N092-095 TITLE: Innovative Analysis Tool for Damage Growth From Loaded Composite Fastener Holes

TECHNOLOGY AREAS: Air Platform, Materials/Processes

ACQUISITION PROGRAM: PMA 261, Heavy Lift Helicopters

OBJECTIVE: Develop a validated analysis tool to predicate damage growth and prognosticate residual life of loaded damaged fastener holes in laminated composites.

DESCRIPTION: Joining of composite laminates to adjoining laminates and substructures is an ever present feature in all Navy platforms that have composite construction. One method of joining uses drilled holes with fasteners. These fastener holes often are points of damage initiation. The damage is introduced either during manufacturing, such as over-tightening of unshimmed holes, or during service. While limited test data has shown that the composites are damage tolerant, not all configurations have been tested. Service life data of multiple Navy platforms has indications of damage modes that were not detected during testing. Currently, there is no validated analysis technique to analyze these damages.

Damage and durability modeling is impeded due to the fact that the composites often fail in multi-modal fashion and also all the damage modes have not been identified completely. A second hurdle in implementing a damage model is the numerical difficulties associated with all progressive damage growth models. The damage growth process creates new surfaces and a precipitous redistribution of forces at the newly created surfaces. This, in turn, causes instability and convergence problems in the solution process.

PHASE I: Develop a preliminary concept for damage growth models applicable to loaded fastener holes. Demonstrate the feasibility of the approach through a numerical example. Propose a conceptual design for a prototype software/tool to perform damage growth and life prediction for loaded holes in composite laminates that have the potential to meet the stated requirements.

PHASE II: Fully develop all the algorithms and integrate them into a tool that can be commercialized. Validate that the prototype software provides appropriate results by correlating it to test data on a selected component.

PHASE III: Implement the validated algorithms in collaboration with a software vendor.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Damage from a loaded fastener is not unique to military aircraft; it is equally prevalent in the civilian sector. Outside the aerospace industry, the automotive industry is increasingly using composite construction. The innovative methods developed in this program have great potential for application in these civilian/commercial areas.

REFERENCES:
1. J. Schon and R. Starikov, "Fatigue of joints in composite structures," in "Fatigue in Composites," Ed. Bryan Harris, pp. 621-643. 2003

2. G. Alfano and M. Crisfield, "Finite element interface models for the delamination analysis of laminated composites - mechanical and computational issues," Int J of Num Meth.of Eng, pp. 1701-1736, 2001.

KEYWORDS: Loaded Fastener Hole; Delamination; Composite: Virtual Crack Closure Technique (VCCT); Finite Element Analysis; Progressive Damage Growth

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