FRAMES-2, A Materials Database Facility for Structural Engineering

T.K. Henriksen, K. Aarhus, M. Van Beijnen

Structures and Mechanisms Division, ESTEC

F. Brunetti

ESPRI Concept (F)

Contractors: ESPRI-Concept (F), Aérospatiale (F)

Funding: Investment Budget

Introduction

At all stages of design and verification of a spacecraft structure, easy access to pertinent materials data, including mechanical and physical properties, is of paramount importance. Selection of materials during the early design phase needs comparative data of material properties, whereas design verification requires accurate data. A comprehensive database of materials information promotes efficient development and verification of mechanical space systems.

Data on the strength of materials have been derived over the years by industry and national agencies, and the need to make this data available for future projects, through a computerised materials data base has been identified. The database, Frames-2, meets this need and will also aid the collection of data about new alloys under investigation for spacecraft applications.

Data requirements

In contrast to classical philosophy of a materials handbook, the structure of Frames-2 is focused on the tests performed to derive materials data for engineering applications. The tests currently considered are listed in Table 1. The definition of requirements and nomenclature for all types of test data complies with ESA PSS-01-745 and is based on standards of national and international bodies such as ISO, EN and ASTM.

In addition to the test data shown in Table 1, general properties and other information such as chemical composition, hardness, heat treatment details, grain size, density, coefficient of thermal expansion or thermal conductivity can be consulted. Three levels have been identified for material data taken from tests:

• raw data;

• "deduced" data;

• design data.

Raw data are taken from measurements performed on test specimens, such as force, displacement, crack length and number of load cycles. Deduced data are produced from raw data by numerical methods, such as curve fitting or differentiation, and represent the expected results of a test. Design data are, in principle derived data, intended for verifying a structural design, such as lower-bound values or the results of curve-fitting.

Figure 1 shows an example of raw data input and the calculation of "deduced" and design data. Raw crack growth data, given as crack length a versus number of cycles N are converted to "deduced" data by the secant method or by polynomial fitting. Solutions for the stress intensity factor K for a range of standard specimen configurations are available, and the stress intensity factor range delta K, is calculated from specified load data. Design data is obtained by fitting the da/dN-delta K points to a selected crack growth model by the least-squares method .

Figure 1. Example of data flow.

Data Bases

In the FRAMES-2 database, the selected standard designations for materials and alloys are referenced by a set of eight digit codes. This may be the standard set, supplied on installation, or the user may define his own alloy and material grouping. In addition to the main data base, each user has a private database.

The material and alloy designations do not follow any single standard, but are taken from what are believed to be the names most commonly used by the aerospace industry. In addition, equivalent names, based on national standards such as DIN, LN, AFNOR, BS and UNS are included, and a material search based on equivalent names can be performed.

For quality, reliability and security reasons, only the system manager has full access to the master data files, which is provided by ESA, and for whose integrity within a single installation he is responsible. Other users are restricted to read-only access.

It is intended that the database supplied to ESA contractors will contain design data accepted by ESA for use in space systems. This will contain the raw and deduced data, needed to develop acceptable design data. These data can be copied into a user's private data base for further evaluation. An example of the referencing of materials and alloys and the selection sequence is shown in Figure 2.

Figure 2. Example of material and alloy selection.

Currently more than 1500 data sets, containing thousands of data points, have been included in the data base; these are primarily static strength, toughness, threshold and fatigue crack growth data for aluminium, titanium and steel alloys.

Data handling facilities

To facilitate data input, evaluation, and consultation the core module of the data base contains functions for:

• data input by, keyboard or ASCII file import;

• data validity checks (at computation level only);

• calculation of "deduced" data;

• calculation of design data (statistical analysis, curve fitting);

• data output (print, plot);

• backup, restore and archiving;

• materials search and selection;

• searches against multiple criteria;

• equivalent name search;

• unit conversion.

The on-line help facility almost eliminates the need for a users' manual.

Several options exist for searching for alloys and their material properties. Materials may be selected by name, as in Figure 2, or by direct input of a material code or set number. Materials may also be selected by an automatic search of their properties or the equivalent name of an alloy used in a national standard. In such cases the search will be performed for design data only. These options provide a significant advantage over classical handbooks. An example of the multi criteria properties search facility is shown in Figure 3.

Figure 3. Searching through properties of materials against multiple criteria.

The baseline unit system is metric, in which stresses are given in megapascals, and lengths in millimetres; however a unit conversion option to units customary in the USA and to SI units is available. This option was implemented to facilitate the introduction of data not available in metric units.

Raw and "deduced" data can be input from an optical character recognition tool, or from an ASCII file. Use of a standard data delivery format simplifies data input considerably.

Conclusions

A database which aids the effective use of engineering materials, tailored to the design and verification of spacecraft structures has been described. The data base contains more than 1500 data sets of data covering typical spacecraft materials. It is intended that ideas implemented in FRAMES-2 should become the baseline for a standardised approach for use in ESA projects.

Table 1. Test types as implemented in FRAMES-2.

Test type                       Test data
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Static strength         - yield stress, ultimate stress, elong. (Sy, Su, A5)
                        - Youngs modulus (E)
                        - shear modulus (G)
                        - Poisson's ratio (v)

Fatigue crack growth    crack growth rate and stress intensity factor
                         range (da/dN-delta K)

Fatigue crack growth     stress intensity factor threshold range (DELTA K(sub th))
threshold

Fracture toughness       plane strain fracture toughness (K(sub Ic))

Fracture toughness,      effective fracture toughness for surface cracks
surface cracks           (K(sub IE)

J Integral               J integral toughness (J(sub IC))

Stress corrosion         minimum stress level (S sub o))

Stress corrosion         threshold stress intensity factor for stress
cracking                 corrosion cracking (K(sub Iscc))

R Curve                  stress intensity factor and crack extension (K(sub R) - DELTA a)

Fatigue                  stress amplitude and number of cycles (S - N)