Ari Caliskan

Discrete Cohesive Zone Model to Simulate Static Fracture in 2D Triaxially Braided Carbon Fiber Composites

A discrete cohesive zone model (DCZM) is implemented to simulate the mode I fracture of two dimensional triaxially braided carbon (2DTBC) fiber composites. The 2DTBC is modeled as an elastic-one-parameter (a66) plastic continuum. The plastic behavior of the 2DTBC was characterized by measuring a66. Mode I fracture tests are carried out by using a modified single edge notch bend (SENB) configuration. Fracture toughness (GIC) as a function of crack extension is measured by a compliance approach. The fracture tests are then simulated by using the DCZM based interface element in conjunction with the commercial software ABAQUS® through a user subroutine UEL. The simulated results, carried out under conditions of plane stress, are compared with the experimental results and also verified for mesh sensitivity. The results presented provide guidelines and a basic understanding to model structural response of non-homogeneous materials, incorporating fracture as a damage mechanism and using constituent level material properties, geometry, and fracture toughness (GIC) as input.

Specimen size effects in the off-axis compression test of unidirectional carbon fiber tow composites

Specimen size effects in the off-axis compression test for unidirectional carbon fiber tow composites were studied by subjecting coupon specimens of different lengths and widths to off-axis compression under static and low velocity impact loading. Specimens were cut such that the fibers are at angles of 15, 30, 45 and 60° to the direction of loading. A modified compression fixture with anti-buckling side supports was used to carry out the tests. Static tests were carried out on a hydraulically activated MTS loading frame, where specimens were subjected to displacement controlled loading. Low velocity impact tests were conducted on a drop tower facility. A three strain gage rosette was used to measure global strains. Load was measured using a load cell. Due to the unique microstructure of the specimens, a modified three parameter orthotropic plasticity characterization of the plastic response was used and the constants associated with this characterization were determined uniquely. It is shown that the orthotropic plastic response of the material is affected by specimen size primarily due to the mechanism of load introduction in the off-axis test and load transfer through the fiber tow/matrix interface that is prevalent in the material.

Strain-Rate Effects on Unidirectional Carbon-Fiber Composites

Strain-rate effects on mode I fracture of unidirectional carbon-e ber tow composites corresponding to crack propagation parallel to the e ber tow direction was investigated. Precracked unidirectional stitched carbon-e ber specimensweresubjected to a staticandlow-velocity-impactthree-pointbend test.Thecrack position asa function oftime and hencethecrack-propagation velocity were measured with thehelp ofspecial crack-propagation gauges and a high-resolution digital camera. Load vs load point displacement was measured for every test. The effect of strain rate on fracture energy was characterized. The Iosipescu shear test under static and low-velocity-impact loading conditions wasused to characterizetherate-dependent shearresponse ofthematerial. In addition, tension and compression responses were characterized using American Society for Testing and Materials standard test cone gurations. It is found that the mode I fracture energy decreases with an increase in the rate of loading. with the failure event. The resin used for the tubes is slightly rate sensitive; however,thisindicatesthat theresin isstiffer and stronger under dynamic conditions. Yet, the dynamically crushed tubes con- sistently absorblessenergythan thestatically crushedtubes,always atalowermeanplateauload.Thus,itappearsthattheratesensitivity of the fracture events warrant a careful examination. The basic building block of braided composite plaques are e ber tows that are braided into different microstructural architectures prior to being infused with resin. Different types of braided archi- tectures are summarized in the text. 1 Prior to studying the fracture propertiesofthebraidedplaques (thebraidedplaquescontainacom- plex internal microstructure ), it is prudent to understand the various fracturemechanisms andfractureproperties of thetows themselves. Fundamental issues related to mode I, mode II, and mixed mode fracture of stitched tow-reinforced composites need investigation. The mode I, mode II, and mixed mode fracture energies are fun- damental properties of a e ber-reinforced composite. Consequently, measurement of these fracture energies is necessary for properly characterizing the response and failure of structures made of these composites. In this paper we present the results of an experimental study that examined the mode I fracture of unidirectional carbon- e ber tow composites with crack propagation along the e ber tow direction. The present investigation of crack growth is limited to low ve- locity impact (LVI) conditions. Under these conditions the dy- namic stress e eld produced by the impact loading subsides, and this transient e eld occurs at the very early stages of loading. In the present experiments the maximum impactor velocity is 4.6 m/s, re- sulting in maximum crack-propagation velocities on the order of 350 m/s. These velocities are a small fraction of the shear wave speed (1540 m/s) and Rayleigh wave speed of the material. Con- sequently, dynamic effects can be neglected. Researchers 7i11 have conducted an extensive experimental and numerical investigation of dynamic crack propagation in unidirectional continuous e ber (prepreg)-laminated composites. The present study examines con- tinuous e ber (e ber tows) unidirectional composites under LVI con- ditions.

Discrete Cohesive Zone Model to Simulate Static Fracture in Carbon Fiber Composites

A discrete cohesive zone model (DCZM) is developed to simulate the mode I and mixed mode fracture. For the mode I case, experimental results generated using a modified single edge notched bend specimen of a 2D triaxially braided composite (2DTBC) are used to verify the DCZM. The 2DTBC is modeled as an elastic one-parameter (“a66”) plastic continuum. The plastic behavior of the 2DTBC is characterized by measuring a66. Fracture toughness (GIC) as a function of crack extension is measured by a compliance approach in the SENB tests. A previously developed mixed mode bending (MMB) fracture test configuration is a useful method to generate fracture envelopes for delamination failure of composites. The DCZM is used to simulate mixed mode fracture of a unidirectional laminated composite loaded using the MMB. The simulated results are compared with selected experimental results and also verified for mesh sensitivity. It is shown that the present DCZM is a versatile tool to study failure of a wide class of composite materials.

Metal Inert Gas (MIG) Welding Process Optimization for Joining Aluminum 5754 Sheet Material Using OTC/Daihen Equipment

The development of lightweight vehicles, in particular aluminum intensive vehicles, require significant manufacturing process development for joining and assembling aluminum structures. Currently, 5xxx and 6xxx aluminum alloys are being used in various structural applications in a number of lightweight vehicles worldwide. Various joining methods, such as MIG, Laser and adhesive bonding have been investigated as technology enables for high volume joining of 5xxx, and 6xxx series alloys. In this study, metal inert gas (MIG) welding is used to join 5754 non-heat-treatable alloy sheet products. The objective of this study is to develop optimum weld process parameters for non-heat-treatable 5754 aluminum alloys. The MIG welding equipment used in this study is an OTC/Daihen CPD-350 welding systems and DR-4000 pulse power supply. The factors selected to understand the influence of weld process parameters on the mechanical properties and metallurgy (weld penetration) include power input (torch speed, voltage, current, wire feed), pulse frequency, and gas flow rate. Test coupons used in this study were based on a single lap configuration. A full factorial design of experiment (DOE) was conducted to understand the main and interaction effects on joint failure and weld penetration. The joint strengths and weld penetrations are measured for various operating ranges of weld factors. Post weld analysis indicates, power input and gas flow rate are the two signficant factors (statistically) based on lap shear load to failure and weld penentration data. There were no 2-way or 3-way interaction effects observed in ths weld study. Based on the joint strength and weld penetration, optimum weld process factors were determined.Copyright

A Novel Capability for Crush Testing Crash Energy Management Structures at Intermediate Rates

The crush performance of lightweight composite automotive structures varies significantly between static and dynamic test conditions. This paper discusses the development of a new dynamic testing facility that can be used to characterize crash performance at high loads and constant speed. Previous research results from the Energy Management Working Group (EMWG) of the Automotive Composites Consortium (ACC) showed that the static crush resistance of composite tubes can be significantly greater than dynamic crush results at speeds greater than 2 m/s. The new testing facility will provide the unique capability to crush structures at high loads in the intermediate velocity range. A novel machine control system was designed and projections of the machine performance indicate its compliance with the desired test tolerances. The test machine will be part of a national user facility at the Oak Ridge National Laboratory (ORNL) and will be available for use in the summer of 2002.

Axial and Lateral Impact Prediction of Composite Structures Using Explicit Finite Element Analysis

The use of composite materials in the automotive industry is growing since these materials exhibit high stiffness, strength and low weight. As such, analytical capabilities must be developed in order for these materials to be used in more structural applications. Previous work in the area of crush performance has concentrated on experimental and empirical studies that have qualitatively characterized the crush process. These studies have shown that the crush process in composite materials is complex, and is dominated by fiber/matrix microcracking, which is the main energy absorption mechanism. In this study, the crush performance of a set of tubular composite structures were modeled using the explicit code RADIOSS™. Unlike many of the other commercially available codes, the composite material model within RADIOSS uses material input parameters that can be easily extracted from basic material test. These tests would include a 0° and 90° tensile and compressive test, as well as an in-plane shear test. The model can also accommodate strain rate effects. As the structure is loaded, the stresses within each element and ply are calculated. Using a Tsai-Wu failure criterion, the material fracture is simulated by removing a failed ply within a given element. As a consequence, the material degradation within and ahead of the crush front is simulated. The results of the study showed that the steady state crush load could be predicted accurately. However, the exact failure mode with the crushed structure was not as accurately represented in the model. In addition, two other case studies one being a 3-point bending on a hexagonal section and composite sandwich plate impact analysis were also performed. The results showed good agreement with experiments in both load levels and failure modes.Copyright

Experimental and Analytical Study of the Crashworthiness for the 2005 Ford GT Aluminum Spaceframe

The use of aluminum structures in the automobile industry have been increasing in the past decade due partly to the demand for light-weight vehicles, and in some instances, lower investment costs. In the case of the 2005 Ford GT, an aluminum spaceframe architecture was chosen. The spaceframe structure consists mainly of extruded 6xxx series aluminum profiles with aluminum castings acting as suspension attachment points. The aluminum castings, located at both the front and rear of the vehicle, also act as nodes to which a number of extrusions are welded. This architecture resulted in a very stiff, yet light-weight vehicle. In addition to stiffness and weight advantages, the use of both aluminum members and the spaceframe construction proved to have good crashworthiness properties for all impact modes. In this paper, the crash performance of the front end of the vehicle consisting of an extruded bumper and double-cell rail system is shown. Once the components were manufactured, specimen level tests were conducted to measure the stress-strain behavior of the extruded material. This information, along with the geometric data of the bumper and rails, was used to create models of the front-end of the vehicle. A series of analyses were conducted using a rigid barrier impact to determine crush loads as well as mode of collapse. Concurrently, the components were assembled and tested using a sled impact facility at speeds comparable to full vehicle impact speeds. The results of the component tests and the analyses showed that the models predicted both the crush loads as well as the crush modes accurately. This validation exercise proved to be key in creating accurate full vehicle models for all the crash modes that are required for certification of the vehicle. As such, development time as well as the number of full vehicle tests was reduced.© 2005 ASME

Fracture of 2D Triaxially Braided Carbon Fiber Composites and Resin Effects on the Energy Absorption

Results from an experimental program to investigate the propagation of damage in 2D triaxially braided carbon fiber textile composites (2DTBC) under static conditions are reported. A methodology is presented in which classical concepts from fracture mechanics are generalized to address damage growth in an orthotropic and heterogeneous structural material. Along with results from the experimental program, a novel numerical technique that employs ideas from cohesive zone modeling and implemented through the use of finite element analysis is also presented. The inputs that are required to implement such a discrete cohesive zone model (DCZM) are identified. Compact tension specimen (CTS) fracture tests were carried out by loading 2DTBC coupons cyclically and monotonically. Load and load point displacement were measured. The crack initiation, propagation and crack path history was recorded using high resolution digital photography. The measurements were used to extract the fracture energy (GIC) as a function of crack tip position. Notched Tension tests were carried out to measure the maximum stress in the composites, which provides the cohesive strength (σ c ) of these composites. The material constants so obtained and the DCZM modeling strategy were independently verified by conducting single edge notched bend (SENB) fracture tests using a modified three-point bend test fixture. The experimental and numerical analyses were carried out for two different types of 2DTBC made from two different resin systems to confirm the usefulness of the proposed methodology.

Effect of Manufacturing and Assembly Process Variations on the Crash Behavior of a Hybrid Aluminum Front-End Structure

In this work, the crash behavior of a hybrid aluminum front-end structure was studied. From the test results, it was shown that overall the structure absorbed the required impact energy. An initial CAE model, which was created prior to the test, predicted the initial portion of the impact pulse. However, beyond the point where weld and joint failure initiated the model over predicted the results. Since the model did not simulate the weld failures, a more detailed model of the front-end structure was created to account for these failures. Unlike steel, the Heat-Effected-Zone (HAZ) around the welds in heat-treatable aluminum alloys exhibit significantly lower yield and ultimate stress than the base material. However, the degree to which the properties are reduced and size of the HAZ vary as a function of welding speed, heat input, weld alloy, etc. A series of analysis using a range of the properties around the HAZ showed a corresponding variation in the crash pulse. The range of properties used was based on coupon level tests of the yield and ultimate strength variation around the HAZ. To better understand the effect of the HAZ and other variations within the structure, a stochastic analysis methodology is proposed. This type of analysis will point to significant source(s) within the structure or components that affect the variations in the crash pulse. It is hoped that this information can be used to improve the manufacturing process or aid in the redesign of the structure to lessen the effect of variations.Copyright