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Residual Property Prediction for Damage Composite Structures


OBJECTIVE: Develop a novel modeling approach for predicting and quantifying the residual strength and stiffness of composite material structures, specifically compression after impact (CAI) strength. DESCRIPTION: Advanced composite material systems are vital to the development of lightweight, multi-functional Army missile systems. In addition to reducing the weight of the structure, these material systems provide the ability to expand the function of the structure by tailoring stiffness and strength characteristics for numerous applications. Carbon fiber-reinforced epoxy structures have become very attractive for applications such as solid rocket motor cases, missile airframes, missile guidance housings, as well as many launch tubes and launcher primary structures. The Weapons Development and Integration Directorate within AMRDEC has identified a need to understand the operational fitness of these types of structures following impact events from a wide range of energy levels. It is normally accepted that a limiting characteristic of thin composite structures of this nature is the response of the impacted material to compressive loads, such as buckling. It is also well documented that delamination is the predominant damage mode in composite materials subjected to impact damage. This delamination significantly affects the residual compressive strength in the structure and there have been numerous studies to characterize the compressive response of the impacted structure using a variety of approaches. Post-impact test-determined material properties, Hertzian law, force-energy relationships, load-rate sensitivity approaches, absorbed-to-impact energy ratio methods, hydro-code and semi-empirical methods are current approaches used to estimate residual strength. Novel advanced composite material design and analysis approaches are sought to minimize the risk of damage by the combined effects of blast and fragmentation from sources such as warhead detonation as well as single low-energy impacts resulting from operations and maintenance impact accidents. Little work has been performed on understanding these combined effects on the residual strength of these structures. Additionally, a better understanding of the low energy single or multi-point impact characteristics on these materials/structures is sought. AMRDEC seeks to advance the state of the art in post-impact determination of residual strength, including compressive residual strength of composite material structures used in defense applications. A successful response to this topic will deliver a computer subroutine that contains novel mathematical/physical approaches that can accurately assess the residual compressive strength in impact-damaged composite material structures. The code would be easily interfaced with existing finite element or finite difference codes to perform design trade studies, preliminary designs, and residual strength and fitness assessment of the structures. The computer model would provide a test-proven method that can be used for design and inspection of composite material structures subjected to single and multi-point impact scenarios with varying impact energy levels. The high-energy events typically result from high velocity debris due to warhead detonation. The low energy events are attributed to accidental tool impact, handling or dropping, and low frequency, low amplitude vibration impacts due to ground transportation. Examples of critical structures are cylindrical solid rocket motor cases and airframes, launcher primary structures, and launch tubes. The goal of a three-phase SBIR process is the delivery of a novel test-proven and verified method for use in determining the residual compressive strength of composite structures for Army missile applications. PHASE I: Phase I will evaluate the technical merit and feasibility of the proposed technology to determine residual compressive strength of advanced composite material structures. The awardee shall present an initial concept method as a required Phase I deliverable; supporting proof-of-principle data may be obtained by correlation with existing data or selected strategic testing. This should include a work flow for the subroutine, major parametric models to be included in the subroutine, and strategies for integrating the model into commercially available analysis codes. Projects seeking Phase II funding should consider: 1) a sound strategy for validating the technology on different material systems and structural configurations; 2) simplicity of use for design and analysis activities; and 3) development of new understanding of critical parameters and their sensitivities on the determination of residual compressive strength. PHASE II: The initial approach verified in Phase I will be further developed and refined with the goal of maturing and expanding the computer model. The second phase will focus on expanding to validate the models using relevant material systems, configurations, impact conditions, and environmental conditions. Any processes/methods developed in Phase I will be expanded to perform more robust correlations based on real world impact and loading scenarios as defined by the appropriate AMRDEC Directorate. The awardee should interface with potential military and commercial customers to guide the scope of a potential Phase III effort. International Traffic in Army Regulation (ITAR) control is required. PHASE III: Phase III will demonstrate a mature analytical computer code/technology. The goal will be a tool that can assess residual strength in relevant composite material systems that are subjected to impact damage scenarios in their operating environment. It will be important also to understand the limitations of the model with respect to the different conditions for which it will be used. The awardee will deliver a product that can easily be integrated with existing computer codes. This will enable transition of the technology to defense and aerospace users. This is considered a pervasive technology but will have application for future Army weight reduction efforts for systems including TOW, Javelin, and JAGM.
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