Failure Analysis of Connecting Rods and Engine Blocks of Small Generators

Three small generators were selected for conducting the failure analyses. The generators tagged FG1(2.7kVA, SG2700), FG2(2.7kVA, TG2700, TIGER) and FG3(2.5kVA ELEPAQ, EC2500CXS) were first dismantled and the components inspected for physical examination. In all the three generators the Connecting Rods were found to have broken into pieces. Two of the engine blocks were pierced by the broken connecting rods. Chemical analysis tests were made on the Connecting Rods and Engine Blocks using XRFNiton analyzer. The tests revealed that all the components were made from Aluminum alloys. The Copper contents for the Connecting Rods were found to be from 1.77% to 2.37% which were below 4.0% minimum requirement for Connecting Rods and other components of high performance engines based on Aluminum Association (AA) and British Standard (BS) specifications. The Connecting Rods also contained up to 2.01% Iron but none of the Connecting Rods had Magnesium which is an important element for increasing strength of Aluminum alloys. The high content of iron coupled with lack of Magnesium resulted in low strength and increased hardness, making the Connecting Rods brittle and highly susceptible to fatigue failure. Hardness tests conducted on the Connecting Rods using Rockwell Hardness Testing machine gave 160,151 and 175 BHN which were much higher than maximum of 105 BHN for AA and BS specifications. Similarly, the hardness values of the Engine Blocks were found to be 128,160 and 140BHN respectively. The corresponding tensile strengths of the Engine Blocks were 167,149 and 152MPa which were lower than the minimum AA and BS specification of 170MPa.The results concluded that the Connecting Rods of the three generators failed due to excessive brittleness.