Examination of a fractured pump drive shaft. 1. Introduction 1.1 A request was received for a examination to be carried out on the subject shaft which, it is understood had recently fractured in service in a catamaran SeaCat. 1.2 The shaft submitted for examination is shown in Plate 01. 2. Examination and Test Results 2.1 Initial visual examination of the shaft in the as submitted condition showed that it had fractured across a diameter at a position outboard of a seal land at the driven (impeller?) end of the shaft where a band of encrusted red and black corrosion products containing metallic particles was present (see Plate 02). There was an area on the seal land which did not extend over the entire circumference of the shaft where a fabric bonded elasomeric material was firmly attached to the seal land (arrowed on Plate 02 - see also Plate 06). Corrosion damage of various degrees was present outboard of the band of elastomeric material but the mating faces of the conical shaft end were not affected. Elsewhere the shaft was in an as new condition except for some localised regions of mechanical damage. A retaining nut, which appeared to have been manufactured from a bronze, was also heavily corroded. 2.2 Examination of the fracture faces showed that both had been smeared over large areas by post fracture contact and that the remaining areas had been corroded to varying degrees (see Plates 03 and 04). The most extensive corrosion was around the periphery, the outer edge of which was heavily pitted and showed evidence of intergranular penetration in places (see Plate 05). In general, the degree of corrosion decreased as the rotational centre of the fracture face was approached. It was clearly evident that the separation was brittle in character. 2.3 The bearing was removed from the pump body end of the shaft and the adjacent fracture face was removed by sawing diametrically across the bearing land. The fracture face and the adjacent shaft surface were briefly washed in mixed acids to remove the majority of the corrosion products. Examination in this condition showed that the fracture had initiated in multiple positions and that the actual sites of initiation had been subsequently removed by corrosion (see Plate 07). Fracture progression inwards from these initiations had merged to form a band in which fatigue progression bands were clearly evident (see Plate 08). The progression rate had increased as the fracture progressed inwards until the shear component of the torque the shaft transmitted in service predominated, resulting in the formation of a series of inclined radial facets, each of which was characterised by fatigue progression bands (see Plate 09). Only the very small central region exhibited overload characteristics indicating that the shaft was much more than adequate to transmit normal service loading. 2.4 The shaft was tested for eccentricity. None of engineering significance was found. The only evidence of possible shaft malalignment was in the form of flattened mechanical damage on the outer rim of the bearing land closest to the fracture (see Plate 10). 2.5 Magnetic tests on the shaft material and on the corrosion products showed that the shaft was not ferromagnetic and that at least some of the corrosion products were. Hardness tests on the shaft material gave values of 293, 299 and 290 HV(30). Indicating that its tensile strength was approximately 900N/mm² (58 tonf/in²). The material was unaffected by 2% nitric acid in alcohol, 25% aqueous nitric acid, and a aqueous acidified copper sulphate solution, indicating that it could be a nickel iron alloy. 3. Conclusions 3.1 It is considered that the fracture in the subject shaft was the result of a rotating bending corrosion fatigue mechanism and that corrosion was the predominant factor in initiating the mechanism. It is also considered that the nature of the corrosion was such as to indicate that the corrosion could have initially followed micrograin boundaries, and that it occurred under galvanic conditions in service.