratory (FPL, or sulfuric acid-sodium dichromate) etched surfaces, both bonded to epoxy adhesives and freely exposed to water or humidity at different temperatures, shows that hydration occurs with the same activation energy and, hence, the same mechanism, independent of whether or not the surface is covered with adhesive. However, the pre-exponential factor in the rate constant is dependent on the concentration of free moisture at the interface so that the hydration rate varies by several orders of magnitude.

Inspection of composite and metal adhesive bonds with an electrochemical sensor

An in-situ sensor, based on Electrochemical Impedance Spectroscopy (EIS), has been used to monitor the health of adhesive bonds constructed from various combinations of aluminum, graphite/epoxy, glass/epoxy, glass/polyester, and glass/vinylester composites and exposed to high humidity and temperature conditions. Modeling of the EIS data as an electric circuit demonstrated that several circuit parameters of the impedance spectra were sensitive to bond performance, as determined by wedge tests and lap shear tests. Moisture absorption by the adhesive and composite was calculated from the circuit capacitance, which was also a function of bonded area and bondline thickness (bondline + composite thickness for glass composites). Material differences, including saturation level of moisture, rate of absorption, and bondline stability, were readily seen among the various materials sets. The sensor electrodes are attached to opposite sides of a bond after fabrication, i.e. they are not embedded. Thus, they are suitable for monitoring existing bonded structures. They have the potential to identify bondlines that are in the early stages of degradation, prior to significant loss of bond strength. As an input to a condition-based maintenance system, they would identify weakening bondlines and allow preventative action to be scheduled and performed.

The Impact of Circulating Cholesterol Crystals on Vasomotor Function: Implications for No-Reflow Phenomenon

OBJECTIVES The purpose of this study was to determine if cholesterol crystals can injure the endothelial surface by their jagged edges altering vasoreactivity and contributing to no-reflow after intervention. BACKGROUND After plaque rupture, cholesterol crystals are released into the circulation and flow downstream contacting the arterial wall. METHODS Both carotid arteries from 22 rabbits were placed in a dual perfusion chamber and challenged with norepinephrine followed by acetylcholine and nitroprusside. Arterial diameters were measured before and after exposure to cholesterol crystals or microspheres and compared with diameters of normal control arteries. Arteries were examined by light, confocal, atomic force and scanning electron microscopy. RESULTS Pre-exposure mean arterial diameter was 2.33 ± 0.27 mm. With baseline norepinephrine there was vasoconstriction of 0.82 ± 0.19 mm, 0.79 ± 0.18 mm, and 0.83 ± 0.16 mm in lumen diameter in the crystal, microsphere, and control groups, respectively. After cholesterol crystals or microspheres, vasoconstriction was significantly less for cholesterol crystals but not for microspheres (0.71 ± 0.28 mm and 0.81 ± 0.15 mm; p < 0.02 and p = 0.68). After acetylcholine in the same artery, there was significantly less dilation before versus after crystals (0.49 ± 0.24 mm vs. 0.38 ± 0.22 mm, p = 0.04) but not with microspheres or in the control group. There was no significant difference after nitroprusside in any group, suggesting endothelial injury. By scanning electron microscopy, cholesterol crystals were found embedded in the intima with endothelial cell tears whereas the microsphere treatment and control groups had minimal or no injury (93% vs. 31% vs. 14%; p < 0.01). By atomic force microscopy, surface roughness was significantly greater with cholesterol crystals compared with microspheres or in control arteries (p < 0.05). CONCLUSIONS Cholesterol crystals damaged the endothelium and reduced vasodilator response, potentially aggravating myocardial ischemia after interventions.

Sulfonation of UHMW-PE fibers for adhesion promotion in epoxy polymers

This paper reports the effects of a controlled sulfonation process using sulfur trioxide in Freon® to treat UHMW-PE fibers in order to improve their adhesion to a thermosetting polymer. Following sulfonation treatment, PE fibers were evaluated by both chemical and physical methods to determine the effectiveness of treatment. Infrared spectroscopy, X-ray photoelectron spectroscopy, titration, and contact angle measurements were used to investigate the chemical pathway of sulfonation on UHMW-PE fibers. The physical effects of sulfonation on fiber topography and strength were investigated using scanning electron microscopy and conducting tensile strength measurements on treated and virgin samples. The effect of sulfonation on adhesion was measured using micromechanical test methods to quantify the level of bonding between sulfonated UHMW-PE fibers and a thermosetting matrix. For comparative purposes, the sulfonated UHMW-PE fibers were compared to plasma and corona treatments. The interfacial shear strength demonstrated an increase in adhesion with longer immersion times in the sulfonating solution which may be an effect of higher levels of mechanical interlocking caused by fiber pitting and etching.

Frequency of Cholesterol Crystals in Culprit Coronary Artery Aspirate During Acute Myocardial Infarction and Their Relation to Inflammation and Myocardial Injury

Cholesterol crystals (CCs) have been associated with plaque rupture through mechanical injury and inflammation. This study evaluated the presence of CCs during acute myocardial infarction (AMI) and associated myocardial injury, inflammation, and arterial blood flow before and after percutaneous coronary intervention. Patients presenting with AMI (n = 286) had aspiration of culprit coronary artery obstruction. Aspirates were evaluated for crystal content, size, composition, and morphology by scanning electron microscopy, crystallography, and infrared spectroscopy. These were correlated with inflammatory biomarkers, cardiac enzymes, % coronary stenosis, and Thrombolysis in Myocardial Infarction (TIMI) blush and flow grades. Crystals were detected in 254 patients (89%) and confirmed to be cholesterol by spectroscopy. Of 286 patients 240 (84%) had CCs compacted into clusters that were large enough to be measured and analyzed. Moderate to extensive CC content was present in 172 cases (60%). Totally occluded arteries had significantly larger CC clusters than partially occluded arteries (p <0.05). Patients with CC cluster area >12,000 µm2 had significantly elevated interleukin-1 beta (IL-1β) levels (p <0.01), were less likely to have TIMI blush grade of 3 (p <0.01), and more likely to have TIMI flow grade of 1 (p <0.01). Patients with recurrent AMI had smaller CC cluster area (p <0.04), lower troponin (p <0.02), and IL-1β levels (p <0.04). Women had smaller CC clusters (p <0.04). Macrophages in the aspirates were found to be attached to CCs. Coronary artery aspirates had extensive deposits of CCs during AMI. In conclusion, presence of large CC clusters was associated with increased inflammation (IL-1β), increased arterial narrowing, and diminished reflow following percutaneous coronary intervention.

Electrochemical impedance spectroscopy inspection of composite adhesive joints

Lap shear joints comprised of aluminum-aluminum, aluminum-glass fiber-reinforced polymer (GFRP), aluminum-carbon fiber-reinforced polymer (CFRP), CFRP-CFRP, CFRP-GFRP and GFRP-GFRP, were exposed to 95% relative humidity (RH) at 50°C, 70°C and 90°C, to salt fog and to a 90°C/95%RH freeze cycle. Electrochemical impedance spectroscopy (EIS) spectra were taken across the whole bonded assembly using an EIS corrosion sensor. Periodically, some specimens were pulled to obtain bond strength as a function of exposure. As expected, the higher the temperature, the faster the bond degradation. The low-frequency impedance correlated with bond strength of the humidity-exposed specimens and showed the same Arrhenius dependence, suggesting that moisture absorption by the adhesive was the limiting factor in bond performance and that EIS has the potential to nondestructively track bond health and warn of deterioration.