Failure Analysis of A Helical Compression Spring - PDF4PRO

International Journal of Modern Trends in Engineering and Research

www.ijmter.com 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

1Dept of Mechanical Engineering,GES’s R.H.Sapat COE,M&R,Nashik [email protected] 2Dept of Mechanical Engineering, GES’s R.H. Sapat COE,M&R, Nashik [email protected]

3Dept of Mechanical Engineering, GES’s R.H. Sapat COE,M&R, Nashik [email protected]

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. 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.

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. 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.

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


2. Surface Imperfection 3. Improper Heat Treatment 4. Corrosion 5. Surface condition and decarburization

2.1 Raw Material Defect

A typical raw material defect is the existence of a foreign material inside the steel, such as non-metallic inclusions. 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 Figure.2.1.1 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.

Figure 2.1.1 Fracture surface of a coil failed early due to an inclusion and Its SEM appearance [ P.S. 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 2.1.2 FEA Model showing inclusion and stress concentration due to the inclusion [ P.S. valsange, 2012]

2.2 Surface Imperfections



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. 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 2.2.1 Part model with imperfection. (Left) and its FEA model (right) [ P.S. valsange, 2012]

Figure 2.2.2 FEA model showing high stress level than nominal at point of imperfection [ P.S. valsange, 2012]

2.3 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. Extensive evaluations are usually needed to identify this problem. The Figure. 2.3.1 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. Tempering induces the decomposition of the retained austenite into mixture of ferrite and carbides



Figure. 2.3.1 Identical raw materials heated with different heating patterns [Manish Dhakore et. al,2013]

2.4 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. In spring design, one should use either low working stress or protect material from corrosive action. Electroplating is one method to protect against corrosion. Cadmium plating and chromium plating is usually used. The FEA models below depict the corrosion in the material. The stress concentration around the place of corrosion is shown in the next FEA model. Finer meshing is used around the corrosion area since a higher stress concentration is expected there. This high stress concentration will cause early spring breakage from this point. One should either protect spring materials from corrosion or else use low working stress. Even then, if corrosion is present, there is no assurance that eventual fatigue failure will not occur, if sufficiently large no of stress reversal take place.

Figure. 2.4.1 FEA Model showing corrosion [ P.S. valsange, 2012]

Figure. 2.4.2 FEA Model showing stress concentration at the place of corrosion [ P.S. valsange, 2012]

2.5 Surface Condition and Decarburisation

Surface conditions in spring steel have a marked effect on the fatigue strength of the material. During heat treatment and forming operations of springs, the surface layer is decarburised to some extent (i.e. there is loss of carbon content form a thin upper layer of surface). Thus there is, in fact, a thin layer of low-carbon steel (which is relatively week) over the body of the spring which is composed of the relatively strong high-carbon steel or alloy steel. Under repeated loading conditions, the weaker low carbon steel on the surface may a develop crack which then spreads across the section as a consequence of the high stress concentration at base of the crack. Actual tests have shown that a



layer of this decarburized material is sufficient to greatly weaken the spring during fatigue. A decarburised layer is shown in the FEA model below. This decarburized layer is weaker than the high carbon layer steel below it. It is prone to crack initiation during stress reversal under dynamic loading conditions.

Figure. 2.5.1 FEA model showing decarburized layer [ P.S. valsange, 2012]

CONCLUSION

It is clear from above analysis that 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. So, it is imperative that a helical spring is designed with great care. Raw material selection for spring manufacturing is the first step where a designer must look for presence of raw material defects as they play an important role in fatigue failure of spring. It is equally important to look for manufacturing blunders like imperfection in coil surface that may develop during coiling of spring and improper heat treatment that may lead to decarburization. If helical spring is to be used in corrosive environments, then corrosive prevention methods like colour coatings, use of grease on the coil surface etc. are to be adhered to. In order that a helical spring functions well during its intended service life, fatigue failure is to be avoided as far as possible. This goal can be fulfilled to a great extend by carefully avoiding the above mentioned factors that lead to fatigue failure of a helical compression spring.

REFERENCES

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Research and Applications (IJERA) pp.513-522 [5] Manish Dakhore and Bhushan Bissa(2013), “ failure analysis of locomotive suspension coil spring using finite

element analysis”, International Monthly Refereed Journal of Research In Management & Technology [6] Youli Zhu, Yanli Wang, Yuanlin Huang(2014), “ Failure analysis of a helical compression spring for a heavy

vehicle’s suspension system” ,Elsevier [7] N.Lavanya1, P.Sampath ,Rao2 M.Pramod Reddy,(2014) “Design and Analysis of A Suspension Coil Spring For

Automotive Vehicle”, Int. Journal of Engineering Research and Applications, , pp.151-157 [8] C.Madan Mohan Reddy, D.Ravindra Naik,Dr M.Lakshmi Kantha Reddy (2014), Analysis And Testing Of Two

Wheeler Suspension Helical Compression Spring IOSR Journal of Engineering (IOSRJEN) (p): 2278-8719 Vol. 04, Issue 06

Failure Analysis of A Helical Compression Spring - PDF4PRO

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