Strain Energy Release Rate in Treated Circumferentially Cracked Spring Steel

The suspension system is a prominent piece of material that plays a vital role in the stability of a vehicle. During the service, the suspension system is subjected to different environmental conditions, at the same time it has to sustain a variety of loads. The damage of the springs is mainly attributed by its load carrying capacity under fatigue loading. Fatigue strength is the most important property for the spring steel. The energy release rate is an important parameter used to predict the life of the springs. In this experimental analysis, the authors investigate the performance of spring steel under the action of fatigue loads. The specimen preparation and the experimentations have been carried out according to the American Society for Testing of Materials (ASTM) standards. From the experiments, the strain energy release rate of the spring steels has been determined. The effects of tempering and cryogenic treatments on the performance of the spring steel have also been determined. The results have revealed that the fatigue strength and the crack growth resistance have increased with quenching and cryogenic treatments. DOI: 10.4018/ijmmme.2012040105 78 International Journal of Manufacturing, Materials, and Mechanical Engineering, 3(2), 77-87, April-June 2012 Copyright © 2012, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Spring steel is the category of medium carbon steel. These steels have high hardness and can be produced by working, quenching or precipitation hardening. A study has been made to investigate the fatigue properties of high-strength spring steel in relation to the microstructural variation via different heat treatments (Wilson & Mintz, 1972; Shin, Lee, & Ryu, 1999). In order to increase the hardenability, elements such as chromium, manganese and silicon are added to these steels. Furthermore, silicon retards the conversion of the carbide to cementite during tempering. It refines the carbides and improves the sag resistance significantly (Nam, Lee, & Ban, 2000). Heat treatment creates a permanent change in the material that alters many characteristics such as fatigue strength (Ardehali Barani, Ponge, & Raabe, 2006). Spring steels are used in the quenched and tempered condition which gives optimum strength and toughness, vibrational damping (Datta, 1981). The change in microstructure and strength after the heat treatment process depends on the cooling rate obtained during quenching (Murakami, Takada, & Toriyama, 1988). Due to operational safety, springs have to meet increasing performance requirements, which concern mechanical properties, Tribological properties as well as fatigue strength (Bensely, Shyamala, Harish, Mohan Lal, Nagarajan, Krzysztof, & Rajadurai, 2009). In the manufacturing process of mechanical springs, high tensile residual stresses are generated which reduces considerably the spring strength and service life. These unfavorable residual stresses are partially eliminated by the heat treatment (Melander & Larsson, 1993). In this process, the spring is heated uniformly below the material transformation temperature (Carneiro, Pereira, Darwish, & Motta, 2002). An experimental investigation has been conducted to assess the stress relief influence on helical spring fatigue properties (Del Llano-Vizcaya, Rubio-Gonzalez, Mesmacque, & BanderasHernández, 2007). Failure of springs generally occurs due to fatigue. The fatigue process involves a competition between different crack initiating defects (Ebara, 2010; Li, Kim, & Lee, 1997). It is often a large defect situated close to the specimen surface and can generate a micro crack early in fatigue life. It is known that the fatigue loading level can have a significant influence on the location of critical defects (Li, 2008). At low loading levels internal defects can cause the majority of failures; at high loading levels defects near the surface can be more important to the fatigue process (Tanaka, Marita, & Akiniwa, 2008). The crack can initiate from the surface or at a depth below the surface depending on the materials processing conditions (Chan, 2010) During quenching, if the quenching rate is not chosen properly, quench crack/distortion may take place leading to tensile residual stress on the surface. The presence of retained austenite is undesirable. It is also found that iron sulphides present in steel along the grain boundaries felicitate the propagation of cracks generated during hardening (Ravi Kumar, Bhattacharya, Das, & Chowdhury, 2000). When a piece of metal is fractured either by tensile or impact loading the facture surface that is formed is rough and irregular. Its shape is affected by the metal’s microstructure as well as by ‘macrostructural’ influences (Molnarova, Mamuzi, Bacso, Fujda, Kodronova, Kuskuli, & Pokorny, 2008). Matract-A method for determining the fatigue notch-size-effect is presented based upon the development of closure in the wake of a newly formed crack growing from a notch (Park & Lee, 2000; Jones, Molent, & Pitt, 2007). In large scale yielding, in the analysis, the elastic plastic fracture mechanics (EPFM) parameter known as the cyclic J-integral, ∆J was adopted to observe the local plasticity at the crack tip and compared with the linear elastic fracture mechanics (LEFM) parameter known as the stress intensity factor range, ∆K (Mcevily & Minakawat, 1987). Shot peening of springs is a common practice to improve fatigue strength by prestressing the surface in compression. However, excessive surface roughening during peening with coarse shot lessens the benefits of peening. The crack growth behavior depends on specimen geometry, load spectra and material type (Li, Hu, & Pan, 1990). 9 more pages are available in the full version of this document, which may be purchased using the "Add to Cart" button on the publisher's webpage: www.igi-global.com/article/strain-energy-release-rate-