A stress singularity approach to failure initiation in a bonded joint with varying bondline thickness

The stress singularity at the theoretical point of maximum stress in an uncracked single lap joint is analysed by a finite element method. By treating the interface corner of a bonded joint (between adherend and adhesive) as a perfectly bonded wedge and using a fracture mechanics method, considerable advantages over other continuum mechanics approaches for investigating the bondline thickness effect on joint strength are shown. This study has essentially two aims: (i) determination of the strength of the singularity by finite element analysis and comparison with the analytical prediction of Bogy for varying bondline thickness; and (ii) determination of stress intensity factors for varying bondline thickness. Good agreement is shown between the numerically-calculated strength of the singularity with the analytical value obtained from Bogy. The calculated stress intensity, after an initial decrease in the low bondline thickness range, is found to increase with increasing bondline thickness. This agrees well with the trends predicted by experiments.     

Shear and peel stress analysis of an adhesively bonded scarf joint

The shear and peel stress distributions in a scarf joint made of two isotropic adherends with blunt adherend tips are analysed using a linear elastic analysis. The limits of the analysis with respect to adherend tip thickness have been investigated. A finite difference method is used to solve the differential equations for the shear and peel stress distributions over the joint. The boundary conditions used limit the analysis to the two adherends having the same thicknesses, lengths, and material properties. The adherends are modelled as plates with extensional and bending stiffnesses bonded together with an elastic interlayer. The stresses across the adhesive layer are assumed to be constant. The current analysis applied to cases known from the literature shows good agreement with the shear stresses but the peel stresses are overestimated.     

Theoretical and experimental studies for the failure criterion of adhesively bonded joints

The objective of this work was to develop a criterion for predicting the failure strength of joints bonded by ductile adhesives. To obtain the criterion, first, fracture tests were carried out on T-peel joints and single-lap joints with various joint geometries, adhesives, and adherend materials. Then using the fracture loads obtained in the tests, a finite element analysis was performed by which the stresses in the adhesive joints were calculated. It is concluded that the failure of an adhesively bonded joint occurs when the maximum of the ratio of the mean to effective stresses exceeds a certain value, which can be considered a new material constant of a ductile adhesive.     

The effects of overlap length and adherend thickness on the strength of adhesively bonded joints subjected to bending moment

In this work, elasto-plastic stress analysis of a Single Lap Joint (SLJ) subjected to bending moment was investigated using 2D non-linear Finite Element Analysis (FEA). The SLJs, consisting of hardened steel as the adherend bonded by two adhesives, one stiff and one flexible, with very different mechanical behaviors were analyzed. In order to determine the effect of geometrical parameters on the performance of the SLJs, four different adherend thicknesses and overlap lengths for each adhesive were used. For verification of the analysis, the FEA results were compared with experimental results. It was observed that there was a significant effect of adherend thickness on the strength of the joint with both adhesives. However, the load carried by the SLJ with the flexible adhesive increased with increasing overlap length.     

Stress analysis and strength evaluation of single-lap band adhesive joints of dissimilar adherends subjected to external bending moments

Single-lap band adhesive joints of dissimilar adherends subjected to external bending moments are analyzed as a four-body contact problem using a two-dimensional theory of elasticity (plane strain state). In the analysis, the upper and lower adherends and the adhesive which are bonded in two regions are replaced by finite strips. In the numerical calculations, the effects of the ratio of Young's moduli of the adherends, the ratio of the adherend thicknesses, and the ratio of the band length to the half lap length on the stress distributions at the interfaces are examined. A method for estimating the joint strength is proposed using the interface stress and strain obtained by the analysis. An elasto-plastic finite element analysis (EP-FEA) was conducted for predicting the joint strength more exactly. Experiments to measure strains and the joint strength were also carried out. The results show that the strength of a single-lap band adhesive joint is almost the same as that of a single-lap adhesive joint in which the two adherends are completely bonded at the interfaces. Thus, the single-lap band adhesive joints are useful in the design of single-lap joints.     

The effects of surface roughness and bond thickness on the fatigue life of adhesively bonded tubular single lap joints

Since the surface roughness of adherends greatly affects the strength of adhesively bonded joints, the effect of surface roughness on the fatigue life of adhesively bonded tubular single lap joints was investigated analytically and experimentally by a fatigue torsion test. The stiffness of the interfacial layer between the adherends and the adhesive was modelled as a normal statistical distribution function of the surface roughness of the adherends. From the investigation, it was found that the optimum surface roughness of the adherends for the fatigue strength of tubular single lap joints was dependent on the bond thickness and applied load.     

An investigation on the initiation and propagation of damaged zones in adhesively bonded lap joints

In this study, the initiation and propagation of damaged zones in the adhesive layer and adherends of adhesively bonded single and double lap joints were investigated considering the geometrical non-linearity and the non-linear material behaviour of the adhesive and adherends. The modified von Mises criteria for adherends and Raghava and Cadell's failure criteria (J. Mater. Sci. 8, 225 (1973) [1]) including the effects of the hydrostatic stress states for the epoxy adhesive were used to determine the damaged adhesive and adherend zones which exceeded the specified ultimate strains. The stiffness of all finite elements corresponding to these zones was reduced so that they could not contribute to the overall stiffness of the adhesive joint. This approach simplifies to observe the initiation and propagation of the damaged zones in both the adhesive layer and adherends. A tensile load caused first the damaged adhesive zones to appear at the right free end of the adhesive-lower adherend interface and at the left free end of the adhesive-upper adherend interface, and then to propagate through the adhesive regions near the adhesive-adherend interfaces (interfacial failure). In the bending test, the damaged zone initiated at the left free end of the adhesive-upper adherend interface in tension, and similarly propagated through the adhesive regions close to the adhesive-adherend interface (interfacial failure). In the double-lap joint subjected to a tensile load, the damaged adhesive zones initiated first at the right free end of the adhesive-middle adherend interface and then propagated through the adhesive region near the adhesive-adherend interface. After the damaged zone reached a specific length it also grew through the adhesive thickness, and the adhesive joint failed. The SEM micrographs of fracture surfaces around the free edges of the overlap region indicated that the failure was interfacial. An additional damaged zone growth was observed in the side adhesive regions due to lateral straining, called the Poisson effect.     

Non-linear stress and failure analyses of adhesively-bonded joints subjected to a bending moment

In this paper, the mechanical behavior of the Single-Lap Joints (SLJs) bonded with two different adhesives (FM 73 and SBT 9244) under a bending moment was analyzed, both experimentally and numerically. Four-point bending experiments for the joints with different overlap lengths were carried out and fracture surfaces of the SLJs were examined with a Scanning Electron Microscope (SEM). After the stress analysis in the SLJs was performed via a finite element method by considering the material non-linearities of the adhesives and adherend (AA2024-T3), the Finite Element Analysis (FEA) results were compared with experimental results. Finally, the stress analyses and experimental results show that the failure in the SLJs subjected to a bending moment probably initiates from the overlap region on the adhesive–upper adherend interface in tension and propagates towards the centre of the overlap. Also, in the joint subjected to a bending moment, it is seen that the load carried by the SLJ with SBT 9244 adhesive with increasing overlap length is more than that of the SLJ with FM 73 adhesive, although in the bulk form FM 73 adhesive is about three times stronger than SBT 9244 adhesive.     

Stress analysis and strength evaluation of bonded shrink fitted joints subjected to torsional loads

This paper deals with the stress analysis and strength evaluation of bonded shrink fitted joints subjected to torsion. The stress distributions in the adhesive layer of bonded shrink fitted joints are analyzed by the axisymmetric theory of elasticity when an external torsion is applied to the upper end of the shaft. The effects of the outer diameter and the stiffness of rings on the interface stress distributions are clarified by numerical calculations. Using the interface stress distributions, the joint strength is predicted. In addition, the joint strength was measured experimentally. It is seen that rupture of the adhesive layer is initiated from the upper edge of the interface when torsion is applied to the upper end of the shaft. The numerical results are in fairly good agreement with the experimental results. It was found that the joint strength of bonded shrink fitted joints is greater than that of shrink fitted joints.     

Analysis of stresses in adhesive joints applicable to IC chips using symbolic manipulation and the numerical method

In this study, a concentrated force is applied to both adherends bonded by an adhesive under pin–pin boundary conditions. First a mathematical model is derived with governing equations and boundary conditions. These complicated, and analytically problematic, coupled equations are solved numerically using symbolic manipulation and singular value decomposition (SVD). Also discussed are the effects of major factors, including the relative thickness of adherends, joint length and the action point of the concentrated force on the peel and shear stresses in the adhesive layer. This study identifies the conditions under which the upper adherend without breakage can be fully separated from the lower adherend. Particularly, it is found that the thickness of the lower adherend should be greater than ten times that of the adhesive layer but less than one-third that of the upper adherend, the adhesive layer should be relatively thin (h _a ≤ 0.01 mm), and the adhesive joint should be relatively short (thickness to length ratio γ ₁ ≥ 0.08).     

The interrelationships between electronically conductive adhesive formulations,substrate and filler surface properties,and joint performance. Part I: the effects of adhesive thickness

Electronically conductive adhesives (ECAs) have received a great deal of attention for interconnection applications in recent years. Even though ECAs have excellent potential for being efficient and less costly alternative to solder joining in electronic components, they still possess a number of problems with respect to durability and design to meet specific needs. One of the issues that requires understanding is regarding the optimum adhesive thickness (AT) to be used. This study addresses this issue in relation to the formulations of the conductive adhesives and their interactions with adherend surfaces. For this purpose, two different adherends varying in surface characteristics were utilized along with three different conductive adhesive formulations with varying particle loadings, and shapes and sizes of conductive nickel fillers. Joints were also prepared with two different AT values, to gain insight into the influence of AT on the joint strength, deformation and joint conductivity. As the AT was increased, only a small reduction in failure load and ultimate displacement values were observed with unetched adherends. With etched adherends, however, a small increase in joint stretchability was evident with higher adhesive thickness tested at a lower crosshead speed. When the AT was increased, we also noted a corresponding increase in the initial joint resistance.     

An experimental study on the dependence of the strength of adhesively bonded joints with thickness and mechanical properties of the adhesives

Adhesive bonding joints are widely applied in many engineering fields. Their overall strength is much dependent on the thickness of adhesive layers. Many previous experimental studies have found that the ultimate failure strength of the bonding structure increases with the decrease of the adhesive thickness. However, few of them consider the effect of adhesive intrinsic material parameters on the relation between the overall strength and adhesive thickness. In the present investigation, the effect of the adhesive thickness on the overall strength of the lightweight metallic adhesive bonding joints was experimentally studied, considering the effect of the adhesive toughness. The results show that the variations of overall strength resulting from the adhesive thicknesses have remarkable discrepancy due to the toughness of the adhesive, which is in agreement with the previous model prediction.     

The effects of diffusion mechanism and void structure on transport rates and tortuosity factors in complex porous structures

Tracer diffusion simulations within random porous structures show that tortuosity factors are independent of diffusion mechanism for all practical void fractions when an equivalent Knudsen diffusivity is correctly defined. Previous studies concluded that tortuosity factors, a geometric property of the void space as defined, increase with increasing Knudsen number, K_n, a measure of the relative number of molecule-surface and intermolecular collisions. The model porous structures in this study consist of random-loose packings of spheres overlapped to achieve a given void fraction and to accurately reflect the void space in practical porous solids. Effective diffusivities were estimated using tracer or flux-based Monte Carlo methods for Knudsen numbers of 10⁻³-10¹⁰; the two methods lead to similar diffusivities for void fractions of 0.06-0.42. Tortuosity factors estimated using the number-averaged distance between collisions, 〈l_p〉, for the characteristic void length scale increased with increasing Knudsen number, even though simulations in infinite cylinders confirmed the accuracy of the Bosanquet equation for all values of K_n. These unexpected changes in a geometric property of the void space become most apparent near the percolation void fraction (∼0.04). For example, the Knudsen tortuosity factor defined in this manner is 1.8 times larger than in the bulk regime for a solid with 0.10 void fraction. Even at high void fractions (∼0.42), the two extreme values of tortuosity factor differ by a factor of ∼1.4. These apparent effects of diffusion mechanism on tortuosities reflect the inaccurate use of number-averaged chord lengths when tracer reflections from random obstacles obey the Knudsen cosine law for diffuse reflection. A corrected length scale, first proposed by Derjaguin, leads to tortuosity factors independent of K_n for void fractions above 0.20; tortuosities differ by only 18% and 4% between Knudsen and bulk regimes even for void fractions of 0.10 and 0.15, respectively. The residual differences at void fractions below 0.10 arise from the increasingly serial nature of the remaining voids. Thus, a long-standing inconsistency between the defined geometric nature of tortuosity factors and their inexplicable dependence on diffusion mechanism is essentially resolved. In practice, these simulations allow the consistent and accurate use of tortuosity factors determined at any value of K_n for all diffusion regimes; they also prescribe, rigorously for void fractions above 0.15 and empirically for lower void fractions, the length scale relevant to diffusion in the Knudsen and transition diffusion regimes.     

焦化苯加氢精制工艺萃取溶剂分析评价

介绍了国内焦化粗苯精制工艺情况,从溶剂的角度分析了不同工艺的特点,对萃取精馏技术中的溶剂NFM、SUL-COS应用进行了比较,提出了工艺选择的建议。萃取精馏工艺投资低,设备单元数少,产品污染风险小,回收率和产品纯度高,能耗和操作费用低,原料适应性宽,更适合焦化苯精制工艺使用。     

In vivo and in vitro evaluation of the cytotoxic effects of Photosan-loaded hollow silica nanoparticles on liver cancer

This study aimed to compare the inhibitory effects of photosensitizers loaded in hollow silica nanoparticles and conventional photosensitizers on HepG2 human hepatoma cell proliferation and determine the underlying mechanisms. Photosensitizers (conventional Photosan-II or nanoscale Photosan-II) were administered to in vitro cultured HepG2 hepatoma cells and treated by photodynamic therapy (PDT) with various levels of light exposure. To assess photosensitizers'' effects, cell viability was determined by 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. In addition, apoptotic and necrotic cells were measured by flow cytometry and the expression of caspase-3 and caspase-9 evaluated by western blot. Finally, the in vivo effects of nanoscale and conventional photosensitizers on liver cancer were assessed in nude mice. Nanoscale Photosan-II significantly inhibited hepatoma cell viability in a concentration-dependent manner and this effect was more pronounced with high laser doses. Moreover, nanoscale photosensitizers performed better than the conventional ones under the same experimental conditions (p < 0.05). Flow cytometry data demonstrated that laser-induced cell death was markedly increased after treatment with nanoscale Photosan-II in comparison with free Photosan-II (p < 0.05). Activated caspase-3 and caspase-9 levels were significantly higher in cells treated with Photosan-II loaded in silica nanoparticles than free Photosan-II (p < 0.05). Accordingly, treatment with nanoscale photosensitizers resulted in improved outcomes (tumor volume) in a mouse model of liver cancer, in comparison with conventional photosensitizers. Hollow silica nanoparticles containing photosensitizer more efficiently inhibited hepatoma cells than photosensitizer alone, through induction of apoptosis, both in vivo and in vitro.     

硫酸工业废热利用现状分析及评价指标研究

介绍了硫酸装置高、中、低温位热能回收利用的现状及技术进展。探讨了废热回收评价指标及利用原则。在废热利用过程中应将热平衡分析与能级分析方法结合,综合热效率与能级差这两个指标分析装置废热回收利用情况,以制定优化废热利用方案及节能措施。     

Analysis of raw and trend removed EN data in time domain to evaluate corrosion inhibition effects of New Fuchsin dye on steel corrosion and comparison of results with EIS

The inhibition effect of New Fuchsin dye on the corrosion of mild steel in 1 M HCl was studied using electrochemical noise measurements (ENM) and the electrochemical impedance spectroscopy (EIS) method. The experimental results indicated that, the New Fuchsin is effective inhibitor even in very low concentration (5 ppm). EIS studies showed that the polarization resistance of mild steel in 1 M HCl increases as the inhibitor concentration increases. Experimental results showed that the DC trend can affect the results obtained from analysis of electrochemical noise data in time domain. DC trend of electrochemical noise data was removed by MAR method. After trend removal, electrochemical noise resistance is confirmed to be strongly correlated to polarization resistance.