Investigating the influence of the chamfer and fillet on the high-cyclic fatigue strength of adhesive joints of steel parts

This paper presents the results of research undertaken to determine the possibility of improving the fatigue properties of peel-loaded adhesive-bonded joints by the constructional modification of the adherend. Fatigue strength tests were carried out on the electromagnetic inductor at the resonance frequency of the adhesive-bonded joint specimens. The tests were carried out on the specimens bonded by means of Bison Epoxy and Epidian 57 epoxy compositions with poly-aluminium chloride hardener. The joined elements were modified by making the chamfer or fillet to enlarge the thickness of the adhesive layer with the aim of reducing the stress concentration in the frontal part of the joint. This modification is the result of a research that confirms the existence of a stress concentration on the short section of the frontal part of an adhesive joint. This phenomenon can lead to the rapid initiation of adhesive joint destruction. The fatigue strength tests revealed a significant improvement in fatigue endurance.     

Modeling the constant amplitude fatigue behavior of adhesively bonded pultruded GFRP joints

The constant amplitude fatigue behavior of adhesively bonded pultruded glass fiber reinforced polymer double-lap joints were modeled by a number of conceptually different phenomenological S–N (cyclic stress vs. number of cycle) formulations. An extended database containing constant amplitude fatigue data under tension (R?=?0.1), compression (R?=?10), and reversed loading (R?=??1) were analyzed in order to investigate whether or not there exists an appropriate fatigue formulation for accurate modeling of the behavior of the examined joints throughout their lifetime, from the very low-cycle fatigue to the high-cycle fatigue regions. Based on an extensive review, appropriate fatigue formulations that take into account the probabilistic nature of lifetime measurements were selected and their fundamental assumptions were examined. The validity of the statistical assumptions of these models was found to be influenced by the applied loading conditions. The modeling results were similar for all selected fatigue formulations with the derived S–N curves exhibiting differences mainly in the low- and high-cycle fatigue regimes. The formulations insensitive to the scatter in the experimental data were found to be the most appropriate models.     

Investigation of debonding propagation in aluminum/composite joints under fatigue loading

In this paper, Fracture Mechanics is used to predict debonding propagation in adhesive joint between aluminum and composite plates. Three types of loadings (λ = 0, λ = 0.5, and λ = 1 when λ is the ratio of lateral to in-plane loading) and two types of glass–epoxy composite sequences: [0/90]_2s and [0/45/?45/90]_s are considered for the composite plate and their results are compared. It was seen that generally the cases with stacking sequence of [0/45/?45/90]_s have much shorter lives than cases with [0/90]_2s. It was also seen that in cases with λ = 0, the ends of the debonding front propagates forward more than its middle, while in cases with λ = 0.5 or λ = 1 it is vice versa. Moreover, regardless of value of λ, the difference between the debonding propagations of the ends and the middle of the debonding front is very close in cases λ = 0.5 and λ = 1. Another main conclusion was that the non-dimensionalized debonding front profile is almost independent of sequence type or the applied load value.     

Moisture and temperature degradation of double cantilever beam adhesive joints

In this work, the double cantilever beam (DCB) test is analysed in order to evaluate the combined effect of temperature and moisture on the mode I fracture toughness of adhesives used in the automotive industry. Very few studies focus on the combined effect of temperature and moisture on the mechanical behaviour of adhesive joints. To the authors’ knowledge, the simultaneous effect of these conditions on the fracture toughness of adhesive joints has never been determined. Specimens using two different adhesives for the automotive industry were subjected to two different ageing environments (immersion in distilled water and under 75% of relative humidity). Once they were fully degraded, they were tested at three different temperatures (?40, 23 and 80 °C), which covers the range of temperature an adhesive for the automotive industry is required to withstand. The aim is to improve the long term mechanical behaviour prediction of adhesive joints. The DCB substrates were made of a high strength aluminium alloy to avoid plastic deformation during test. The substrates received a phosphoric acid anodisation to improve their long term adhesion to the adhesive. Results show that even though a phosphoric acid anodization was applied to the adherends, when the aged specimens were tested at room temperature and at 80 °C, they suffered interfacial rupture. At ?40 °C, however, cohesive rupture was observed and the fracture toughness of the aged specimens was higher.     

Mechanical behavior of polyurethane adhesive bonded joints as a function of temperature and humidity

The temperature and humidity were found to be the most effective parameters in the behavior of polyurethane flexible adhesive bonded aluminum joints. In order to obtain the effect of environment on bond strength, toughness, failure displacement, joints stiffness and failure model, in this work, aluminum single-lap joints were tested under various temperatures (25, 40, 60 and 80 °C) and relative humidity (RH, 55, 65, 75, 85, 95 and 99%) using an environmental chamber. The results showed that as the humidity increased from 55 to 99%, bond strength decreased as linear function. As the temperature increased from 25 to 80 °C, the bond strength decreased as exponential function. The joints stiffness reduced gradually with the increase of temperature and humidity. The analysis of the failure section of the ageing joints showed that the humidity caused the transition of the failure model, and the increase of the temperature promoted the change of the failure model. Besides, at low humidity (55 and 65%), failure displacement decreased gradually with the increase of temperature, and at high humidity (95 and 99%), failure displacement increased. This study will help engineers design a reliable, safe and effective bonding structure. And it is conducive to solve the problem of joint strength degradation in the hygrothermal environment.     

Impact of multiwall carbon nanotubes on the fatigue strength of adhesive joints

This paper presents the results of research undertaken to determine the possibility of improving the fatigue properties of peel-loaded adhesive joints by dispersing multiwall carbon nanotubes (MWCNTs) into epoxy-based adhesives. The fatigue strength tests were carried out on an electromagnetic inductor with the resonance frequency of the adhesively bonded joint specimen. The tests were conducted for three types of epoxy adhesives whose properties were modified through the introduction of multiwalled carbon nanotubes, into their structure. Carbon nanotubes were synthesized by means of the Chemical Vapour Deposition (CVD) method with Fe-Co catalysts. A quantity of 1 wt.% of the dried material was dispersed into the epoxy adhesives. The results of the fatigue strength tests revealed a significant improvement of the fatigue lifetime of adhesive joints due to MWCNT introduction as filler for epoxy adhesives. In the case of the Epidian 57/PAC adhesive composition, a more than twofold increase in the fatigue lifetime was obtained (an increase of 106.8%). For the Bison Epoxy adhesive composition, the fatigue lifetime increased by 69.3%. The fatigue strength for the best result increased by about 13%.     

Assessment of the fatigue life of aluminium spot-welded and weld-bonded joints

In modern machinery and automobile structures weight reduction and increased durability are the main issues in design. In these applications, lap welded and/or bonded joints are widely used; therefore, tools are needed to accurately predict their fatigue life. This paper is concerned with the fatigue strength of single lap joints formed with thin plates of 6082-T6 aluminium alloy using a high strength two-component epoxy adhesive (Araldite 420 A/B from Hunstman). Experimental S–N curves were obtained for resistance spot-welded and weld-bonded lap joints. The fatigue lives of weld-bonded joints were significantly higher than those of resistance spot-welding joints. In addition, fatigue lives were predicted with Morrow's modified Manson–Coffin (M/M–C) and the Smith–Watson–Topper (S–W–T) damage equations. Elastic–plastic numerical models were developed, replicating the experimental work, in order to obtain local stress and strain fields. An acceptable agreement was obtained between the numerical predictions and the experimental results. The M/M–C damage equation diverged from experimental results for relatively long fatigue lives, while the S–W–T equation gave good predictions for all fatigue lives.     

Study on the fatigue strength of AA 6082-T6 adhesive lap joints

A research study on the fatigue behaviour of aluminium alloy adhesive lap joints was carried out to understand the effect of surface pre-treatment and adherends thickness on the fatigue strength of adhesive joints. The adherend material used for the experimental tests was an aluminium alloy 6082-T6 in the form of thin sheets, and the adhesive used was a high strength epoxy (Araldite 420 A/B). The surface preparation included an abrasive preparation (AP joints) and sodium dichromate–sulphuric acid etch (CSA joints).A maximum fatigue strength was obtained for the CSA surface treatment with a 1.0 mm adherends’ thickness. The fastest fatigue damage was related with a high surface roughness and a high stress perpendicular to adhesive surface, which helps to promote the adhesive failure. A numerical analysis was also performed to understand the effect of the adherends thickness on the stress level. Results showed an increase of the out-of-plane peak stresses with the increase of adherends thickness.     

Influence of mechanical surface treatment on fatigue life of bonded joints

Adhesively bonded joints can support a longer fatigue life if compared to conventional joining techniques, provided that a set of requirements is fulfilled. One of the most important requirements is the mechanical preparation of the bonded joint surface, which improves the joint interface adhesion. The aim of this work is to investigate the influence of surface roughness of mild steel substrates on fatigue behavior in adhesive bonded plates. To accomplish this objective, three different surface treatments were used on A36 steel substrate specimens, namely sand blasting, grit blasting, and bristle blasting. Bonded plate specimens, using end-notched flexure format, with a thin adhesive epoxy layer were manufactured and tested, under mode II loading condition, in both static and dynamic tests. The results confirm the importance of surface treatment of the substrate on the fatigue life, confirming that adhesively bonded joints have significant performance differences when subjected to static and dynamic loadings.     

Effect of vibration fatigue on modal properties of single lap adhesive joints

As most existing studies focus on developing models and theories describing the static strength of adhesive joints as a function of the fatigue loading, there is a lack of understanding on how the fatigue of the adhesive joint affects dynamic modal properties of the bonded structure. In applications such as automobile components, modal properties are critical in determining their dynamic performances. To investigate the relationship between modal properties of single lap joints (SLJs) and the cyclic-vibration-peel loading, this study first carries out vibration fatigue tests and subsequent modal response measurements using steel–aluminum SLJ specimens. It is experimentally demonstrated that modal frequencies of the SLJ structure tend to decrease with increasing vibration fatigue cycles. Furthermore, it is also shown that this trend is related to the fatigue characteristics of the adhesive layer. The fatigue degradation effects of Young's modulus and contact area between the adhesive and the adherends on modal frequencies are then investigated using a finite element model. Simulation results reveal that dramatic reductions in modulus and contact area values are required to result in the modal frequency shifting observed in experiments, which may not be always realistic. Although the findings in this study are informative, more research effort is needed to further identify the critical reason(s) for the experimental trend of decreasing modal frequencies with increasing vibration fatigue cycles.     

Process variable effect on the strength of autoclave-bonded film adhesive joints

This study investigates the effect of five independently-controlled process variables and variable combinations on the shear strength of autoclave-bonded film adhesive joints. Studied variables include the cure temperature, cure pressure and their respective ramp rates, as well as the duration of cure time. A full factorial design of experiment (DoE) at two levels for each variable is conducted with 3 replicas of each test. Test coupons are made of two layers of polycarbonate lexan that are autoclave-bonded using aliphatic polyether film adhesive (Huntsman PE399). Two set of test joints are used for generating test data on shear strength and failure mode. Bonded joints in the first set are tested prior to any environmental cycling, in order to generate baseline data on joint shear strength and failure mode. However, samples from the second set of autoclave-bonded joints were heat-cycled, in an environmental chamber at high relative humidity, prior to testing for shear strength. Test data on shear strength and failure mode is statistically analyzed using ANOVA.     

Environmental damage and degradation of FRP composites: A review report

Fiber‐reinforced plastics (FRP) composites are prime choice materials in various civil engineering structures and high performance aerospace components. They exhibit superior mechanical properties than their metallic counterparts. However, they are susceptible to environmental damages and degradations. To utilize the full potential of FRP composite materials their physical, chemical, and mechanical behavior in different environmental conditions should be well established. Present review is an attempt at highlighting different damage/degradations that may be caused to the FRP composite on its exposure to various environmental conditions. POLYM. COMPOS. 36:410–423, 2015. © 2014 Society of Plastics Engineers     

Modelling adhesive joints with cohesive zone models: effect of the cohesive law shape of the adhesive layer

Adhesively-bonded joints are extensively used in several fields of engineering. Cohesive Zone Models (CZM) have been used for the strength prediction of adhesive joints, as an add-in to Finite Element (FE) analyses that allows simulation of damage growth, by consideration of energetic principles. A useful feature of CZM is that different shapes can be developed for the cohesive laws, depending on the nature of the material or interface to be simulated, allowing an accurate strength prediction. This work studies the influence of the CZM shape (triangular, exponential or trapezoidal) used to model a thin adhesive layer in single-lap adhesive joints, for an estimation of its influence on the strength prediction under different material conditions. By performing this study, guidelines are provided on the possibility to use a CZM shape that may not be the most suited for a particular adhesive, but that may be more straightforward to use/implement and have less convergence problems (e.g. triangular shaped CZM), thus attaining the solution faster. The overall results showed that joints bonded with ductile adhesives are highly influenced by the CZM shape, and that the trapezoidal shape fits best the experimental data. Moreover, the smaller is the overlap length (L_O), the greater is the influence of the CZM shape. On the other hand, the influence of the CZM shape can be neglected when using brittle adhesives, without compromising too much the accuracy of the strength predictions.     

Flexural fatigue behavior of injection-molded composites based on poly(phenylene ether ketone)

Flexural fatigue tests were conducted on injection-molded short fiber composites, carbon fiber/poly(phenylene ether ketone) (PEK-C) and glass fiber/PEK-C (with addition of polyphenylene sulfide for improving adhesion between matrix and fibers), using four-point bending at stress ratio of 0.1. The fatigue behavior of these materials was presented. By comparing the S-N curves and analyzing the fracture surfaces of the two materials, the similarity and difference of the failure mechanisms in the two materials were discussed. It is shown that the flexural fatigue failure of the studied materials is governed by their respective tensile properties. The matrix yielding is main failure mechanism at high stress, while at lower stress the fatigue properties appear fiber and interface dominated. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1857–1864, 1997     

Coupling effect of non-relaxing and high temperature on fatigue properties of carbon-black filled isoprene rubber

This paper presents an extensive experimental investigation on fatigue properties of carbon-black filled isoprene rubber under complex loadings. A basic life prediction model taking strain energy density as fatigue parameter is first proposed based on fatigue crack growth tests and uniaxial tension fatigue tests under relaxing loads at room temperature. A database of fatigue life including relaxing and non-relaxing is established through a great number of fatigue tests under five temperatures. Based on the database, two empirical parameters, that is, temperature factor (N_RT/N_HT) and life reinforcement factor (Re), are introduced to quantitatively characterize the coupling effects of high temperature and non-relaxing loads on fatigue life. The fatigue mechanisms under different conditions are compared via wide-angle x-ray diffraction tests and postmortem analysis. It reveals that the weakening of life reinforcement at high temperatures is closely related to the strain induced crystallization behavior of rubber.     

Effect of notch size on the fatigue damage behaviour of toughened epoxy adhesive specimens

ABSTRACT

In the present work the influence of notch size on the fatigue damage behaviour of toughened epoxy adhesive specimens is investigated. Notched and un-notched bulk adhesive specimens were fatigue tested at room temperature under tension-tension cyclic loading at a stress ratio of 0.1. The investigation was based on the analysis of fatigue life (SN) and stiffness degradation curves, which were correlated with notch size and applied stress. Finite element analysis (FEA) was carried out in order to evaluate the notch-dependent stress concentrations. Fatigue results evidenced a reduction of lifespan with increased applied stress amplitude and a possible relationship between the inverse slope of SN curves and notch size. Most notched samples exhibited lower fatigue strength in comparison to un-notched, except in the low cycle fatigue range where un-notched and notched samples had similar fatigue strength. Stiffness degradation showed a correlation with applied stress, i.e. an increase in applied stress was accompanied by faster and stronger degradation. For higher loads, un-notched and 0.2 mm notch samples presented greater stiffness degradation prior to failure than other notched samples.     

Experimental and numerical analysis on fatigue durability of single-lap joints under vibration loads

Fatigue is one of the most common yet complicated failures that can cause damage to mechanical structures. Structural adhesively bonded joints are not exempt from this deleterious phenomenon and have to be assessed under vibration loads. In this work, fatigue characteristics of single-lap joints (SLJ) made of steel and carbon fibre reinforced plastic (CFRP) laminates under vibration loads are primarily investigated by experiments. The aim of this work is to analyze the changes in the ultimate load of the SLJ under vibration loads. The experimental results showed that SLJ will face cohesive failure after the uniaxial tensile loading test. In addition to the increase of vibration cycles, the ultimate load and failure displacement gradually decrease. In order to model the adhesive between joint components and simulate the damage propagation, a new traction–separation law called the embedded process zone (EPZ) and a damage factor are introduced and developed within the framework of cohesive zone Modeling (CZM) techniques. Meanwhile, the stress variations in the adhesive layer of SLJ in different vibration cycles are researched using the finite element method in ABAQUS.     

Bonding kinetics of thermosetting adhesive systems used in wood-based composites: the combined effect of temperature and moisture content

—The processes operative within wood-based composites during hot pressing are briefly reviewed, and the role of adhesion in the system is considered. A new method is described for investigating how temperature and moisture content together affect the kinetics of thermosetting adhesive bond strength development. For this purpose, powdered phenol formaldehyde to wood bonds were formed under highly controlled, nearly steady-state conditions of temperature (between 90 and 120°C) and moisture content (between 4 and 16%); after a range of curing times, the bonds were immediately destructively tested in tension. Plotting accumulated strength versus forming time enables construction of strength-development curves, which reveal an initial delay before the onset of strength development, a period of nearly constant strength development, a small discontinuity in which the bonding rate is reduced, and, finally, a period of well-defined bonding at an ever decreasing rate. All the stages of these curves were found to be affected interactively by local temperature and moisture content. The term 'reactivity index' was coined to describe the dependence of strength-development rates on temperature for a specified moisture content. The derived relationships may be used in simulation models to estimate the development of bonding within composites as hot pressing proceeds. The technique may also be used more generally to aid in the development of adhesive systems with bonding characteristics tailored for specific curing conditions.     

The effect of primers on adhesive properties and strength of adhesive joints made with polyurethane adhesives

The paper presents selected aspects of the effect of primers on adhesive properties and strength of aluminium sheet adhesive joints, made using polyurethane adhesives. The strength of adhesive joints was determined based on two cure time variants: 15 and 64 h. It was found that the longer cure time at a humidity of 33% is more desired, as it leads to a substantial increase in strength of the tested adhesive joints. In addition, two variants of surface preparation were applied: degreasing and degreasing followed by the application of a primer (a pro-adhesive agent). It was observed that the primer application prior to the application of an adhesive leads to a significant increase in strength compared to the variant where the adhesive application is preceded only by degreasing. Moreover, the aluminium sheet surface that was subjected to cataphoretic painting and priming exhibits better adhesive properties. It has a higher value of both surface free energy and its dispersion and polar components compared to the surface that was only subjected to degreasing.