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HIGH-TEMPERATURE JOINTS
Bolted joints subjected to cyclic loading perform best if
an initial preload is applied. The induced stress mini-
mizes the external load sensed by the bolt, and reduces
the chance of fatigue failure. At high temperature, the
induced load will change, and this can adeversely affect
the fastener performance. It is therefore necessary to
compensate for high-temperature conditions when
assembling the joint at room temperature. This article
describes the factors which must be considered and illus-
trates how a high-temperature bolted joint is designed.
In high-temperature joints, adequate clamping force
or preload must be maintained in spite of temperature-
induced dimensional changes of the fastener relative to
the joint members. the change in preload at any given
temperature for a given time can be calculated, and the
affect compensated for by proper fastener selection and
initial preload.
Three principal factors tend to alter the initial
clamping force in a joint at elevated temperatures,
provided that the fastener material retains requisite
strength at the elevated temperature. These factors are:
Modulus of elasticity, coefficient of thermal expansion,
and relaxation.
Coefficient of Expansion: With most materials, the
size of the part increases as the temperature increases.
In a joint, both the structure and the fastener grow with
an increase in temperature, and this can result,depending on the materials, in an increase or decrease in the
clamping force. Thus, matching of materials in joint
design can assure sufficient clamping force at both
room and elevated temperatures. Table 16 lists mean
coefficient of thermal expansion of certain fastener
alloys at several temperatures.
Relaxation: At elevated temperatures, a material
subjected to constant stress below its yield strength will
flow plastically and permanently change size. This phenomenon is called creep. In a joint at elevated temperature, a fastener with a fixed distance between the bearing
surface of the head and nut will produce less and less
clamping force with time. This characteristic is called
relaxation. It differs from creep in that stress changes
while elongation or strain remains constant. Such
elements as material, temperature, initial stress,
manufacturing method, and design affect the rate of
relaxation.
Relaxation is the most important of the three factors.
It is also the most critical consideration in design of
elevated-temperature fasteners. A bolted joint at 1200°F,
can lose as much as 35 per cent of preload. Failure to
compensate for this could lead to fatigue failure through
a loose joint even though the bolt was properly tightened
initially.
If the coefficient of expansion of the bolt is greater
than that of the joined material, a predictable amount of
clamping force will be lost as temperature increases.
Conversely, if the coefficient of the joined material is
greater, the bolt may be stressed beyond its yield or even
fracture strength. Or, cyclic thermal stressing may lead to
thermal fatigue failure.
Changes in the modulus of elasticity of metals with
increasing temperature must be anticipated, calculated,
and compensated for in joint design. Unlike the coefficient of expansion, the effect of change in modulus is to
reduce clamping force whether or not bolt and structure
are the same material, and is strictly a function of the
bolt metal.
Since the temperature environment and the materials of the structure are normally "fixed," the design
objective is to select a bolt material that will give the
desired clamping force at all critical points in the operating range of the joint. To do this, it is necessary to
balance out the three factors-relaxation, thermal expansion, and modulus-with a fourth, the amount of initial
tightening or clamping force.
In actual joint design the determination of clamping
force must be considered with other design factors such
as ultimate tensile, shear, and fatigue strength of the fastener at elevated temperature. As temperature increases
the inherent strength of the material decreases.
Therefore, it is important to select a fastener material
which has sufficient strength at maximum service
temperature.