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.     

Modeling the curing kinetics of two thermoset adhesives under varying temperature conditions

This paper presents detailed curing kinetics models for two thermoset adhesives. The cure kinetics were characterized using differential scanning calorimetry in both anisothermal and isothermal modes. The Sestak–Berggren autocatalytic model was applied to describe the anisothermal cure kinetics of the two adhesives with the Malek and undetermined coefficients methods determining their kinetic parameters. The Kamal autocatalytic model was adopted for the isothermal curing processes with the Kenny analytical-graphical method determining the kinetic parameters. A modified Kamal model was developed by introducing a concept of the maximum degree of cure (DOC) and temperature-depended kinetic parameters to describe the isothermal cure kinetics of the adhesive with a typical exothermic peak, and an extended Kamal model was further proposed by adding an initial-phase-control term to the modified Kamal model to describe the isothermal cure kinetics of the adhesive with two exothermic peaks. The results showed that the presented curing kinetics models with the determined parameters can precisely predict the evolutions of the DOC of the two thermoset adhesives in both anisothermal and isothermal modes.     

An alternative model for predicting the cure kinetics of a high temperature cure epoxy adhesive

The purpose of this work was to develop a cure kinetics model for a commercially available high temperature cure epoxy adhesive commonly used in the aerospace industry. While there are several phenomenological cure kinetic models commonly used in the literature for describing the rate of conversion of thermosetting epoxy adhesives as a function of degree of conversion, none of these models adequately depicts the adhesive used in this work over the entire range of conversion. Hence, by curve fitting empirical data collected using differential scanning calorimetry and refinement of existing models, an alternative model is proposed. The form of the present model suggests that chemical curing is the result of the combination of two autocatalytic reactions. The model is able to account for both the chemically controlled and diffusion controlled regimes of the cure. This paper also describes a novel iterative approach for predicting kinetics parameters as a function of isothermal cure temperature. Excellent agreement between experimental measurements and model predictions has been demonstrated over the entire range of conversion.     

Effect of Bondline Thickness on Mixed-Mode Debonding of Adhesive Joints to Electroprimed Steel Surfaces

Structural applications of adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for techniques to assess adhesive joint strength, particularly along bondline interfaces where compliant adhesives contact more rigid metallic surfaces. The present study investigates the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel adherend surfaces. Three bondline thicknesses, representative of structural automotive joints, were evaluated for unprimed and primed bondlines. Experimental results for static load versus debond extension were input to finite element analyses for computing debond parameters (strain energy release rates). The debonds always initiated at a through-the-thickness location that had the greatest peel component of strain energy release rate. The total strain energy release rate values correlated well with trends in joint strength as a function of bondline thickness.     

Fatigue Crack Growth in Epoxy/Aluminum and Epoxy/Steel Joints

The fatigue crack growth rate within epoxy/aluminum and epoxy/steel joints was evaluated as a function of a) type of surface pretreatment, b) water soak, c) fatigue cycle rate (Hz), d) adhesive thickness and e) type of epoxy adhesive.

For both adherends, aluminum and steel, a significant improvement in the fatigue behavior was obtained by use of a mercaptoester coupling agent. After an 8-day, 57°C water soak, the metal surfaces which were pretreated with coupling agent (CA) or by phosphoric acid anodization (PAA) still resulted in cohesive failure, while the controls had higher crack growth rate and showed greater scatter. The room-temperature cure matrix with CA-treated aluminum showed a less dramatic improvement, probably because of a known difference in the application procedure. For the steel joints and room-temperature adhesive the improvement in the fatigue behavior of CA-treated samples was maintained after the 8-day hot water soak. No significant change was found in the fatigue crack growth rate over a frequency range of 1 to 5 Hz, but a significant change was found as a function of the bondline thickness. The room temperature curing adhesive evaluated herein exhibited a much lower fatigue resistance than a heat-cured commercial structural adhesive FM-73.     

Behaviour of Bi-adhesive Joints

The use of relatively low modulus adhesive at the ends of overlap in a bi-adhesive bondline of a bonded joint can reduce the stress concentration significantly and, therefore, potentially lead to higher strength of the joint. This study presents the two-dimensional and three-dimensional nonlinear (geometric and material) finite element analyses of adhesively bonded single lap joints having modulus-graded bondline under monotonic loading conditions. The adhesives were modelled as an elasto-plastic multi-linear material, while the substrates were regarded as both linear elastic and bi-linear elasto-plastic material. The computational simulations have been performed to investigate the bondline behaviour by studying the stress and strain distributions both at the mid-plane as well as at the interface of the bondline. It has been observed that the static strength is higher for joints with bi-adhesive bondlines compared to those with single adhesives in bondline. Higher joint strength has also been observed for optimum bi-adhesive bondline ratio through parametric studies. Effects of load level, and bondline thickness on stress distribution in the bi-adhesive bondline have also been studied. 3D analysis results reveal the existence of complex multi-axial stress/strain state at the ends of the overlap in the bondline which cannot be observed in 2D plane strain analysis. About 1/3rd of the width of the joint from the free edge in the width direction has 3D stress state, especially in the compliant adhesive of the bondline. Magnitudes of longitudinal and lateral stress/strain components are comparable to peel stress/strain components. It has also been analytically shown that the in-plane global stiffness of the joint remains unaffected by modulus gradation of the bondline adhesive.     

Fatigue Crack Growth in Epoxy/Aluminum and Epoxy/Steel Joints

The fatigue crack growth rate within epoxy/aluminum and epoxy/steel joints was evaluated as a function of a) type of surface pretreatment, b) water soak, c) fatigue cycle rate (Hz), d) adhesive thickness and e) type of epoxy adhesive.

For both adherends, aluminum and steel, a significant improvement in the fatigue behavior was obtained by use of a mercaptoester coupling agent. After an 8-day, 57°C water soak, the metal surfaces which were pretreated with coupling agent (CA) or by phosphoric acid anodization (PAA) still resulted in cohesive failure, while the controls had higher crack growth rate and showed greater scatter. The room-temperature cure matrix with CA-treated aluminum showed a less dramatic improvement, probably because of a known difference in the application procedure. For the steel joints and room-temperature adhesive the improvement in the fatigue behavior of CA-treated samples was maintained after the 8-day hot water soak. No significant change was found in the fatigue crack growth rate over a frequency range of 1 to 5 Hz, but a significant change was found as a function of the bondline thickness. The room temperature curing adhesive evaluated herein exhibited a much lower fatigue resistance than a heat-cured commercial structural adhesive FM-73.     

CRH3高速动车组空调通风口胶接结构设计

通过理论分析和计算确定了动车组空调通风口部件与铝合金车体胶接用胶粘剂的强度指标。介绍了胶粘剂的选择及胶接结构的设计原则,考查了搭接长度、搭接宽度、胶层厚度和被粘接材料厚度等对胶接件粘接强度的影响。结果表明:车体与空调通风口部件的胶接接头选择受剪切应力作用的搭接接头较适宜,并且搭接接头的承载能力随搭接长度或宽度增加呈先快速上升后趋于稳定态势;当搭接长度为10 mm、胶层厚度为6 mm、铝合金板厚度为5 mm且常温湿固化型单组分PU(聚氨酯)胶粘剂的剪切强度超过0.23 MPa时,搭接接头的承载能力相对最大。     

Investigation of structural bond lines in wind turbine blades by sub-component tests

In the present study, the structural fatigue performance of an adhesive joint, imitating the connection of the shear web to the spar caps of a wind turbine blade, is investigated. An asymmetric three point bending test has been developed from early design steps to manufacturing and testing. The sub-component is supposed to fail in the bondline, behavior consistently validated after a comprehensive testing campaign. Similar failure patterns to the full scale wind turbine blades have been observed in the bond line during the fatigue tests. Tensile cracks propagated transverse to the adhesive longitudinal axis, approximately at 10% of the total lifetime. Their growth was hindered from the spar cap and web laminate. After around 60% of the specimen life, cracks changed their direction resulting in disbonds between the adhesive–adherend interface.Shear stress variations and artificial defects seemed to have a relatively low impact on the sub-component fatigue performance. At the same time it was shown that cover-laminates drastically increase the fatigue performance of the adhesive joint.     

A shear-lag model for functionally graded adhesive anchors

A shear-lag model for stress transfer through an adhesive layer of variable stiffness joining an anchor rod and the concrete is presented and the effect of such an inhomogeneous bondline on interfacial shear stress distribution in comparison with that of a homogeneous bondline anchor subjected to monotonic axial tension is investigated. A closed-form solution is presented for arbitrary distribution of shear stiffness of the bondline considering both bonded and debonded embedded-end conditions of the anchor. Subsequently, the specific cases of linear and constant distribution of stiffness are discussed in detail, and it is shown how the general solution can be simplified for these examples. For validation, the distribution of shear stress along the bondline for the aforementioned cases is compared with that of equivalent axisymmetric Finite Element (FE) models and the results are found to be in good agreement. The theoretical solution developed can be readily used to evaluate the pull-out performance of post-installed adhesive anchors. Variable stiffness adhesive interfaces deserve an interest in practical applications either to estimate the effect of loss of interface stiffness, due to degradation of the adhesive material, or to engineer the interface with optimum distribution of stiffness so as to maximize the structural performance of bonded systems.     

Moisture-Curing Kinetics of Isocyanate Prepolymer Adhesives

The reaction between isocyanate-terminated prepolymers and atmospheric moisture produces urea linkages and results in a hydrogen-bonded network of linear high molecular weight polymers with adhesive properties. This study describes the synthesis of isocyanate-terminated prepolymers and investigates the use of in situ infrared spectroscopy as a technique for monitoring the chemistry of the polymerization reaction kinetics. In situ FTIR was successfully used as a means to monitor the residual isocyanate levels and the extent of the polymerization reaction. Frequency dependent dielectric sensing (FDEMS) using a thin, planar sensor has been used to monitor the reaction kinetics by monitoring changes in the mobility of ions in the reacting medium. A direct correlation of the extent of prepolymer cure was found using the normalized FTIR isocyanate absorbance spectrum and FDEMS imaginary permittivity at 500 Hz for the duration of the cure cycle. The results of this investigation demonstrate that FDEMS is an effective online method to monitor the extent of moisture cure in the bulk as well as in a coating or adhesive bondline.     

2007-2008年国外环氧树脂工业进展

综述了国外环氧树脂工业2007-2008年市场需求,公司经营,原料(甘油法环氧氯丙烷)生产和新产品(环氧树脂、固化剂、水性脱模剂等助剂和填料)开发情况并介绍了环氧树脂应用领域如胶粘剂、涂料、光固化产品和复合材料(如风力发电机叶片配套产品)的发展。     

Moisture-Curing Kinetics of Isocyanate Prepolymer Adhesives

The reaction between isocyanate-terminated prepolymers and atmospheric moisture produces urea linkages and results in a hydrogen-bonded network of linear high molecular weight polymers with adhesive properties. This study describes the synthesis of isocyanate-terminated prepolymers and investigates the use of in situ infrared spectroscopy as a technique for monitoring the chemistry of the polymerization reaction kinetics. In situ FTIR was successfully used as a means to monitor the residual isocyanate levels and the extent of the polymerization reaction. Frequency dependent dielectric sensing (FDEMS) using a thin, planar sensor has been used to monitor the reaction kinetics by monitoring changes in the mobility of ions in the reacting medium. A direct correlation of the extent of prepolymer cure was found using the normalized FTIR isocyanate absorbance spectrum and FDEMS imaginary permittivity at 500 Hz for the duration of the cure cycle. The results of this investigation demonstrate that FDEMS is an effective online method to monitor the extent of moisture cure in the bulk as well as in a coating or adhesive bondline.     

Aromatic copolyester thermosets: High temperature adhesive properties

The adhesive properties of a new family of crosslinkable aromatic copolyester resins are described in this paper. A unique method for forming an adhesive bond in the solid state through high temperature interchain transesterification reactions (ITR) between cured thin films of these copolyester thermosets is indicated. Bonds formed via this procedure yield average lap shear strengths of roughly 13 MPa to titanium adherends and invariably exhibit failure at the polymer-metal interface and not within the polymer. Various procedures to increase the strength of the bond at the interface between the polymer and the metal were tested such as the effect of surface pretreatments, primers, fillers, and the role of bondline thickness on the lap shear strength of the joints. Of these, the use of surface pretreatments and thinner bondlines has the greatest effect, raising the observed average lap shear strength of the joints to 16.5 and 20.7 Mpa, respectively. Some of the adhesive properties pertaining to the use of this new thermoset as a matrix in a graphite fiber composite are also presented.     

风电叶片复合材料固化工艺优化研究

近年来,随着风电叶片越做越大,叶片的生产周期也随之变长。为了有效缩短叶片的生产周期,降低企业的生产成本,必须从环氧树脂固化工艺方面进行研究,在保证产品力学性能的前提下,尽可能的缩短生产周期。本文选用四组不同的工艺方法进行比较,研究结果表明,可以通过力学和热学手段来评估固化过程并通过合理的调控升温程序来缩短生产周期。     

Experimental characterization of autoclave-cured glass-epoxy composite laminates: Cure cycle effects upon thickness,void content,and related phenomena

This article presents results from over 100 experimental autoclave curing fiber-glass-epoxy composite laminate curing runs. The primary objective was to verify shrinking horizon model predictive control—SHMPC—for thickness and void content control, using readily available secondary measurements. The secondary objective was to present and analyze the extensive experimental results obtained through this verification. Seven series of curing runs (16 per series) were performed, with cure settings governed by partial- or full-factorial orthogonal array based design of experiments. Through t-tests and two-way analysis of variance, it was found that pressure magnitude had the largest influence on laminate thickness and void content, while first hold duration/temperature, pressure application duration, and run delay influenced void content more than thickness. Thinner laminates with lesser void contents resulted from pressure application before the second temperature ramp. Prepreg age also affected thickness and void content. Photomicrographs revealed not one large void, but void clusters. Interrupted autoclave cure cycles revealed that significant laminate thickness reduction occurred during all curing cycle stages. The percentage of resin weight loss through laminate sides increased with pressure magnitude and application duration. Ten test curing runs indicated that SHMPC met difficult thickness targets while minimizing void content.     

A Fracture Mechanics Approach to Characterizing Cyclic Debonding of Varied Thickness Adhesive Joints to Electroprimed Steel Surfaces

Applications of adhesive bonding for automotive structures have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for design techniques to assess the influence of bondline thickness on adhesive joint strength. One design approach currently being used is based on limiting shear stresses in the adhesive while designing to eliminate peel stresses. Another design approach is based on fracture mechanics and accounts for shear and peel stresses and both static and fatigue modes of failure. The present study applies fracture mechanics to investigate the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel surfaces. Three bondline thicknesses equal to 0.254, 0.813, and 1.27 mm were evaluated for unprimed and primed bondlines. For the experimental portion of the study, debond growth rates (da/dN) were measured using a remote imaging system over a range of applied cyclic loads. Corresponding changes in the strain release rates (ΔG) were calculated, through finite element analyses, as a function of debond length and applied load level. The computations for ΔG applied a finite element formulation to determine both the peel component, ΔG_i, and the shear component, ΔG_ii. When computed ΔG values were plotted against the measured debond growth rates, da/dN, the results showed a power law relationship which characterizes the debond behavior of a given material system and bondline thickness.     

Three-dimensional simulations of the curing step in the resin transfer molding process

The curing step in resin transfer molding process involves heat transfer coupled with the curing reaction of thermoset resin. In order to examine the curing behavior under a specified cure cycle in the resin transfer molding process, numerical simulations are carried out by three-dimensional finite elements method. An experimental study for isothermal cure kinetics of epoxy resin is conducted by using differential scanning calorimetry. Kinetic parameters based on the modified Kamal model are determined from the calorimetric data for the epoxy system, and by using these parameters, numerical simulations are performed for a hat-shaped mold. It is found from the simulation results that the temperature profile and the degree of cure are well predicted for the region inside the mold. This numerical study can provide a systematic tool in the curing process to find an optimum cure cycle and a uniform distribution of the degree of cure.     

Smart cure cycle to improve tensile load capability of the adhesively bonded joint

Generally, the thermal residual stress which is generated during the curing process of an adhesive significantly decreases the tensile load capability of adhesively bonded joints. In this work, a smart cure cycle with abrupt cooling and postcuring at room temperature was devised to eliminate thermal residual stress and to produce sufficient interfacial wetting. For monitoring and controlling the curing reaction, dielectrometry was used, where the dissipation factor of the adhesive joint was measured. These results showed that the tensile load capability of the adhesively bonded joint fabricated by the smart cure cycle was greatly enhanced because the thermal residual stress was reduced, and sufficient interfacial wetting between the adhesive and adherend was achieved.