The study of redistribution in residual stresses during fatigue crack growth

Numerical and experimental study was conducted on fatigue crack growth (FCG) of metallic components to investigate the redistribution of mechanical residual stresses during FCG. To this end, the compact tension specimens of an aluminium alloy were used. In addition, mechanical residual stresses were introduced near the crack tip by applying compressive and tensile loads, followed by visually observing the side-surface of the specimens to estimate the crack growth length. In the numerical simulation, cyclic J-integral was used as the crack growth fracture parameter and a good agreement was observed between the numerical and experimental results. The results of the finite element method demonstrated a clear redistribution of mechanical residual stresses during FCG. After a few cycles, the residual stress field around the crack tip reached a lower magnitude value confined in a smaller zone, although this zone was stable during the remaining fatigue process. Finally, present study evaluated the effect of stress ratio, load amplitude, and initial residual stresses level on the redistribution of residual stresses. It was observed that the residual stresses are mainly released during the first steps of fatigue loading.

Modeling Brittle Fractures in Epoxy Nanocomposites Using Extended Finite Element and Cohesive Zone Surface Methods

Linear elastic fracture modeling coupled with empirical material tensile data result in good quantitative agreement with the experimental determination of mode I fracture for both brittle and toughened epoxy nanocomposites. The nanocomposites are comprised of diglycidyl ether of bisphenol A cured with Jeffamine D-230 and some were filled with core-shell rubber nanoparticles of varying concentrations. The quasi-static single-edge notched bending (SENB) test is modeled using both the surface-based cohesive zone (CZS) and extended finite element methods (XFEM) implemented in the Abaqus software. For each material considered, the critical load predicted by the simulated SENB test is used to calculate the mode I fracture toughness. Damage initiates in these models when nodes at the simulated crack tip attain the experimentally measured yield stress. Prediction of fracture processes using a generalized truncated linear traction–separation law (TSL) was significantly improved by considering the case of a linear softening function. There are no adjustable parameters in the XFEM model. The CZS model requires only optimization of the element displacement at the fracture parameter. Thus, these continuum methods describe these materials in mode I fracture with a minimum number of independent parameters.

Modeling Brittle Fracture in Epoxy Nanocomposites using Extended Finite Element and Cohesive Zone Surface Methods

Linear elastic fracture modeling coupled with empirical material tension data result in good quantitative agreement with experimental measurements of fracture failure for both brittle and tough epoxy nanocomposites. The nanocomposites comprise diglycidyl ethers of bisphenol A cured with O,O’ bis (2-aminopropylpropylene glycol) (Jeffamine D230) and doped with rubber nanoparticles of varying concentrations. Toughness, critical load, and critical displacement in quasi-static single edge-notched three-point bending are predicted accurately using both surface-based cohesive zone (CZS) and extended finite element (XFEM) methods implemented in Abaqus software. Fracture initiation within a crack is taken at the yield stress from uniaxial tension data. Prediction of fracture processes using a generalized truncated linear traction-separation law was significantly improved by considering the case of a linear softening function. There are no adjustable parameters in the XFEM model. The CZS model requires only optimization of the element displacement at fracture parameter. Thus, these continuum methods describe these materials in mode I fracture with a minimum number of independent parameters.

Novel Model to Predict Critical Strain Energy Release Rate in Semi-Circular Bend Test as Fracture Parameter for Asphalt Mixtures Using an Artificial Neural Network Approach

Growing use of recycled asphalt materials in asphalt pavement means the current volumetric-based Superpave mixture design may not address durability concerns arising from replacement of a proportion of virgin binder with recycled ones. To address this limitation, performance-based testing is introduced to supplement conventional volumetric mixture design in assessing cracking performance of asphalt mixtures. Louisiana Department of Transportation and Development’s Specifications for Roads and Bridges specify a criterion for the critical strain energy release rate, Jc, obtained from semi-circular bend (SCB) test as a complement of current practice to evaluate cracking resistance of asphalt mixtures. Quality control/assurance practices, however, require SCB samples to be long-term aged for five days at 85°C, which is a time-consuming process. Therefore, it is beneficial to be able to estimate SCB Jc for long-term aged asphalt mixtures based on SCB Jc measured from plant-produced asphalt mixtures. Asphalt mixture aging is complex, and various variables are involved in the aging process, including volumetric properties of asphalt mixture and chemical/rheological characteristics of asphalt binder. With the capability of artificial neural network (ANN) to address complex relationships between input and output variables, this study aims to predict the fracture parameter, SCB Jc, of asphalt mixtures using ANN. A total of 34 asphalt mixtures were selected for this study. SCB fracture test and asphalt binder tests for chemical and rheological characterization were conducted. Stepwise regression analysis was used to determine the significant parameters in the correlation with SCB Jc. With determined significant parameters, ANN using the gradient descent backpropagation approach was then applied to develop and validate the predictive model. It was shown that the developed ANN model was able to predict the fracture parameter, SCB Jc, of asphalt mixtures more accurately than linear and non-linear regression models.

Effect of Stress Interactions on Effective Elasticity and Fracture Parameters in the Damage Zones

Elastic interactions between fractures will greatly affect the effective elasticity, which, in turn, will reshape the effective fracture parameters. The disturbance will be more complex in the fault zone due to the complicated fracture distributions. This problem is addressed by the comparison of two types of solutions: one containing the stress interaction while the other one excluding the stress interaction. The gap between the two solutions allows the quantitative estimation of stress interactions on elasticity. Furthermore, based on the orthorhombic assumption for fracture clustering in the damage zone, the effect of stress interaction on the equivalent fracture parameter is estimated. We first characterize the fracture parameters in the fault damage zone considering more realistic distributions of fractures. Then, a series of numerical simulations are conducted to study the effective parameters of the fractured model. Finally, assuming the orthorhombic system of the fracture clustering, we invert the crack density and validate the accuracy of the inversion through the incidence angle seismic velocities. Our numerical results suggest that the size of fractures will determine the dominant type of stress interactions, and thus significantly reshape the effective properties of the models regardless of the spatial distribution of the fracture. Furthermore, the stress interactions tend to underestimate the fracture density for models containing long fractures but generate a relatively satisfactory inverted fracture density for short fractures.

Investigation of Surrogate Performance Related Tests for Fatigue Cracking of Asphalt Pavements

This paper studies the relationship between laboratory measurements of fatigue performance and fracture performance of conventional asphalt mixtures, asphalt mixtures with reclaimed asphalt pavement (RAP), and rubberized asphalt mixtures. The existing four-point bending (4PB) test was developed to evaluate the fatigue performance of asphalt pavements; however, it is not necessarily appropriate for use in routine job mix formula approval and is too slow and expensive for quality control/quality assurance (QC/QA). In this paper, the semi-circular bending test and indirect tensile asphalt cracking test (IDEAL-CT) were evaluated for their potential to serve as a simple and fast surrogate fatigue performance related test for QC/QA on routine projects and routine mix design. Multiple representative fracture parameters were obtained from the Illinois flexibility index test and the IDEAL-CT. The coefficient of variation revealed that the lowest variability from both tests was in fracture strength. In addition, the linear regression analysis between fracture parameters and fatigue performance indicated that slopes, fracture toughness, and strength from fracture tests have good correlations with the initial flexural stiffness from 4PB tests, while 4PB initial stiffness is well correlated with fatigue life. The direct correlation between fracture properties and fatigue life was not as good. The fracture parameter “strength” also showed the capability of discriminating among asphalt materials with low RAP content.