Question

What may be a simple test method to differentiate between brittle and ductile fracture? Discuss the effects of temperature and loading rate

Answer 1

A), The simple test for ductile materials is tensile test or bend test. Stress strain curve is plotted and yield point is evident.

And for brittle materials the test is compression test and no yield point is evident on stress-strain curve.

B) Effect stress and temperature are creep which is discussed.

Creep is a time-dependent and permanent deformation
of materials when subjected to a constant load at a high
temperature (> 0.4 Tm). Examples: turbine blades, steam
generators.

Stages of creep
1. Instantaneous deformation, mainly elastic.
2. Primary/transient creep. Slope of strain vs. time
decreases with time: work-hardening
3. Secondary/steady-state creep. Rate of straining is
constant: balance of work-hardening and recovery.
4. Tertiary. Rapidly accelerating strain rate up to failure:
formation of internal cracks, voids, grain boundary
separation, necking, etc.

Parameters of creep behavior
The stage of secondary/steady-state creep is of longest
duration and the steady-state creep rate .
is the most important parameter of the creep behavior
in long-life applications.
Another parameter, especially important in short-life creep
situations, is time to rupture, or the rupture lifetime, tr.
/ t ε&s
= Δε Δ
tr
Δε/Δt

Creep: stress and temperature effects
With increasing stress or temperature:
¾ The instantaneous strain increases
¾ The steady-state creep rate increases
¾ The time to rupture decreases

The stress/temperature dependence of the steady-state
creep rate can be described by

ε = σ − RT
Q K exp n c
s 2 &
where Qc is the activation energy for creep, K2 and n
are material constants.
(Remember the Arrhenius dependence on temperature for
thermally activated processes that we discussed for diffusion)