2,808
edits
Changes
→5.1.7.2 Fatigue Strength
selected copper
materials
(Wieland)
=====5.1.7.3 Bendability=====
The measure for bendability of a strip material is the smallest possible bending radius r of a sample piece of given material thickness s without appearance of surface cracking. Bending tests are performed as either 90 degree bends according to ISO 7438 or as defined forth-and-back bending. The bendabilty of naturally hard copper alloys is significantly better perpendicular to the rolling direction than parallel to it ''(Figs. 5.41 and 5.42)''.
Fig. 5.41:
Smallest possible bend radii
for 90° bends as a function of the 0.2%
yield strength R – bend line p0.2
perpendicular to the rolling direction
(Wieland)
Fig. 5.42:
Smallest possible bend radii
for 90° bends as a function
of the 0.2% yield strength Rp0.2 –
bend line parallel
to the rolling direction (Wieland)
=====5.1.7.4 Softening Behavior=====
Through thermal activation at elevated temperatures the original mechanical material strength achieved by cold working or precipitation hardening can be reversed completely. The start of softening is mostly defined as the temperature at which a 10% reduction of mechanical strength is reached. It is dependent on the degree of initial cold working and the annealing temperature and time. For higher initial degrees of cold working degrees the softening temperature becomes lowered.
As expected, the softening temperature for pure copper is rather low. CuNi9Sn2 and CuSn1CrNiTi exhibit high softening temperatures ''(Fig. 5.43)''.
Fig. 5.43:
Softening behavior
for selected
copper-based
materials after 40%
cold working
(Wieland)
=====5.1.7.5 Relaxation Behavior=====
Tension relaxation is defined as the loss of tension of an elastically stressed material as a function of time and temperature. The causes for the relaxation are thermally activated processes which are comparable to creep behavior. As a measure for the relaxation the percentage decrease in the bending tension compared to the initial one is used. Temperature increase is a stronger influencing factor on the relaxation of the spring force than growing operational times. Through suitable annealing processes the relaxation degree can be significantly reduces.
For the measurement of tension relaxation different test procedures are used, based on the ASTM E-32-86.
Fig. 5.44 illustrates the different relaxation behavior of some copper alloys. Good behavior is shown for CuNi3Si1Mg and CuCrSiTi while CuZn30 and
CuSn6 exhibit a less favorable relaxation tendency.
Fig. 5.44:
Relaxation behavior
of selected copper-based
materials.
Starting tension: 100% of
spring bending limit;
Stress duration: 100 hrs
(Wieland)
