AFRL researchers developed a unique design and structural analysis tool for composite materials, and they subsequently transitioned their product to manufacturers of helicopters and other rotorcraft. The new tool, the B-Spline Analysis Method (BSAM), makes it quicker and less expensive to characterize and predict the behavior of flaws or damage in the structures used to build these aircraft. AFRL developed the technology in coordination with the University of Dayton Research Institute (UDRI) and the United Technologies Research Center (UTRC) and then transitioned it to Sikorsky Aircraft Corporation.

Graphical depiction of cracked composite part under tensile loading

Air Force initiatives to make composite aerospace structures more affordable have concentrated on the design and manufacture of highly unitized structures, a focus that eliminates joints and fasteners and thereby lowers manufacturing costs.1 The use of adhesively bonded joints in primary aircraft structures and critical load paths is constrained both by the lack of an established nondestructive evaluation technique for quantifying the integrity of the adhesive bond and by the inability to predict the long-term performance and damage tolerance of the adhesively bonded joint under environmental or mechanical service loading. These complex composite materials pose major challenges to current modeling techniques, particularly in assessing the damage tolerance of composite structures. Traditional damage assessments rely on risk reduction testing and detailed finite element models (FEM). AFRL scientists developed BSAM as a tool that engineers can apply early in the design process to rapidly assess component damage tolerance, a capability that shortens design timelines and decreases risk reduction activities.

BSAM employs a revolutionary numerical approach in modeling solidmechanics problems. It analyzes the three-dimensional stress behavior within a layered composite material, operating at the composite's ply level and maintaining the continuity of strains and stresses throughout a homogeneous ply while allowing strain discontinuity at ply interfaces. A general-purpose solidmechanics analysis method, BSAM employs an innovative method of assembling B-spline approximations of deformation in a numerical format, which enables the code to efficiently solve complex mechanics problems. Comparative analysis of composite materials with fastener holes indicates that BSAM demonstrates excellent agreement with experimental results and is more efficient than state-of-the-art FEM methods.

AFRL researchers have used BSAM primarily as a research tool for modeling a variety of composite configurations, ranging from bolted and bonded joints to fracture mechanics problems (see figure). They have also employed it to test new theories on the deformation, strength, and durability of advanced composite materials, including the emerging complex fiber architectures envisioned by aerospace manufacturers.

AFRL scientists, in a collaborative effort with UDRI and UTRC, recently transitioned the BSAM tool to Sikorsky Aircraft Corporation. Sikorsky engineers will use the code to design and evaluate key composite parts for future Department of Defense and civilian rotorcraft. Sikorsky is an established leader in the design and manufacture of advanced helicopters for commercial, industrial, and military uses and provides aircraft to all 5 branches of the US military, as well as to military and commercial operators in more than 40 nations.

With technology transition in mind, AFRL and its government and aerospace industrial partners also formed an alliance to guide the development of BSAM-related technology. Thus far, the emphasis has been on understanding and accurately predicting the strain distribution near open fastener holes (i.e., the hole size effect) and bolted joints (i.e., the laminate stacking sequence effect). As a result of this technology's transition, the BSAM code is rapidly evolving into a functional design tool that more efficiently addresses a broader diversity of fiber-reinforced composite materials problems than standard FEM analysis is able to accommodate.

Dr. David H. Mollenhauer and Dr. Peter S. Meltzer (General Dynamics), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information visit http://www.afrl.af.mil/techconn_index.asp. Reference document ML-H-04-37.

Reference

1 Mollenhauer, D. H., Schoeppner, G. A., and Iarve, E. V. "Experimental/Analytical Examination of Residual Strains in Composite Bonded Joints." Society of Manufacturing Engineers (Feb 04): 1.


Air Force Research Laboratory Technology Horizons Magazine

This article first appeared in the December, 2006 issue of Air Force Research Laboratory Technology Horizons Magazine.

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