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Adhesive bonding has become a fundamental manufacturing technology in the railway industry by allowing lightweight design of structures, which is beneficial in terms of energy, environmental and cost efficiency. Due to the polymeric nature of adhesive materials, the presence of stress multiaxiality has a strong effect on their mechanical behaviour under static and fatigue conditions. The present work deals with an investigation of the multiaxial behaviour of an elastic (polyurethane-based) and a structural (epoxy-based) adhesive for railway applications. Samples with different stress multiaxialities (butt joint, scarf joint, and thick adherend shear test joint) were tested under static and fatigue conditions. The stress multiaxiality was defined as the ratio between the hydrostatic stress (p) and the von Mises equivalent stress (q). Finite Element Analysis showed that the mechanical properties of adhesives have a strong influence of the stress multiaxiality distribution of joints with elastic adhesive reaching higher levels of stress multiaxiality. Static tests revealed that elastic adhesive joints are more sensitive to multiaxiality (i.e. higher hydrostatic stresses) than their structural counterparts, especially in samples with larger hindering of lateral contraction. From fatigue test results of both adhesives, namely SN curves, it was demonstrated how multiaxial p-q fatigue diagrams can be constructed. One of the main advantages of this approach is the possibility of predicting the fatigue lifetime of joints regardless of their multiaxial stress state.

The development of accurate, robust, and reliable methods for fatigue lifetime prediction is a key aspect to safe and efficient design of adhesively bonded structures. Here, a method for lifetime prediction of structural adhesive joints is proposed. The method is based on the stress-life approach and the Theory of Critical Distances (TCD) to account for the effects of stress concentrations. Two types of structural adhesives (epoxy-based and PU-based) are considered for the investigations. Stress assessment is carried out by standard 2D-FEA using continuum elements and elastoplastic material behaviour. A novel process for the determination of the critical distance for fatigue of adhesives is used and validated. The robustness of the method is evaluated by predicting the fatigue lifetime of three notched bulk specimens (2.52

Basalt fiber‐reinforced polybenzoxazines (BFRP) were manufactured through vacuum infusion using resorcinol bis (diphenyl phosphate) and poly‐(m‐phenylene methylphosphonate) together with bisphenol‐F and aniline based benzoxazine. Different types and loadings of flame‐retardant additives showed to have catalysis or dilution effects in viscosity measurements. BFRPs show well‐penetrated fibers and near‐zero porosity. Additive addition did not influence tensile properties, while apparent interlaminar shear strength decreased indicating a lower adhesion between fiber and matrix. BFRP's heat and smoke release properties increased, though time to ignition increased and flammability behavior improved by decreasing delamination yielding oxygen indices in between 72 and 91%.

Bisphenol F and aniline-based benzoxazine monomers were selected to fabricate basalt, glass and carbon fiber reinforced polybenzoxazine via vacuum infusion, respectively. The impacts of the type of fiber reinforcement on the resulting material properties of the fiber reinforced polymers (FRPs) were studied. FRPs exhibited a homogenous morphology with completely impregnated fibers and near-zero porosity. Carbon fiber reinforced polybenzoxazine showed the highest specific mechanical properties because of its low density and high modulus and strength. However, regarding the flammability, fire, smoke and toxicity properties, glass and basalt reinforced polybenzoxazine outperformed carbon fiber reinforced polybenzoxazine. This work offers a deeper understanding of how different types of fiber reinforcement affect polybenzoxazine-based FRPs and provides access to FRPs with inherently good fire, smoke and toxicity performance without the need for further flame retardant additives.

Under working conditions, adhesively bonded structures are exposed to variable cyclic loading and temperature variation. However, there is still limited number of works dealing with their relationship. The present study investigates the combined effect of variable amplitude stress and variation of temperature on the fatigue damage behaviour of adhesively bonded scarf joints. Initially, experiments at constant amplitude stress, which showed reduction of fatigue strength with increasing temperature, were carried out to establish base SN curves for calculations of cumulative damage. Then, tests under variable amplitude fatigue (VAF) were performed with constant temperature and with two, four and six temperature changes. Temperatures for VAF experiments were −10, 23 and 50 °C. VAF at constant temperature demonstrated: (i) damage retardation at −10 and 23 °C and damage acceleration at 50 °C and (ii) no clear effect on the fracture mode. Conversely, VAF with temperature variation showed: (i) high damage acceleration with an increase proportional to the number of temperature changes and (ii) a trend of more adhesive failure. For lifetime predictions made with a linear damage rule, good correlation was obtained at constant temperature. However, under variable temperature conditions caution must be taken since predictions overestimated fatigue lifetime.

The present work provides a comparative investigation between different methods of manufacturing basalt fibre reinforced polybenzoxazines (BFRP), including vacuum infusion, hand laminating, dynamic fluid compression moulding and autoclave curing processes. In comparison to the high pressure based autoclave-cured and compression-moulded BFRPs, vacuum-infused BFRPs showed similar or even higher mechanical properties. Despite the low pressure curing, vacuum infusion yielded BFRPs with a 10% higher tensile strength and a 24% higher strain at failure compared to its autoclave-cured counterparts. Thus, it is possible to gain BFRPs with near-zero porosity and high mechanical properties without the need of high pressure curing methods.

The effect of stress concentrations on the fatigue behaviour of two structural adhesives, relevant materials in lightweight applications, was investigated regarding fatigue strength, crack initiation lifetime, damage evolution and fracture behaviour. Internally and externally notched adhesive bulk specimens were tested under constant amplitude loading with a stress ratio of 0.1. Majority of fatigue lifetime (60–95%) was spent on crack initiation, which was shortened by an increasing stress concentration factor. Crack initiation causes a sudden damage increase near to failure. A model was proposed correlating the fatigue strength of notched specimens with highly stressed region and stress concentration factor.

Polybenzoxazines-based composites are an excellent alternative in lightweight design due to their outstanding properties in terms of chemical stability, inherent flame resistance, as well as mechanical performance. The present work provides an investigation of different joining techniques for polybenzoxazine-based composites, namely adhesively - and hybrid bonded joining. The effects of fibre reinforcement (carbon or basalt), and substrate thickness (2 or 3 mm) on the lap-shear strength of joints are taken into consideration. Here, basalt (BFRP) and carbon fibre reinforced plastics (CFRP) were manufactured by vacuum infusion technique followed by a convection oven curing. Mechanical characterisation of substrates included tensile and interlaminar shear strength testing. Hybrid joints were fabricated by rivet insertion in cured joints bonded with a structural adhesive. Adhesive bonding showed to be a proper joining technique presenting a good adhesion with substrates, and adequate cohesive strength. Hybrid bonded joints had a fail-safe fracture and higher energy absorption prior to failure than their adhesively bonded counterparts. No effect of substrate thickness on lap-shear strength was observed. The dominating failure mechanism for the joints was the delamination of the topmost-layer due to peel stresses. CFRP joints exhibited higher lap-shear strength than BFRP joints, which was consistent with interlaminar shear strength results.

In this work, three types of structural modified epoxy adhesives were used to investigate the effect of stress concentrations on the fatigue behavior of notched bulk specimens. SN curves of un-notched and notched specimens were determined at constant amplitude and R = 0.1 in the range between Nf = 103 (LCF) and Nf = 106 (HCF). The following key conclusions were made: (i) fatigue strength was reduced due to the presence of notches, especially at the HCF; (ii) adhesives showed different values of notch sensitivity with values for the adhesives lower than typical values of metals; (iii) for un-notched samples fatigue strength was between 62 and 78% of tensile strength for Nf = 103 and around 50% for Nf = 106; (iv) for notched samples fatigue strength was between 67 and 78% of the tensile strength for Nf = 103 and around 40% for Nf = 106; (v) fractography evidenced the presence of voids and shear yielding around the notches, (vi) unnotched samples showed the same fracture behavior for both LCF and HCF with crack formation at the external surface. For notched samples there was a significant distinction between LCF and HCF with cracks forming at the notch root.

The influence of stress concentrations on the tensile static strength and fracture behaviour of notched bulk specimens was investigated by comparing the response of two epoxy-based structural adhesives (a rubber-toughened and a polyurethane-toughened). Numerical predictions of failure stress were carried out using a 3D-FEA model with a hydrostatic dependent elastoplastic material behaviour and the equivalent plastic strain for failure assessment. The ductile adhesive, which plastically deforms more under high stresses, provided experimental evidence of a notch strengthening effect. Conversely, the less ductile adhesive has shown a reduction in tensile strength compared to un-notched samples. For both adhesives fracture surface analysis showed the presence of stress whitening and voids close to the notch regions. These regions could be correlated to higher values of stress triaxiality using numerical simulation. The more ductile adhesive underwent widespread stress whitening prior to failure, whereas the response of the less ductile adhesive was more localised. Numerically based predictions showed excellent agreement with experimental results with average error of 5.1% for different notch types in both adhesives.

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.

In the present work the influence of material model and multiaxiality on the fatigue lifetime predictions of scarf and single-lap joints under uniaxial tension-tension cyclic loading is investigated. Lifetime predictions, from 103 to 106 cycles, were performed using a stress-life approach taking into account for the effect of multiaxiality by means of hydrostatic pressure (p) and von Mises stress (q). These stresses were calculated employing 2D-FEA with linear-elastic and elastoplastic (von Mises and Drucker-Prager) material models. Effective stresses were obtained using the theory of critical distances. Findings showed that material model affected significantly the multiaxiality distribution of the single lap joint (SLJ), especially at singularity regions. For scarf joints, this effect was noticeable for higher stresses. With the exception of the SLJ, by excluding the singularity regions close to the free edges, the value of multiaxiality was nearly constant and decreased with increasing bondline angles (higher shear effects). With regards to lifetime predictions, overall accuracy for the scarf joint was higher than for the SLJ. Linear-elastic material models provided satisfactory accuracy for the scarf joint, but not for the SLJ. Elastoplastic material models were able to improve predictions and to provide suitable accuracy for both joint configurations.

The use of adhesive bonding for designing lightweight load-bearing components has increased in recent decades. In this paper the influence of plasticity on the lifetime prediction of bonded joints using the stress-life approach was investigated. The adhesive was a toughened epoxy for structural applications. Stress calculations were performed using finite element analysis. Three material models were employed, a linear-elastic model and two elastoplastic models: Von Mises (pressure independent) and Drucker-Prager (pressure dependent). Effective stress was calculated using the theory of critical distances. Lifetime predictions were based on SN curves from literature for scarf and single-lap joints at four different temperatures (−35, −10 °C, RT, +50 °C). The material properties were acquired from uniaxial tensile quasi-static experiments on bulk adhesive specimens. These experiments showed a reduction in the values of Young's modulus and yield stress with increasing temperature. A model was proposed based on an Arrhenius-type equation in order to fit the yield stress as a function of temperature. The model showed good agreement to the experimental findings. Regarding lifetime predictions (a) the influence of critical distance was higher for single-lap joints than scarf joints and (b) the prediction errors were lower for elastoplastic modelling than linear-elastic modelling, especially for single-lap joints.

Toughened epoxy adhesives are widely used for structural applications, especially in the design of lightweight products. During their service life, bonded joints are exposed to cyclic loading under a broad range of temperatures. In this study the influence of temperature on the fatigue behaviour of a toughened epoxy adhesive was investigated by experimental determination of SN curves under tension-tension cyclic loading. Bulk adhesive specimens and single lap, thick adherend and scarf joints were tested at a stress ratio of R = 0.1 and a frequency of f = 7 Hz at five different temperatures (-35 degrees C, -10 degrees C, RT, + 50 degrees C, + 80 degrees C). Dynamic mechanical analysis (DMA) was conducted to evaluate the temperatures of the phase transitions in the adhesive. Based on the experimental findings an interpolation method was proposed for estimating SN parameters at temperatures unaffected by phase transitions. The fatigue tests showed there was a reduction in fatigue lifetime with increasing temperature. DMA results indicated the adhesive underwent two phase transitions. The results from interpolations for different types of samples showed excellent agreement with the experimental findings, confirming the validity of the method, providing no phase transitions are within or close to the interpolation range.

The fatigue lifetime of joints bonded with a toughened epoxy adhesive has been estimated using the stress-lifemethod, based on experimental SN curves. Five temperatures (35 degrees C, -10 degrees C, RT, + 50 degrees C, + 80 degrees C) were considered. The estimations were performed using a direct approach, for joints having near-homogeneous stress distributions (scarf joints), and; an optimised approach, for joints having inhomogeneous stress distributions (single lap joints). The stress distributions were calculated considering linear-elastic material behaviour using analytical methods and finite element analysis. For inhomogeneous stress distributions the effective stress was defined in terms of the theory of critical distances. The calculations were performed considering: (i) uniaxial stress, evaluating the maximum principal stress, and considering (ii) multiaxial stress, for the influence of the relationship between hydrostatic and deviatoric stresses. The estimations were validated with fatigue experiments under tension-tension loading at a stress ratio of 0.1 and a frequency of 7 Hz. The experimental results showed a reduction of fatigue lifetime with increasing temperature. The estimated values indicate a better quality of prediction for the multiaxial stress and that it is more preferable to use a base SN curve having a stress distribution as similar as possible to the stress distributions of the components to be designed.