It is not necessary to cool and reheat the furnace between tests.
An apparatus for positioning and holding material specimens is a major subsystem of a system for impact testing of the specimens at temperatures up to 1,500 °C. This apparatus and the rest of the system are designed especially for hot impact testing of advanced ceramics, composites, and coating materials.
The apparatus includes a retaining fixture on a rotating stage on a vertically movable cross support driven by a linear actuator. These components are located below a furnace wherein the hot impact tests are performed (see Figure 1). In preparation for a test, a specimen is mounted on the retaining fixture, then the cross support is moved upward to raise the specimen, through an opening in the bottom of the furnace, to the test position inside the furnace. On one side of the furnace there is another, relatively small opening on a direct line to the specimen. Once the specimen has become heated to the test temperature, the test is performed by using an instrumented external pressurized-gasdriven gun to shoot a projectile through the side opening at the specimen.
Advantageous features of the design and operation of this apparatus include the following:
- All parts of the retaining fixture are made of silicon carbide to withstand high test temperatures.
- The simplest version of the retaining fixture (see Figure 2) includes a tube, into which are machined tapered slots to accommodate a flat specimen and a side hole for admitting a projectile. (In a more complex version, there are slots for two specimens and two corresponding projectile holes at diametrically opposite locations.) The specimen is held in place by silicon carbide wedges inserted in the tapered gaps remaining between the specimen and the slots.
- Among the alternative versions of the retaining fixture are versions that offer a choice between full support or span support of the specimen. If full support is needed, then one can choose a version having slots wide enough to support not only the specimen but also a solid backing plate.
- To some extent, by partially enclosing the specimen, the retaining fixture provides some protection of the furnace insulation and heating elements against flying debris from a specimen and projectile. Shielding separate from the retaining fixture can added in cases in which more protection is needed.
- The rotational stage enables adjustment of the angle of impact — a feature that is desirable for impact testing of vanes under realistic conditions. Alternatively or in addition, if the retaining fixture is of the two specimen type described above, then the rotational stage can be used to expose both specimens in succession without removing them from the furnace.
- The provision for inserting and removing specimens through the opening in the bottom of the furnace eliminates the need to cool and reheat the furnace between tests, thereby saving substantial amounts of test time.
- When multiple impacts at different positions along a lengthened specimen are required, the retaining fixture can be modified to lengthen the tapered slots and side holes at the additional impact positions, and the linear actuator can be used to place the specimen at the various impact positions. In such a case, the modifications can reduce the shielding effect of the retaining fixture, thereby making it desirable to add separate shielding as mentioned above.