Nonlocal Modeling in High-Velocity Impact Failure of 6061-T6 Aluminum

In this paper, we present numerical simulations with local and nonlocal models under dynamic loading conditions. We show that for finite element (FE) computations of high-velocity, impact problems with softening material models will result in spurious post-bifurcation mesh dependency solutions. To alleviate numerical instability associated within the post-bifurcation regime, a characteristic length scale was added to the constitutive relations based on calibration of the series of different notch specimen tests. This work aims to assess the practical relevance of the modified model to yield mesh independent results in the numerical simulations of high-velocity impact problems. To this end, we consider the problem of a rigid projectile moving at a range of velocities between 89-107 m/s, colliding against a 6061-T6 Aluminum disk. A material model embedded with a characteristic length scale in the manner proposed by Pijaudier-Cabot and Bazant (1987), but in the context of concrete damage, was utilized to describe the damage response of the disk. The numerical result shows that the addition of a characteristic length scale to the constitutive model does eliminate the pathological mesh dependency and shows excellent agreements between numerical and experimental results. Furthermore, the application of a nonlocal model for higher strain rate behavior shows the ability of the model to address intense localized deformations, irreversible flow, softening, and final failure

Finite element modeling of weld solidification cracking in 6061-T6 aluminum - applicability of strain-based failure criteria

Finite element simulations using an internal state variable constitutive model are used to study weld solidification cracking of 6061-T6 aluminum. Stress and strain histories at the weld centerline for two types of specimen are studied with regards to application of strain-based failure criteria for predicting weld solidification cracking. 11 refs., 7 figs., 2 tabs

Application of internal state variable plasticity and damage models to welding

An internal state variable constitutive model coupled with a ductile void growth model is applied in two finite element simulations of welding. Shrinkage of a 304 L stainless steel pipe due to multipass gas tungsten arc welds is presented as an example of tracking distortion far from the weld. Weld solidification cracking in Al-6061 disks is presented as an application of the plasticity model coupled with the damage model

Thermal-mechanical modeling and experimental validation of weld solidification cracking in 6061-T6 aluminum

Finite element simulation using an internal state variable constitutive model coupled with a void growth and damage model are used to study weld solidification cracking of 6061-T6 aluminum. Calculated results are compared with data from an experimental program determining the locations of failure as a function of weld process parameters and specimen geometry. Two types of weld solidification cracking specimen were studied. One specimen, in which cracking did not occur, was used to evaluate finite element simulations of the thermal response and calculations of average strain across the weld. The other specimen type was used to determine the location of crack initiation as a function of weld process parameters. This information was used to evaluate the finite element simulations of weld solidification cracking. A solidification model which includes dendrite tip and eutectic undercooling was used in both thermal and mechanical finite element analyses. A strain rate and temperature history dependent constitutive model is coupled with a ductile void growth damage model in the mechanical analyses. Stresses near the weld pool are examined to explain results obtained in the finite element analyses and correlated with experimental observations. Good agreement is obtained between simulation and experiment for locations of crack initiation and extent of cracking. Some effects of uncertainties in material parameters are discussed

Percentile reference values for anthropometric body composition indices in European children from the IDEFICS study

INTRODUCTION: To characterise the nutritional status in children with obesity or wasting conditions, European anthropometric reference values for body composition measures beyond the body mass index (BMI) are needed. Differentiated assessment of body composition in children has long been hampered by the lack of appropriate references. OBJECTIVES: The aim of our study is to provide percentiles for body composition indices in normal weight European children, based on the IDEFICS cohort (Identification and prevention of Dietary-and lifestyle-induced health Effects in Children and infantS). METHODS: Overall 18 745 2.0-10.9-year-old children from eight countries participated in the study. Children classified as overweight/obese or underweight according to IOTF (N = 5915) were excluded from the analysis. Anthropometric measurements (BMI (N = 12 830); triceps, subscapular, fat mass and fat mass index (N = 11 845-11 901); biceps, suprailiac skinfolds, sum of skinfolds calculated from skinfold thicknesses (N = 8129-8205), neck circumference (N = 12 241); waist circumference and waist-to-height ratio (N = 12 381)) were analysed stratified by sex and smoothed 1st, 3rd, 10th, 25th, 50th, 75th, 90th, 97th and 99th percentile curves were calculated using GAMLSS. RESULTS: Percentile values of the most important anthropometric measures related to the degree of adiposity are depicted for European girls and boys. Age-and sex-specific differences were investigated for all measures. As an example, the 50th and 99th percentile values of waist circumference ranged from 50.7-59.2 cm and from 51.3-58.7 cm in 4.5-to < 5.0-year-old girls and boys, respectively, to 60.6-74.5 cm in girls and to 59.9-76.7 cm in boys at the age of 10.5-10.9 years. CONCLUSION: The presented percentile curves may aid a differentiated assessment of total and abdominal adiposity in European children

A new approach for determination of material constants of internal state variable based plasticity models and their uncertainty quantification

a b s t r a c t Physically-based plasticity models such as the BCJ model include internal state variables that represent the current state of the material and allow capturing strain rate and temperature history effects as well as the coupling of rate-and temperature-dependence with material hardening. However, the inclusion of internal state variables increases significantly the number of unknown material constants that need to be found through fitting of the model to experimental stress-strain data at different strain rates and temperatures. This makes the fitting process extremely challenging and increases the uncertainty in the material constants. The paper presents a physics-guided numerical fitting approach that reduces the associated difficulties and uncertainties involved in determining the material constants of the BCJ plasticity model. The approach uses experimental data from monotonic and reverse loading stress-strain curves at different temperatures and strain rates to determine the 18 material constants of the model. An evidential uncertainty quantification approach is used to determine uncertainties rooted in experimental data, selection of stress-strain curves at different loading conditions, variability of material properties, numerical aspects of the fitting method and mathematical formulations of the BCJ model. The represented uncertainty of the BCJ material constants based on mathematical tools of evidence theory is propagated through Taylor impact simulations of a 7075-T651 aluminum alloy cylinder. Uncertainty quantification results verify the presented numerical fitting approach for the BCJ model and its potential applicability to other similar material models

Predicting weld solidification cracking using damage mechanics -- LDRD summary report

This report summarizes the efforts to develop and validate a finite element based model to predict weld solidification cracking behavior. Such a model must capture the solidification behavior, the thermal behavior in the weld pool region, the material mechanical response, and some failure criteria to determine when solidification cracking will occur. For such a program to be successful, each aspect of the model had to be accurately modeled and verified since the output of one portion of the model served as the input to other portions of the model. A solidification model which includes dendrite tip and eutectic undercooling was developed and used in both the thermal and mechanical finite element analysis. High magnification video techniques were developed to measure strains for validation of the mechanical predictions using a strain rate and temperature dependent constitutive model. This model was coupled with a ductile void growth damage model and correlated with experimental observations to determine capabilities of predicting cracking response. A two phase (solid + liquid) material model was also developed that can be used to more accurately capture the mechanics of weld solidification cracking. In general, reasonable agreement was obtained between simulation and experiment for location of crack initiation and extent of cracking for 6061-T6 aluminum. 35 refs

Validation of thermal-mechanical modeling of stainless steel forgings

A constitutive model for recrystallization has been developed within the framework of an existing dislocation-based rate and temperature-dependent plasticity model. The theory has been implemented and tested in a finite element code. Material parameters were fit to data from monotonic compression tests on 304L steel for a wide range of temperatures and strain rates. The model is then validated by using the same parameter set in predictive thermal-mechanical simulations of experiments in which wedge forgings were produced at elevated temperatures. Model predictions of the final yield strengths compare well to the experimental results