Adhesive Creep Research Articles

Wheel-rail contact and friction models: A review of recent advances

Rail transportation is regarded as a reliable, quick, and secure mode of transportation. The wheel-rail contact interaction is crucial to the railway operation since it is responsible for supporting, traction, braking, and steering of railway vehicles. Improper wheel-rail interactions may produce or exacerbate wheel-rail interface issues such as rolling contact fatigue (RCF) and wear, which can threaten the vehicle’s running safety and stability. A review of the evolution and recent literature on wheel-rail contact mechanics and tribology is presented here. Topics covered include the basics of wheel-rail contact problem and methodologies for modeling both the normal contact (Hertzian and non-Hertzian) and tangential contact (Kalker’s theories including CONTACT and FASTSIM algorithms, Polach’s theory, USETAB program, etc.). The paper also reviewed various effects of contaminants and environmental conditions (water, leaves, sand, temperature, humidity, etc.) in wheel-rail contact. Various wheel-rail empirical adhesion models like the Water-induced low adhesion creep force model (WILAC) model and adhesion models based on elastohydrodynamic lubrication (EHL) theory (Greenwood-Tripp [GT] and Greenwood-Williamson [GW] models) are also reviewed. Lastly, the paper discusses the need and challenges for developing and integrating the wheel-rail non-Hertz contact model and adhesion model, as well as open areas for further research.

Impact of the load level on the creep behaviour of adhesively bonded fastener in tension: experimental investigations, analytical and numerical modelling

The use of structural adhesive bonding is increasing in structural assembly. It has indeed several advantages compared to other assembly techniques such as welding and riveting: uniform stress distribution, assembly of materials with different natures, no heat production. This last aspect is particularly interesting for the offshore activities. The firm Cold Pad has developed adhesively bonded fastener solutions, whose aim is to replace welding by structural bonding for the reinforcement of steel structures. Yet, some issues remain about the appraisal of the durability for such assembly and particularly creep. To address this issue, experimental creep investigations were carried out at real scale for one geometry of a fastener bonded with a methacrylate adhesive, and at different tension load levels. An inverse identification method has been developed in a previous study to identify the material creep properties from these investigations, independently of the fastener geometry. Considering the industrial applications of this fastener, the aim of this modelling approach is to describe correctly the primary and secondary creep stages. In this article, this identification method was applied to all the experimental results, using the Burger creep model. The dependency of the parameters with the load level is highlighted, which led to a nonlinear model. These results were then compared to finite element investigations to verify the consistency of the method and its ability to correctly identify the adhesive creep behaviour. This offers good perspectives in the transposition of the study to alternative fastener geometries using simple analytical design approaches. Highlights Bonded fastener’s creep behaviour experimentally investigated under different load levels. Creep modelling using a nonlinear and analytical approach with Burger model Identification of the material creep properties from experimental investigations at real scale on the bonded assembly. Numerical model developed to assess the consistency of the analytical approach.

Creep of an FRP-strengthened metallic beam under cyclic temperature and cyclic load

Metallic beams can be strengthened using fibre reinforced polymer (FRP) plates that are adhesively bonded to their soffit. This relies critically upon the bonded adhesive joint, and because the adhesives are viscoelastic it is known that creep at warm services temperatures can potentially result in large slips that can reduce the effectiveness of the strengthening. This paper examines the impact of adhesive creep upon the performance of an FRP-strengthened metallic beam when subjected to daily temperature cycles and daily load cycles. A finite element (FE) study was conducted that incorporates a linear viscoelastic constitutive model for the adhesive, temperature and load cycles of up to 50 years were considered, and the behaviour of the strengthened beam was examined. Differential thermal expansion between the FRP plate and metallic beam is shown to be significant because it results in both (a) additional instantaneous shear stress across the adhesive joint and (b) the shear stress due to sustained differential thermal expansion results in additional creep of the adhesive that can further reduce the effectiveness of the strengthening. An equivalent cumulative time approach is demonstrated to predict the creep response of the strengthened metallic beam without the need for a complex cyclic analysis.

Bond stress distribution in adhesive anchor systems: Interplay of concrete and mortar creep

The safe design of fastening systems, especially considering the multi-decade performance, relies on a thorough understanding of mechanisms and processes that lead to excessive deformations or even failure in course of time. According to current design guidelines and standards, adhesive anchor system are designed based on the uniform bond model. The uniform bond model is a generally good approximation of the real stress distribution during short-term testing and for loads close to the pull-out capacity of the system. However, both concrete and adhesive mortar are characterized by pronounced time-dependent deformation, especially at elevated temperatures. Thus, noteworthy shear stress redistributions are expected due to creep in course of a structure’s life-time. Depending on the level of stress redistributions potentially critical stress levels may be reached locally, triggering progressive failure. Therefore, it is quintessential to understand bond stress redistribution phenomena in adhesive anchor systems under sustained load. Unfortunately, it is experimentally impossible to decouple the intertwined effects of concrete and adhesive creep. Thus, in this study a numerical approach is adopted. The response of concrete is modeled by a discrete meso-scale damage model in combination with the micro-prestress solidification theory coupled to a hygro-thermal chemical analysis while the adhesive layer is represented by a visco-elastic shear stress–slip law. After separate calibration of concrete creep and creep of the visco-elastic interface the obtained model is validated on independent experimental data on anchor systems. Finally, the established computational framework is utilized to virtually isolate concrete creep, adhesive creep and study their mutual interaction. Results show strong antagonistic redistribution mechanisms driven by concrete and adhesive creep with varying dominance depending on the investigated time-scale.

A nonlinear viscoelastic\u2013viscoplastic model for adhesives

We consider the nonlinear viscoelastic–viscoplastic behavior of adhesives. We develop a one-dimensional nonlinear model by combining Schapery’s nonlinear single integral model and Perzyna’s viscoplastic model. The viscoplastic strain was solved iteratively using the von Mises yield criterion and nonlinear kinematic hardening. The combined viscoelastic–viscoplastic model was solved using Newton’s iteration and implemented into a finite element model. The model was calibrated using creep-recovery data from bulk adhesives and verified from the cyclic behavior of the bulk adhesives. The finite element model results agreed with experimental creep and cyclic responses, including recoverable and permanent strain after load removal. Although the contribution of the viscoplastic strain was small, both viscoplastic and viscoelastic components of strain response were required to describe the adhesive creep and cyclic response.

Improved modelling of trains braking under low adhesion conditions

ABSTRACT Predicting the behaviour of trains when braking under low adhesion conditions presents considerable challenges. This paper describes an approach to the problem using a model of the full train braking system known as LABRADOR (Low Adhesion Braking Dynamic Optimization for Rolling Stock) and an improved method for representing the creep force–creepage behaviour when low adhesion is presently known as WILAC (Water Induced Low Adhesion Creep Force Model). The development of these models and their integration are summarized and a number of test cases are presented to demonstrate the improvements which can be gained from this approach. A number of suggestions are made for future enhancements with the aim of providing brake engineers and systems integrators with reliable simulation tools for optimizing train braking performance when low adhesion is present.

Analysis of nano-creep deformation of epoxy adhesive in optical fiber connector for long-term reliability prediction

Abstract We describe a comprehensive analysis of the nano-creep deformation of an epoxy adhesive used in physical-contact optical fiber connectors. To design a highly reliable multicore fiber connector and predict the long-term reliability of commercially installed optical connectors, we focus on the change in fiber withdrawal, which is a key parameter for achieving physical-contact connection. We also propose an adhesive creep model that includes long-term time dependence by detailing through a finite element analysis the parameters that influence the fiber withdrawal, such as shear stress with connection, thermal stress, residual stress, swelling, and cure shrinkage. The measurement results for fiber withdrawal under several environmental conditions provided in this paper support the behaviors considered in our proposed model. From our obtained short-term results and rheological approach, we predict the nano-creep deformation observed in the connector over the long term by applying the time-temperature superposition principle, and discuss the correspondence with exposure test results. Our verified mechanism and estimated values of creep deformation prove the high reliability of the optical connectors and also provide guidelines for determining the need to update installed connectors and for designing future connection devices such as physical-contact multicore fiber connectors.

Low-stress creep deformation in two opto-electronic glass-epoxy joints: Part I – adhesive creep data

This two-part paper presents measurements and modelling of creep deformation at relatively low stress of two epoxy adhesives used in opto-electronic applications, one having a relatively high modulus and the other a low modulus. Part I describes the bulk tensile creep tests at various temperatures (75–95 °C) and stress levels representative of those in opto-electronic devices. A novel laser-based method of non-contact creep strain measurement was used to eliminate errors associated with adhesive specimen contact. The high modulus epoxy, tested at stress levels between 5 and 20 MPa, displayed non-linear behavior at high stress levels but was linear over the range of interest. The low modulus adhesive, tested between 0.25 and 0.5 MPa, was linearly viscoelastic over the full range of tested stress levels. Part II assesses the accuracy of finite element models to predict the creep deformation in two different glass-epoxy joints designed to represent stress states in some opto-electronic applications.

Low-stress creep deformation in two opto-electronic glass-epoxy joints: Part II – Joint measurements and FEA

The creep deformation of two glass-epoxy adhesive joints, representative of joint designs used in opto-electronic devices, was measured at various temperatures and relatively low stress levels using a novel, ultra-sensitive method employing optical interference fringes. The adhesive creep models obtained in Part I were then used as the constitutive equations in finite element models of the joints with the objective of assessing the accuracy of the predictions of creep deformation at the low stresses typical of opto-electronic applications. Good agreement was obtained for both adhesives; however, the higher modulus adhesive was more complicated, exhibiting highly non-linear creep behavior as the stress levels approached zero. The lower modulus adhesive was shown to be linearly viscoelastic over the entire range of stresses encountered in the opto-electronic joints.

Time dependent behavior of FRP-strengthened RC beams subjected to preload: Experimental study and finite element modeling

This paper deals with the influence of preload and FRP stress-lagging on the time dependent behaviors of strengthened concrete beams. For this purpose, three concrete beams were fabricated: one (B1) strengthened directly without any preload; one (B2) preloaded, fully unloaded and then strengthened; one (B3) preloaded by the same value of force as B2, but strengthened with the preload remaining. Sustained loads were applied to test beams and their deformations and cracking patterns were recorded for comparison. According to the test results, B1 presented the largest long-term deformation with the most evident crack propagation in load sustained period, followed by B2 and B3. Accordingly, non-linear finite element (FE) analyses were performed. The validity and accuracy of the FE models were demonstrated by comparing theoretical and experimental results. The effect of adhesive creep was also investigated on the basis of the proposed FE approach. It could be concluded that adequate development of cracks before sustaining loads may have positive influence on better controlling the long-term deformations, while higher level of shear stress in bonding interfaces may present negative effects.

Wheel-rail creep force model for predicting water induced low adhesion phenomena

A computationally efficient engineering model to predict adhesion in rolling contact in the presence of water is presented which may be implemented in multibody dynamics software or in braking models to study train performance and braking strategies. This model has been developed in a project funded by the Rail Safety and Standards Board (RSSB) and Network Rail. It is referred to as the water-induced low adhesion creep force (WILAC) model. The model covers a wide range of conditions from dry over damp to wet. Special emphasis is put on little amounts of water which can cause low adhesion without any oil or grease. Such conditions may be encountered in humid weather or at the onset of rain. The model is parameterised based on experimental results from a tram wheel test rig. Adhesion values as low as 0.06 are observed at high creep with only wear debris and little water present in the contact. The model results also agree with experimental data from locomotive tests in dry and wet conditions.

Creep Testing of One-Component Polyurethane and Emulsion Polymer Isocyanate Adhesives for Structural Timber Bonding*

Abstract Moisture hardening one-component polyurethane (PU) adhesives and emulsion polymer isocyanate (EPI) adhesives have recently gained a considerable market share in structural timber gluing, especially in Europe. The creep behavior of bond lines between solid timber adherends with said adhesives, specifically nine PU and five EPI products, along with two phenolic resorcinol-formaldehyde (PRF) adhesives used for reference purposes, were studied. Most of the adhesives investigated are approved for gluing of structural timber components in Europe. All of the reported 34 bending creep test series (with 340 specimens), were performed with two-part laminated spruce (Picea abies) specimens according to the European structural adhesive creep test standard EN 15416‐3. The standard addresses the ratio RC of creep between PU and EPI adhesives' bonds versus the minimally creeping bond lines of matched specimens glued with a reference PRF adhesive. Mainly glue line thicknesses of 0.3 mm were investigated, with some additional tests having been performed with 0.2- and 0.5-mm-thick glue lines as well. The investigations provide a substantial insight into the creep behavior of the two regarded polyaddition adhesive families. Additionally, the evaluation proves, for PUs and EPIs for test glue line thicknesses of 0.3 mm and less, that it is well justified to reduce the presently prescribed creep periods of 26 and 52 weeks down to 13 and 26 weeks, respectively. The results will serve as a basis for a change of the European test and requirement standards on structural PU and EPI adhesives.

Rheological modeling and finite element simulation of epoxy adhesive creep in FRP-strengthened RC beams

We have shown that a significant creep occurs at the concrete–fiber reinforced polymer (FRP) interface based on double shear long-term test. The primary test parameters were the shear stress to ultimate shear strength ratio, the epoxy curing time before loading as well as the epoxy thickness. The test results showed that when the epoxy curing time before loading was earlier than seven days the shear stress level significantly affected the long-term behavior of epoxy at the interfaces, and in particular the combined effect of high shear stress and thick epoxy adhesive can result in interfacial failure if subjected to high-sustained stresses. In this paper, based on the previous experimental observations, an improved rheological model was developed to simulate the long-term behavior of epoxy adhesive at the concrete–FRP interfaces. Furthermore, the newly developed rheological creep model was incorporated in finite element (FE) modeling of a reinforced concrete (RC) beam strengthened with FRP sheets. The use of rheological model in FE setting provides the opportunity to conduct a parametric investigation on the behavior of RC beams strengthened with FRP. It is demonstrated that creep of epoxy at the concrete–FRP interfaces increases the beam deflection. It is also shown that consideration of creep of epoxy is essential if part or the entire load supported by FRP is to be sustained.

Antiskid control of railway train braking based on adhesion creep behavior

In modern trains wheelset skidding leads to the deterioration of braking behavior, the degradation of comfort, as well as a boost in system hazards. Because of the nonlinearity and unknown characteristics of wheelset adhesion, simplifications are widely adopted in the modeling process of conventional antiskid controllers. Therefore, conventional antiskid controllers usually cannot perform satisfactorily. In this paper, systematic computer simulation and field tests for railway antiskid control system are introduced. The operating principal of antiskid control system is explained, which is fundamental to the simulation of antiskid brakes, and the simulation model is introduced, which incorporates both the adhesion creep curve and a pneumatic submodel of antiskid control system. In addition, the characteristics of adhesion curves and the simulation target are also provided. Using DHSplus, the pneumatic submodel is created to analyze the performance of the different control strategies of antiskid valves. Then the system simulation is realized by combining the kinematical characteristics of railway trains and the pneumatic submodel. The simulation is performed iteratively to obtain the optimized design of the antiskid control system. The design result is incorporated in the hardware design of the antiskid control system and is evaluated in the field tests in Shanghai Subway Line 1. Judging by the antiskid efficiency, the antiskid braking performance observed in the field tests shows the superiority of the optimized design. Therefore, the proposed simulation method, especially in view of its ease of application, appears to be a useful one for designing railway antiskid control systems.

Viscoelastic and fatigue properties of model methacrylate‐based dentin adhesives

The objective of the current study is to characterize the viscoelastic and fatigue properties of model methacrylate-based dentin adhesives under dry and wet conditions. Static, creep, and fatigue tests were performed on cylindrical samples in a 3-point bending clamp. Static results showed that the apparent elastic modulus of the model adhesive varied from 2.56 to 3.53 GPa in the dry condition, and from 1.04 to 1.62 GPa in the wet condition, depending upon the rate of loading. Significant differences were also found for the creep behavior of the model adhesive under dry and wet conditions. A linear viscoelastic model was developed by fitting the adhesive creep behavior. The developed model with 5 Kelvin Voigt elements predicted the apparent elastic moduli measured in the static tests. The model was then utilized to interpret the fatigue test results. It was found that the failure under cyclic loading can be due to creep or fatigue, which has implications for the failure criterion that are applied for these types of tests. Finally, it was found that the adhesive samples tested under dry conditions were more durable than those tested under wet conditions.

Short and long-term bond performance of prestressed FRP sheet anchorages

This paper discusses experimental results of the short and long-term behavior of the anchorage zones of externally bonded prestressed fiber reinforced polymer (FRP) sheets. An experimental program was conducted to investigate seven beams bonded with prestressed FRP sheets including anchored or unanchored FRP sheet ends. Using different layers of FRP sheet, the prestress level of FRP sheets varied from 20% to 40% of the guaranteed tensile strength. The experimental observation was conducted in an outdoor environment and lasted about twenty months when temperatures were in the 7–30 ∘C range. This study provides significant data on the development of the effective bonding lengths, as well as the initiation and propagation of debonding along the FRP-concrete interface due to creep effect of the adhesive layer. Although adhesive creep leads to debonding propagation at higher shear stress, this creep is favored at low shear stress because it increases the effective bond length which improves the bond capacity of FRP-concrete interface and prevents premature failure of the anchorages. The effective bonding length was found to increase to 50% due to creep of the adhesive layer. The anchored end of the FRP sheets using steel plates and anchor bolts is an effective solution to enhance the bond capacity of FRP-concrete interface for short and long-term loading.

Ultrasonic Study of Environmental Damage Initiation and Evolution in Adhesive Joints

Abstract This article reports on an experimental study of environmental degradation of adhesive joints by an ultrasonic angle-beam technique. The technique is based on measurements of the frequency response of the reflection of obliquely incident ultrasonic signals from a joint bondline. Ultrasonic measurements were performed using a special ultrasonic goniometer with only one ultrasonic transducer. By this method, the degradation of single lap adhesive joints was studied as a function of exposure in NaCl solutions at 68°C under static tensile load. It was found that joint degradation is accompanied by a shift of the ultrasonic reflection spectrum minimum to a lower frequency. Two stages of adhesive joint environmental degradation can be distinguished: a) a relatively slow adhesive joint degradation dominated by adhesive creep, and b) delamination along the adhesive/adherend interface, leading to failure. Several degradation mechanisms are found in the first stage to affect the position of the spectral minimum. The first is adhesive creep caused by normal-to-bond-plane stress concentration at the joint overlap edges. This mechanism is found to have the dominant effect on the ultrasonic signature. Second, changes of the effective density and elastic moduli of the adhesive layer also affect the spectrum of the reflected signal. The third mechanism is the degradation of the adhesive-adherend interface. In the second stage of the joint degradation process, delamination along the adhesive/adherend interface occurs and is followed by joint failure. While the time span of the first stage changes significantly from joint to joint, the time span of the second stage (failure by delamination) in our conditions is about 30–40 h. The delamination results in a significant additional spectral minimum shift to a lower frequency that can be used as an indicator of failure initiation.

Ultrasonic Study of Environmental Damage Initiation and Evolution in Adhesive Joints

This article reports on an experimental study of environmental degradation of adhesive joints by an ultrasonic angle-beam technique. The technique is based on measurements of the frequency response of the reflection of obliquely incident ultrasonic signals from a joint bondline. Ultrasonic measurements were performed using a special ultrasonic goniometer with only one ultrasonic transducer. By this method, the degradation of single lap adhesive joints was studied as a function of exposure in NaCl solutions at 68°C under static tensile load. It was found that joint degradation is accompanied by a shift of the ultrasonic reflection spectrum minimum to a lower frequency. Two stages of adhesive joint environmental degradation can be distinguished: a) a relatively slow adhesive joint degradation dominated by adhesive creep, and b) delamination along the adhesive/adherend interface, leading to failure. Several degradation mechanisms are found in the first stage to affect the position of the spectral minimum. The first is adhesive creep caused by normal-to-bond-plane stress concentration at the joint overlap edges. This mechanism is found to have the dominant effect on the ultrasonic signature. Second, changes of the effective density and elastic moduli of the adhesive layer also affect the spectrum of the reflected signal. The third mechanism is the degradation of the adhesive-adherend interface. In the second stage of the joint degradation process, delamination along the adhesive/adherend interface occurs and is followed by joint failure. While the time span of the first stage changes significantly from joint to joint, the time span of the second stage (failure by delamination) in our conditions is about 30–40 h. The delamination results in a significant additional spectral minimum shift to a lower frequency that can be used as an indicator of failure initiation.

Ultrasonic Study of Environmental Damage Initiation and Evolution in Adhesive Joints

This article reports on an experimental study of environmental degradation of adhesive joints by an ultrasonic angle—beam technique. The technique is based on measurements of the frequency response of the reflection of obliquely incident ultrasonic signals from a joint bondline. Ultrasonic measurements were performed using a special ultrasonic goniometer with only one ultrasonic transducer. By this method, the degradation of single lap adhesive joints was studied as a function of exposure in NaCl solutions at 68°C under static tensile load. It was found that joint degradation is accompanied by a shift of the ultrasonic reflection spectrum minimum to a lower frequency. Two stages of adhesive joint environmental degradation can be distinguished: a) a relatively slow adhesive joint degradation dominated by adhesive creep, and b) delamination along the adhesive/adherend interface, leading to failure. Several degradation mechanisms are found in the first stage to affect the position of the spectral minimum. The first is adhesive creep caused by normal-to-bond-plane stress concentration at the joint overlap edges. This mechanism is found to have the dominant effect on the ultrasonic signature. Second, changes of the effective density and elastic moduli of the adhesive layer also affect the spectrum of the reflected signal. The third mechanism is the degradation of the adhesive—adherend interface. In the second stage of the joint degradation process, delamination along the adhesive/adherend interface occurs and is followed by joint failure. While the time span of the first stage changes significantly from joint to joint, the time span of the second stage (failure by delamination) in our conditions is about 30—40 h. The delamination results in a significant additional spectral minimum shift to a lower frequency that can be used as an indicator of failure initiation.

The Dimensional Stability of Adhesives, Zirconia, and Silica in Optical Connector Ferrules and their Impact on Optical Performance

AbstractIn order to achieve the optimum optical performance of fiber optic connectors, it is critical that there be good physical contact at the fiber-to-fiber interface. Loss of physical contact results in increased insertion loss and reflectance. Today's connectors are designed to have mating surfaces with tightly controlled endface geometries to ensure physical contact during temperature variations and repeated matings. However, material degradation of the fiber, ferrule, or the adhesive which holds the fiber into the ferrule, can change the endface geometry and cause eventual failure of the connector. Tests have been developed or are under development to quantify mechanical changes in these materials, including adhesive creep, fiber pistoning, ferrule deformation and roughening, and fiber deterioration. This paper will describe the latest test results acquired from applying these procedures.