Failure Analysis Of A Diesel Generator Connecting Rod

Introduction

It is a discussion on the outcomes of an investigation on the failure analysis done from a diesel engine’s connecting rod used to generate electrical energy. A detailed analysis of the fracture zone and the con-rod material was also included in the investigation. This investigation comprised of the following testing techniques and experimental procedures:

• Visual inspection
• Magnetic particle inspection
• Chemical analysis
• Fractography
• Microanalysis
• Metallography
• Tensile & harness testing
• Particle inspection.

The connecting rod had been constructed of a low alloy steel of the AISI/SAE 4140 specification. The microstructure, mechanical properties, and chemical composition were perfect for the said application. The fracture in the connecting word was in a part near the head. Plus, the fracture’s origin was situated on the lubrication channel of the con-rod. The lubrication channel displayed an area comprising a material made of tungsten, perhaps from one of the machine tools, attached in its surface due to a poor quality manufacturing process. The area functioned as a nucleation site for fissures, which transmit through the section of the connecting rod through a fatigue mechanism, decreasing its section and eventually creating its catastrophic failure.

Research and Theory on the Problem

A critical component representing a complex motion is a high performance connecting rod responding to an internal combustion engine. It is subjected to tensile load because of the moving components’ inertia, as well as, compressive load because of combustion force. If not manufacture or designed with caution, the component becomes highly susceptible to fatigue failure and the loadings are cyclic. It is precisely for this reason why connection rods are manufactured and designed with high precision for endless loading cycle. However, connecting rod failures are frequently reported that is related to assembly faults, bearing failure, bending, and fatigue. The experiment addresses one of the connecting rod failures reported during the course of fatigue testing. The occurrence of failures took place at about one million fatigue loading cycle vis-à-vis the loading cycles of five million, which is its target life.

This study discusses an investigation conducted for an engine connected rod and also tries to identify and do the RCA (Root Cause Analysis). There are many factors that can affect failure such as material type, structural design, as well as, dynamic loads, which were reviewed using the technique of standard failure analysis. These include metallurgical testing, visual observation, fractography analysis, magnetic particle testing, grain flow analysis, microstructure analysis, inclusion analysis, tensile testing, Brinell hardness testing, chemical analysis, dimensional inspection, and residual stress analysis. According to the outcomes of this investigation, the failure of the connecting rod occurred because of improper drilling/machining of chamfer and oil hole at the connecting rod’s small end that had created rough marks of the tool at the inner diameter, material chip off, and serration marks. This, in turn, led to damaging tensile residual stresses that induced early fatigue failure

Specific Theory – Scientific/Engineering

The connecting rod was visually inspected and an analysis of the surface of the fracture was done at the first stage. Thereafter, a detailed characterization was conducted for the con-rod material. Samples for investigation were extracted from the fracture zone in order to conduct tests like a tensile test, metallographic analysis, hardness test, and chemical analysis. A Bruker Magellan Q8 atomic emission spectrometer was used to conduct chemical analysis. Longitudinal samples were used to conduct the tensile tests according to a standard called ASTM A370-15. The samples were situated at the body of the con-rod’s mid ratio. A standard called ASTM E384-11e1was used to conduct hardness testing while employing a hardness tester called Zwick Roell ZHV30. Finally, the ASTM E407-07(2015) e1 and ASTM E3-11 standards were used to prepare the samples needed to conduct the metallographic analysis. The mounting of the samples was done in epoxy resin. They were then subjected to polishing and grinding processes with the aid of diamond and sandpaper paste individually. The microstructure of the samples was exposed after a chemical etching took place with a two percent Nital solution. An optical microscope (Leica M2) was used with a digital camera to carry the analysis.

Specific Theory – Risk and Management

The investigation also constituted the fracture zone’s comprehensive analysis. Samples were procured from the fracture to conduct hardness testing, metallographic analysis, inspection, and magnetic particle inspection. The lubrication channel was cut longitudinally to procure the samples needed for this analysis. First two samples were taken from the surface area of the fracture while sample 3 was taken from a section away from it. Hardness and metallographic tests were done as explained above. A standard called ASTM E709-15 was followed to perform the magnetic particle testing. AC magnetic field is created with fluorescent particles and a Magnaflux Magnetic Yoke. As mentioned earlier, the samples’ microanalysis was done using an energy dispersive spectrometer and a powerful microscope (FEI Quanta 650 scanning electron microscope) that operates at 30 kV.

Link between Case Evidence and Theory

Visual inspection as well as fractography

The body of the connecting rod was fractured on the surface by cutting a portion perpendicular to the axis, keeping close to its head. Earlier investigations have also concluded that the connecting rods are subject to suffer from failure in this section as the highest level of stresses and deformation occur in that particular area as per the finite element simulations.

The fatigue pattern with its beach marks was seen. The cracking started in many places at the lubrication channel while spreading to the connecting rod’s external part. On close observation, it was found that over 75 percent of the section of the connecting rod was covered by the fatigue area. Thus, the pattern indicated normal stresses of low magnitude. The remaining portions of the fracture pattern highlight an overload pattern. While a fatigue fracture can be typically found in the connecting rod failure cases, it was not a common sight to find the observed pattern that ran from the middle to the exterior.

Analysis of fracture zone

According to the magnetic particle examination, secondary racks were found that ran parallel to the surface of the fracture. Thus, it becomes evident that the cracks spread to the connecting rod’s exterior surface from the lubrication channel as suggested for a secondary crack.

According to the metallographic samples, examination for the first two samples, a white layer was present on the lubrication channel’s surface. It is the area where fissures nucleate according to what was observed. The cracks were found to spread towards the connecting rod’s exterior. A wide-ranging thickness was exhibited by the white layer. Plus, a zone affected by heat was also identified which had a varied size. On the other hand, when an analysis was done for the third sample on a distance away from the rod’s fracture area, there is no proof of any heat affected zone or the higher layer at all. However, it was observed that the channel had a coarse machine finishing in those areas.

The proof of being exposed to temperature and the tungsten’s presence were large enough for promoting its fixture in the steel base material. The creation of the neat affected zone as observed in the investigation show compatibility with the electrical discharge machining or EDM process. A material that is electrically conducive is machined by creating a localized and controlled electric spark between a work piece and an electrode in the process of EDM. The temperatures in the process can go as high as 12, 000 °C while materials made of tungsten are used as electrodes.

Thus, one can presume that the process of EDM was used for machining the lubrication channel, as well as, the creation of the heat affected zone got produced due to its substandard application.