Mohit Garg

Cohesive Zone Model for Surface Cracks using Finite Element Analysis

A debonding comparative study for accuracy and software convergence among the Finite Element based non-predetermined crack growth strength/strain based Progressive Failure Analysis (PFA) and pre-determined crack growth fracture mechanics based Continuous/Discrete cohesive zone model (CZM) is performed. It is concluded that for typical 2D/3D crack growth or debonding, a combination of PFA and CZM will result in then most accurate predicted load-displacements in comparison with test. PFA requires the fiber/matrix or lamina material input and is mesh sensitive at the crack tip, necessitating a mesh convergence effort. PFA generates an accurate crack path and the load displacement curve (up-to load peak value). Continuous/Discrete CZM requires the fracture toughness, correlation of cohesive strength and predetermined crack path/interfaces. Several comparative studies of these methodologies against test data were performed. In particular the Composite Storage Module (CSM) adhesively bonded joint tests revealed the pre-mature failure of the bonded area as possible combination of both adhesive and cohesive failure. Adhesive joint failure was caused by a) non-clean surface preparation, b) thick bond line, and c) existence of voids in the adhesive bondline. The problem was analyzed using two approaches: a) Progressive failure analysis with complete and partial bond void representation and b) Virtual Crack Closure Technique with complete and partial bonding. Results indicated full load carrying capability of the joint using PFA and VCCT and a complete bondline approach and a significant reduction in the load carrying capability due to improper bonding.

Predicting Bearing Strength of Fiber Metal Laminates Via Progressive Failure Analysis

Fiber metal laminates (FML), such as GLARE and CentrAL, can offer better structural performance compared to monolithic metal alloys for many aircraft components. Experiments show that the FML’s improve fatigue, residual strength, impact and corrosion resistance. Because of these enhanced capabilities, FML’s are finding their way into aircraft fuselage and wing sections. The next-generation super jumbo Airbus A380 selected GLARE for its fuselage sections. The fuselage sections are generally constructed using mechanical joints that contain rivets and bolts that contribute to the bearing strength issues. Bearing tests clearly show that bearing strength depends highly on the pin/fastener diameter and its distance from the edge. Numerically evaluating the bearing strength of coupons made from FML materials is difficult because of the combination of isotropic metallic layers with highly anisotropic composite layers. The complexity arises from the mixing of different material systems and the ability to track different failure mechanisms, such as such as net-tension, shear-out and bearing. A multi-stage progressive failure analysis, based on finite element analyses, was used to predict and validate the bolted joint structural performance under tension loading. Excellent correlation between test and prediction was observed.

Failure Analysis of Composite Bolted Joints in Tension

The failure of preloaded cross-ply laminated composite has been studied through finite element simulation embedded in Progressive Failure Analysis (PFA). Two modeling strategies including lowand high-fidelity models have been considered for this investigation. The high-fidelity FE model consists of fixture components (bolts and washers). It has been shown that both lowand high-fidelity FE models are capable of predicting the experimentally observed failure modes of bolted joints that depends on the geometric parameters with reasonable accuracy. Two catastrophic failure loads, net-tension and shearout can be predicted using both lowand high-fidelity model while the failure load of bearing mode can only be predicted via high-fidelity model that considers the applied preload of the bolt. However, the overall stiffness in the actual experiment is lower than that of predicted via finite element simulation.

Validation of Class of Applications Using Progressive Failure And Discrete Cohesive Zone Model for Line And Surface Cracks

Combination of multi-scale and multi-physics Progressive Failure Analysis (PFA) methodology and linear elastic fracture mechanics is introduced. The approach combined approach detects crack-path or multi-site crack path and predicts the complete load-displacement curve correctly. Though more accurate, LEFM based methodologies of Discrete Cohesive Zone Modeling (DCZM) and Virtual Crack Closure Technique (VCCT) alone seem to be test duplication methodologies rather than test prediction for several complex problems. This is primarily because the structural finite element (FE) models need pre-defined crack path information from the test, in addition, to fracture toughness and cohesive material properties. This seriously limits the capability of both DCZM and VCCT approaches. The proposed approach in this paper focuses mainly to use DCZM and VCCT approach as test prediction methods rather than test duplication methodology. The methodology works in two stages: first, analyzing an FE model with the strength-/strain-based PFA; second, performing crack propagation analysis with either DCZM or VCCT approach. The approach is designed as a test prediction/reduction strategy for analyzing crack initiation and propagation problems. The first step of simulating a problem with PFA helps determine the expected crack-path initiation and propagation information that both DCZM and VCCT methodologies require solely based on material stiffness and strength properties. The second step helps improve the simulation results and overcome the stress singularity issues predominate in crack propagation fracture analysis problems (associated with removal of elements in FE simulations). The methodology was validated by simulating the delamination process (initiation and propagation) in composite structural components such as joints and z-pinned reinforced composite beam structures. The two step strategy has been applied to a skin/stringer structural joint debonding analysis subjected to a tension and a threepoint bending load. The FE models are a three dimensional surface crack problem analyzed using PFA and DCZM approach consisting predominantly of Mode II (tension) and Modes I and II (3 point bending) failure. All simulation results are in close agreement with the test data. It is noted that strength-based PFA approach alone gave results which are in good agreement with the test data and was able to show secondary failure mechanisms not visible when DCZM alone was used to analyze the joint. On the other hand, the usefulness of the DCZM methodology is apparent for simulating the crack propagation in z-pinned unidirectional composite double cantilever beam (here crack path is well defined). The results are within reasonable agreement with the test data.

Technical Approach for Coupled Reliability-Durability Assessment of Army Vehicle Sub-Assemblies

Abstract : The US Army is seeking to advance simulation methods for assessing the performance and reliability of ground vehicles. The reliability is defined as the probability that the Army vehicle performs its function over a specified period of time and under specified loading conditions; it can be viewed as a measure of successful performance of the component, sub-assembly and eventually whole vehicle. For the structural reliability calculation to be meaningful, it must be coupled with durability evaluation. The durability describes the ability of the structure to endure or resist applied loading. Maximum benefit would be obtained when both the reliability and durability are maximized. Such an outcome is highly desired, especially if it is achieved at low cost and low weight.

Fatigue Life Prediction of Center Cracked CentrAl Stiffened Panel Subject to Spectrum Loading

Fiber Metal Laminates (FML), such as GLARE and CentrAL, offer improved structural performance as compared to that obtained from monolithic materials. FML are expected to enhance durability and damage tolerance (D&DT), and fatigue life of aircraft structures. Weight reduction is another anticipated benefit from FML. A building block verification strategy involving testing and analytical simulation of FML structures was conducted to demonstrate the FML technology. Notched and un-notched coupons made from CentrAL material subjected to static and fatigue loadings were tested and analyzed using multi-scale progressive failure analysis (MS-PFA). Analytical predictions were validated with tests. The results from analyses were in very good agreement with those obtained from tests. The D&DT approach predicted conditions that would produce damage initiation and propagation, fracture initiation and propagation and final failure. It also calculated the residual strength of the FML structure. The MS-PFA approach was also applied to simulate crack growth behavior for a five-stringer stiffened panel designed for a lower wing section subjected to fatigue spectrum loadings. The prediction results matched test within 10%.

Advanced Multi-Scale Composites Material Characterization for Fracture Toughness and Impact Resistance Applications

A multi-scale micromechanics approach is devised and applied to characterize the fracture toughness and impact resistance of glass composites enriched with nano particles. The technical approach integrates multi-scale mechanics with finite element based progressive failure analysis (PFA) for damage tracking and fracture. Implicit finite element solution scheme is used for damage and fracture evaluation under static loading while explicit scheme is used for dynamic (impact) loading simulation. The methodology was validated by simulating published test data [1,2] for E-glass fiber and DGEBA epoxy enriched with silica nanoparticles. Briefly, the process entailed characterizing the constituent properties through a dedicated reverse engineering approach using test unidirectional stiffness and strength as input. In addition to unidirectional specimens under tension and compression loading, mode I and mode II fracture properties for double cantilever beam (DCB) and end notched flexure (ENF) test were also generated using combined PFAVirtual crack closure technique approach. Impact analysis was performed as well to determine the damage modes and damage footprint under low energy impact. The methodology [3] was validated by comparing simulation results against test data. The error difference ranged from 4 to 10%. The analysis was repeated without the use of silica nanoparticles to assess anticipated benefits from advanced multiscale material as compared to conventional composite materials.

Structural Evaluation of a Nickel Base Super Alloy Metal Foam Via NDE and Finite Element

Cellular materials are known to be useful in the application of designing light but stiff structures. This applies to various components used in various industries such as rotorcraft blades, car bodies or portable electronic devices. Structural application of the metal foam is typically confined to light weight sandwich panels, made up of thin solid face sheets and a metallic foam core. The resulting high-stiffness structure is lighter than that constructed only out of the solid metal material. The face sheets carry the applied in-plane and bending loads and the role of the foam core is separate the face sheets to carry some of the shear stresses, while remaining integral with the face sheet. Many challenges relating to the fabrication and testing of these metal foam panels continue to exist due to some mechanical properties falling short of their theoretical potential. Hence in this study, a detailed three dimensional foam structure is generated using series of 2D Computer Tomography (CT) scans, on Haynes 25 metal foam. Series of the 2D images are utilized to construct a high precision solid model including all the fine details within the metal foam as detected by the CT scanning technique. Subsequently, a finite element analysis is then performed on an as fabricated metal foam microstructures to evaluate the foam structural durability and behavior under tensile and compressive loading conditions. The analysis includes a progressive failure analysis (PFA) using GENOA code to further assess the damage initiation, propagation, and failure. The open cell metal foam material is a cobalt-nickel-chromium-tungsten alloy that combines excellent high-temperature strength with good resistance to oxidizing environments up to 1800 °F (980 °C) for prolonged exposures. The foam is formed by a powder metallurgy process with an approximate 100 pores per inch (PPI).

Sizing of Composite Metal Lined Tanks for Space Propulsion Applications

A computational method is described to evaluate the structural performance of composite over-wrapped metal lined LH2 tanks. This work was performed in support of the human space exploration initiative undertaken by NASA. The method is a judicious combination of available computer codes for finite elements, composite mechanics, durability, damage tracking, and damage tolerance. To illustrate the effectiveness of the analytical approach, composite over-wrapped LH2 core tanks of the Bimodal Nuclear Thermal Rocket (BNTR) were sized parametrically using launch loads and burst test requirements. The benefits and debits of inserting advanced composite technology into existing LH2 tank design concepts are evaluated in the paper. Results obtained indicate that LH2 tanks made from tape placement carbon fiber in a toughened epoxy matrix backed by a metallic liner for hermiticity are able to: (1) sustain micro-cracking in the matrix of the composite system prior to liner failure, (2) offer significant weight savings as compared to present technology (up to 31%), and (3) use unified design and weight configuration to support both launch loads and burst test requirements. The structural performance and sizing evaluation was performed for composite tanks varying in length from 10m to 28m. Weight calculations for the composite over-wrapped tanks show that the larger the tank length, the larger the weight savings (compared to those of traditional metallic tanks).