Detection of Disbonds in Multi-layer Structures by Laser-Based Ultrasonic Technique

Adhesively bonded multi-layer structures are frequently used, mostly in the aerospace industry, for their structural efficiency. Nondestructive evaluation of bond integrity in these types of structures, both after manufacturing and for periodic inspection during service, is extremely important.

A laser-based ultrasonic technique has been evaluated for non-contact detection of disbonds in aluminum multi-layer structures. Two configurations have been used to detect disbonded areas: pitch-catch with unidirectional guided wave scan and through-transmission with bidirectional scan. Guided wave scanning was done with a laser line source and air-coupled transducer sensing at 500 kHz, 1 MHz, and 2 MHz. Signals showed attenuation of the main frequency component and frequency shift on disbonded areas, whereas, a regular and standard waveform is seen outside disbonds. In through-transmission the longitudinal wave at normal incidence was monitored with a 1 MHz probe. One sample showed, besides the introduced inserts, other disbonded areas. After the ultrasonic measurements the sample was cut to visually check adhesive and interfaces.

The guided wave pitch-catch scan allowed fast inspection and quick indication of disbonded zones, while the through-transmission C-Scan provided better definition of defects but was slower and required access from both sides of the test part.     

Finite element analysis and experimental investigation of ultrasonic testing of internal defects in SiCp/Al composites

Revealing the interactions of sound waves with both SiC particles and internal defects is crucial for facilitating the detectability of internal defect features in SiCp/Al by using ultrasonic testing (UT). In the present work, we demonstrate the feasibility of UT of internal flat-bottom holes with diameters ranging from 0.2 mm to 2 mm in SiCp/Al composites through the combination of finite element (FE) simulations and experiments. Specially, a 2D FE model of UT of SiCp/Al with consistent geometrical features of SiC particles with experimental one is established, the accuracy of which is validated by theoretical and experimental characterizations of P-wave velocity and ultrasonic attenuation coefficient of SiCp/Al. Subsequently, the propagation behavior of sound waves in the SiCp/Al specimen with pre-existing defects under UT, in particular the impact of defect boundary on the scattering behavior of sound waves, is revealed in detail by FE simulations and also validated by corresponding experiments. Furthermore, the UT limit of detectable size of the internal defects is revealed jointly by FE simulations and experiments, based on which a correlation map between defect size and echo signal amplitude is established. Current study provides theoretical and practical guidance for the UT of internal defects in SiCp/Al composites.     

Effect of the Etching Duration and Ultrasonic Cleaning on Microtensile Bond Strength Between Feldspathic Ceramic and Resin Cement

This study assessed the effect of different etching durations of feldspathic ceramic with hydrofluoric acid (HF) and ultrasonic cleaning of the etched ceramic surface on the microtensile bond strength stability of resin to a feldspathic ceramic. The research hypotheses investigated were: (1) different etching times would not affect the adhesion resistance and (2) ultrasonic cleaning would improve the adhesion. Ceramic blocks (6 × 6 × 5 mm) (N = 48) were obtained. The cementations surfaces were duplicated in resin composite. The six study groups (n = 8) were: G1—Etching with 10% aqueous HF (30 s) + silane; G2—10% HF (1 min) + silane; G3—10% HF (2 min) + silane; G4—10% HF (30 s) + ultrasonic cleaning (4 min) in distilled water +silane; G5—10% HF (1 min) + ultrasonic cleaning + silane; G6—10% HF (2 min) ultrasonic cleaning + silane. The cemented blocks were sectioned into microbars for the microtensile test. The etching duration did not create significant difference among the groups (p = .156) but significant influence of ultrasonic cleaning was observed (p = .001) (Two-way ANOVA and Tukey's test, p > 0.05). All the groups after ultrasonic cleaning presented higher bond strength (19.38–20.08 MPa) when compared with the groups without ultrasonic cleaning (16.21–17.75 MPa). The bond strength between feldspathic ceramic and resin cement was not affected by different etching durations using HF. Ultrasonic cleaning increased the bond strength between ceramic surface and resin cement, regardless of the etching duration.     

Numerical stress analysis of carbon-fibre-reinforced epoxy composite single-lap joints

A numerical strategy based on a finite element method is developed in order to model the stress distribution in single-lap adhesive joints. The joints were manufactured from unidirectional carbon-fibre-reinforced epoxy composites joined by an epoxy adhesive layer. Experimental parameters are used as a reference to allow for the numerical validation of the proposed analysis. Additionally, joints with different types of defects in the lap region were modelled with both two-dimensional and three-dimensional finite elements. The models include defects that vary in format (straight or circular) and position (centred or dispersed). The influenced spew fillets in the adhesive layer were also examined. Although the computational cost is higher, the results of the three-dimensional model are more compatible with the experimental results than those of the two-dimensional model. The effect of defects in the joints was adequately modelled, and the proposed methodology can be used to accurately assess the integrity of the joints since the defect has been successfully detected.     

Optimization of Ethanol-Ultrasound-Assisted Destabilization of a Cream Recovered from Enzymatic Extraction of Soybean Oil

A novel method using ethanol and ultrasound to extract oil from cream obtained from enzyme-assisted aqueous extraction of soybean oil was developed. To evaluate the relationships between operating variables and free oil yield and to maximize the free oil yield, response surface methodology was introduced in this work. The developed regression model was fitted with R ² = 0.9591. Optimized variables were: ethanol concentration of 73 %, ethanol addition volume of 0.55 L/kg, ultrasound power of 427 W, ultrasound time of 47 s, and ultrasound temperature of 53 °C. The free oil yield from the cream under the above conditions was 92.6 ± 3.4 %. Scanning electron microscopy (SEM) was used to evaluate the effect of ultrasonic treatment on ethanol-treated cream, and the SEM images clearly showed that the ultrasound treatment affected dispersing and fracturing of the microstructure of ethanol-treated cream.     

纤维-金属层板粘接质量的非线性超声评价研究

传统超声检测利用超声波传播中遇到缺陷时波的反射、散射等特征进行缺陷检测和评价,实际上表征的是缺陷和周围介质的声阻抗差别。当超声波在材料中传播时,位错等微小缺陷会与其发生非线性相互作用,非线性超声检测就是利用这些非线性响应信号进行材料性能的评估和微小缺陷的检测,本质上反映的是微小缺陷对材料非线性的影响。文中从非线性超声理论出发,提出运用2阶和3阶非线性系数进行纤维-金属粘接层的评价,实验结果表明非线性系数可以表征粘接缺陷。     

The Impacts of Heat Treatment on Lap Joint Shear Strength of Black Pine Wood

This study was conducted to determine the impacts of heat treatment on lap shear strength, density, and mass loss of black pine wood. In the study, black pine wood boards bonded with polyurethane were subjected to temperatures of 160, 180, and 200°C for durations of 2 and 6 hours. Specimens having two layers were prepared from untreated and treated wood for mechanical testing of bond lines. Data were analyzed using variance analysis and Tukey's test to determine the impacts of changes in density and mass of heat-treated black pine wood on lap shear strength. The results indicated that the lap shear strength of black pine wood decreased as the intensity of heat treatment increased. The results also indicated that the minimum and maximum percentage decreases of lap shear strength were approximately 27% for 160°C and 2 hours and 78% for 200°C and 6 hours.     

Improving the strength of adhesively bonded joints through the introduction of various surface treatments

The aim of this paper is to model an interface adhesion and failure mechanism of single lap joints, subjected to tensile loading, focusing on the effects of various surface treatments, including surface characterization parameters, such as surface roughness and contact angle of adherend surfaces. The applied surface treatments are sandblasting, etching, anodic oxidation and hybrid processes. The influence of surface treatment techniques and conditions on single lap joint strength and interfacial properties is investigated by performing a static tensile test. A numerical approach, which is a cohesive zone model, is implemented using ABAQUS? and introduced to create a correlation between maximum interface traction and surface processing parameters, such as surface roughness and work of adhesion. As a result of experiments, an etching plus sanding process was found to provide the best single lap joint performance (8726 N), having surface roughness of Ra = 2.93 μm and work of adhesion, W_a = 119.4 mJ/m². Based on numerical solutions, a correlation between maximum interface traction and type of surface treatment process has been established, taking certain assumptions into consideration.     

The effect of curing temperature on thermal,physical and mechanical characteristics of two types of adhesives for aerospace structures

The effects of cure temperatures on the thermal, physical and mechanical characteristics of two types of thermosetting structural epoxy film adhesives were determined in detail. The aim of this paper is to assess the effect of cure temperatures (82–121 °C) on the degree of cure of the two adhesives and the relevant void formations that need to be addressed in bonded part production and repair. Two thermal parameters were used to characterize the advancement of the reaction, such as degree of cure and glass transition temperature. The joint properties with respect to the cure temperatures were characterized by void content and bond-line thickness measurements and lap shear strength tests. Experimental results presented that all lap shear strengths were well within minimum shear strength (29 MPa) required by the specification of the film-type adhesive. However, the lap shear strength testing after aging at 82 °C and 95%R.H for 1000 h showed that the improved durability when the adhesive is cured at 121 °C did not occur for the 82 °C cure. Low curing conversion (75–77% degree of cure) combined with high voids (over 2 areal%) has a catastrophic effect on the bonding qualities at the metal-adhesive interface and due to lack of cohesion in the adhesive. The changes in the interface caused by the low temperature curing may contribute to an increased susceptibility of the bonded joint to moisture and consequent bond-line degradation.     

Real‐time monitoring of injection molding for microfluidic devices using ultrasound

Real‐time process monitoring of the fabrication process of microfluidic devices using a polymer injection molding machine was carried out using miniature ultrasonic probes. A thick piezoelectric lead‐zirconate‐titanate film as an ultrasonic transducer (UT) was fabricated onto one end of a 4‐mm diameter and 12‐mm long steel buffer rods using a sol gel spray technique. The center frequency and 6 dB bandwidth of this UT were 17 MHz and 14 MHz, respectively. A signal‐to‐noise ratio of more than 30 dB for ultrasonic signals reflected at the probing end was achieved. The probe can operate continuously at 200°C without ultrasonic couplant and cooling. Clear ultrasonic signals were obtained during injection molding of a 1‐mm‐thick part having test patterns on its surface. Shrinkage of the molded part and part detachment from the mold were successfully monitored. Surface imperfections of the molded parts due to a lack of the sufficient holding pressure is discussed with regard to the ultrasonic velocity obtained. The presented ultrasonic probes and technique enable on‐line quality control of the molded part by optimizing the holding pressure and improvement of process efficiency by reducing the cycle time. POLYM. ENG. SCI., 45:606–612, 2005. © 2005 Society of Plastics Engineers     

Annealing Effect on Microhardness and Elastic Constants of PMMA

Abstract

The ultrasonic velocities of both longitudinal and shear waves were measured in thermoplastic discs of poly methyl methacrylate (PMMA) as a function of annealing temperatures ranging from 23°C to 95°C (23°C, 45°C, 60°C, 75°C, 80°C, and 95°C). Ultrasonic velocity measurements were taken at 2 MHz ultrasonic frequency using the pulse echo method. From the experimental data, the longitudinal modulus, rigidity, and Young's modulus are derived. Effect of annealing on microhardness of PMMA was studied using ultrasonic pulse echo method as well as mechanical measurements. The critical stress intensity factor was also determined at the same annealing conditions. In all events, the microhardness and the elastic constant are found to be changed at annealed temperature named 80°C.     

Development of Polyaniline/Epoxy Composite as a Prospective Solder Replacement Material

In this paper we present a novel application of a conducting polymer, polyaniline, as a conductive filler for the development of isotropically conductive adhesives. We have developed isotropically conductive adhesives using protonic acid-doped polyaniline as the conducting filler in an anhydride-cured epoxy system. Fundamental material characterization like DSC, TGA and SEM of the samples was conducted to study their properties. Conductivity of these materials was measured by the four probe method while impact properties were studied by lap shear and drop tests. Samples were aged at 85°C/～100% RH for more than 500 h and the effects of aging were studied. Conductivity value of 10^?3 S/cm was obtained at 25% PANI filler concentration. These results demonstrate the potential of such systems to function as isotropically conductive adhesives.     

Preparation of epoxy-based insulator and optimization of its thermal property by Taguchi robust design method in double base propellant grain application

Taguchi method (orthogonal array, OA₉) was used to design an epoxy insulator by evaluating its glass transition temperature (T _g) for using in a double base (DB) propellant grain. In this design method, three epoxy resins based on diglycidylether bisphenol A (DGEBA), three polyamine curing agents and a DGEBA-based reactive diluent agent were used. The curing process of epoxy resins with polyamines was studied by Fourier transform infrared spectroscopy. The results showed that the curing process was completed at room temperature. The effects of four parameters including resin type, curing agent type, curing agent concentration and diluent quantity were investigated to design a resin formulation with a highest T _g after curing. The obtained results were quantitatively evaluated by the analysis of variance (ANOVA). The results of ANOVA showed that the highest T _g of 86.0 ± 9.0 °C was obtained for the optimum formulation of MANA POX-95 as epoxy resin, H-30 as curing agent and 52 phr H-30. The T _g measured by the experiment was 78.0 ± 0.9 °C. In addition, the single lap shear strength (adhesion strength) of the optimized insulator was measured at 13.66 ± 1.02 MPa. Pull-off test performed on the surface of DB propellant resulted a 1.935 ± 0.003 MPa adhesion strength.     

Strain rate dependence of adhesive joints for the automotive industry at low and high temperatures

In this study the impact and quasi-static mechanical behaviour of single lap joints (SLJ) using a new crash resistant epoxy adhesive has been characterized as a function of temperature. Single lap adhesive joints were tested using a drop weight impact machine (impact tests) and using a universal test machine. Induction heating and nitrogen gas cooling was used in order to achieve a homogeneous distribution of temperature along the overlap of + 80 °C and ?20 °C, respectively. Adherends made of mild steel, similar to the steel used in automobile construction, were chosen in order to study the yielding effect on the strength of the SLJ. Results showed that at room temperature (RT) and low temperature (LT), failure was dictated by the adherends due to the high strength of the adhesive. At high temperature (HT), a decrease was found in the maximum load and energy absorbed by the joint due to the reduced strength of the adhesive at this temperature. The results were successfully modelled using the commercially available finite element software Abaqus^®. Good correlation was found between experimental and numerical results, which allows the reduction of experimental testing.     

Optimization of Ultrasound-Assisted Extraction of Artemisinin from Artemisia annua L. by Response Surface Methodology

Artemisinin is a compound extracted from Artemisia annua L. with a remarkable curative effect against malaria. It can be extracted using ultrasound-assisted extraction (UAE) and then detected via HPLC. In this study, response surface methodology (RSM) was used to optimize UAE conditions for obtaining the maximum yield of artemisinin. Three independent variables (ratio of solvent to material, extraction temperature, and ultrasonic power) were evaluated using the Box-Behnken experimental design, with the yield of artemisinin as a response variable. Experimental data were highly fitted to a mathematical-regression model using multiple linear regression (MLR). Based on response surface plots, the three independent variables exhibited interactive effects on the yield of artemisinin. The optimal extraction conditions were as follows: 42.71 mL/g ratio of solvent to material, 41.86°C extraction temperature and 120 W ultrasonic power. The predicted yield of artemisinin by model was 0.7848%, whereas the actual yield in the extracts was 0.7826% ± 0.0790% in adjusted optimal conditions, with a relative error of 0.28%. The results undoubtedly demonstrated that RSM could be used to explore the optimum conditions of artemisinin extraction.     

Effects of Ultrasonic Waves on Vapor-Liquid Equilibrium of an Azeotropic Mixture

Abstract

Azeotropic and extractive distillation techniques used to separate azeotropic mixtures are among the most challenging separation processes in the chemical industry. In this work, an innovative distillation technique which employed ultrasonic waves was proposed to intensify the conventional multi-column azeotropic distillation method into a single-column alternative. The effects of ultrasonic intensity on the vapor-liquid equilibrium (VLE) of methyl-tert-butyl-ether (MTBE)-methanol was investigated at 50, 100, 200, and 250 W/A·cm² and at a fixed frequency of 40 kHz. Studies were also done to examine the effects of ultrasonic frequency on the VLE data at 25 and 68 kHz frequencies. It was found that ultrasonic waves at 50 W/A·cm² intensity and 25 kHz frequency gave the highest relative volatility (α) at 2.654 and completely eliminated the MTBE-methanol azeotrope, thereby allowing highly pure MTBE to be recovered in just a single distillation column. The results revealed that ultrasonic waves had the potential to favorably manipulate α, and hence, the VLE of an azeotropic mixture.     

Highly hydrothermally stable Al-MCM-41 with accessible void defects

Mesoporous aluminosilicates (Al-MCM-41) with high hydrothermal stability were synthesized via self-assembly of nanoseed precursors, obtained from alkali-treatment of ZSM-5. Characterized by N₂ sorption, Transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and Mercury intrusion, as-prepared Al-MCM-41 possessed a large number of void defects, enhancing the connectivity of MCM-41. The effects of alkali-treatment degree, including time and concentration, on volume of void defects were investigated and discussed. It is revealed that volume of void defects decreased in the severe condition of alkali-treatment, and void defects representing the intraparticular pores account for a volume of 0.138 cm³/g in Al-MCM-41, prepared under the condition of 1.0 mol/L of NaOH and 1 h of stirring time. A tentative proposed mechanism for interpreting the formation of void defects was presented. Aggregated secondary building units in the precursors not only provided Si (Al) sources, but also functioned as templates for the development of void defects. Al-MCM-41 with void defects would be beneficial to diffusion and mass transportation.     

Exploring point defects and trap states in undoped SrTiO3 single crystals

The defect chemistry and electronic trapping energies in undoped single crystalline SrTiO₃ were examined by electrochemical impedance spectroscopy at low (25–160 °C) and intermediate (500–700 °C) temperatures. Electronic and ionic conductivity as well as chemical capacitance values were obtained with a transmission line equivalent circuit. Impedance spectroscopy at low temperatures was used to quantify trapping energies of main ionic defects. Particularly the chemical capacitance is shown to be a highly valuable, though hardly used tool for establishing a defect model based solely on electrochemical measurements. It is very sensitive for minority charge carriers and can thus unveil otherwise hardly accessible defect concentrations. The chemical capacitance analysis yields a valence dependent acceptor concentration in the ppm range for the investigated samples. Complementary positron annihilation lifetime spectroscopy (PALS) suggests existence of Ti vacancies and both methods (chemical capacitance and PALS) agree in their quantification of the corresponding vacancy concentration (6 ppm). Beyond successfully predicting acceptor defect concentrations in undoped SrTiO₃, the method is sensitive for electronically relevant defects in sub-ppm concentrations.     

Comparison of osteogenesis in poly(L-lactic acid)-coated and non-coated porous hydroxyapatite scaffolds

To improve the biocompatibility and mechanical strength of porous hydroxyapatite (HA) scaffolds that have high osteoconduction, we coated them with 1, 5, or 10 wt% poly(L-lactic acid) (PLLA). In the 5 and 10 wt% PLLA-coated groups, osteoblast proliferation rates were higher than those in non-coated and 1 wt% PLLA-coated groups. The alkaline phosphatase (ALP) activity was highest in the 5 wt% PLLA-coated group. In addition, the porosity was highest in the non-coated HA group (82 %), whereas it was 72, and 41 % in the 5, and 10 wt% PLLA-coated groups, respectively. Scaffold strength increased in proportion to the concentration of PLLA coating. Overall, the 5 wt% PLLA scaffold had best characteristics when considering osteoblast proliferation, ALP activity, porosity, and compressive strength. We next tested this scaffold in an in vivo alveolar defect rabbit model. Multi-detector row computed tomography showed that non-coated and 5 wt% PLLA-coated HA groups had a similar high density. Adequate osteogenesis was observed by histological analysis in the non-coated HA and 5 wt% PLLA-coated HA groups. In addition, sufficient mineralization was observed in the coated scaffolds by X-ray fluorescence spectroscopy without significant adverse effects. Therefore, based on our findings, a PLLA-coated porous HA scaffold may be a suitable bone substitute for the correction of bone defects.     

A Combined Experimental and Theoretical Approach to Study Temperature and Moisture Dynamic Characteristics of Intermittent Paddy Rice Drying

Intermittent drying of paddy rice is fully investigated both theoretically and experimentally. A model is developed to describe simultaneous heat and mass transfer for the drying stages and mass transfer for the tempering ones. The model is considered for both cylindrical and spherical geometries. The model excels in considering non-constant paddy rice and air physical properties as well as surface vaporization and convection. The consequent equations are numerically solved with finite-difference method of line using implicit Runge–Kutta. Furthermore, a set of experiments is conducted in a laboratory-scale fluidized bed dryer to estimate the moisture diffusivity of rice and evaluate the effects of different parameters. Two correlations for moisture diffusivity are derived for each geometry based on the experimental results. It is noteworthy that the geometry choice leads to significantly different moisture diffusivities. As a result, the diffusivity values obtained for spherical presentation is 2.64 times greater than that of cylinder. Moreover, the cylindrical model fits the experimental results more precisely, especially for tempering stage (AARD_cyl = 1.03%; AARD_sph = 1.53%). Model results reveal that thermal equilibrium is quickly reached within the first 2 min. Air velocity shows no influential effect on drying upon establishment of fluidized condition. In addition, drying rate is drastically improved after applying the tempering stage. A definition for tempering stage efficiency is also proposed which shows that 3 h tempering will be 80% efficient for the studied case. Rising temperature significantly improves the drying rate, while it does not contribute much in the tempering efficiency.