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→Bendability
<figure id="fig:Direction_dependence_of_the_spring_bending_limit">
[[File:Direction dependence of the spring bending limit.jpg|rightleft|thumb|Figure 1: Direction dependence of the spring bending limit of selected copper materials (Wieland)]]
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====<!--5.1.7.2-->Fatigue Strength====
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<figure id="fig:Woehler curves for selected copper based materials">
[[File:Woehler curves for selected copper based materials.jpg|rightleft|thumb|Figure 2: Woehler curves for selected copper based materials. Strip samples: 0.3 mm thick, cold worked; Testfrequency; 1,500 / min (Wieland)]]
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<figure id="fig:Ranges of fatigue strength for selected copper materials Wieland">
[[File:Ranges of fatigue strength for selected copper materials Wieland.jpg|rightleft|thumb|Figure 3: Ranges of fatigue strength for selected copper materials (Wieland)]]
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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 [[#figures10|(Figs. 4 – 7)]]<!--(Figs. 5.41 and 5.42)-->.
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<figure id="fig:Smallest possible bend radii for 90 bends as function">
[[File:Smallest possible bend radii for 90 bends as function.jpg|rightleft|thumb|Figure 4: Smallest possible bend radii for 90° bends as a function of the 0.2% yield strength R<sub>p0.2</sub> – bend line perpendicular to the rolling direction (Wieland)]]
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<figure id="fig:Smallest possible bend radii as a functionbend line parallel to the rolling direction">
[[File:Smallest possible bend radii as a functionbend line parallel to the rolling direction.jpg|rightleft|thumb|Figure 5: Smallest possible bend radii for 90° bends as a function of the 0.2% yield strength R<sub>p0.2</sub> – bend line parallel to the rolling direction (Wieland)]]
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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. At higher initial degrees of cold forming, the softening temperature is lowered.
As expected, the softening temperature for pure copper is rather low. CuNi9Sn2 and CuSn1CrNiTi exhibit high softening temperatures (<xr id="fig:Softening behavior for selected copper based materials"/><!--(Fig. 5.43)-->).
<figure id="fig:Softening behavior for selected copper based materials">
[[File:Softening behavior for selected copper based materials.jpg|rightleft|thumb|Figure 6: Softening behavior for selected copper-based materials after 40% cold working (Wieland)]]
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====<!--5.1.7.5-->Relaxation Behavior====
<figure id="fig:Relaxation behavior of selected copper based materials">
[[File:Relaxation behavior of selected copper based materials.jpg|rightleft|thumb|Figure 7: Relaxation behavior of selected copper-based materials. Starting tension: 100% of spring bending limit; Stress duration: 100 hrs (Wieland)]]
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==References==