Frank Abdi

Composite army bridges under fatigue cyclic loading

Military composite bridges offer many unique advantages to the army—for example their high strength-to-weight ratio and superior corrosion and fatigue resistance properties—compared to current steel and aluminum bridges. This paper presents the results of part of a comprehensive, on-going research program sponsored by the US Army to develop innovative field repair techniques for military composite bridges. The virtual tests were performed on the composite treadway under four different loading cases: (i) maximum shear static loading case, (ii) maximum bending static loading case, (iii) fatigue progressive failure analysis for the moving load case, and (iv) fatigue progressive failure analysis for the maximum flexural loading case. Results of virtual testing and progressive failure analysis (PFA) simulation conducted on a composite army bridge (CAB) prototype demonstrated a good match with the full-scale laboratory test results conducted in an earlier study. For instance, the variation between the maximum deflections predicted by the GENOA simulation for the maximum shear and those obtained from the full-scale tests was only 3.2%. In addition, the location and type of damages at the ultimate load were very close to those obtained from the full-scale laboratory tests.

The Residual Strength of Composite Army Bridge after Fire Exposure

Composite Army Bridges (CAB) offer many unique advantages to the military including their lightweight (high strength-to-weight ratio), compared to current steel and aluminum bridges, as well as their superior corrosion and fatigue resistance properties. This paper presents the results of a part of a comprehensive on-going research program sponsored by the US Army to develop innovative field repair techniques for military composite bridges. The program included 1) calibrating the materials constituent properties of the CAB to demonstrate and virtually verify the smart repair technology, 2) employ GENOA virtual testing and progressive failure analysis to confirm behavior of the composite treadway under two different loading schemes, and 3) simulate and predict the post-fire strength of CAB subject to maximum shear loading. The final task employed several sub-steps to generate the required result. The first step subjected the deck of CAB to a thermal load, in order to simulate the effects of fire, and undertook a thermal analysis with MSC Nastran to obtain the corresponding temperature distribution. The second step used the temperature distribution as an input load to calculate damage distribution and pattern due to fire exposure with the aid of a multi-factor interaction model (MFIM), which served as the composite material degradation model with respect to temperature. The third step utilized the damage distribution and damage pattern to update the CAB model, which was submitted to the GENOA progressive failure analyzer for the case of maximum shear loading. In this manner, the final post-fire residual strength of CAB was eventually evaluated as the ultimate load of the third step. The results of the simulation show that fire will dramatically reduce the stiffness and strength of composite army bridge, such that 15% strength remains for CAB under maximum shear loading after fire exposure.

Advanced Composite Wind Turbine Blade Design Based on Durability and Damage Tolerance

The objective of the program was to demonstrate and verify Certification-by-Analysis (CBA) capability for wind turbine blades made from advanced lightweight composite materials. The approach integrated durability and damage tolerance analysis with robust design and virtual testing capabilities to deliver superior, durable, low weight, low cost, long life, and reliable wind blade design. The GENOA durability and life prediction software suite was be used as the primary simulation tool. First, a micromechanics-based computational approach was used to assess the durability of composite laminates with ply drop features commonly used in wind turbine applications. Ply drops occur in composite joints and closures of wind turbine blades to reduce skin thicknesses along the blade span. They increase localized stress concentration, which may cause premature delamination failure in composite and reduced fatigue service life. Durability and damage tolerance (D&DT) were evaluated utilizing a multi-scale micro-macro progressive failure analysis (PFA) technique. PFA is finite element based and is capable of detecting all stages of material damage including initiation and propagation of delamination. It assesses multiple failure criteria and includes the effects of manufacturing anomalies (i.e., void, fiber waviness). Two different approaches have been used within PFA. The first approach is Virtual Crack Closure Technique (VCCT) PFA while the second one is strength-based. Constituent stiffness and strength properties for glass and carbon based material systems were reverse engineered for use in D&DT evaluation of coupons with ply drops under static loading. Lamina and laminate properties calculated using manufacturing and composite architecture details matched closely published test data. Similarly, resin properties were determined for fatigue life calculation. The simulation not only reproduced static strength and fatigue life as observed in the test, it also showed composite damage and fracture modes that resemble those reported in the tests. The results show that computational simulation can be relied on to enhance the design of tapered composite structures such as the ones used in turbine wind blades. A computational simulation for durability, damage tolerance (D&DT) and reliability of composite wind turbine blade structures in presence of uncertainties in material properties was performed. A composite turbine blade was first assessed with finite element based multi-scale progressive failure analysis to determine failure modes and locations as well as the fracture load. D&DT analyses were then validated with static test performed at Sandia National Laboratories. The work was followed by detailed weight analysis to identify contribution of various materials to the overall weight of the blade. The methodology ensured that certain types of failure modes, such as delamination progression, are contained to reduce risk to the structure. Probabilistic analysis indicated that composite shear strength has a great influence on the blade ultimate load under static loading. Weight was reduced by 12% with robust design without loss in reliability or D&DT. Structural benefits obtained with the use of enhanced matrix properties through nanoparticles infusion were also assessed. Thin unidirectional fiberglass layers enriched with silica nanoparticles were applied to the outer surfaces of a wind blade to improve its overall structural performance and durability. The wind blade was a 9-meter prototype structure manufactured and tested subject to three saddle static loading at Sandia National Laboratory (SNL). The blade manufacturing did not include the use of any nano-material. With silica nanoparticles in glass composite applied to the exterior surfaces of the blade, the durability and damage tolerance (D&DT) results from multi-scale PFA showed an increase in ultimate load of the blade by 9.2% as compared to baseline structural performance (without nano). The use of nanoparticles lead to a delay in the onset of delamination. Load-displacement relationships obtained from testing of the blade with baseline neat material were compared to the ones from analytical simulation using neat resin and using silica nanoparticles in the resin. Multi-scale PFA results for the neat material construction matched closely those from test for both load displacement and location and type of damage and failure. AlphaSTAR demonstrated that wind blade structures made from advanced composite materials can be certified with multi-scale progressive failure analysis by following building block verification approach.

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.

Durability and Reliability of Wind Turbine Composite Blades Using Robust Design Approach

The paper describes a computational simulation approach for durability, damage tolerance (D&DT) and reliability of composite wind turbine blade structures in presence of uncertainties in material properties. This computer-based prediction methodology combines composite mechanics with finite element analysis, damage and fracture tracking capability, probabilistic analysis and a robust design algorithm to reduce weight of turbine bales without loss in structural durability and reliability. A composite turbine blade was first assessed with finite element based multi-scale progressive failure analysis to determine failure modes and locations as well as the fracture load. Analysis D&DT results were validated with static test performed at Sandia National Laboratories. The work was followed by detailed weight analysis to identify contribution of various materials to the overall weight of the blade. The methodology ensured that certain types of failure modes, such as delamination progression, are contained to reduce risk to the structure. Probabilistic analysis indicated that composite shear strength has a great influence of the blade ultimate load under static loading. Weight was reduced by 12% with robust design without loss in reliability or D&DT. It was achieved by replacing a small volume of key materials with foam.

Robust Design of Composite Stiffened Panels under Discrete Source Damage, a Combined Durability- Reliability Evaluation

A computational simulation approach was developed to maximize the Durability and Damage Tolerance (D&DT) and reliability of Stitched/Resin Film-Infused (S/RFI) composite stiffened panels in the presence of material, fabrication and geometric uncertainties. Stitched/Resin Film-Infused (S/RFI) composites have been identified as low-weight and cost-effective materials. However, the application of S/RFI composites in aircrafts yields more challenges to aircraft designers due to a lack of understanding failure mechanisms and the damage tolerance of composite systems. As part of the NASA Advanced Composite Technology (ACT) project [Ref 1], the load carrying capability of S/RFI stiffened panels under Discrete Source Damage was investigated. The predicted results were then validated against test data. Subsequently robust design optimization was used to maximize the ACT panel structural durability without loss in reliability. The applied computational process ensures that certain types of failure modes, due to crack propagation in the structure, are contained to reduce risk to the structure. The application of coupled optimization-probabilistic approach to ACT panels shows that the structural reliability and durability can be simultaneously improved with little or no weight penalty.

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.

Health Monitoring of Composite Structures Using Advanced Diagnostic Systems

6and Rashid Miraj University Of California, Irvine, CA, 92697 Alpha STAR Corporation and the University of California (UCI), under US Army contract, devised a comprehensive program of innovative Diagnostic Prognostic System (DPS) and a structural evaluation of Composite Army Bridge (CAB) system. The approach requires the application of physics based durability and damage tolerance to track damage and fracture numerically while monitoring and streaming strain response. The process starts with a simplistic micro-mechanics based computational simulation to assess composite material behavior. The methodology reverse engineers fiber, matrix and interface constituent properties by iteratively solving micro-mechanics equations using un-notched laminate test data as input. Un-notched longitudinal tension (LT), longitudinal compressive (LC), transverse tension (TT), transverse compression (TC), and in-plane shear (IPS) ASTM tests are simulated with finite element-based Progressive Failure Analysis (PFA). The reverse engineering/calibration approach determines the constituents’ linear and non-linear properties. The calibrated properties can then be used with confidence to evaluate the performance of any structure made from the same composite material system. This technical approach was used to characterize the constituents for biaxial and triaxial carbon-based composite material architectures. The materials were representative of a composite Army bridge beam structure subjected to 4-point bending load. Use of the calibrated constituent properties opened the way for successful a priori prediction of the bridge structure experimental behavior. The progressive failure analysis of the bridge structure, using the calibrated composite constituent properties was in very good agreement with those from experimental test results (less than 8% difference). Results from the DPS for strain measurements compared well with strain data predicted by the simulation.

Multilevel Optimization Techniques for Aircraft

A multilevel decomposition methodology for which design variables have relatively equal impact at each level is discussed as an approach to largescale system optimization. Multidisciplinary optimization techniques utilizing sensitivity derivatives and a global sensitivity matrix to combine the levels are used to develop a total optimum design. A hypothetical hypersonic vehicle is used to illustrate the process when considering trajectory issues as well. An advanced fighter wing example is used to illustrate the potential of improved design optimization in a multidisciplinary environment as compared to conventional sequential design development.