Developed Material Model Research Articles

Cure-dependent thermo-chemical modelling and analysis of the manufacturing process of an aircraft composite frame

This study contributes to the understanding of the mechanism behind thermo chemical aspects related to the resin transfer moulding manufacturing process of a composite part. The aim is to comprehend the phenomena, to identify related parameters and to get knowledge-based methods for the process development. Therefore, the first part of this study is an experimental study about the behaviour of material properties during the manufacturing process of the single component and the composite. It concludes with constitutive equations for single-process parameters and their associated homogenisation approach for the composite properties. During the manufacturing process, material values of the matrix are changing and influenced by a high number of effects. In the second part, a simulation strategy is been derived. This developed material model integrates a dependency of the time–temperature–polymerisation and fibre volume content. The model is validated in a test case of a manufacturing process of an aircraft component, a fuselage frame.

Neural network based material models with Bayesian framework for integrated materials and product design

Integrated Materials and Products Design (IMPD) is a new system-based design approach. This emerging method focuses on designing a product and its materials at the same time to further enhance product performances. In the process of IMPD, material models that predict material properties with given inputs of material processing parameters play an important role in numerous design optimization iteration loops. In this work, a material model for predicting the tensile strength of austenitic stainless steels is developed based on the neural network with Bayesian framework. Using the Bayesian framework, we quantify the degree of uncertainty, originated from lack of data or the architecture of employed neural network, in the prediction of material properties. This quantification is very important for the later use in robust design optimization. Developed material model is validated based on the two different types of austenitic stainless steels, AISI 316L and AISI 347H, subjected to prior heat treatment processes. Comparing the predicted results with experimental results, we observe our material model predicts the tensile strengths of AISI 316L steels heattreated at various temperatures with higher levels of accuracy. The predicted tensile strengths of AISI 347H steels tested at different temperatures are reasonably close to the experimental results.

Ultimate Limit Load in Welded Joints and in Net Sections of High Strength Steels with Yield Stress 960 MPa

Ultra-high strength steels are becoming more common, particularly in lifting machinery and mobile equipment. However, elon- gation is a problem in ultra-high strength steels, which has been reported to be low in traditional material tests. Although in these tests, ultra-high strength steels have exhibited good local strain properties, which suggests that traditional material test procedures are inappropriate.In this paper, notched specimens were used to develop material models and to define failure criteria. The results were verified using welded components and tensile tests. The failure point and capacity levels were determined in welded and net section tests. We conclude that the strain properties of ultra-high strength steels are good, where the full capacity of the net section stress was reached in the tensile tests. The results also indicate that the current elongation requirements in different codes require adjustments or a new formulation.

Mechanical response of AZ31B magnesium alloy: Experimental characterization and material modeling considering proportional loading at room temperature

Tension and compression experiments have been performed to characterize the mechanical response of 1.57mm AZ31B-O sheet at room temperature. Five different sheet orientations were used to characterize the in-plane anisotropy under tensile loading conditions while cubic samples consisting of adhesively-bonded layers of sheet samples were used for compression testing along four sheet directions. During uniaxial tensile testing, the axial and transverse strain components were measured using two independent extensometers. A digital image correlation system was used to measure the strain components during compression testing. Both instantaneous and cumulative r-values were measured as they evolved with plastic strain. A strong, evolving asymmetry is observed. An evolving anisotropic/asymmetric continuum-based material model based on a Cazacu–Plunkett–Barlat (CPB)-type yield function is proposed to fit the material behavior as a continuous function of plastic strain. Considerable improvement in the representation of the material behavior is achieved as the number of stress transformations used in the CPB yield surface formulation is increased. To capture the evolution of the envelope of the subsequent yield surfaces, the anisotropy and asymmetry parameters are replaced with functions expressed in terms of plastic strain. The evolution parameters are found by minimizing the difference between the model predictions and the experiments at discrete plastic strain levels, using gradient search methods. A strain rate-independent elastic–plastic material model incorporating the evolving envelope of subsequent yield surface formulation has been developed and implemented within a commercial finite element package. The model reproduces the experiments initially used for fitting. The predictions of the developed material model are compared with the measured load–displacement and strain distributions from a three-point bending experiment. Improvement in the prediction of strain and forming forces is observed compared to the previously available non-evolving material models.

Correlation between simulations and cavitation-induced erosion damage in Spallation Neutron Source target modules after operation

An explicit finite element (FE) technique developed for estimating dynamic strain in the Spallation Neutron Source (SNS) mercury target module vessel is now providing insight into cavitation-induced erosion patterns observed on interior surfaces of SNS targets during post-irradiation examination. The technique uses an empirically developed material model for the mercury that describes its volumetric stiffness combined with a tensile pressure cut-off limit to approximate the threshold and effect of cavitation. The longest period each point in the mercury is at the tensile cut-off threshold is denoted as “saturation time”. Patterns of saturation time can be obtained from the FE simulations and are being positively correlated with observed damage patterns as a qualitative measure of damage potential. Saturation time has been advocated by collaborators at the Japan Proton Accelerator Research Complex (J-PARC) as a factor in predicting bubble nuclei growth and collapse intensity. Larger ratios of maximum bubble-size-to-nucleus result in greater bubble collapse intensity; longer saturation times correlate to greater ratios. With the recent development of a user subroutine for the FE solver, saturation time is now provided over the entire mercury domain. Saturation time contour maps agree with patterns of damage seen on the SNS inner vessel beam window and elsewhere. The other simulation result which seems to correlate with observed damage patterns is the local mercury velocity. Related R&D has provided evidence that damage is mitigated by flow velocity. Surfaces which are near regions of low mercury velocity appear to be more vulnerable to damage than those where the mercury flow is strong and sustained. By combining the patterns of saturation time and velocity a viable explanation for observed damage patterns is presented.

FE-simulation of machining processes with a new material model

In the field of materials mechanics the influence of the state of stress on the plastic deformation behavior of metals is known since decades. However, the state-of-stress influences are usually not considered in structural or processing simulations. Nevertheless, its application in the numerical investigation of manufacturing processes seems very promising since, for example, machining processes are characterized by complex states of stress. Consequently, its incorporation in the computation of the workmaterial's flow stress may increase the physical conformity and accuracy of cutting FE-analysis.This paper presents the creation and experimental validation of a 3D-FEM model of the longitudinal turning process with an extended modified Bai–Wierzbicki material model (extended MBW model). This newly developed material model evaluates the influence of state of stress as well as damage on the strain hardening behavior. In addition, it takes temperature and strain rate effects into consideration, whose influences are both typically higher in cutting processes than in structural–mechanical problems.For the validation of the proposed material model, longitudinal turning experiments were conducted on AISI 1045 steel. Four different cutting tools and process conditions were investigated, which cover a broad range from finishing to roughing. A high speed camera was used to film the chip formation and chip flow in order to compare it to the simulation results. The three cutting forces components were also collected. Measured chip temperatures were taken from the literature. The validation showed that the implementation of the selected material model results in a close agreement between experimentally obtained and predicted chip geometries, cutting forces and chip temperatures.

Finite element modeling of ballistic impact on multi-layer Kevlar 49 fabrics

This paper presents a material model suitable for simulating the behavior of dry fabrics subjected to ballistic impact. The developed material model is implemented in a commercial explicit finite element (FE) software LS-DYNA through a user defined material subroutine (UMAT). The constitutive model is developed using data from uniaxial quasi-static and high strain rate tension tests, picture frame tests and friction tests. Different finite element modeling schemes using shell finite elements are used to study efficiency and accuracy issues. First, single FE layer (SL) and multiple FE layers (ML) were used to simulate the ballistic tests conducted at NASA Glenn Research Center (NASA-GRC). Second, in the multiple layer configuration, a new modeling approach called Spiral Modeling Scheme (SMS) was tried and compared to the existing Concentric Modeling Scheme (CMS). Regression analyses were used to fill missing experimental data – the shear properties of the fabric, damping coefficient and the parameters used in Cowper-Symonds (CS) model which account for strain rate effect on material properties, in order to achieve close match between FE simulations and experimental data. The difference in absorbed energy by the fabric after impact, displacement of fabric near point of impact, and extent of damage were used as metrics for evaluating the material model. In addition, the ballistic limits of the multi-layer fabrics for various configurations were also determined.

Efficient finite element analysis of large‐scale structures based on a phenomenological approach to RC membrane behaviour

Purpose – The purpose of this paper is to present the implementation in a finite element (FE) code of a recently developed material model for the analysis of cracked reinforced concrete (RC) panels. The model aims for the efficient nonlinear analysis of large‐scale structural elements that can be considered as an assembly of membrane elements, such as bridge girders, shear walls, transfer beams or containment structures.

Mechanical, biological and structural characterization of in vitro ruptured human carotid plaque tissue

Recent experimental studies performed on human carotid plaques have focused on mechanical characterization for the purpose of developing material models for finite-element analysis without quantifying the tissue composition or relating mechanical behaviour to preoperative classification. This study characterizes the mechanical and biological properties of 25 human carotid plaques and also investigates the common features that lead to plaque rupture during mechanical testing by performing circumferential uniaxial tests, Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) on each specimen to relate plaque composition to mechanical behaviour. Mechanical results revealed large variations between plaque specimen behaviour with no correlation to preoperative ultrasound prediction. However, FTIR classification demonstrated a statistically significant relationship between stress and stretch values at rupture and the level of calcification (P=0.002 and P=0.009). Energy-dispersive X-ray spectroscopy was carried out to confirm that the calcium levels observed using FTIR analysis were accurate. This work demonstrates the potential of FTIR as an alternative method to ultrasound forpredicting plaque mechanical behaviour. SEM imaging at the rupture sites of each specimen highlighted voids created by the nodes of calcifications in the tissue structure which could lead to increased vulnerability of the plaque.

Modelling of size effects in microforming process with consideration of grained heterogeneity

Size effect is a special phenomenon in metal micro-forming process. As the deformation process is scale down to micro/mesoscale, the characteristics of single grain involved in the deformed region play a significant role on the material mechanical behaviours resulting in the invalidation of classical theories in microforming. This paper presents a newly developed material model in microscale on the basis of the grained heterogeneity (e.g. grain size, shape and deformability) and specimen dimension. Voronoi tessellation has been employed to describe the polycrystalline aggregate. The grain shape is controlled by the centroidal-voronoi algorithm to drive grains into steady state. Hardness of the grains obtained from Nano-indentation is used to identify the scatter of the grained deformability. Applying the new material model, the micro-compression test of pure copper is numerically simulated by finite element method (FEM). The influences of grain size and feature size on the deformation behaviours are discussed. The numerical simulation results are in good agreement with the experimental results in terms of the flow stress curves and profile of deformed parts. Based on the novel material model, a FE model of microcross wedge rolling is established and the obtained results show the strain of specimen core region increases with the magnification of grain size.

Three-dimensional model for analysis of high performance fiber reinforced cement-based composites

High performance fiber reinforced cement-based composites (HPFRCCs) are distinguished from regular cement materials by their strain hardening behavior under uniaxial tension as well as significantly enhanced durability, damage tolerance, and shear resistance. This paper presents a three-dimensional constitutive model for the nonlinear finite element analysis of HPFRCC structures. The proposed material model is an orthotropic hypoplastic model. Based on the smeared rotating crack approach, the directions of orthotropy are assumed to coincide with the principal strain directions. The constitutive model is capable of accounting for the general nonlinear behavior of HPFRCC, including the effects of multiple narrow cracking, crack localization, and compression crushing. The performance of the proposed model is demonstrated using data from experimental tests carried out on a punching shear slab and a two-span continuous beam. Comparisons between numerical solutions and experimental results suggest that the developed material model is able to represent the observed experimental behavior with reasonable accuracy.

Response of semi-crystalline thermoplastic polymers to dynamic loading: A finite element study

Mechanical behaviours and properties of polymers under dynamic loading conditions differ significantly from those under quasi-static loads. Consequently, for dynamic numerical analysis, a linear elastic/hyperelastic material model has its limitations and can produce inaccurate results. In order to obtain an adequate response to high-speed loading in simulations, a viscoelastic material model has been developed. Simulations with such a model were performed using the finite-element software Abaqus and employed to calculate the Prony series, characterising the material’s time-dependent mechanical behaviour. The initial material models, developed starting from creep and Dynamic Mechanical Analysis test data, failed to provide adequate results; hence, a customised relaxation function was suggested based on experimental tests at different strain rates. The developed material model was validated in static and dynamic simulations with good results for loads below the yield point.

EPR-based material modelling of soils considering volume changes

In this paper an approach is presented for developing material models for soils based on evolutionary polynomial regression (EPR), taking into account its volumetric behaviour. EPR is a recently developed hybrid data mining technique that searches for structured mathematical equations (representing the behaviour of a system) using genetic algorithm and the least squares method. Stress–strain data from triaxial test are used to train and develop EPR-based material models for soil. The developed models are compared with some of the well known conventional material models. In particular, the capability of the developed EPR models in predicting volume change behaviour of soils is illustrated. It is also shown that the developed EPR-based material models can be incorporated in finite element (FE) analysis. Two geotechnical examples are presented to verify the developed EPR-based FE model (EPR-FEM). The results of the EPR-FEM are compared with those of a standard FEM where conventional constitutive models are used to describe the material behaviour. The results show that EPR-FEM can be successfully employed to analyse geotechnical engineering problems. The advantages of the proposed EPR models are highlighted.

Hot‐embossing experiments of polymethyl methacrylate across the glass transition temperature with variation in temperature and hold times

AbstractHot‐embossing (HE) experiments were conducted on polymethyl methacrylate (PMMA) across its glass transition temperature from 92 to 142°C. The glass transition temperature (Tg) of the PMMA used in this study was ∼ 102°C. The polymer samples were embossed to a depth of 0.8 mm (800 μm). The experiments were carried out at various temperatures for different hold times of 30, 90, and 180 sec during the embossing process. A few additional experiments were conducted at 142°C with cooling of the samples as well. The force required for embossing and the final depth of the embossed features were analyzed. Polymers, including PMMA, show significantly different material behavior around and above Tg. The same was seen in the aforementioned tests; the trends observed for the force as well as the final depth changed considerably around 122°C (Tg + 20). These findings will be used in developing material models for use in simulating the hot‐embossing process. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Process‐chain simulation for prediction of the distortion of case‐hardened gear blanks

AbstractIn this study, a process‐chain simulation model is presented for the prediction of distortion of low‐pressure gas carburised SAE 5120 (EN 20MnCr5) steel gear blanks. For this purpose, the evolution of the banded microstructure stemming from the continuous casting process was traced by computer simulations of subsequent shape rolling, forging and machining steps. Then, the simulated local orientation angles of the deformed banded microstructure are transferred to heat treatment simulation module as an input for the recently developed material model that exploits the “Anisotropic Transformation Strain (ATS)“ concept to reproduce the dishing behaviour which cannot be reproduced by former models. The results indicate that the suggested procedure provides quite good predictions of the dishing directions and dishing‐free cutting strategy, whereas; the dishing magnitude is predicted fairly reasonably considering large scatters in the experiments.

Modelling and testing of ageing of short fibre reinforced polymer composites

A study in accelerated humidity–temperature ageing and it is numerical modelling for short fibre reinforced polymer composites (SFRPC) based on poly(butylene terephthalate) (PBT) is reported. Authors described experimental results of humidity–temperature ageing of PBT reinforced with glass fibres and proposed a novel computation method of strength and durability analysis for SFRPC parts. Experimental results showed different ageing behaviours, which depend on fibre alignment, e.g. a decrease of Young’s modulus in longitudinal fibre alignment in tension after ageing, an increase of Young’s modulus in transverse direction in tension after ageing, and the increase of the shear modulus and decrease of shear strength after ageing in both directions. Proposed modelling procedure takes the fibre orientation from mould filling analysis as an independent material orientation, using a developed ageing dependent material model, based on tensile, compressive, and shear properties for longitudinal and transverse fibre alignments, and calculates failure criteria as a function of the ageing time and fibre alignment. An innovative approach is to create a fibre alignment dependent material ageing model which takes into account changes of material properties depending on the direction of the reinforcement. This methodology was extended to arbitrary models and validated on real parts made of SFRPC.

The Nonlinear Viscoelastic Behavior for Asphalt Mixture Materials

Compressive behavior of asphalt mixture is studied in creep and strain recovery tests observing large nonlinear viscoelastic strains. The nonlinear viscoelastic material model for asphalt mixture is presented, based on a modified version of Schapery’s constitutive relationship. For the description of the nonlinear viscoelastic response of the material, simple creep and recovery tests for different stress levels were executed. An analytical method and a nonlinear fitting procedure by the least square method are developed to determine nonlinear viscoelastic stress dependent parameters. Constant stress creep testing were also performed to validate the developed material model. The model successfully describes the main features for asphalt mixture and shows good agreement with test data within the considered stress range.

On the thermodynamically consistent fractional wave equation for viscoelastic solids

Since the well-known methods for the computation of harmonically induced LAMB waves in elastic plates cannot be applied directly to viscoelastic material, an enhanced material model is developed. It is based on fractional time derivatives and incorporates a fractional KELVIN–VOIGT model. A proof of the thermodynamic consistency of this model is given. On the basis of the developed material model, the fractional wave equation is derived which allows for the calculation of LAMB waves in viscoelastic solids.

On the influence of kinematic hardening on plastic anisotropy in the context of finite strain plasticity

The paper discusses the application of a newly developed material model for finite anisotropic plasticity to the simulation of earing formation in cylindrical cup drawing. The model incorporates Hill-type plastic anisotropy, nonlinear kinematic and nonlinear isotropic hardening. The constitutive framework is derived in the context of continuum thermodynamics and represents a multiplicative formulation of anisotropic elastoplasticity in the finite strain regime. Plastic anisotropy is described by means of second-order structure tensors which are used as additional tensor-valued arguments in the representation of the yield criterion and the plastic flow rule. The evolution equations are integrated by a form of the exponential map that fullfils plastic incompressibility and preserves the symmetry of the internal variables. The numerical examples investigate the influence of the hardening behaviour on an initially anisotropic yield criterion. In particular, the influence of using the kinematic hardening component of the model in addition to isotropic hardening in the earing simulations is examined. Comparisons with test data for aluminium and steel sheets display a good agreement between the finite element results and the experimental data.

Viscoplastic behaviour of asphalt mixture in compression

Compressive behaviour of asphalt mixture is studied in creep and strain recovery tests observing large irreversible viscoplasticity strains. The viscoplastic material model for asphalt mixture is presented, based on a modified version of Schapery’s constitutive relationship where a viscoplastic term is added. For the description of the non‐linear viscoplastic behaviour of the material, repeated creep and recovery strain tests at a fixed stress level and a fixed loading time were conducted. The viscoplastic strain is described by the strain hardening model. Constant stress creep testing was also performed to validate the developed material model. The model successfully describes the main features for asphalt mixture and shows good agreement with test data within the considered stress range.