LITERATURE SURVEY
2.6 BOLTED JOINT PERFORMANCE OF COMPOSITE MATERIALS
Joint strength between leaf spring and eye end must have superior strength than that of the designed leaf spring so that composite leaf springs can be a viable suspension system (Yu and Kim, 1988). Bolted joints were found to be more useful than adhesive bonded joints, due to the superior load transfer and less sensitive to the surface preparation characteristics (Camanho and Matthews, 1997). Shokrieh and Rezai (2003) proposed four different types of end joints for the leaf spring application and highlighted the manufacturing easiness and significance of bolted joints. Bolted joints and adhesively bonded joints are commonly being utilized for joining the composite structures. Common failure observed in mechanical joints of polymer composite parts are tension, shear, bearing, cleavage and pull-through failures (ASTM D5961, 2005).
Various aspects of joint performance such as geometric configurations, fastener preload, failure mode under static and fatigue conditions, effects of service conditions
(temperature and moisture) on joint performance and non-destructive evaluation techniques have been carried out in the past (Thoppul et al., 2008).
2.6.1 Composite Joint Design
Geometric configuration of joint significantly alters the bearing strength and failure mode. Collings (1977) investigated the influence of geometric parameters viz. width, hole diameter, end distance and thickness on the joint strength performance and reported that the bearing strength of carbon fiber reinforced plastics can be improved by increasing the lateral compressive stress around the loaded hole. Godwin et al.
(1982) used glass fiber reinforced polyester material for multi-bolt joints and reported pitch as an important design parameter in the bolted joint performance. Kretsis and Matthews (1985) observed the change in failure mode from bearing to tension when the specimen width decreases for glass fiber reinforced epoxy laminates.
Chen and Lee (1995) investigated the effect of friction, clamping pressure, clearance and stacking sequence of pin loaded joints using finite element technique on glass fiber epoxy laminates. The maximum load that can be sustained before failure was predicted using the maximum stress theory and later confirmed with the experimental results. It was also observed that the dependency of failure mode was dependent on lay-up characteristics rather than fiber/resin combination. Similar observation was substantiated by Park (2001), Okutan (2002), and Lim et al. (2006) for E-glass epoxy laminate. McCarthy et al. (2005) developed a three dimensional finite element model for multi-bolt, double-lap composite joints and addressed the progressive damage induced due to the effect of clearance and studied the damage mechanisms of the joint. The influence of traction forces by two parallel rigid pin for two parallel circular holes in woven glass fiber - vinylester was investigated by
Karakuzu et al. (2008) and reported the influence of end distance on failure mode.
Yavari et al. (2009) evaluated the stress distribution around the hole of epoxy laminates and reported that the higher friction and smaller plate width would offer safe joint. Atkas et al. (2009) observed good agreement between experimental and numerical predictions during the joint performance for glass-epoxy laminate and observed maximum bearing load for the glass fiber epoxy composite plates in double pinned joints, when the ratio of end distance to the hole center was greater than four.
2.6.2 Influence of Clamping, Clearance and Notch on Composite joint
Clamping torque alters the static and fatigue performance of bolted joints. Stockdale and Matthews (1976) investigated the effect of clamping pressure on bolt bearing loads for glass fiber epoxy polymer parts. Static and fatigue strength of the graphite- epoxy laminate was found to be improved with higher clamping torque (Crews, 1981). Sun et al. (2002) observed the effect of clamping area on improving the failure load of the graphite epoxy laminate joint. Khashaba et al. (2006) investigated the effect of washer size on the strength of composite bolted joints made of glass fiber reinforced epoxy laminates. For a constant clamping torque, the stiffness and the failure load of the joint decreases with the increase in washer diameter due to the reduction in contact pressure with increasing clamping area. Pakdil et al. (2007) reported different failure mode at various clamping torque of glass fiber epoxy laminated plate and highlighted the importance of increasing the preload for obtaining safe bolted joint characteristics.
Clearance plays an important role in altering the stiffness and damage encountered in bolted composites (Naik and Crews, 1986). Clearance fits should be avoided due to reduced contact area between the bolt and the bolt-hole, which causes
higher compressive contact stresses on the bearing surface (Scalea et al., 1998).
McCarthy et al. (2005) investigated the bolt hole clearance effect on single bolted, single lap carbon fiber epoxy joint and reported the reduction in joint stiffness due to the increase in clearance.
Material discontinuities due to hole in bolted composite joints produce areas of high stress concentrations which reduce the load-carrying capability of the composite structure. Composite constituent properties play a significant role in deciding the notched-strength performance. Carlsson et al. (1989) confirmed higher notch sensitivity in graphite polyetherketone composite than graphite epoxy composite and highlighted the importance of stress relieving mechanism in reducing the notch sensitivity. Swanson et al. (1993) observed superior performance of open hole under fatigue loading for toughened matrix system and also reported the enhancement in retaining the residual strength. Pinnell (1996) observed higher open and filled hole tensile strength in thermoplastic than thermoset composites and reported a greater stress relief at the hole edge in thermoplastic material. It was also reported that higher notch sensitivity of thermoset than thermoplastic material provides lower bearing strength to the composite for the same amount of reinforced graphite fibers. Ferreira et al. (1997) confirmed that stress concentration was found be more prominent in static and low cycle fatigue for fiber reinforced polypropylene material.