513.8-11 scope

513.8-1.1 Fixture Design Considerations

An installation design in which centrifugal force tends to hold the test item against the machine or fixture as shown in Figures 513.8A-1 and 513.8A-2 is generally preferred for unusually severe acceleration conditions, since this type of installation tends to minimize the possibility of accidental loss of the test item during a test. In this case, a compressive stress at the test item attachment location results from the normal or centripetal acceleration. A centrifuge equipped with an adjustable mounting table has definite advantages over a machine with a fixed mounting surface as being adjustable means greater versatility in the test installation. For example, an adjustable mounting table that may be rotated relative to the axis of the centrifuge arm might allow a test installation of the type referred to above or allow a choice of more than one test item axis for exposure to the acceleration vector without detaching the test item to reorientate it for each axis tested. Difficulties in such operations as installation, checkout, servicing, and removal of the test item can be reduced by using a mounting table that allows a change in position relative to the centrifuge arm.

The testing of small items, or items that are difficult to set up, may be expedited by using a fixture that allows exposure of each axis to the acceleration vector without removal of the test item from the fixture. In this procedure, the fixture (with the test item attached to it) is re-oriented. One of the simpler fixtures of this type holds the test item at a central location so that any number of fixture faces may be attached to the centrifuge mounting table depending upon the item orientation required. Installations of this type are usually bolted to the centrifuge. At centrifuge sites where numerous tests requiring re-orientation of the test object are conducted, fixture versatility means reduced costs in test programs and less time to complete tests.

The decision in favor of a particular fixture design may be affected by such considerations as:

• a. The scope of the test program.
• b. The complexity of test requirements.
• c. Physical characteristics of the test item.
• d. Centrifuge design.

The economics of conducting a centrifuge test are often primary considerations. If the test program is a large one requiring a test to be duplicated for a number of like test items, an elaborate fixture design that minimizes the installation and test time for each test item may be required. The design and cost of the fixture, in this case, might be justified by a reduction in the cost of the program such that the cost of fixture design and fabrication is a fraction of the total amount saved. Conversely, a small number of tests might be conducted more economically by using a simple installation in which the test item is unfastened from the centrifuge and re-orientated for each part of a test.

Knowledge of the ability of supporting bracketry to carry the required loads is an important factor in the preparation for a centrifuge test. A detailed analysis may not be necessary, if a previously used mounting bracket is to be exposed to loads known to be less severe than those for which it was designed; however, a preliminary design investigation, including a force and stress analysis, usually is required in conjunction with a new test installation. Basic forces imposed on the test item by centrifuge accelerations are shown in Figures 513.8A-1, 513.8A-2, and 513.8A-3.

Free-body diagrams showing the forces at critical locations under various load conditions are commonly used in making the force analysis. After the forces have been identified as to point of application, direction, and magnitude, the stress analysis is undertaken. The analysis may require consideration of as many as four separate loading cases: axial force, transverse force (shear), bending, and torsion. In a bracket under complex loading conditions, it is possible that more than one of these conditions will exist. Loading conditions, that appear to be relatively simple, are sometimes required to be broken down into idealized conditions. After each loading condition has been analyzed to determine stresses and deflections, the results are combined to determine total strength and deflection characteristics.

MIL-STD-810H

METHOD

513.8-1.2 Fixture Materials and Construction

In selecting the material for a fixture, two important factors to be considered are the stress to which the fixture will be subjected, and the weight the centrifuge arm can support. Other factors that should be taken into account are machinability and fabrication qualities. The material giving the lowest cost, yet having the properties needed, generally is considered the best engineering material. However, test schedule and material availability influence the choice of materials to be used. Aluminum and magnesium combining lightness with good mechanical properties to give a high strength-to-weight ratio are frequently used for centrifuge test fixtures. Both metals are available in a variety of forms including standard sheet, plate, bar stock, and miscellaneous shapes, and both have generally desirable fabrication qualities. Most of the fusion and mechanical fastening methods common to the metal working trades may be used in the fabrication of fixtures, however, the designer should be aware of the characteristics of each material under his design conditions. Inserts may be used to reinforce the fixture base metal. In bolted connections, they increase the resistance to severe loading conditions and/or to thread wear due to repeated use of the fixture. A bolted fixture design may be found desirable because of the versatility of this fabrication method in new fixtures, as well as in the adaptation of fixtures previously used for other tests. The fixture may either bolt directly to the centrifuge platform or, if necessary, to an adapter plate that, in turn, is bolted to the centrifuge arm.

513.8-1.3 General Considerations in Centrifuge Testing

Although testing by means of the centrifuge appears to be simple when compared with other types of testing, the test engineer may encounter numerous issues that vary in magnitude depending upon the complexity of the test. Typical issues encountered are those associated with the generation of required test conditions, data acquisition, test item servicing and handling, and miscellaneous support of the type supplied at the test site.

The generation of acceleration conditions other than those required at the test item location may be objectionable. An acceleration gradient along the axis of the centrifuge arm and a tangential acceleration exist at the test item location in varying degrees of intensity during the operation of a centrifuge.

Centrifugally-produced forces are not uniform along a test specimen on a centrifuge because of the proportionality of acceleration to the radius. The normal acceleration (along the length of the centrifuge arm) varies directly with the radius and by the square of the angular velocity (a n = rω ² ). The effect of the incremental variation of acceleration along the radius of a centrifuge arm may be undesirable if a test item is required to be subjected to an acceleration value within specified tolerances at more than one location, and test item dimensions along the centrifuge arm are such that the difference in acceleration between these locations is excessive. The importance of a centrifuge with a large radius is appreciated in such a situation, since the incremental variation of acceleration over the test item is less if the item was installed at the end of a centrifuge arm of greater length.

In tests requiring acceleration values to be maintained within close tolerances on a large object, it may be desirable to adjust the centrifuge rotational speed so the required acceleration is obtained at the location (radius of gyration) of a critical test item component.

The effect of tangential acceleration ( $\mathrm{a}_{\mathrm{t}}=\mathrm{r} \alpha$ ) on the resultant acceleration vector should not be overlooked. This acceleration occurs in a direction perpendicular to that of the normal acceleration, and may be large enough to cause a considerable change in direction and magnitude of the resultant acceleration vector. Because of centrifuge design and power requirements, the tangential acceleration usually encountered on a large centrifuge is relatively low. However, the tangential acceleration generated by changes in rotational speed of smaller centrifuges may become

MIL-STD-810H METHOD

1.3.1 Acceleration

As addressed in this Method, acceleration is a load factor (inertia load, "g" load) applied slowly enough and held steady for a period of time long enough such that the materiel has sufficient time to fully distribute the resulting internal loads, and such that dynamic (resonant) response of the materiel is not excited. Where loads do not meet this definition, more sophisticated analysis, design, and test methods are required.

1.3.2 Aerodynamic Loads

Materiel mounted such that any or all surfaces are exposed to aerodynamic flow during platform operations are subject to aerodynamic loads in addition to inertia loads. This method is not generally applicable to these cases. Materiel subject to aerodynamic loads must be designed and tested to the worst case combinations of these loads. This often requires more sophisticated test methods usually associated with airframe structural (static and fatigue) tests.

1.3.3 Acceleration versus Shock

Acceleration loads are expressed in terms of load factors that, although dimensionless, are usually labeled as "g" loads. Shock environments (Methods 516.8 and 517.3) are also expressed in "g" terms. This sometimes leads to the mistaken assumption that acceleration requirements can be satisfied by shock tests or vice versa. Shock is a rapid motion that excites dynamic (resonant) response of the materiel, but with very little overall deflection (stress). Shock test criteria and test methods cannot be substituted for acceleration criteria and test methods or vice versa.

1.3.4 Climatic Conditioning

Special provisions for performing combined acceleration/climatic environment tests (e.g., acceleration tests at high or low temperatures) may be required for certain operational tests. Guidelines found in the climatic test methods may be helpful in setting up and performing combined environment tests.