1. By; Noor Azira binti Mohd Noor
CHAPTER 7 CREEP
By;
Noor Azira binti Mohd Noor

2. Introduction
Creep is the permanent elongation of a component under a static load maintained for a period of time.
It is a phenomena of metals and of certain nonmetallic materials eg thermoplastic and rubbers.
It can occur at any temperature.

3. Example
Lead - creeps under a constant tensile load at room temperature
metals and their alloys – creep of any significance occur at elevated temp.
aluminum alloy - 200°C
refractory alloys °C
the mechanism of creep at elevated temp in metals is generally attributed to grain-boundary sliding

4. Why creep is important?
Creep is especially important in high-temperature application eg;
gas-turbine blades
component in jet engines and rocket motors.
high-pressure steam lines
nuclear-fuel element
tools and dies that subjected to high stresses at elevated temp. during hot-working operation- forging and extrusion

5. Creep test
Consist of subjecting a specimen to a constant tensile load at a certain temp.
And measure the changes in length at various time increments.
A typical creep curve usually consist of primary, secondary and tertiary stage (refer Fig below).

6. The specimen fails by necking and fracture called rupture or creep rupture.
The creep increase with;
Temp
Applied load

7. Important of creep
Design against creep usually involve knowledge of the secondary (linear) range and its slope.
The creep rate can be determined reliably only when the curve has a constant slope.
Generally, resistance to creep increase with the melting temp of a material.
Stainless steel, superalloys, refractory metals and alloys are commonly used in applications where resistance to creep is required.

8. Stress relaxation Closely related to creep.
The stress resulting from a loading of structural component decrease in magnitude over a period of time even though the dimensions of the component remains constant.
Example;
the decrease of tensile stress of a wire in tension between 2 fixed ends
Rivets, bolts, guy wires and similar parts under tension, compression or flexure.

9. Result from Creep test

12. 1.Primary or Transient Creep
The material experience an increase in creep resistance or strain hardening.
Occur quickly
Can be treated in much the way that elastic deflection is allowed for in a structure.

14. 2.Secondary or Steady-State Creep
As the name itself, the line became linear or constant.
The most longest duration.
Steady-state creep rate (∆ε/∆t) is the most important parameter from a creep test.
Engineer must consider creep curve for long-time applications (such as nuclear power plant component) that is scheduled to operate for several decades, and when failure or too much strain is not an important.

15. 3.Tertiary Creep
There is an acceleration of the rate and ultimate failure.
Termed as rupture, or fracture.
If the test is using tensile loads, a ‘neck’ may form.
For many relatively short-life creep situation (e.g. turbine blades, rocket motor nozzles, etc), time of rupture, tr, is the dominant design consideration.

16. Stress and Temperature Effects
Influence the creep characteristic.
These characteristic are:
The instantaneous strain at the time of stress application increase.
The steady-state creep rate is increase.
The rupture lifetime is diminished.
The result of creep rupture test commonly presented as the logarithm of stress vs the logarithm of rupture lifetime.

18. Empirical relationships have been developed in which the steady-state creep rate as a function of stress and temperature is expressed.
ε = K1σn
Where K1 and n are material constant. A plot of the logarithm of ε vs the logarithm of σ yields a straight line with the slope of n.