Finite element simulations of three laminates in open-hole and unnotched configurations subjected to tension and compression quasi-static loading are investigated as part of the Damage Tolerant Design Principles program organized by the Air Force Research Laboratory. The coupons are made from unidirectional IM7/977-3 plies, which are a composite material composed of intermediate modulus carbon fibers and a toughened epoxy matrix. Blind simulations of coupon stiffness, nominal coupon stress at failure and damage evolution are benchmarked against experimental measurements and X-rays. The blind simulations are followed by a second round of simulations where the modeling strategy is modified to improve agreement between the simulations and experiments. In the present article, the commercial software Autodesk Helius PFA is used to model the non-linear response of the composite material. Within Helius PFA, failure is evaluated at the constituent level by extracting the fiber and matrix volume average stress state from the homogenized composite stress state. The relationships between the composite and constituents are developed using multicontinuum theory and a high-fidelity micromechanics model.
This work was motivated by previous cryogenic-cycling experiments using liquid nitrogen and associated analysis of a balanced and symmetric cross ply laminate with polished edges made from IM7 fibers and CYCOM® 5250-4 bismaleimide matrix. Moisture effects were studied both experimentally and analytically in an attempt to explain differences in crack occurrences between inner and outer plies and between otherwise nearly identical experiments. It was shown that moisture locally diffused into a surface layer has a three-dimensional effect on the laminate stress distribution near the edges where cracks initiate. These effects are strongly ply dependent and help explain ply-to-ply variations in crack densities observed in experiments.
Finite element simulations of three laminates in open-hole configuration subjected to constant amplitude tension–tension fatigue loading are investigated as part of the Damage Tolerant Design Principles program organized by the Air Force Research Laboratory. All coupons were made from unidirectional IM7/977-3 plies, which are composed of intermediate modulus carbon fibers and a toughened epoxy matrix. Government furnished experimental data from an assortment of fatigue loaded unnotched coupons were used to characterize the behavior of the composite material in the simulations. The commercial software Autodesk Helius PFA was used to model the non-linear response of the material. Blind simulations of coupon stiffness and damage at several cycle numbers and residual coupon tensile and compressive strengths are benchmarked against experimental measurements and X-rays. Upon review of the experimental results, a second round of simulations was performed where the modeling strategy was updated to improve correlation to experiment.
IntroductionIntegrated Computational Materials Engineering (ICME) is a game-changing AFRL and industry vision to reduce the material and process development cycle time and cost, simultaneously bringing optimized material systems to the war fighter tailored to the needs of both the airframe and propulsion systems [Ref: National Materials Advisory Board]. The nearterm path to achieving these goals is through integration of material modeling capabilities. AFRL is currently working on two Foundational Engineering Problems (FEPs), one for metallic aircraft applications, and one for composites. GE Aviation and Lockheed Martin Aeronautics (LM Aero) have teamed to work the composites FEP, called "Integrated Computational Methods for Composite Materials (ICM2)" specifically targeting integration of composites processing, micromechanics, and damage progression modeling codes to address composite material development and application issues. GE is focused on engine applications, whereas LM Aero is focused on airframe applications.For the airframe specific ICM2 FEP, LM Aero is targeting the fundamental issues that drive the design of acreage composite materials on the next generation airframes. In order to meet composite airframe future needs, large scale airframe manufacturing will target larger, unitized composite assemblies with increased use of bonding and reduced part-count. Process automation will be utilized to reduce costs through reductions in touch labor. Improvements in composite design allowables are critical to optimal airframe weight (and hence performance) and must be obtained through use of higher performance resins and fibers along with reduced variability in key sizing properties. The ICM2 program intends to integrate composite process and design modeling codes to streamline the development cycle time and reduce the cost to implement such new high performance materials on next generation aircraft. For the ICM2 program's demonstration purposes LM Aero is studying the IM7/M65 bismaleimide (BMI) system for application to large acreage wing skin and web applications. M65 is an established BMI system (MRL ≥5) well suited to manufacturing using high speed automated fiber placement (AFP).BMI systems have experienced increased usage on fighter aircraft due primarily to key structural design properties such as open hole compression (OHC) and compression-strengthafter-impact (CSAI), the values for which exceed epoxies at max service temperature and moisture conditions [Rousseau et al.] These key properties often "size" the acreage of the aircraft composite skins. Bolted joint strength and acreage repair criteria are most closely related
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