Failure Analysis of A Helical Compression Spring

1 International Journal of Modern Trends in Engineering and Research e-ISSN No.:2349-9745, Date: 28-30 April, 2016 @IJMTER-2016, All rights Reserved 877 Failure Analysis of a Helical Compression Spring Amitesh1, Prof V. C. Kale2, Prof K. V. Chandratre3 1 Dept of Mechanical Engineering,GES s COE,M&R,Nashik 2 Dept of Mechanical Engineering, GES s Sapat COE,M&R, Nashik 3 Dept of Mechanical Engineering, GES s Sapat COE,M&R, Nashik Abstract- Helical compressions springs function as an energy absorbing machine element. They absorb vibration and protect structure from damage. They act as shock absorber in a vehicle suspension system and thus help in giving comfortable ride by mitigating the transfer of vibration from road irregularities to the vehicle and the rider in turn.

2 It has been observed that most of the Compression coil springs fracture at the transition position from the bearing coil to the first active coil in service, while the nominal stress here should always be much less than that at the inside coil position of a fully active coil. This paper aims at Analysis of the reasons for Failure of a Helical Compression Spring . The prime focus is on fatigue Failure because this is the mode of Failure for most of Helical Compression Spring in dynamic loading conditions. Keywords- Raw Material Defect, Surface Imperfection, Improper Heat Treatment, Corrosion, Shot peening and decarburization.

3 I. INTRODUCTION Most of the vehicles have Helical Compression Spring as one of the primary elastic members in their suspension systems. They act as an energy absorbing machine element. The prime aim of a vehicle suspension system is to connect the wheel to the body. A Helical Compression Spring being an important part of the vehicle suspension system absorb energy and smooth out shocks that are received by the wheel from road irregularities. Thus, it helps in giving comfortable ride by mitigating the transfer of vibration from road irregularities to the vehicle body and the rider in turn.

4 Fatigue Failure of the suspension Spring often results in a variety of ways owing to the dynamic service loading conditions. The reasons for fatigue Failure of suspension Spring are raw materials defects, surface imperfections, improper heat treatment, corrosion, surface conditions and decarburisation. It is the combined act of some the above mentioned reasons or all of them that lead to fatigue Failure . The inner surface of an active coil of the Helical Spring is the position of maximum stress. Raw materials defects, surface imperfections, improper heat treatment, corrosion, surface conditions and decarburisation act as stress raiser and lead to Failure of Spring usually at the inner surface of an active coil of the Helical Spring .

5 II. COMMON REASONS FOR Spring Failure Some of the common reasons that lead to Failure of a Helical Compression Spring in service are as listed below. They are the prime factors that lead to Failure of Spring under dynamic loading conditions. 1. Raw Material Defect International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 3, Issue 4, [April 2016] Special Issue of ICRTET 2016 @IJMTER-2016, All rights Reserved 878 2. Surface Imperfection 3. Improper Heat Treatment 4. Corrosion 5. Surface condition and decarburization Raw Material Defect A typical raw material defect is the existence of a foreign material inside the steel, such as non-metallic inclusions.

6 In general, there are two types of foreign materials that can become trapped inside the steel solution: large imperfections such as spinells, and smaller imperfections such as inclusions that are caused by alloying elements. The shows a raw material defect that is usually very difficult to find after a coil is formed. This type of defect is easy to detect during the cold drawing process of coil manufacturing preparation. An ideal raw material has the form of ferrite pearlite. However, a raw material can also have local bainite inside the ferrite pearlite matrix. Due to a hardness difference, such raw materials may exhibit internal cracking.

7 Figure Fracture surface of a coil failed early due to an inclusion and Its SEM appearance [ valsange, 2012] The Figureure below depicts FEA model of inclusion inside the material and stress concentration near the inclusion. The stress at this inclusion is higher than at other positions in the material. These stress concentration leads to Failure of material during dynamic loading conditions. Figure FEA Model showing inclusion and stress concentration due to the inclusion [ valsange, 2012] Surface Imperfections International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 3, Issue 4, [April 2016] Special Issue of ICRTET 2016 @IJMTER-2016, All rights Reserved 879 Surface imperfections can occur as small hardening cracks, tool marks, scale embedded to the base material during cold drawing or surface flaws inherited by the raw material.

8 Poorly shotpeened surfaces can also be classified as surface imperfections. A crack alongside of the centerline of the wire is shown in the part model and its FEA model below. The stress distribution is also shown in the next FEA model. One can observe a high stress concentration at the crack location which is much higher than the outer surface stress level. Therefore, the product would likely fail from this point. Figure Part model with imperfection. (Left) and its FEA model (right) [ valsange, 2012] Figure FEA model showing high stress level than nominal at point of imperfection [ valsange, 2012] Improper Heat Treatment Improper heat treatment can be easily overlooked since a temperature difference in heating does not relate directly to the hardness of the material.

9 Extensive evaluations are usually needed to identify this problem. The Figure. shows a typical example of an improper heat treatment. Prolonged heating can cause the prior austenite grain size to grow significantly. Improper heat treatment can also result in the microstructure becoming pearlite instead of the required martensite. This type of defect is easier to identify due to the clear difference in hardness. The Figureure shows two different coils of the same product with varying microstructure. This defect usually occurs when the heating system does not operate normally. Again, referring to the Figureure, the left hand side coil has a much lower lifetime than that of the right side.

10 Tempering induces the decomposition of the retained austenite into mixture of ferrite and carbides International Journal of Modern Trends in Engineering and Research (IJMTER) Volume 3, Issue 4, [April 2016] Special Issue of ICRTET 2016 @IJMTER-2016, All rights Reserved 880 Figure. Identical raw materials heated with different heating patterns [Manish Dhakore et. al,2013] Corrosion In case where springs are subject to even mildly corrosive action while under fatigue stressing, the endurance limit for most ordinary material is reduced greatly. Higher value of endurance limits are achieved on corrosion resistant steels like chromium steel.