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The leaf spring manufacturer must supply high quality raw material at required strength for ensuring endurance rig tests. It's very important to maintain both inside and surface cleanliness of raw material. This study presents micro crack effects on material surface by evaluating residual stress values. Residual stress values on leaf springs are measured with X-ray diffractometer and different residual stress values are classified on the same raw material batch which have also the same material failures. Finally, micro cracks are measured in metrics. Micro crack standardization is performed considering the residual stress values and rig tests. The outputs in metrics which correlated with endurance rig tests can be taken as reference by the manufacturers of leaf spring and original equipment manufacturers.

Leaf springs are mainly used for absorbing energy associated with road outputs and they release energy coming from the road. If the complete leaf stiffness and each leaf stress distribution can be calculated carefully, while ensuring safety, required ride comfort will also be maintained. The desired vehicle ride height (attitude) under load must be taken into consideration during calculations of leaf spring design. In addition to providing the loads coming from the vehicle, leaf springs are significantly controllers of windup effects. Braking windup causes rotation at the lateral axis. When vertical F jounce load and braking moment are applied to the leaf spring simultaneously, von misses stress value on leafs will increase to a higher level which is very close to and sometimes higher than the tensile strength. The designers must ensure safety and endurance for any case during working conditions. The most critical point is front axle windup stopper position.

Leaf springs are used as the suspension elements for the front and rear axles of light and heavy commercial vehicles in the automotive key industry. While the vehicle is running empty or with a load under on-road or off-road conditions, it catches the loads transmitted from the wheels i.e., the loads from the ground to the hub axle, and working together with the damper it helps absorption of such loads and prevents them from being transmitted to the chassis. As a result, vehicle comfort is achieved. In addition to this function, leaf springs act as a safety part that continuously hold the chassis and axle together under static and dynamic loads. While the vehicle is runs under rough road conditions, the impact loads from the road not only cause negative impacts on the vehicle but also damage the drivetrain. Such impact loads substantially disturb both the vehicle and the passengers.

Decarburization occurs on heat treated components. At leaf spring manufacturers, the occurrence of decarburization, which has a negative impact on fatigue, starts at hot rolled leaf spring steel raw materials' production process. The decarburization rate of the raw material, which is taken through a heat treatment, increases during the leaf spring manufacturing process. Through metallographic analyses and experimental tests, this study manifests how leaf spring's resistance to fatigue is affected by the correlation of ferrite structure occurred by decarburization on heat treated leaf springs, the surface hardness, and the permanent surface tension to be occurred during the shot peening process. This study conveys the techniques that can be applied to obtain low decarburization in leaf spring manufacturing, and the improvements achieved through optimization of high temperature treatment times of materials through use of thermographic survey at heat treatment furnaces.