Changes

====5Important for the usage as spring contact components are, besides mechanical strength and electrical conductivity, mainly the typical spring properties such as the maximum spring bending limit and the fatigue strength as well as the bendability.1During severe thermal stressing the behavior of spring materials is determined by their softening and relaxation.7The following briefly describes these material properties.1 Spring Bending Limit====

The spring bending limit is defined as the boundary condition under which a standar====<!--dized spring sample retains a deformation of 0.05 mm after initial bend stressing and subsequent force removal. The measurement is performed according to the standard EN 12384. The spring bending limit is strongly dependent on the direction of stressing with regard to the strip rolling orientation ''(Fig. 5.38)''1. Higher values are obtained if bending is perpendicular to the rolling direction as compared to parallel. This has to be considered when designing contact springs7.1-->Spring Bending Limit====

Fig. 5.38: Direction dependence of the The spring bending limit of selected copper materials (Wieland)[[File:Direction dependence of is defined as the boundary condition under which a standardized spring bending limitsample retains a deformation of 0.05 mm after initial bend stressing and subsequent force removal.jpg|right|thumb|Direction dependence of The measurement is performed according to the standard EN 12384. The spring bending limit is strongly dependent on the direction of selected copper materials stressing with regard to the strip rolling orientation (Wieland<xr id="fig:Direction_dependence_of_the_spring_bending_limit"/><!--(Fig. 5.38)-->)]]. Higher values are obtained, if bending is perpendicular to the rolling direction, as compared to parallel. This has to be considered when designing contact springs.

<figure id====5"fig:Direction_dependence_of_the_spring_bending_limit">[[File:Direction dependence of the spring bending limit.jpg|left|thumb|Figure 1.7.2 Fatigue Strength===: Direction dependence of the spring bending limit of selected copper materials (Wieland)]]</figure><br style="clear:both;"/>

The fatigue strength is a measure of maximum alternating bending force, symmetrical to the zero position, which a sample – for example a relay spring – can be exposed to for an “unlimited” number of cycles without breaking. (Rule of thumb: Fatigue strength = 1/3 of Tensile strength). The measurement is conducted using so===<!--called Woehler– diagrams 5. With increasing bending force 7 the number of alternating cycles before breaking decreases1. Above 10 cycles the influence of further cycling numbers becomes insignificant and therefore 7 the force value reaching 2x10 cycles can be used to define the fatigue strength.The multi2--component alloys CuZn23Al3.5Co and CuSn1CrNiTi show high values of fatigue strength while CuFe2P and CuZn30 exhibit low ones ''(Figs. 5.39 and 5.40'').>Fatigue Strength====

Fig. 5.39: Woehler curves The fatigue strength is a measure of maximum alternating bending force, symmetrical to the zero position, which a sample – for example a relay spring – can be exposed to for selected copper based materialsan “unlimited” number of cycles without breaking. Strip samples(Rule of thumb: 0.3 mm thick, cold worked; Testfrequency; Fatigue strength = 1,500 / min (Wieland3 of Tensile strength). The measurement is carried out with so-called Wöhler diagrams. With increasing bending force the number of alternating cycles before breaking decreases. Above 10 cycles the influence of further cycling numbers becomes insignificant and therefore the force value reaching 2x10 cycles can be used to define the fatigue strength.The multi-component alloys CuZn23Al3.5Co and CuSn1CrNiTi show high values of fatigue strength while CuFe2P and CuZn30 exhibit low ones [[File:Woehler curves for selected copper based materials.jpg#figures9|right|thumb|Woehler curves for selected copper based materials. Strip samples: 0(Figs.2 – 3 mm thick, cold worked; Testfrequency; 1,500 / min (Wieland)]]<!--(Figs. 5.39 and 5.40)-->.

Fig. 5.40<div class="multiple-images"><figure id="fig: Ranges of fatigue strength Woehler curves for selected copper based materials (Wieland)"> [[File:Ranges of fatigue strength Woehler curves for selected copper based materials Wieland.jpg|rightleft|thumb|Ranges of fatigue strength Figure 2: Woehler curves for selected copper based materials . Strip samples: 0.3 mm thick, cold worked; Testfrequency; 1,500 / min (Wieland)]]</figure>

<figure id====5.1.7"fig:Ranges of fatigue strength for selected copper materials Wieland"> [[File:Ranges of fatigue strength for selected copper materials Wieland.jpg|left|thumb|Figure 3 Bendability===: Ranges of fatigue strength for selected copper materials (Wieland)]]</figure></div><div class="clear"></div>

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 51.42)''7.3-->Bendability====

Fig. 5.41: Smallest The measure for bendability of a strip material is the smallest possible bend radii for 90° bends as bending radius r of a function sample piece of given material thickness s without appearance of the 0surface cracking.2% yield strength R – bend line p0Bending tests are performed as either 90 degree bends according to ISO 7438 or as defined forth-and-back bending.2 The bendabilty of naturally hard copper alloys is significantly better perpendicular to the rolling direction(Wieland)than parallel to it [[File:Smallest possible bend radii for 90 bends as function.jpg#figures10|right|thumb|Smallest possible bend radii for 90° bends as a function of the 0.2% yield strength R_p0(Figs.2 4 – bend line perpendicular to the rolling direction (Wieland7)]]<!--(Figs. 5.41 and 5.42)-->.

Fig. 5.42<div class="multiple-images"><figure id="fig: Smallest possible bend radii for 90° 90 bends as a function of the 0.2% yield strength Rp0.2 – bend line parallel to the rolling direction (Wieland)"> [[File:Smallest possible bend radii for 90 bends as a functionbend line parallel to the rolling directionfunction.jpg|rightleft|thumb|Figure 4: Smallest possible bend radii for 90° bends as a function of the 0.2% yield strength R_p0.2 – bend line parallel perpendicular to the rolling direction (Wieland)]]</figure>

<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|left|thumb|Figure 5: Smallest possible bend radii for 90° bends as a function of the 0.1.72% yield strength R_{p0.4 Softening Behavior===2} – bend line parallel to the rolling direction (Wieland)]]</figure></div><div class="clear"></div>

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====<!--5. 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. 51.43)''7.4-->Softening Behavior====

FigThrough thermal activation at elevated temperatures the original mechanical material strength achieved by cold working or precipitation hardening can be reversed completely. 5The start of softening is mostly defined as the temperature at which a 10% reduction of mechanical strength is reached.43: Softening behavior for selected copper-based materials after 40% It is dependent on the degree of initial cold working (Wieland)and the annealing temperature and time. At higher initial degrees of cold forming, the softening temperature is lowered.[[File:Softening behavior As expected, the softening temperature for selected pure copper based materialsis rather low.jpg|right|thumb|CuNi9Sn2 and CuSn1CrNiTi exhibit high softening temperatures (<xr id="fig:Softening behavior for selected copper-based materials after 40% cold working "/><!--(WielandFig. 5.43)]]-->).

<figure id====5.1.7"fig:Softening behavior for selected copper based materials">[[File:Softening behavior for selected copper based materials.5 Relaxation Behavior===jpg|left|thumb|Figure 6: Softening behavior for selected copper-based materials after 40% cold working (Wieland)]]</figure><br style="clear:both;"/>

Tension relaxation is defined as the loss of tension of an elastically stressed material as a function of time and temperature====<!--5. The causes for the relaxation are thermally activated processes which are comparable to creep behavior1. As a measure for the relaxation the percentage decrease in the bending tension compared to the initial one is used7. 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 E5-32-86.>Relaxation Behavior====

FigTension relaxation is defined as the loss of tension of an elastically stressed material as a function of time and temperature. 5The causes for the relaxation are thermally activated processes which are comparable to creep behavior.44 illustrates 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 different relaxation behavior of some copper alloysthe spring force than increasing operating times. Through suitable annealing processes the relaxation degree can be significantly reduced. Good behavior is shown for CuNi3Si1Mg and CuCrSiTi while CuZn30 andCuSn6 exhibit a less favorable For the measurement of tension relaxation tendency, different test procedures are used, based on the ASTM E-32-86.

<xr id="fig:Relaxation behavior of selected copper based materials"/><!--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. <figure id="fig:Relaxation behavior of selected copper based materials">[[File:Relaxation behavior of selected copper based materials.jpg|left|thumb|Figure 7: Relaxation behaviorof selected copper-basedmaterials.Starting tension: 100% ofspring bending limit;Stress duration: 100 hrs(Wieland)]]</figure><br style="clear:both;"/>