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A numerical model of fatigue crack propagation based on the Theory of Critical Distances has been developed in a previous paper (Zheng et al., 2013 [1]). The present study aims to investigate the capability of the crack propagation model to predict the effects of overloads and underloads. The plastic energy density at the critical distance ahead of crack tip is affected by the changes in the plasticity-induced crack closure levels and has been used as a measure of the fatigue damage. It is concluded that the predicted crack retardation and acceleration after the overload and underload, respectively, are in satisfactory agreement with the experimental observations.

In most current works related to the structure analysis of cured woven composites, the influence of yarn angle variation on the local material properties is neglected. In this paper, its influence was studied, and an integrated preforming-performance simulation model, accounting for the influence of yarn angle variation, was proposed. The multi-scale modeling approach was adopted to predict the material properties of cured woven composites with different yarn angles. The results were compared with experiments to validate its effectiveness. In the integrated preforming-performance simulation model, non-orthogonal constitutive law was used in the preforming simulation to compute yarn orientations and yarn angle distribution. To demonstrate the capabilities of the integrated simulation model, simulations were conducted and compared with experiments. The results show that the proposed simulation model has more accurate prediction than the simulation model without considering the influence of yarn angle, and the impact of yarn angle shouldn’t be neglected.

A numerical model based on the theory of critical distances has been developed to simulate the fatigue crack propagation for cast aluminum alloys under mode I loading. The accumulated plastic energy density at the critical distance point ahead of the crack tip is used as a measure of the fatigue damage. The effect of different R -ratios on the crack propagation rate is correctly predicted using the proposed methodology. For the 319 cast aluminum alloys investigated in the present paper, the critical distance is of the same order of magnitude as the secondary dendrite arm spacing (SDAS).

The variability in fatigue life for a cast aluminum alloy has been investigated under both high-cycle and low-cycle fatigue. The fatigue cracks have initiated mostly near the surface from pores or oxide inclusions. The size of the initiation sites has been evaluated for all specimens and the statistical distribution has been determined. In addition, probabilistic fatigue life regression models have been used to characterize the observed scatter in fatigue life. The cumulative density function (CDF) of fatigue life was found to be approximately equal to the complementary value of the CDF of the near surface fatigue initiator size. Therefore, it has been concluded that the observed scatter in fatigue life is directly related to the distribution of casting defects within the alloy.

Most cast aluminum-engineered components are water quenched after the solution-treatment cycle of the casting process. This rapid water quenching has the potential to induce high residual stresses in regions of the castings. Reducing the amount of residual stress can have a promising effect on the life of the component. This study was conducted to quantify how aging affects the amount of residual stress in an aluminum casting. An engineered high residual stress test sample and quenching technique has been developed, and a relaxation study has been completed. The study focused on four different temperatures: 463 K, 493 K, 513 K, and 533 K (190 °C, 220 °C, 240 °C, and 260 °C) and a range of aging times (0.3 to 336 hours). The aging data were used to verify a stress relaxation model. The results indicated that as the aging temperature increased, the amount of relaxation of the residual stress increased.

In this paper the fatigue behavior of the friction stir spot welded (FSSW) coupons of magnesium AZ31 alloy manufactured under different welding process conditions is investigated. Two sets of lap-shear coupons were made based on variation in plunge depth, tool rotation speed and tool geometry. Metallographic analysis of the untested FSSW coupons revealed differences in weld geometry and microstructure. Interfacial hooking of the faying surface was found to vary significantly between the two sets of process conditions. Microhardness test data revealed a large gradient in hardness profile. Results from the load controlled cyclic tests showed that lower tool rotation and shallower shoulder plunge depth led to better fatigue performance. Optical fractography of the fatigued coupons revealed that fatigue cracks initiated at the weld interface in both sets of coupons. However, the fracture mode showed variability between the two process conditions. The fractography analysis suggests that the effective top sheet thickness, interfacial hook and microstructure, which are largely determined by the process conditions, significantly affect the fatigue behavior of the friction stir spot welds in magnesium alloys.

This study investigates the interaction between high-cycle fatigue (HCF) and low-cycle fatigue (LCF) in cast aluminum alloys. Initially, the baseline HCF and LCF lives have been determined under constant amplitude loading; LCF–HCF interaction fatigue tests were subsequently carried out through the application of periodic underloads. To account for the inherent scatter in fatigue life, probabilistic fatigue life curves were employed to establish the baseline HCF and LCF fatigue lives. The interaction fatigue damage was found to increase with the stress amplitude of the HCF cycles. Larger underloads (LCF cycles) also resulted in larger interaction damage. The impact of different numbers of HCF cycles per block (between underloads) was also studied. It was also found out that there is an additional damage caused by frequent applications of LCF cycles, which is related to the damaged Si particles in the large reversed plastic zone generated by the LCF cycles. The evolution of the crack opening stress following the application of an underload has been proposed as the major mechanism for the load interaction effect. An interaction damage model is proposed to account for these observed effects.

The residual stress profile in dissimilar metal sheets joined by a self-piercing rivet is simulated and compared to experimental measurements. Simulation of joining aluminum alloy 6111-T4 and steel HSLA340 sheets by self-piercing riveting (SPR) is performed using a two-dimensional axisymmetric model with an internal state variable (ISV) plasticity material model. Strain rate and temperature dependent deformation of the base materials is described by the ISV material model and calibrated with experimental data. Using the LS-DYNA simulation package, an element erosion technique is adopted in an explicit analysis of the separation of the upper sheet with maximum shear strain failure criterion. An explicit analysis with dynamic relaxation technique was then used for springback and cooling down analysis following the riveting simulation. The residual stress profile of SPR experimental joint with same configuration is characterized using neutron diffraction, and good agreement was found between the simulation and residual stress measurements.

The warpage of injection molded part strongly depends on the shrinkage property, which is highly dominant by the Pressure-Volume-Temperature (PVT) curves. Accurate description of PVT curves is essential for running the simulation of warpage. The standard PVT curves are measured at different pressure for the isotactic polypropylene, which are modified with the help of crystallizing curves obtained by the Differential Scanning Calorimetry (DSC) tests at different cooling rate. The user subroutine UEXPAN is coded to describe the modified PVT curves and implemented into the FEA model for warpage prediction in Abaqus. The simulated deflection of a plaque is performed and compared with the one measured by the coordinate measuring machine (CMM). It is concluded that the specific volume at high cooling rate varies slower with the temperature than that at low cooling rate and thus the predicted deflection with modified PVT data decreases significantly with increasing cooling rate. This numerical observation matches well with the experimental result. The warpage decreases when the crystallinity increases from 33 to 60 %. The plaque has the lowest peak deflection when the cooling rate is 60 °C/min. The system is built successfully for the warpage prediction of injection molded part.

The mechanical properties of thermoplastic olefin (TPO) composites with rubber content ranged from 17 to 23 wt% and talc content ranged from 19.5 to 25.5 wt% have been investigated at different strain rates and temperatures. Tensile tests were conducted at temperature of −30, 23, and 80°C, respectively, and strain rate from 10−3 to 10−1/s, respectively. The results showed that that the mechanical properties were modified significantly with the addition of rubber and talc, strain rate and temperature. The necking process of the TPO was rate dependent. At low strain rate, the deformation was close to the isothermal drawing.

The deformation behavior of a thermoplastic olefin (TPO) during tensile loading has been studied by a finite element method (FEM). Dumbbell shaped specimens of the TPO were tested under various strain rates and temperatures. The test results were used to characterize the mechanical properties of this material. To capture the nonlinear rate-dependent response of the TPO, a physically based constitutive model [M.C. Boyce, D.M. Parks, A.S. Argon, 1988] for large strain deformation was used. The model includes rate and temperature dependent yield, strain softening, and strain hardening. The constitutive model was incorporated into a finite element code and was validated by comparison of predicted with measured results. A good correlation was found between the theoretical model results and the experimental data. Finite element analysis was also used to study neck propagation during tensile tests. Using the implemented constitutive model, necking was simulated and compared to experimental observations.

A tangent constitutive model was developed in this article to address the nonlinear viscoplastic behavior of compound grade thermoplastic olefin (TPO). The TPO was commonly blends of polypropylene matrix, rubber, and inorganic filler. The constitutive model for TPO was obtained from the combination of the mechanical behavior of the matrix and fillers. In a multiphase material, the rate-dependent behavior of polypropylene matrix was presented by a physically based constitutive model for large strain deformation, while the deformation behavior of rubber and talc were captured by Hooke's law. The average strain of each phase, as well as the strain of the voids caused by cavitation of rubber and debonding of talc, was determined by the Mori-Tanaka method, in conjunction with a tangent modulus approach. To test the applicability of the developed model, it was applied to calculate the rate dependent stress-strain relations of TPO. The model was predictive of the initial rate-dependent stiffness, yield, and strain hardening response in large strain deformation. The constitutive model was incorporated into a finite element code to predict the large strain deformation behavior of TPO. The initiation of necking and neck propagation were obtained and confirmed by experimental observation. POLYM COMPOS., 2010. © 2009 Society of Plastics Engineers

The aim of this study was to investigate the mechanical behavior and to develop a micromechanical model for thermoplastic olefin (TPO). The TPO with varying ratios of polypropylene (PP)/rubber/talc were prepared in a twin-screw extruder. The mechanical behavior was studied through a series of tensile tests at different strain rates. Considering TPO as a composite consisting of PP matrix, rubber phase, and talc phase, the effective self-consistent method was used to predict material Young's modulus, and an averaging method was used to predict yield stress. Comparisons between predictions and experiments showed that the models capture the elastic behavior of TPO accurately. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

The ability to quantify the material damage at different length scales is critical in the multiscale analysis of material behavior from nanoscale to macroscale. In this article, on the basis of the equivalence of complementary elastic energy we propose a multiresolution rule that transforms different levels of material defects to the equivalent degradation of material properties. It facilitates a sequential memory-efficient processing of massive material defects in a multiresolution framework, and also supports a functionality of partial damage conversion to serve different needs in subsequent numerical analyses. Numerical simulation was conducted with different settings of material defects. The analysis results indicate the efficacy of the proposed method, offering a potential (i) to interface between multiscale material defects and (ii) as an effective method of homogenization for the determination of the damage variable in continuum damage mechanics.

A probabilistic approach is taken to provide a means to predict variability in fatigue properties given a measured or predicted distribution of a critical microstructural feature. Under the assumption that local fatigue life is a monotonically decreasing function of the size of the local microstructural feature responsible for fatigue crack initiation we obtain analytically the cumulative fatigue strength distribution, cumulative life distribution and their corresponding distribution density functions. The theory has been used to study the variability of cast aluminum alloys, which are widely used in the automotive industry for engine components. In this case, the mean fatigue strength and its variation are based on the distribution of micropores that are ubiquitous in cast aluminum. The impact of the size of the highly stressed area, represented by local surface area S and the local density of the microstructural feature is found to have a critical role on the fatigue strength distribution. The influence of other parameters, such as mean and standard deviation of the distribution of the microstructural feature, on fatigue distributions is discussed.

While Raman spectroscopy is often used to measure stresses, the analyses are almost always limited to cases with simple stress states (uniaxial, equibiaxial). Recently we provided an experimental methodology to determine the full state of stress in Si wafers. Here we extend that methodology to interrogate stress states in Si particles embedded in an Al–Si alloy. Such determinations will ultimately be valuable for predicting ductility of cast Al, since a primary source of damage is cracking of eutectic Si particles. We combine electron back-scattered diffraction with the frequency shift, polarization and intensity of the Raman light to determine stress states. Stress states are measured both in the as-received residually stressed state and under in situ uniaxial loading. Comparison with finite element calculations shows good agreement. As an application of the technique, we show the determination of strength of an individual Si particle and compare the stress evolution with various models.

In this paper, we present the results of fatigue testing and analysis of friction stir linear welded dissimilar aluminum-to-magnesium alloys in lap-shear configuration. The overlap linear welds were created by joining AA6022 aluminum alloy to AM60 magnesium alloy. In general, the test data exhibited significant scatter in the fatigue life results and the corresponding failure modes. In fact, observations from fractography analysis revealed two distinct modes of failure. In the first mode of failure observed, fracture occurred when the dominant fatigue crack propagated into either the magnesium or aluminum sheet in a kinked crack formation. Interestingly, frettinglike debris was observed at the initiation sites for this failure mode. In the second mode of failure observed, fracture occurred by interfacial weld separation. In this mode, fractography analysis suggests that the fatigue cracks initiated at weld defects and then propagated through the intermetallic phase.

The automotive product design and manufacturing community is continually besieged by Hercule an engineering, timing, and cost challenges. Nowhere is this more evident than in the development of designs and manufacturing processes for cast aluminum engine blocks and cylinder heads. Increasing engine performance requirements coupled with stringent weight and packaging constraints are pushing aluminum alloys to the limits of their capabilities. To provide high-quality blocks and heads at the lowest possible cost, manufacturing process engineers are required to find increasingly innovative ways to cast and heat treat components. Additionally, to remain competitive, products and manufacturing methods must be developed and implemented in record time. To bridge the gaps between program needs and engineering reality, the use of robust computational models in up-front analysis will take on an increasingly important role. This article describes just such a computational approach, the Virtual Aluminum Castings methodology, which was developed and implemented at Ford Motor Company and demonstrates the feasibility and benefits of integrated computational materials engineering.

The fatigue behavior of self-piercing riveting (SPR) joint, joining aluminum alloy 6111T4 and steel HSLA340 sheets with lapshear geometry was investigated in this paper. Sheet crack was the dominant failure mode, while unexpected rivet shank failure tended to occur under high loading level. Fretting wear was observed at interface of aluminum and steel layers, as well as at rivet and sheets under sinusoidal cyclic tension-tension loading. An energy dispersive X-ray (EDX) analysis of fretting debris revealed the presence of oxide of aluminum and zinc. Fretting was shown to be critical as crack initiation spot. In sheet failure mode, micro cracks were found to initiate at early stage, and crack initiation life was much shorter than crack growth life.Copyright © 2014 by ASME

This paper presents the experimental results of benchmark coupon testing of monotonic and cyclic conditions on friction stir spot welded coupons of Mg AZ31 alloy. The results presented here are a product of a collaborative multinational research effort involving research teams from Canada, China, and the United States. Fatigue tests were conducted in load control at R=0.1 at two different maximum loads: 1kN and 3kN. Good agreement was found between the participating labs regarding the number of cycles to failure. Differences in the failure modes were observed for the two different loading conditions tested. At the higher load, fatigue failure was caused by interfacial fracture. However, at the lower load, fatigue cracks formed perpendicular to the loading direction, which led to full width separation. For additional comparison, the monotonic and cyclic results of the friction stir spot welds are compared to resistance spot welded coupons of similar nugget size.

Magnesium alloys are the lightest structural metal andrecently attention has been focused on using them forstructural automotive components. Fatigue and durabilitystudies are essential in the design of these load-bearingcomponents.In 2006, a large multinational research effort, MagnesiumFront End Research & Development (MFERD), waslaunched involving researchers from Canada, China and theUS. The MFERD project is intended to investigate theapplicability of Mg-alloys as lightweight materials forautomotive body structures. The participating institutions infatigue and durability studies were the University of Waterlooand Ryerson University from Canada, Institute of MetalResearch (IMR) from China, and Mississippi StateUniversity, Westmorland, General Motors Corporation, FordMotor Company and Chrysler Group LLC from the UnitedStates. This paper presents the results of benchmark coupontesting that were obtained for monotonic and cyclicconditions on extruded AM30 alloy samples. Tests wereperformed independently in Canada, China, and the US. Ingeneral, the results reported by different institutions were ingood agreement.Microstructure analyses revealed strong material texture witha unique orientation of extension twinning with respect to theinitial basal plane. The cyclic deformation, therefore, wasseen to be dominated by twinning and detwinning. Theunusual asymmetric hysteresis of AM30 observed for fullyreversed cyclic loading is attributed to twinning undercompression in the extrusion direction, detwinning uponunloading from compression and dislocation slip in tension.The monotonic tests were performed under different strainrates and at room temperature or 125°C. Cyclic tests wereperformed under strain controlled conditions. Two strainamplitudes were considered, 0.3% and 0.6% and all fatiguetests were performed under standard laboratory conditions.Raising the temperature from standard laboratory conditionsto 125°C had a significant effect under monotonic loading:both the yield and tensile strength dropped by about 25%,while ductility increased by 300%. Under fatigue loading atroom temperature, extruded AM30 exhibits asymmetricalcyclic behavior at a strain amplitude of 0.6%, whereas thecyclic behavior at 0.3% was symmetric. The material showedsignificant plastic strain recovery, cyclic hardening, and aclear endurance limit.

Integrated computational materials engineering refers to the use of computer simulations that integrate mathematical models of complex metallurgical processes with computer models used in component and process design. This article outlines an example of a computer-aided engineering tool, such as virtual aluminum castings (VAC), developed and implemented for quickly developing durable cast aluminum power train components. It describes the procedures for the model development of the VAC system. These procedures include linking the manufacturing process to microstructure, linking microstructures to mechanical properties, linking material properties to performance prediction, and model validation and integration into the engineering process. The article discusses the benefits of the VAC system in process selection, process optimization, and improving the component design criteria.

People typically use geographic names instead of coordinates to find geographic information on the web through a search engine. But the current keyword-based web search engines are poorly adapted to help people find information that relates to a particular geographic name, because they don't incorporate the geospatial semantic during the search process. The Semantic Web is a new semantic-based information-retrieval environment. We propose the information organization framework of the GIS semantic data according to the architecture of the Semantic Web, that is, the ontology, the metadata and the data source. Then we deal with the organization of the semantic data based on the three-layered framework respectively. As a focus, we present a novel method to disambiguate geographical name based on the ontology of the place.

Presented is a simulation of an engine cylinder head undergoing water quenching process using a recently developed approach for modeling quenching cooling of metal parts (Wang et al., 2002). The approach is based on the AVL SWIFT Eulerian two-fluid method with special emphasis on handling high mass exchange rate associated with quenching. A tetrahedral grid of 830,000 cells is generated for the computational domain, which includes the solid part of the cylinder head immersed in the fluid. Detailed vapor and temperature distributions are obtained which offer valuable information for the thermal stress analysis. It is observed that the temperature field within the cylinder head is highly non-uniform. The computed cylinder head monitoring point temperature versus time is compared with that registered by the thermal couple measurement. Reasonable agreement is observed. The simulation exercise may potentially be used to identify the cause of cracks often encountered in quenching heat treatment thereby lead to a better design of the process.Copyright © 2003 by ASME

A numerical approach is presented for modeling quenching cooling of solid metal parts. The approach is based on the Eulerian two-fluid model with modifications toward specific needs of boiling heat transfer. Numerical aspects are discussed on handling high phase change rate of boiling, flux formulation in the presence of strong body force, and conjugate heat transfer of solid immersed in a liquid bath. The modeling approach is assessed for a quenching cooling problem with a simple geometry. Comparisons are made between the computed temperature history within the solid and that of measured data using thermocouples. Reasonably well agreement is observed. The study demonstrated that the present modeling approach is able to meet the numerical challenges entailed in quenching cooling and able to predict the quenching cooling phenomena with satisfactory accuracy. Areas for further improvement are also noted.