Characterization of Notched Composites Strength with Empirical and Analytical Methods

Bau, H., Hoyt, D.M.

NSE Composites Phase II Final Report to USAF for STTR AF96T009 (AF Contract # F33615-97-C-5013), July 2000.

STTR Phase II Summary:
Structural laminated composites are used extensively in new aircraft applications. Notched strength design criteria are often used for composite structures in aircraft as a simple method to account for free edge effects, repair and damage. As a result, a significant area of structure away from bolted joints as well as bolted joint areas of composite skin structures in aircraft are often sized using notched strength allowables. Expensive large test programs are conducted to generate composite strength allowables because existing analytical methods are not accurate. Therefore, improvements in analytical methods to predict notched strength may result in substantial cost and weight savings in new composite aircraft designs, and may result in increased allowable damage or increased life in existing designs.

NSE Composites Stress Services (NSE), the University of Dayton Research Institute (UDRI), and Acellent/Leading Aeronautical Technologies Inc. (LAT), were teamed together for this STTR Phase II to characterize notched strength behavior in composite materials. The STTR Phase II program involved a comprehensive approach to improving methods for predicting notched strength. This included an empirical evaluation of existing notched tension test data from DoD aircraft allowables programs, a static/incremental x-ray test program which included uniaxially notched tension and compression specimens in four IM6/3501-6 aerospace layups, and development of analytical tools including progressive damage compression analyses and the addition of clamp-up effects to a splined stress analysis model, SVELT, developed by the US Air Force. These combined efforts have resulted in significant advancements in the understanding of bolted joints behavior and improved analysis methods to predict notched strength.

The major underlying problem with analytically-based strength allowables is the lack of validated methods to predict composites strength at the laminate level. A major obstacle to developing accurate analytical strength methods is the lack of a comprehensive knowledge of the physical behavior of laminate failure mechanisms. This is due to the high level of complexity in the failure mechanisms for composites and the expense of testing to produce a database of those failure mechanisms. Development of a strong link between theory and test data is a prerequisite for improving the accuracy of analytical composite strength methods as well as for the validation of these theories. In this program, NSE provided links between theory and test data to a degree that had not previously been accomplished. This STTR program has achieved results in the following areas:

Integrated test results from multiple aircraft programs for several graphite tape laminate material systems. Demonstrated a new method to correlate Kt stress concentrations to notched tension strength. Determined sub-critical failure modes and loads in open and filled hole tension specimens using incremental x-ray testing. Identified failure mechanisms and corresponding sub-critical damage modes for open and filled hole tension test configurations. Generated characteristic equations for layup variables for notched tension failure mechanisms which can be used to characterize nearly all of the DoD notched tension baseline test data, including open and filled configurations at every tested environment. Demonstrated methods to use characteristic equations for particular failure mechanisms as the basis for calculating statistical allowable strength values. Correlated progressive damage model predictions of ply failure sequence and modes to test data. Characteristic equations that correlated to notched tension strength with theory, sub-critical damage, and extensive test data were successfully generated from this program. When fully developed, these characteristic equations can be used to predict strength trends and develop structural allowables with far less testing, thus significantly reducing the cost of developing allowables for new composite material systems.