Diagnosis of environmentally induced failures is greatly facilitated by metallographic analysis. As an example, a failure analysis of ASTM A574 material grade bolts is presented. The bolts served as bonnet screws in underground valves and failed due to stress-corrosion cracking. Metallographic methods were used to diagnose and provide solutions for the service failure. Included are photos showing crack propagation morphology and fracture surface appearance.

A 20 ton polar crane motor fell during a 3400 kg (7500 lb) lift, narrowly missing personnel working beneath the crane. Witnesses reported that the motor fall was preceded by a falling oil mass, and it was believed that the motor was intact prior to impact. The maintenance history of the crane showed that the motor had been removed, repaired, and reinstalled 2 years prior to the failure. Observations of oil leakage were noted yearly up to the failure. The motor casing was held onto the adapter plate by eight 14-20 UNC x 25 mm (1 in.) long hex socket cap screws. Examination of the motor adapter plate, motor casing shards (aluminum), the gear side of the motor housing, and seven fractured cap screws (ASTM A574) showed that the motor casing was intact at the time of “uncontrolled descent” and that the screws had failed by high nominal stress reverse bending load fatigue, which was probably the result of insufficient torque on the bolts.

Socket head cap screws used in a naval application were failing in service due to delayed fracture. The standard ASTM A 574 screws were zinc plated and dichromate coated. Investigation (visual inspection, 1187 SEM images, chemical analysis, and tension testing) of both the failed screws and two unused, exemplar fasteners from the same lot supported the conclusion that the cap screws appear to have failed due to hydrogen embrittlement, as revealed by delayed cracking and intergranular fracture morphology. Static brittle overload fracture occurred due to the tension preload, and prior hydrogen charging that occurred during manufacturing. The probable source of charging was the electroplating, although postplating baking was reportedly performed as well. Recommendations included examining the manufacturing process in detail.

Six ASTM A-574 steel cap screws from a hydraulic coupling failed after 3 months in service. The screws were replacements for smaller-diameter cap screws that had been installed during an outage. Six new cap screws were examined along with the failed screws. Eight fracture locations were identified—three at the head-to-shank fillet, four at the eighth thread root from the cap, and one at the sixth thread root from the cap. Fracture surfaces were examined using a stereomicroscope and SEM, and the fracture mode was shown to be transgranular. EDS on the fracture surfaces showed sulfur and chlorine in the surface deposits. The observations indicated that the screws had failed by fatigue. Insufficient preloading was considered to be the most likely cause of the fatigue cracking. It was recommended that the proper preload on the screws be verified and maintained.

This article aims to identify and illustrate the types of overload failures, which are categorized as failures due to insufficient material strength and underdesign, failures due to stress concentration and material defects, and failures due to material alteration. It describes the general aspects of fracture modes and mechanisms. The article briefly reviews some mechanistic aspects of ductile and brittle crack propagation, including discussion on mixed-mode cracking. Factors associated with overload failures are discussed, and, where appropriate, preventive steps for reducing the likelihood of overload fractures are included. The article focuses primarily on the contribution of embrittlement to overload failure. The embrittling phenomena are described and differentiated by their causes, effects, and remedial methods, so that failure characteristics can be directly compared during practical failure investigation. The article describes the effects of mechanical loading on a part in service and provides information on laboratory fracture examination.