Spring durability and spring fatigue
A spring is regarded as being durable if it does not loose force or when subjected to a certain spring travel, shortens under a given force or breaks to that the function is disrupted or disappears.
If a spring is to be durable, the stress in the material must not be greater than the strength of the material permits.
When a load is applied to a piece of metal it deflect first elastically and if the load is high enough also plastically. Elastic deflection is when the material after unloading return to its previous shape/geometry. At plastic deflection the metal receives a permanent deformation and do not return to its previous geometry after unloading. It is the elastic part of the deflection which is used to receive the deflection in durable springs. The function stress and deflection is described in Hooks law where the elastic module E for normal and G for shear stress is described. These E and G modulus are the figures describing elastic behaviour for metals. A low figure means more deflection than a high figure for the same stress. A metal with high strength can normally be loaded to a higher stress without passing the limit where the deflection starts to be plastic hence more elastic deflection. The stress limit when the metal starts to deflect plastically is the materials yield strength. The ultimate tensile strength is the stress at which the metal breaks.
Two more material properties are of great interest for springs it is the creep/relaxation and the fatigue. One physical phenomenon with metals is that at stress below the yield strength of the material a very slow plastic deformation take place. In the spring branch this is called creep when a spring under constant load loose length and it is called relaxation when a spring under constant compression lose load. How much creep/relaxation is depends on the temperature, the stress in the metal, the metals yield strength and the time. Increased temperature, stress and time also increase the creep/relaxation. Especially the temperature and stress have a big influence. Lesjöfors regularly does relaxation and creep tests on different materials and different types of springs, we provide information and knowledge about this important property, creep/relaxation. The diagram below show how the relaxation depends of the time, for a certain material, stress and temperature.
One other phenomenon with metals is that, at pulsated stress below the yield strength of the spring materials, the materials can break because of fatigue.
A spring fatigue problem starts with the development of a micro fatigue crack which grows for every pulsation. When the stress in the remaining material reaches the ultimate tensile strength the spring will break. The risk for fatigue depends among other factor very much on the designed stress in the spring, the amplitude of the pulsation and the material ultimate tensile strength. It should be noted that material with the same chemical analysis and strength can have different fatigue property because also other material factors have an influence. Also the temperature and corrosion affect the fatigue strength of a spring. When testing the fatigue strength the spread makes its necessary to use statistic methods to get reliable results and to calculate the risk levels for breakages or security factors. Lesj"fors regularly does fatigue tests on various materials and types of springs and we provide knowledge and information about this important spring property, fatigue. The diagram below show how the fatigue strength in number of pulsations for different stress levels, for a certain material and temperature.