Enabling Technologies for Unified Life-Cycle Engineering of Structural Components (1991)

The cost of materials currently contributes only a small fraction to the overall cost of structural components in use in military applications. The cost factor and current rapid advances in materials combine to form a high-leverage item for improving systems performance, maintainability, and supportability. Advances in materials are occurring on a wide front. Composites with either metal, ceramic, or polymer matrices reinforced with a broad spectrum of fibers or particles allow for development and design in an ever-increasing number of applications. Metal alloys with very refined microstructure produced by rapid solidification processing provide enhanced mechanical properties and corrosion resistance, and toughened ceramics are finding uses in high-temperature bearings. The synthesis of new polymers has led to the development of materials with improved mechanical properties and resistance to solvent degradation.

Validated experimental measurements of materials properties are the materials data most needed by designers. Indeed, the important structure-property correlations derived over the past few decades show that materials properties cannot now be predicted with the accuracy needed for design purposes and even interpolation must be done conservatively. Therefore, larger materials data bases are needed now and will continue to be required in the future (NMAB, 1983). The cost of developing the data for a data base on a new alloy sufficiently detailed to be useful for design is estimated to be upwards of $100 million. This cost estimate will encourage the continued use of existing alloys rather than new ones offering only minor gains. There have been no results from current efforts to construct a national materials data base.

The materials data needed for design in a life-cycle engineering framework do not now exist for structural applications except for those under the simplest of conditions (e.g., uniaxial tension stress states in an inert environment). The principal reasons for the absence of these data are (a) inadequate specification of service environment, (b) the dependence on inexpensive simple tests rather than multivariable tests with only an approximate conceptual framework to extrapolate from the test conditions to the service conditions, and (c) the duration and high cost of long-term tests (exceeding 25,000 hours).

At present, materials are selected from a list of available alloys, compounds, and composites. Metals are attractive because their parameters have less variability than those of the newer composite materials. In some cases new materials are used in design without adequate experience data to characterize their performance completely. Application of new materials in initial design can be a high-risk, high-payoff (or high-loss) design decision. Variability of material properties produces uncertainties in design, which can result in conservative parameter values being applied.

Substantially more effort has been devoted to developing structure-property correlations than processing-structure-property correlations. This is partly because it is more difficult to model changes in structure caused by the manufacturing or forming process. Nondestructive evaluation (NDE) is not sufficiently quantitative in characterizing either materials microstructure or defects and needs to be coupled to real-time process control to upgrade product quality and yields.

Experience gained in the laboratory on research of either processing or properties of new materials is often not readily transferrable to materials processing engineers, equipment designers, and manufacturing staff, due to inadequate knowledge of the effects of part and process scale-up and the reduced process control capability of many manufacturing operations. Although "limit" criteria exist for various processing operations, it is not possible in all cases to predict materials properties after single processing sequences, much less those involving multiple operations. To a large extent, material behavior is not sufficiently well understood to allow prediction of mechanical properties or their time-dependence, particularly for complex loading sequences. The time required for "diffusion" of materials experience delays the successful application of advanced materials.

The repair function currently utilizes existing materials processing and joining techniques such as welding, riveting, and adhesive bonding. Damaged parts are often completely replaced. Most of the repair function is done manually. The increasing complexity of materials and joining techniques is putting increased pressure on the maintenance and repair function for greater use of NDE techniques both through in-service monitoring and on a periodic basis during maintenance. Although a substantial part of this inspection process uses manual NDE techniques, automated techniques are being introduced.

Current research in materials science tends to focus on a specific material or class of new materials. Common first principles and unifying theories are only slowly emerging from the research community. Information flow to the materials community from the design, manufacturing, and support communities is weak and irregular.

The driving forces to develop and use advanced materials will continue to be strong in the future. In fact, there are many indications that the pace of advancement will increase. Revolutionary advances will occur in materials technology brought about by a combination of new requirements, computer modeling techniques, and the rapidly expanding knowledge base in

materials science and engineering. Materials will need to be tailored more closely for a wider variety and combination of properties and applications.

The expected emergence of hypervelocity aircraft with orbital capability will create many new materials challenges. These vehicles will require extraordinarily light materials that also possess resistance to high temperatures (3000°F for leading edges and 1500°F for large expanses of primary structure). The requirement for reusability for repeated space missions will lead to new problems not previously encountered for materials used in spacecraft or rocket booster systems; for example, fatigue, repairability, and resistance to runway foreign object damage. Materials of primary interest for these applications will be metal matrix composites, carbon-carbon laminates, and toughened ceramics. For example, composites will provide directional strength and stiffness as well as greater reliability and high-temperature capability than currently available. Reduced density, greater temperature resistance, increased toughness, enhanced repairability, greater damage tolerance, etc., will be designed into materials in various degrees for each unique application. Weight savings of the order of 20 percent over present-day materials are expected.

Design requirements will continue to drive the materials specifications, but there will be a unifying theory available to estimate the life-cycle properties of a new (developmental) material before testing and physical analysis are complete. The design process will be facilitated by the availability of a common materials property data base. This data base will still be incomplete, especially for advanced composites (metallic, ceramic, and polymeric matrix), and this could retard application of these materials. The development of more powerful models and constitutive equations of materials behavior will compensate for the incomplete data bases to some extent.

Substantial gains in two areas underpinning development of structure-property-processing correlations are occurring now, and more are anticipated in the next 20 years. First is the development of probes based on NDE sensors for characterizing materials during processing in terms of their microstructure, geometry, and chemistry. Second is the development of more sophisticated and realistic models of materials behavior during processing. These areas will make key contributions to the technology base needed for predictive or intelligent processing capabilities.

The revolutionary advances in materials will lead to higher costs for materials and a proportionately greater cost of materials in new systems. Costs will be somewhat controlled and quality greatly enhanced by automating the processing of materials and by automating parts fabrication. In many cases these materials will be custom-designed to provide unique properties tailored to a specific application blurring the distinction between laboratory scale and production processing.

Suppliers will assume a greater responsibility for materials certification. This in turn will require that test methods be developed and agreed on, especially for advanced composites.

Progress in sensors and process models will lead to automated or intelligent processing of materials (Yolken and Mordfin, 1986). This intelligent processing of materials will utilize a feedback system consisting of NDE sensors operating in the framework of a process model and in an expert system that drives the process controls; a valid process data base will be essential. This automation approach, which could be called intelligent processing of materials, will change.

The implementation of intelligent processing of materials will result in improved productivity and quality, increased uniformity of properties, and, if the knowledge base for the expert system were started during the research stage, shorter time from research to application. Intelligent materials processing will also allow for improved scale-up from small batches to larger production runs.

In the future there will be a need for continuous monitoring techniques for critical components of weapons systems. The NDE equipment will have to be built into the system and for many applications equipment weight reduction will be vital. NDE techniques in current use include ultrasonics, x-ray radiography and tomography, eddy currents, dye penetrants, magnetic particles, and thermography. NDE techniques of the future might include electrical measurements of dielectric materials (ceramics and polymer composites) utilizing microwaves, capacitance probes, direct dielectric measurements with electrodes, and a.c. spectroscopy; nuclear resonance; acoustic emission; neutron techniques; and laser techniques for holography, surface finish, and thermal wave imaging.

Novel new repair techniques will need to be developed for advanced materials, especially for advanced composites. Materials processing utilizing microwaves or ultrasonic bonding might be employed. There will be an increased need for this type of repair as system components become increasingly larger. Partial automation of repairs will also become more common in the future.

The emphasis on maximizing performance will continue to foster the development of high-performance materials; however, the need to more completely characterize these materials to assure their suitability over the complete product life cycle will inhibit premature application of new materials. The list of currently available materials is sufficiently lengthy that, even in today's environment, it challenges the ability of the materials engineer to make the most effective material selection. The quantity of empirical data which must be generated to adequately characterize materials in the ULCE environment and the amount of empirical data which must be digested and weighed during materials specification in the ULCE environment is so vast that the empirical approach is clearly impractical. Thus the role of the materials science community must be to develop models or (less satisfactorily) empirical correlations (Ashby, 1989) so that the material data can be made available in compact form. Future research must focus on unifying theories that will permit rapid evaluation and estimation of parameters for new and untested materials. A major requirement will be the capability of predicting material behavior in complex and time-varying load, temperature and chemical environments and the ability to model the interaction between these parameters will be crucial to successful materials design. It will also be necessary to more completely characterize and control manufacturing processes to assure that the material, as fabricated and applied, is well described by the material model used.

Ashby, M. F. 1989. "On the Engineering Properties of Materials." Acta Metallurgica Vol. 37 pp.1273–1293.

National Materials Advisory Board. 1983. NMAB-405, Materials Properties Data Management—Approaches to a Critical National Need, National Academy Press.

Yolken, H. Thomas, and Leonard Mordfin. 1986. Automated processing of advanced materials. ASTM Standardization News. No. 2, p. 4.