Question

4. The following data were obtained from a creep test for a specimen having an

initial gage length of 2.0 in. and an initial diameter of 0.6 in. The initial stress

applied to the material is 10,000 psi. The diameter of the specimen after fracture

is 0.52 in.

Length Between Gage Marks ( (in.)	Time (h)	Strain (in./in.)
2.004	0	0.002
2.01	100	0.004
2.02	200	0.006
2.03	400	0.008
2.045	1000	0.012
2.075	2000	0.020
2.135	4000	0.035
2.193	6000	0.050
2.23	7000	0.070
2.30	7200 (fracture)	0.078

Determine:

1. The load (lb_f) applied to the specimen during the test
2. The approximate length of time (h) during which linear creep occurs
3. The creep rate in in./in. h and %/h, and
4. The true stress acting on the specimen at the time of rupture

Answer 1

(a) The Load (lbf) applied to the specimen during the test:

Load = Stress X area of specimen

(b) The approximate length of time during which linear creep occurs:

Length during which linear creep occurs = 6000 h -1000 h= 5000 h

thus linear creep is observed during 1000 h to 6000h.

(c) Creep Rate:

creep rate is the slope of the linear portion of strain vs time graph

thus the slope of line AB will give creep rate.

creep rate (%/h )= 7.6* 10-6 /100

creep rate (%/h )= 7.6* 10-8 %/h

(d) The true stress acting on the specimen at the time of rupture:

The true stress acting on the specimen at the time of rupture= Load/area of specimen after fracture