June_AMP_Digital

A D V A N C E D

M A T E R I A L S

P R O C E S S E S | J U N E

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esigners of ultra-efficient aircraft,

automobiles, and power genera-

tion systems need materials with

high strength-to-weight ratios as well

as those that can withstand high oper-

ating temperatures for extended time

periods. In both cases, fuel efficiency

is the goal. To achieve this, researchers

must accurately and precisely mea-

sure material properties at elevated

temperatures.

However,

elevated temperature

means different things to different

researchers. In general, there are three

distinct temperature ranges for materi-

als with the highest strength-to-weight

ratios. The first range, 200°-425°C,

applies to polymer matrix composites

(PMCs). The second range, 800°-1200°C,

is used for metals. The third range is

suitable for ceramic matrix composites

(CMCs), which are tested to 1500°C. For

PMCs, traditional use temperature is lim-

ited by the glass transition temperature

) of the matrix resin, where the matrix

becomes soft and rubbery. Aerospace

materials generally use epoxy resinswith

a T

of approximately 200°C or lower.

Composites that use polyimide

resins with much higher T

values report

use at temperatures as high as 371°C.

For metals, many mechanisms can

define high temperature because the

traditional use temperature is limited by

loss of strength, onset of creep deforma-

tion, change in material microstructure,

or the appearance of high temperature

corrosion. Single crystal Ni-base alloys

and some refractory alloys are used in

the air to roughly 1200°C. For the most

advanced CMC applications, associ-

ated testing requirements reach nearly

1500°C, with even higher temperatures

envisioned for the future. In each range,

there are tradeoffs that test engineers

need to carefully consider in order to run

effective tests, measure material prop-

erties at elevated temperatures, and

acquire high-quality results.

These tradeoffs directly affect the

accuracy and precision of mechani-

cal test data, because any object that

needs to hold, touch, or be placed near

the specimenmay increase data scatter.

In other words, grips, extensometers,

furnaces, and environmental chambers

are potential sources of experimental

error. Variability that arises from these

components tends to be systemic, so

solving an issue with one tends to raise

issues with another.

HIGH-TEMPERATURE

SPECIMEN EXAMINATION

To understand how these inter-

related issues manifest during test

setup, consider a typical specimen.

PMC and CMC specimens are flat and

cannot be gripped in the same way as

round, threaded, or button-head metal-

lic specimens. For PMCs, cost effective

and easy-to-use hydraulic wedge grips

are usually appropriate. PMC speci-

mens often lack compressive strength

across their thinnest cross section, and

the evenly applied pressure from the

hydraulic wedges protects the fibers

in the polymer matrix. These hydraulic

grips not only prevent the fibers from

being crushed, but also help maintain

correct pressure as the chamber and

grip wedge head heat up.

The tradeoff is that grip wedge

heads are relatively large, and for best

results, must fit inside an environmen-

tal chamber. The chamber for these

lower temperature PMC tests is often

larger than the furnaces required for

higher temperature metal or CMC tests.

Although larger equipment usually is

less efficient, the thermal mass of the

grips and chamber leads to very stable

test environments.

On the other hand, the larger

chamber makes using inexpensive

contact extensometers difficult. With a

smaller furnace, extensometers can be

situated outside the chamber, allow-

ing them to translate motion from the

contact arm to the capacitor plates or

strain-gaged beam. But with a larger

chamber, the motion is not effectively

translated because the arms become

too long and cause additional measure-

ment variability. A short arm extensom-

eter needs to be positioned inside the

chamber, but the elevated temperature

would damage its sensitive electronics.

One way to solve this problem is

with video extensometry and digital

image correlation, which can be situated

outside the chamber. A chamber with

a window lets these technologies look

inside and measure specimen motion

Blue LEDs illuminate chamber during

PMC test.

Temperature profiling of a button-head

metallic specimen in 1200°C grips.

Remotely actuated hydraulic grips for

PMC specimen at temperatures to 425°C