Saber DorMohammadi

Damage tolerant composite design principles for aircraft components under static service loading using multi-scale progressive failure analysis

The overall objective of this effort was to provide theoretical prediction for damage development for a set of laminated composites using Alpha STAR Corporations’ commercial code GENOA (GENeral Optimization Analyzer) for the Air Force Research Laboratory program entitled “Damage Tolerance Design Principles (DTDP)”. Damage progression and prediction for advance composite benchmarks were done under static and fatigue service loading using test data from Lockheed Martin Aeronautics and Air Force Research Laboratory. In the current paper, the results for the static analysis are presented. Emerging and innovative multi-scale (MS) modeling using computational structural mechanics and progressive failure analysis were proven to address the Air Force’s vision to perform predictive evaluation of composite materials using a building block validation strategy and certification process. Three layups were tested in tension and compression for unnotched and openhole configurations. Calibration of the fiber and matrix properties was performed using in plane, 3pt bend and DCB test data. After this, mesh convergence, solver selection based on CPU time, and mesh sensitivities was performed. The static blind simulations of strength showed an average error of 12.9% between simulation and the test data. For stiffness, the percent difference was found to be 23.5% on average. Although the focus was on the ability to blindly predict test data, recalibration efforts show an average of 9.2% difference between simulation and test for strengths and 12.4% for stiffness computations. Damage at ∼60–75% and ∼90% of max loading was comparable with X-ray observations of specimens set aside solely for that purpose. All simulations used the same set of inputs (constituents, voids, fiber waviness, etc.) except for the noted analysis setting differences between blind and recalibration simulations. The method is consistent and follows a building block simulation approach that has an advanced yet simplistic theoretical multi-scale progressive failure analysis approach all contained in the commercial GENOA software. The method was demonstrated to work having GENOA directly run sequential NASTRAN simulations and, post project completion, with the ABAQUS solver using GENOA as a material subroutine.

Crush simulation of automotive chopped fiber composite structures by de-homogenization multi-scale computational method

A de-homogenization multi-scale computational method is proposed for the virtual performance simulation of chopped fibers composites under crush loadings. The novel approach led to the development of a multi-scale material characterization procedure for composite systems, comprising of: (1) chopped fibers homogenization based on the Eshelby and Mori–Tanaka inclusion theories, (2) orientation tensor stiffness averaging technique, (3) micro- and macro-mechanics damage and failure theories, and (4) crush resistance evaluation, as a part of the durability and damage tolerance analysis. The chopped fibers material model developed in this work is then employed in a finite element analysis, interfaced with a multi-scale progressive failure technique to track damage and fracture evolution. Comparison of the simulation results obtained for two tubes, manufactured by injection and compression molding respectively, shows good agreement with the crush test data within 10% accuracy. The proposed de-homogenization method offers a superior load resistance prediction over commercially available techniques. Furthermore, the implementation of the proposed approach in our in-house software provides traceable damage evolution and visualization of the contributing failure mechanisms, valuable sources for the design and development of new composite structures.

Prediction Validation of Thermal Aging Performance of Military Composite Bridges

Short-term aging properties of a tri-axial polymer composite material for mobile bridges operating in high heat environment were analyzed with progressive failure simulation software and physically tested for verification. The work focused on thermal cycling, impact damage and ultra violet (UV) exposure effects on degradation of the composite material tensile and compressive strengths as well as fatigue life. Tests and simulations showed that short-term thermal cycling and UV exposure did not have significant effects on the composite mechanical properties. Simulation results agreed well with the test data.

A-B Basis Allowable Test Reduction Approach and Composite Generic Basis Strength Values

Traditionally engineers have tested materials and coupons in various forms in order to obtain the necessary “material allowable”, consider scatter in composite material laminates, and support scale up according to the “Building Block Testing Approach”. Unfortunately, the cost of testing per the Building Block approach is costly, because it requires a robust or reduced sampling of ASTM coupons. The determination of strength allowable by means of testing, even if the material is produced at one facility, is costly and time consuming, as a large number of samples needs to be tested at different environments. A new methodology is required to develop, qualify and introduce advanced composite materials into the mainstream of future applications within a shorter time span by determining A-B Basis allowables: 1) number of required tests, cause of scatter from material and manufacturing process. The proposed paper validates IM7-8552 material system using multi-scale progressive failure integrated with probabilistic and statistical approach that consists of: a) Constituent variation method (CVM) and Bayesian statistics; and b) generic basis strength values for polymer composites using NIAR. Its purpose is aimed at reducing tests and risk associated with the use of composites in aerospace structures. This is achieved by lowering the probability of failure of primary structures through the use of A-basis or B-basis strength allowables as design values considering (i) the composite scale up, (ii) use of different facilities and (iii) structural redundant load carrying capability. The methodology combines probabilistic methods with advanced multi-scale multi-physics progressive failure analysis and Bayesian statistics to reduce the number of tests needed for determination of strength allowables. The test reduction analysis process produces random variable vectors of fiber/matrix/lamina properties and fabrication variables such as fiber content, void, and ply thickness. The random variables are then fitted to normal distribution and distribution parameters are computed. Cumulative distribution functions, probabilistic sensitivities, and A and B Basis Allowables for unidirectional and several soft, quasi, and hard mixed layups (10/80/10, 25/50/25, and 50/40/10) in tension and compression for un-notched, open-hole and filled-hole are validated with physical testing for HEXCEL 8552 IM7 tape. All simulations are within an acceptable amount of error.