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Journal Article

Derivation of Effective Strain-Life Data, Crack Closure Parameters and Effective Crack Growth Data from Smooth Specimen Fatigue Tests

Small crack growth from notches under variable amplitude loading requires that crack opening stress be followed on a cycle by cycle basis and taken into account in making fatigue life predictions. The use of constant amplitude fatigue life data that ignores changes in crack opening stress due to high stress overloads in variable amplitude fatigue leads to non-conservative fatigue life predictions. Similarly fatigue life predictions based on small crack growth calculations for cracks growing from flaws in notches are non-conservative when constant amplitude crack growth data are used. These non-conservative predictions have, in both cases, been shown to be due to severe reductions in fatigue crack closure arising from large (overload or underload) cycles in a typical service load history.
Journal Article

Predicting Failure during Sheared Edge Stretching Using a Damage-Based Model for the Shear-Affected Zone

Hole expansion of a dual phase steel, DP600, was numerically investigated using a damage-based constitutive law to predict failure. The parameters governing void nucleation and coalescence were identified from an extensive review of the x-ray micro-tomography data available in the literature to ensure physically-sound predictions of damage evolution. A recently proposed technique to experimentally quantify work-hardening and damage in the shear-affected zone is incorporated into the damage model to enable fracture predictions of holes with sheared edges. Finite-element simulations of a hole expansion test with a conical punch were performed for both a punched and milled hole edge condition and the predicted hole expansion ratios are in very good agreement with the experiment values reported by several researchers.
Technical Paper

A Review Study of Methods for Lithium-ion Battery Health Monitoring and Remaining Life Estimation in Hybrid Electric Vehicles

Due to the high power and energy density and also relative safety, lithium ion batteries are receiving increasing acceptability in industrial applications especially in transportation systems with electric traction such as electric vehicles and hybrid electric vehicles. In this regard, to ensure performance reliability, accurate modeling of calendar life of such batteries is a necessity. In fact, potential failure of Li-ion battery packs remains a barrier to commercialization. Battery pack life is a critical feature to warranty and maintenance planning for hybrid vehicles, and will require adaptive control systems to account for the loss in vehicle range, and loss in battery charge and discharge efficiency. Failure not only results in large replacement costs, but also potential safety concerns such as overheating or short circuiting which may lead to fires.
Journal Article

Impact Testing of a Hot-Formed B-Pillar with Tailored Properties - Experiments and Simulation

This paper presents the numerical validation of the impact response of a hot formed B-pillar component with tailored properties. A laboratory-scale B-pillar tool is considered with integral heating and cooling sections in an effort to locally control the cooling rate of an austenitized blank, thereby producing a part with tailored microstructures to potentially improve the impact response of these components. An instrumented falling-weight drop tower was used to impact the lab-scale B-pillars in a modified 3-point bend configuration to assess the difference between a component in the fully hardened (martensitic) state and a component with a tailored region (consisting of bainite and ferrite). Numerical models were developed using LS-DYNA to simulate the forming and thermal history of the part to estimate the final thickness and strain distributions as well as the predicted microstructures.
Technical Paper

Weldability Improvement Using Coated Electrodes for RSW of HDG Steel

The increased use of zinc coatings on steels has led to a decrease in their weldability. Weld current and time need to be increased in order to achieve sound welds on these materials compared to uncoated steels, and electrode tip life suffers greatly due to rapid alloying and degradation. In this work, typical uncoated Class II electrodes were tested along with a TiC metal matrix composite (MMC) coated electrode. Tests were conducted to study the weldability and process of nugget formation for both electrodes on HDG (hot dipped galvanized) HSLA (high strength low alloys) steels. Current and time ranges were constructed for both types of electrodes by varying either the weld current or weld time while holding all other parameters constant. Analysis of weld microstructures was conducted on cross-sectioned welds using SEM (scanning electron microscopy). Using the coated electrodes reduced weld current and times needed to form MWS (minimum weld size) on the coated steels.
Journal Article

Estimating the Strain-Based FLC of a Tube from Straight Tube Hydroforming Experiments and Numerical Models

The Extended Stress-Based Forming Limit Curve (XSFLC) failure criterion has been shown to provide good qualitative and quantitative predictions of failure (necking) in straight tube hydro forming when the on the level of end-feed (EF) used during hydro forming, the failure criterion has a tendency to over predict failure pressure at low Keeler-Brazier (K-B) approximation is used to define the XSFLC failure curve. Depending EF and under predict failure pressure for high EF. The over/under predictions suggest that the strain-space εFLC, which the XSFLC is based on, has too high of a plane-strain intercept (FLCo), when it is obtained using the K-B approximation (developed for sheet metal).
Technical Paper

Damage and Formability of AKDQ and High Strength DP600 Steel Tubes

Using standard tensile testing methods, the material properties of AKDQ and DP600 steels tubes along the axial direction were determined. A novel in-situ optical strain mapping system ARAMIS® was utilized to evaluate the strain distribution during tensile testing along the axial direction. Microstructural and damage characterization was carried out using microscopy and image analysis techniques to compare the damage evolution and formability of both materials. Failure in both steels was observed to occur via a ductile failure mode. AKDQ was found to be the more formable material as it can achieve higher strains, total elongations and thinning prior to failure than the higher strength DP600.
Technical Paper

Numerical and Experimental Investigation of 5xxx Aluminum Alloy Stretch Flange Forming

Stretch flange features are commonly found in the corner regions of commercial parts, such as window cutouts, where large strains can induce localization and necking. In this study, laboratory-scale stretch flange forming experiments on AA5182 and AA5754 were conducted to address the formability of these aluminum alloys under undergoing this specific deformation process. Two distinct cracking modes were found in the stretch flange samples. One is radial cracking at the inner edge of flange (cutout edge) while the other is circumferential cracking away from the inner edge at the punch profile radius. Numerical simulation of the stretch flange forming operations was conducted with an explicit finite element code-LS-DYNA. A coalescence-suppressed Gurson-based material model is used in the finite element model. Void coalescence and final failure in stretch flange is simulated through measured second-phase particle fields with a so-called damage percolation model.
Technical Paper

Application of Damage Models in Bending and Hydroforming of Aluminum Alloy Tube

This paper examines the application of damage models in tube bending and subsequent hydroforming of AlMg3.5Mn aluminum alloy tubes. An in-house Gurson-based damage model, incorporated within LS-DYNA, has been used for the simulations. The applied damage model contains several void nucleation and growth parameters that must be determined for each material. A simpler straight tube hydroforming process was considered first to check the damage parameters and predicted ductility. Then the model was applied to a sequence of bending and hydroforming. The damage history from pre-bending was mapped to the hydroforming stage, to allow prediction of the overall ductility. The applied forming parameters in the simulation were based on data extracted during the experimental tests. Finally, the numerical results were compared to the experimental data.
Technical Paper

Multi-Scale FE/Damage Percolation Modeling of Ductile Damage Evolution in Aluminum Sheet Forming

A so-called damage percolation model is coupled with Gurson-based finite element (FE) approach in order to accommodate the high strain gradients and localized ductile damage. In doing so, void coalescence and final failure are suppressed in Gurson-based FE modeling while a measured second phase particle field is mapped onto the most damaged mesh area so that percolation modeling can be performed to capture ductile fracture in real sheet forming operations. It is revealed that void nucleation within particle clusters dominates ductile fracture in aluminum alloy sheet forming. Coalescence among several particle clusters triggered final failure of materials. A stretch flange forming is simulated with the coupled modeling.
Technical Paper

Simulation of Electromagnetic Forming of Aluminum Alloy Sheet

Electromagnetic forming of aluminum alloys provides improved forming limits, minimal springback and rapid implementation. The ability to predict the minimum energy required in electromagnetic forming is essential in developing an efficient process. Understanding the development of the strain distribution over time in the blank is also highly desired. A numerical model is needed that offers insight into these areas and the electromagnetic forming process in general that cannot easily be extracted from experiments. To address these concerns, ANSYS/EMAG is used to model the time varying currents that are discharged through the coil in order to obtain the transient magnetic forces acting on the blank. The body forces caused by electromagnetic induction are then used as the boundary condition to model the high velocity deformation of the blank with LS-DYNA, an explicit dynamic finite element code.
Technical Paper

Damage Characterization and Damage Percolation Modelling in Aluminum Alloy Sheet

Tessellation methods have been applied to characterize second phase particle fields and the degree of clustering present in AA 5754 and 5182 automotive sheet alloys. A model of damage development within these materials has been developed using a damage percolation approach based on measured particle distributions. The model accepts tessellated particle fields in order to capture the spatial distributions of particles, as well as nearest neighbour and cluster parameter data. The model demonstrates how damage initiates and percolates within particle clusters in a stable fashion for the majority of the deformation history. Macro-cracking leading to final failure occurs as a chain reaction with catastrophic void linkage triggered once linkage beyond three or more clusters of voids takes place.
Technical Paper

Fatigue Life Prediction for Variable Amplitude Strain Histories

This paper presents a model for fatigue life prediction for metals subjected to variable amplitude service loading. The model, which is based on crack growth and crack closure mechanisms for short fatigue cracks, incorporates a strain-based damage parameter, EΔε*, determined from the effective or open part of a strain cycle along with a fatigue resistance curve that takes the form: EΔε* = A(Nf)b, where E is the elastic modulus, Nf is the number of cycles to failure, and A and b are experimentally determined material constants. The fatigue resistance curve is generated for a SAE 1045 steel and the model is used successfully to predict the fatigue lives of smooth axial specimens subjected to two variable amplitude strain histories. The model is also used to predict the magnitude of non-damaging cycles that can be omitted from the strain histories to accelerate fatigue testing.
Journal Article

The Influence of the Through-Thickness Strain Gradients on the Fracture Characterization of Advanced High-Strength Steels

The development and calibration of stress state-dependent failure criteria for advanced high-strength steel (AHSS) and aluminum alloys requires characterization under proportional loading conditions. Traditional tests to construct a forming limit diagram (FLD), such as Marciniak or Nakazima tests, are based upon identifying the onset of strain localization or a tensile instability (neck). However, the onset of localization is strongly dependent on the through-thickness strain gradient that can delay or suppress the formation of a tensile instability so that cracking may occur before localization. As a result, the material fracture limit becomes the effective forming limit in deformation modes with severe through-thickness strain gradients, and this is not considered in the traditional FLD. In this study, a novel bending test apparatus was developed based upon the VDA 238-100 specification to characterize fracture in plane strain bending using digital image correlation (DIC).