Fig shows the optical photomicrographs

Fig. 11 shows the optical photomicrographs of shank portion of the failed shear screw showing crack initiation near the thread root (Fig. 11(a)). Fig. 11(b) shows the morphology of cracks with extensive branching that has initiated from a pit. Fig. 12(a) and b show the scanning ctap Supplier micrographs showing surface of unfailed shear screw taken from the same batch stored under identical conditions to that of the failed one. Scanning electron microscopic observations revealed the surface of the unfailed shear screws having pit throughout their surfaces. Rough patches were also observed just below the head portion and at the threads. Optical microscopic observations did not reveal the presence of any defects in the thread portion. Microstructure of the shear screw revealed tempered martensite, typical of M250 maraging steel.
Discussion Service failures of maraging steel components and parts have been reported by many researchers [10–13]. Stress corrosion cracking (SCC) is a serious problem in high strength steels. SCC is the cracking/failure induced to the susceptible material by the combined effect of tensile stress and a corrosive environment. Thus for SCC to occur, three conditions must be simultaneously satisfied: a susceptible material, a corrosive environment and tensile stress. In the absence of either tensile stress above some minimum threshold value or corrosive environment, cracking does not occur. It is reported that the SCC resistance of high strength steels decreases as strength increases in many corrosive media. Kenyon et al. [14] reported that steels with yield strength of above 1400 MPa are susceptible to cracking in corrosive environments [14]. Diwakar et al. [15] have studied the KISCC of maraging steel in water. They found that that the KISCC deteriorates to as low as 8 MPa m1/2 in water. This implies that exposure to moist environment can drastically deteriorate the mechanical properties of M250 maraging steel. Further, SCC of maraging steel fabricated parts and components has been studied by Jha et al. [10–13]. They established that the cracking in this material was intergranular in nature. In the present case too, the cracking was found to be typically intergranular in nature.
Conclusions
Acknowledgment
Introduction In the field of system reliability and maintenance engineering, a typical series–parallel system consists of multiple subsystems in series, which is composed of several components in parallel. As an important kind of multi-component system, such model can describe distinctly the basic features of a complex system, such as aircraft engine [1], naval vessel [2], machining line in automobile engine shop [3] and coal transportation system in power station [4]. However, exon is impossible to perform all necessary maintenance activities before the next mission for the limited maintenance resources, such as time, budget, staff, spare parts and workshop. Therefore, decision-makers have to determine a subset from the complete set of all available maintenance activities. Only the selected maintenance activities are to be completed by a maintenance team during a mission interval, while the rest of the activities will be carried out for the next chance. The main purpose of such tough choice for maintenance activities is that the overall performance and reliability of the system should be restored as much as possible on the premise of meeting the maintenance resource requirements. Such problem, called selective maintenance problem (SMP), falls into maintenance resources allocation and mission planning. It was first proposed by Rice et al. in 1998 [5]. The original selective maintenance model for series–parallel system was extended following two ways in Refs. [6,7]. First, each independent subsystem in a complex system was composed of inhomogeneous components, which could be connected in any way. Second, it was also extended to a general case where both maintenance time and budget were constrained. Three alternative selective maintenance models were defined and themost appropriate one could be determined by decision-makers based on the interested performance index. One possible objective was to maximize system reliability, the second possible objective was to minimize total repair cost, and the last possible objective was to minimize total repair time. After that, in order to make the maintenance model more feasible and exercisable in engineering applications, the component life was assumed to follow Weibull distribution in series–parallel system, and the decision-makers could also perform different maintenance policies [8].