Compact tension test for fracture characterization of thin bonded asphalt overlay systems at low temperature

Asphalt overlays provide an economical means for treating deteriorated pavements. Thin bonded overlay (TBO) systems have become popular options for pavement rehabilitation. In addition to functional improvements, these systems ensure a high degree of waterproofing benefits. Conventional asphalt concrete fracture tests were developed for pavements with homogeneous asphalt concrete mixtures, and typically their thicknesses exceed 50?mm (2?inch). The use of spray paver technology for construction of TBO leads to continuously varying asphalt binder content, up to approximately one-third of the layer thickness. Commonly utilized fracture test geometries for asphalt concrete include the single-edge notched beam, SEN[B], the disk-shaped compact tension, DC[T], and the semi-circular bend, SC[B]. The SEN[B] test geometry is not preferable for use in pavement systems due to difficulties in procuring beam samples from the field. Applications of the other established test geometries, the DC[T] and SC[B] tests, are limited because of the material nonhomogeneity caused by nonuniform distribution of asphalt binder and smaller as-constructed thicknesses of TBO, which are usually less than 25?mm (1?inch) for gap-graded and 50?mm (2?inch) for dense-graded hot mix asphalt (HMA) mixtures. Both the DC[T] and SC[B] tests simulate movement of the crack fronts in transverse or longitudinal directions in the pavement. Use of these tests on field-procured samples of TBO yields a crack front that encounters nonhomogeneous material through the specimen thickness. The crack moves perpendicular to the axis of material nonhomogeneity, which makes data interpretation and fundamental material fracture characterization challenging. In addition, the crack in the specimen is correlated to a crack channeling across the pavement width rather than a bottom-up or top-down direction, which is more desirable from the standpoint of coupling experimental results with currently available simulation models. This paper proposes a test procedure for fracture characterization of graded asphalt pavement systems that have significant material property gradients through their thicknesses. Suitable specimen geometry and testing procedures were developed using ASTM E399 and ASTM D7313-07b as a starting point. Laboratory tests were performed using an optimized compact tension, or C[T], test geometry for field cores as well as laboratory-fabricated composite specimens. Laboratory testing using the proposed procedure clearly showed distinction in the fracture characteristics for specimens prepared with varying material compositions. The capability of distinguishing different materials combined with stable crack growth makes the proposed testing procedure ideal for fracture characterization of thin and graded pavement systems. Statistical analysis of test data revealed that the proposed C[T] test procedure is capable of detecting differences in fracture energy results across a wide range of pavement systems and yields a low test variability. Finite element simulations of the test procedure further indicate the suitability of the test procedure as well as demonstrating a procedure for extraction of fundamental material properties.     

Phenomenological fracture model for biaxial fibre reinforced composites

This paper proposes a new mathematical fracture model (FM) applicable to a biaxial reinforced composite material. The mathematical model provides predictions about the limit state of composite material. It is applicable both in uniaxial and biaxial requests. The mathematical model is validated by comparing its predictions with the experimental data obtained by authors. The studied composite material is composed by carbon fibre in epoxy matrix. The process used for obtaining the composite materials plates is vacuum forming.     

A fracture mechanics model for fatigue in concrete

A theoretical model based on fracture mechanics is developed. The tensile failure of plain concrete during both monotonic and cyclic loading conditions can be described. The results of a numerical study utilizing the model are compared with experimental results.     

A coupling model of reinforced concrete fracture

In this paper, a coupling model is presented for analysis of reinforced concrete fracture. An effective approach, the Boundary Collocation Method, and an iteration procedure are employed in the analysis. Based on the assumed suitable stress functions, stress intensity factors, displacements of the crack surfaces and forces of the steel reinforcement can be easily calculated. Further analyses, such as determination of the ultimate load and direction of the crack propagation, can also be carried out on the basis of the present calculation and fracture criteria.

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Résumé Dans cet article nous présentons un modèle de couplage pour l'analyse de la rupture du béton armé. Pour l'analyse, on a utilisé une approche basée sur la méthode spécifique de traitement des conditions aux limites, et une procédure itérative. En supposant des fonctions de contrainte convenables, on a pu calculer les facteurs d'intensité de contrainte, les déplacements des surfaces des fissures et les efforts dans l'acier des armatures. Des analyses complémentaires, telle que la détermination de la charge ultime et la direction de la propagation des fissures, peuvent également être menées sur la base de ces calculs et les critères de rupture.

     

A statistical model for fatigue fracture under constant-amplitude cyclic loading

A statistical approach to fatigue fracture under constant load is presented. Crack initiation due to detrimentally-acting external factors is considered, with a time-dependent rate assumed for an ensemble of growing stochastically-independent fatigue cracks. For crack increments under cyclic loading, a proportional-effect model is applied. It is shown that at low rates of surface crack initiation and low scatter of the crack increments, the median value of the fracture stress appreciably exceeds the one predicted by the deterministic model, whose estimate is conservative. At high rates and high scatter, the picture is reversed.     

An accumulation damage model for fatigue fracture of ferroelectric ceramics

The accumulated plastic displacement criterion for crack propagation in traditional materials is extended to develop equations to predict the fatigue crack growth of ferroelectric ceramics subjected to combined electromechanical loads. The crack-line is perpendicular to the poling direction of the medium. An electric saturation zone and a stress saturation zone are assumed to develop at the crack tips when the medium is subjected to external electromechanical loads. This assumption makes it possible to obtain the accumulated plastic deformation in closed form. A fatigue crack growth law, which is a fourth-power function of the effective stress intensity factor, similar to the well-known Paris law, is derived. Graphical results for the effect of electric load on the effective crack tip stress intensity factor and crack growth rate are provided.     

Mortar fatigue model for meso-mechanistic mixture design of ravelling resistant porous asphalt concrete

This paper presents the development of a practical mortar fatigue model on the basis of the dissipated energy concept. A specially designed test setup was developed for characterization of mortar fatigue at meso-scale by means of dynamic shear rheometer. Test results showed that mortar fatigue models based on the dissipated energy concept can be developed for the purpose of life predictions under complicated loading conditions. The dissipated energy per cycle in the initial phase of fatigue tests is a practical indicator for fatigue life determination purposes than the total energy dissipated during a fatigue test. Since a mortar fatigue model based on the initial dissipated energy per cycle was adopted, effects of random stress and strain signals on mortar fatigue can be taken into account.     

A static fatigue model for the durability of glass fibre reinforced cement

This paper presents a model for predicting service lives for glass-fibre reinforced cement (grc) components using hot-water accelerated ageing. It improves on previous models, being derived from consideration of a specific proposed micro-mechanical strength loss mechanism based on static fatigue principles and can be applied from time = 0. The model fitted well to all available strength vs. time data pertaining to various grc formulations. The activation energies thus derived for the strength loss process (80–90 kJ mol^–1) were consistent with those derived previously and those proposed for general glass dissolution mechanisms. Updated acceleration factors for predicting service lives for grc are advanced. The model was also applied to grc made with modified cement matrices. For metakaolin modified matrices, the activation energy appeared similar to that for OPC-grc, thus the use of similar acceleration factors appears justified. There is some evidence that calcium sulphoaluminate modified grc degrades according to a different activation energy. More data are required for modified matrix grcs if the model is to be applied thereto with confidence.     

Probabilistic model of fatigue fracture of carbon nanostructures

Presented evaluation of risk of fatigue fracture of carbon nanostructures (CN) is based on: (a) the criterion of the threshold of the dissipated energy of tensile stresses, and (b) simulation of stochastic character of fracture by the failures in queueing system. The assessments of probability of fatigue fracture of CN in conditions of uniaxial tension, plane torsion and uniaxial compression are done.     

A plastic fracture mechanics model for characterization of multiaxial low-cycle fatigue

The near-tip stress and strain fields of small cracks in power-law hardening materials are investigated under plane-stress, general yielding, and mixed mode I and II conditions by finite element analyses. The characteristics of the near-tip strain fields suggest that the experimental observations of shallow surface cracks (Case A cracks) propagating in the maximum shear strain direction can be explained by a fracture mechanics crack growth criterion based on the maximum effective strain of the near-tip fields for small cracks under general yielding conditions. The constant effective stress contours representing the intense straining zones near the tip are also presented. The results of the J integral from finite element analyses are used to correlate to a fatigue crack growth criterion for Case A cracks. Based on the concept of the characterization of fatigue crack growth by the cyclic J integral, the trend of constant J contours on the Γ-plane for Case A cracks compares well with those of constant fatigue life and constant crack growth rate obtained from experiments.     

Multiaxial fatigue of a short glass fibre reinforced polyamide 6.6 – Fatigue and fracture behaviour

The multiaxial fatigue behaviour of a short glass fibre reinforced polyamide 6.6 (PA66-GF35) is investigated on hollow tubular specimens in the range of fatigue lives between 10² and 10⁷ cycles. Fatigue experiments included pure tension, pure torsion, combined tension–torsion at different biaxiality ratios and phase shifting angles between the stress components. Tests were carried out with load ratio R = 0 and R = −1 at room temperature as well as at 130 °C. The influence of biaxiality ratio, phase angle between load components and load ratio is discussed.An extensive analysis of the fracture behaviour is performed on the specimens to recognise the crack nucleation and propagation mechanisms; failure modes were evaluated via optical and scanning electron microscopy.     

Fatigue analysis of post-weld fatigue improvement treatments using a strain-based fracture mechanics model

Untreated and post-weld treated (peened) fatigue details common to welded steel structures are analyzed herein using a strain-based fracture mechanics model. The model is first described and then evaluated by comparison with data from several test-based studies as well as analytical results obtained using two linear elastic fracture mechanics (LEFM) models. The strain-based model is then used to perform several parametric studies. Based on the results of these studies, loading conditions are identified for which ignoring the nonlinear material behaviour may lead to overestimation of the post-weld treatment benefit measured as in increase in the fatigue life of the weld.     

A low cycle fatigue model of a short-fibre reinforced 6061 aluminium alloy metal matrix composite

A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin–Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al₂O₃ Saffil short-fibres of a volume fraction of 20 vol.% and test temperatures from −100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al₂O₃ fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.     

A micro-damage healing model that improves prediction of fatigue life in asphalt mixes

The focus of the current paper is on the development and validation of a micro-damage healing model that improves the ability of an integrated nonlinear viscoelastic, viscoplastic, and viscodamage constitutive model based on continuum damage mechanics for predicting the fatigue life of asphalt paving mixtures. The model parameters of the continuum-based healing model are related to fundamental material properties. Recursive-iterative and radial return algorithms are used for the numerical implementation of viscoelasticity and viscoplasticity models respectively, whereas the viscodamage and micro-damage healing models are implemented using the concept of the effective undamaged-healed natural configuration. Numerical algorithms are implemented into the well-known finite element code Abaqus via the user material subroutine UMAT. Finally, the model is validated by comparing its predictions with experimental data on an asphalt mix that include repeated creep-recovery tests for different loading times and rest periods in both tension and compression. The significant enhancement of the ability of the constitutive model to predict fatigue life due to inclusion of the micro-damage healing is clearly demonstrated.     

A bilinear cohesive zone model tailored for fracture of asphalt concrete considering viscoelastic bulk material

A bilinear cohesive zone model (CZM) is employed in conjunction with a viscoelastic bulk (background) material to investigate fracture behavior of asphalt concrete. An attractive feature of the bilinear CZM is a potential reduction of artificial compliance inherent in the intrinsic CZM. In this study, finite material strength and cohesive fracture energy, which are cohesive parameters, are obtained from laboratory experiments. Finite element implementation of the CZM is accomplished by means of a user-subroutine which is employed in a commercial finite element code (e.g., UEL in ABAQUS). The cohesive parameters are calibrated by simulation of mode I disk-shaped compact tension results. The ability to simulate mixed-mode fracture is demonstrated. The single-edge notched beam test is simulated where cohesive elements are inserted over an area to allow cracks to propagate in any general direction. The predicted mixed-mode crack trajectory is found to be in close agreement with experimental results. Furthermore, various aspects of CZMs and fracture behavior in asphalt concrete are discussed including: compliance, convergence, and energy balance.     

Probability of cleavage fracture for a crack propagating by fatigue

Standard fracture toughness tests use fatigue pre-cracked specimens loaded monotonically from zero to failure. Scatter in toughness (cleavage) occurs because steel is metallurgically inhomogeneous, and because each specimen has its crack tip in a different local microstructure. A probability of fracture toughness distribution can be obtained by conducting multiple repeat tests on the same steel. This is often used to make probabilistic structural fracture predictions for combinations of crack length and applied load. However, it is likely the true structural situation involves gradual extension of a fatigue crack under a cyclic load. The question then arises as to how often the probability of fracture for the structure needs to be re-calculated. It could be argued that each fatigue load cycle moves the crack tip to a new position and gives a different instantaneous probability of fracture. But if this were the case, the predicted cumulative probability of fracture would quickly tend to unity. This paper describes cold temperature, wide plate fatigue tests designed to investigate this apparent contradiction. The steel is 15 mm thick, grade A, ship plate and the tests involve propagation of a fatigue crack from 300 mm to 650 mm length under a constant amplitude fatigue cycle of 10-100 MPa at −50 °C. The cold temperature fatigue tests do not show an obviously increased probability of fracture compared with the standard monotonic load tests. Nevertheless, in view of uncertainties surrounding the issue, a cumulative probability of fracture determined at 5 mm intervals through the steel is recommended for safe structural predictions.     

Developing an aging model to evaluate engineering properties of asphalt paving binders

Oxidative aging of asphalt is a primary cause of binder hardening in pavements, thus contributing to various forms of pavement failures. An essential element of predicting long-term pavement performance is to understand binder oxidative aging and its effect on engineering properties. Five asphalt binders were evaluated relative to their changes in engineering and chemical properties in pavement service. Laboratory rolling thin-film oven test (RTFOT) and pressure aging vessel (PAV) test were conducted to simulate the in-situ pavement aging. In addition, a test road was constructed for this study to investigate the real aging process in the field. Comparable data were shown between field binders and laboratory binders aged at temperature 60°C under pressure 20 kg/cm². The aging time of asphalts in PAV depended on how long pavements were used in the field. This paper was to determine the temperature and pressure used for PAV to simulate aging condition in the field. A good correlation between field-service and laboratory aging during test road project was found. An aging model was proposed to predict the changes in paving binder’s properties during field age hardening. Results were shown to give a close fit with experimental data from both laboratory and field aging tests. This model allowed highway engineers to quantify two essential characteristics of binder aging: the aging rate and the ultimate degree of changes in binder properties due to aging.