N121-042 TITLE: High-Fidelity Residual Strength and Life Prediction Tool for Adhesively Bonded Composite Structures

TECHNOLOGY AREAS: Air Platform, Materials/Processes

ACQUISITION PROGRAM: PMA 261

OBJECTIVE: Develop a multiphysics-based tool that assesses the residual strength and bondline-damage propagation of as-built and service-disbonded adhesively bonded composite joints subjected to significant thermal/mechanical/environmental loads.

DESCRIPTION: Adhesively bonded joints are an increasingly popular alternative to mechanical joints in aerospace engineering applications and provide many advantages over conventional mechanical fasteners, including increased strength-to-weight ratios, design flexibility, ease of fabrication, cost efficiency, integrity, and durability. These attributes make them attractive for use in military and aircraft structural components.

To ensure the integrity and durability of the bonded primary structure, adhesively bonded joints need to be evaluated to determine their strength and life expenditure. However, it is difficult to evaluate bond strength due to the process-driven bonding properties, service-driven failure modes, and damage initiation and propagation. Bonding processes involve critical steps such as surface preparation, adhesive mixing, adhesive application, and curing time, as well as critical parameters such as curing temperature and pressure. Service- and environment-induced damages include crack initiation and propagation along the interface, crackling within the adhesive, and failure of the adherend.

Currently, the bond strength of adhesively bonded joints is evaluated using a variety of methods�all of which entail problems of some sort. Coupon-level test data, for example, cannot ideally represent the bonding properties of a large-scale bonded structure at field due to the non-uniform applied temperature and different curing time experienced. Analysis simplification by smearing a finite adhesive layer with a cohesive model either through a cohesive element or a cohesive interface interaction is also problematic because the adhesive's physical properties and nonlinear material behavior, as well as the discrete damage in the adhesive layer, are lumped into a few parameters to describe the constitutive behavior of the cohesive model. The use of a conventional finite element discretization of the adhesive layer is burdensome because an extremely higher mesh density has to be introduced to maintain an adequate element aspect ratio for the adhesive layer of the thickness between 0.03 mm and 0.5 mm.

The goal of this project is to develop an innovative, high-fidelity multiphysics-based tool along with standardized material characterization to evaluate the residual strength and life expectancy of adhesively bonded composite structures.

PHASE I: Develop and demonstrate the technical feasibility of creating a tool that will evaluate processing-dependent bonding properties, mechanical performance, residual strength, and life expectancy of adhesively bonded composite structures. Conduct validation at the subcomponent level to illustrate the accuracy and modeling efficiency of the tool.

PHASE II: Based on the approach selected in Phase I, design, develop, and validate a prototype that integrates all of the algorithms developed. 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 house.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The use of adhesives for bonding structures is steadily growing and finding new applications in areas such as electronics, construction, and packaging. Bond integrity is critical to both commercial and military aerospace structures. This tool will be applicable to both sectors for optimizing design and maximizing performance. It is increasingly being used in automotive applications where there is a need to join sheets of dissimilar materials to produce lightweight car bodies.

REFERENCES:
1. Hoyt, D. M., Ward S. H., & Minguet, P. J. (2002). Strength and fatigue life modeling of bonded joints in composite structure. Journal of Composites Technology and Research, 24(3), 188-208.

2. Adams, R. D., Comyn J., & Wake, W. C. (1997). Structural adhesive joints in engineering (2nd ed.). London, England: Chapman & Hall.

KEYWORDS: bonded structures; durability; damage tolerance; composite fatigue; disbond growth; model validation

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