A- and B-Basis Allowables for Composite Materials

Issue #12 - January 28th, 2008

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This Week's Feature Composite Example

A Novel Approach to Determine A- and B-Basis Allowables for
Composite Materials 

Figure 1 - V-Notched Shear Test (Equivalent to ASTM D5379-Isopescu Test)

Many aircraft and spacecraft have advanced designs based on a well-matured and systematic testing technology; however, the manufacture and service life risks still remain. In production, testing to reduce risks due to manufacturing defects is performed at many steps up to and including the entire assembly / integration system levels. Even with all this testing, costly failures still occur and these are increasingly prone to public scrutiny. Table 1 shows the possible mission outcomes given a test prior to service. These tests could be at the part, sub-component, component, sub-assembly, assembly and product levels. As the system moves through production, the cost of executing the mitigation strategy increases due to the increased time and labor required for disassembly and reassembly. Composites are particularly vulnerable to increased risk from the execution of this mitigation strategy from both the additional testing that the adjacent structure will be subjected to and the risk of damage due to increased handling. Sometimes the additional damage done by repeated ground tests is not considered in the overall "remaining life" assessment of the system.

 

Table 1.  Possible Outcomes of the Item Tests [1]

A-Basis and B-Basis allowable values are of great importance in the engineering industries involving composite structures. In most cases, strength Allowables are defined using "design point strategy," meaning that Allowables will be obtained for certain predefined configurations, after which only these configurations are allowed for use in the design. In order to reduce cost, limited amount of configurations will be tested, reducing the choice in configurations and with that the optimization possibilities. A solution to this problem would be a "design space strategy" where few tests are needed to calibrate and validate a failure analysis method, after which analysis can be used to analyze a wide range of possible configurations.

The use of A-Basis and B-Basis allowable properties recognizes that material properties are statistical in nature. The two statistically based tolerance bounds are:

A-Basis or T99: At least 99% of the population of material values is expected to equal or exceed this tolerance bound [2-3] with 95% confidence (single point catastrophic failure with no-load redistribution)

B-Basis or T90: At least 90% of the population of material values is expected to equal or exceed this tolerance bound with 95% confidence (redundant load path with load redistribution)  

These allowable values are important for reducing risk in structural designs. Allowable determination is a time consuming and expensive process, since a large number of tests are required. In order to reduce cost and product lead-time, Virtual Testing is used to reduce necessary physical tests by replacing them with analysis. The objective is to reproduce scatter from multiple sources: manufacturing, material properties, and test. This is done analytically using combined multi-scale Progressive Failure Analysis and Probabilistic Analysis methods to generate reliably the scatter (variability) in material and coupon strength. To assure high accuracy, the multi-scale analysis is used based on a hierarchical analysis, where a combination of macro-mechanics and micro-mechanics are coupled with finite element analysis, damage tracking and fracture, and material degradation capability to analyze structures in great detail [4]: The procedure for predicting material Allowables is: 

1. Calibration of Constituents (fiber and matrix) to verify the lamina/laminate properties

2. Selection of Random Variables: appropriate variables are selected and reasonable assumptions are made to reproduce natural scatter in the material strength (i.e., void and fiber volume fractions). (See Reference 4 for further details.)

3. Use Virtual Simulation capability in GENOA Software to Predict Probabilistic Scatter in the Failure Load for Each Type of Test: combine progressive failure, as shown in Figure 1, analysis with probabilistic methods

4. Plot and Compare Generated Cumulative Distribution and Probability Density Functions for each test category, as shown in Figure 2.

5. Obtain A and B Basis Allowables from 0.01 and 0.1 Probabilities with 95% confidence, as shown in Figure 2. 

Figure 2. GENOA Results: Cumulative Distribution Function Vs. Test Data [3]

There are three distinct capabilities (options) for generating the A- and B-Basis Allowables using GENOA Software using:

1. User Provided Test Data - Approach combines deterministic and statistical methods as specified in: 

- MIL HDBK-17E: Military Handbook for Polymer Matrix Composites

- FAA CFR 14: Aeronautics and Space

2. Limited Test Data (1 coupon from each batch) - Approach combines progressive failure analysis with probabilistic methods. Reduce the number of tests for material qualification. 

3. Without Test Data - For trade study approach combines progressive failure analysis with probabilistic methods (assume scatter based on standard practices).

Figure 3. GENOA Probabilistic Sensitivities of Random Variables

Results & Discussions

Example of generating the A- and B-Basis Allowables using GENOA Virtual Simulation, based on limited test data [option 2] is shown in Figure 2. The material Allowables are generated and compared with published test data using 3 compressive fiber-resin woven composite coupon tests (1st coupon of the 1st panel of the 1st batch). In addition, the software generates the sensitivity of design parameters to the Allowables (Figure 3). The selected variables in simulation were: fiber longitudinal modulus, fiber compressive strength, matrix modulus, matrix compressive strength, fiber volume ratio and void volume ratio. The computational capability presented here results in three major benefits:

1. Reduction in the number of coupon tests for material qualification

2. Accelerate the trade-off studies in the down-selection process for certifying new materials

3. Identification of critical material and manufacturing variables to maximize material performance for a given application

References:

1. Frank Abdi, Tina Castillo, Edward Shroyer "Risk Management of Composite Structure" Book Chapter 45, CRC Handbook, January 2005. Click here to read technical publication. 

2. R. Rice, R. Randall, J. Bakuckas, S. Thompson., "Development of MMPDS Handbook Aircraft Design Allowables". Prepared for the 7th Joint DOD/FAA/NASA Conference on Aging Aircraft, September 8-11, 2003, New Orleans, LA. Click here to read technical publication.

3. DOT/FAA/AR-03/19, Final Report, "Material Qualification and Equivalency for Polymer Matrix Composite Material System: Updated Procedure" Office of Aviation Research, Washington, D.C. 20591, U.S. Department of Transportation Federal Aviation Administration, September, 2003. Click here to read technical publication.

4. M. R. Talagani, Z. Gurdal, and F. Abdi, S. Verhoef "Obtaining A-Basis and B-Basis Allowable Values for Open-Hole Specimens Using Virtual Testing" AIAAC-2007-127, 4. Ankara International Aerospace Conference, 10-12 September, 2007 - METU, Ankara. 
Click here to read technical publication.

Click here to read the full technical product data sheet of GENOA. 

This software feature will be available in our upcoming GENOA 4.3 release and utilized through our on-line web service.

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