====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====

FigThe spring bending limit is defined as the boundary condition under which a standardized spring sample retains a deformation of 0. 505 mm after initial bend stressing and subsequent force removal.38:Directiondependence of The measurement is performed according to thestandard EN 12384. The spring bending limit is strongly dependent on the direction ofselected coppermaterialsstressing 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====

FigThe 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. 5(Rule of thumb: Fatigue strength = 1/3 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.39:Woehler curves forselected copperbased materialsThe multi-component alloys CuZn23Al3.Strip samples:05Co and CuSn1CrNiTi show high values of fatigue strength while CuFe2P and CuZn30 exhibit low ones [[#figures9|(Figs.2 – 3 mm thick, coldworked;Testfrequency;1,500 / min )]]<!--(WielandFigs. 5.39 and 5.40)-->.

Fig<div class="multiple-images"><figure id="fig:Woehler curves for selected copper based materials"> [[File:Woehler curves for selected copper based materials. 5.40jpg|left|thumb|Figure 2:Ranges of fatiguestrength Woehler curves forselected copperbased 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 radiifor 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.2The bendabilty of naturally hard copper alloys is significantly better perpendicular to the rolling directionthan parallel to it [[#figures10|(Figs. 4 – 7)]]<!--(WielandFigs. 5.41 and 5.42)-->.

Fig<div class="multiple-images"><figure id="fig:Smallest possible bend radii for 90 bends as function"> [[File:Smallest possible bend radii for 90 bends as function. 5.42jpg|left|thumb|Figure 4:Smallest possible bend radiifor 90° bends as a functionof the 0.2% yield strength Rp0R_p0.2 –bend line parallelperpendicular to the rolling direction (Wieland)]]</figure>

=====<!--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 At higher initial degrees of cold working degrees forming, the softening temperature becomes 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)''-->).

Fig<figure id="fig:Softening behavior for selected copper based materials">[[File:Softening behavior for selected copper based materials. 5.43jpg|left|thumb|Figure 6:Softening behaviorfor selectedcopper-basedmaterials after 40%cold working(Wieland)]]</figure><br style="clear:both;"/>

=====5Tension relaxation is defined as the loss of tension of an elastically stressed material as a function of time and temperature.1The causes for the relaxation are thermally activated processes which are comparable to creep behavior.7As 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 increasing operating times. Through suitable annealing processes the relaxation degree can be significantly reduced.For the measurement of tension relaxation, different test procedures are used, based on the ASTM E-32-86.5 Relaxation Behavior=====

Tension relaxation is defined as the loss <xr id="fig:Relaxation behavior of tension of an elastically stressed material as a function of time and temperatureselected copper based materials"/><!--Fig. 5. The causes for 44--> illustrates the different relaxation are thermally activated processes which are comparable to creep behaviorof some copper alloys. As a measure Good behavior is shown for the relaxation the percentage decrease in the bending tension compared to the initial one is used. Temperature increase is CuNi3Si1Mg and CuCrSiTi while CuZn30 and CuSn6 exhibit 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 less favorable relaxation different test procedures are used, based on the ASTM E-32-86tendency.

Fig. 5.44 illustrates the different relaxation <figure id="fig:Relaxation behavior of some selected copper alloys. Good based materials">[[File:Relaxation behavior is shown for CuNi3Si1Mg and CuCrSiTi while CuZn30 andCuSn6 exhibit a less favorable relaxation tendency. Fig. 5of selected copper based materials.44jpg|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;"/>