Failure analysis of vehicle water cooling pumpABSTRACT
Internal combustion engines must be kept at a certain temperature in order to work efficiently. During the design of the engines, cooling systems are used to keep them in the temperature ranges determined. Water pumps are used in the engines for the circulation of the water used in the cooling systems. Failure of these pumps can cause enormous damage to the engine. In this study, the damage caused by the water pump of an engine operating according to the otto principle was investigated. In damage analysis studies, metallographic samples of the broken surfaces of the circulation pump were prepared. Spectroscopic analyses, metallographic analyses, and hardness measurements were made. Additionally, the solid model was drawn, and stress analyses were carried out using the finite element method. Spherical cementites were observed in optical microscope images of the material. Ferrite-rich regions were detected towards the inner parts of the material.In this study, it was observed that intergranular morphology coalescence occurred due to fatigue, but dense corrosion deposit was observed at the crack initiation. As a result, the fatigue strength of the material decreased due to some errors made during production and rapid breakage occurred due to the corrosive effect of the working environment.
Keywords:
Circulation pump; Damage Analysis; Failure Analysis; Water cooling pump
1. INTRODUCTION
Internal combustion engines perform optimally at a specific temperature range. When engines are cold, their components wear out more quickly and produce higher emissions. Thus, an essential role of the cooling system is to bring the engine to its operating temperature swiftly and then maintain a stable temperature. The primary purpose of the cooling system is to ensure the engine remains at its ideal operating temperature. Should any part of the cooling system malfunction, it can lead to the engine overheating, which may result in significant damage.
A water cooling pump is essential for the functioning of internal combustion engines. It facilitates the movement of coolant through the engine block, cylinder head, hoses, and radiator, ensuring the engine maintains its optimal operating temperature. Typically powered by a belt connected to the crankshaft pulley, the water cooling pump uses impeller blades and centrifugal force to propel the coolant through the various channels and hoses of the cooling system. Once the coolant has circulated through the engine, it travels to the radiator via the hoses. Radiators, usually positioned at the front of vehicles, cool the hot liquid as it passes through by utilizing the airflow moving over it [1].
The purpose of damage analysis is to investigate the mechanism and reasons that cause unexpected damage to parts [2, 3]. Most of the damage encountered in engineering is due to fatigue. However, studies on water cooling pumps can cover different topics. Most of the studies on the cooling system involve thermodynamic analyses on the efficiency of the cooling system or parts [4, 5, 6, 7–8]. In the damage analysis study on the industrial-type water cooling pump shaft, they concluded that the material was broken due to heat treatment during manufacturing [9]. In another study [10] on the circulation pump, it was found that during the material joining process, it was broken due to the material characterization in the weld area [11]. Many studies have been conducted on the damage analysis of automotive and machine parts. Many studies have been carried out on the axle shafts of vehicles in automotive and rail systems [12, 13, 14–15]. There are studies on gears [16, 17 –18], springs [19, 20, 21, 22 –23], and vehicle pistons [24, 25, 26–27].
Chemical analysis, mechanical properties, microstructure and breaking surface examinations (fractography), and EDX analyses are examined in damage analysis examinations [28, 29–30]. Many researchers have also used this method to study the model [31, 32]. Similar analyses were carried out in this study.
In this study, damage analysis studies were carried out on the broken water cooling pump of an internal combustion engine. The vehicle was brought to the service due to an overheating problem and has a mileage of approximately 210,000 km. According to the owner of the vehicle, this problem occurs especially during long journeys and causes a decrease in engine performance. The service teams have initiated a detailed examination to determine the source of the problem. At the end of the examination, it was determined that the shaft of the water cooling pump was broken. The location of the analysed water cooling pump on the engine is shown in Figure 1a, and the damaged (removed from the engine) state is shown in Figure 1b.
View of the circulation pump a) On the engine, b) Damaged water cooling pump (Removed from the engine).
2. EXPERIMENTAL METHODS
The average water cooling pump has a lifespan of 60,000–100,000 kilometers. If the water cooling pump fails, it cannot circulate the cooling water cooling in the vehicle in sufficient quantities. This can cause the engine to overheat [33]. In addition, your vehicle may lose water cooling because the felt and gaskets in the worn water cooling pump may leak water cooling over time. Such failures are symptoms that indicate the end of its life. However, the breaking of the shaft of the water cooling pump under analysis is an unexpected and undesirable type of damage.
In this research, detailed failure analysis was conducted using chemical assessments, mechanical property evaluations, microstructural and fractographic examinations [34], SEM and EDX analyses, and finite element (stress) analysis [35, 36–37]. Optical emission spectrometry provides an industry-standard analytical solution for metallurgical applications. In particular, it is used in the analysis of a wide range of metals and alloys. This method allows the elemental composition of alloys to be analyzed down to trace levels with minimal impact on operating efficiency. Therefore, during damage analysis, it plays an important role in determining whether the chemical elements used in the production phase of the damaged part comply with the standards.
Specimens taken from the front axle were prepared for chemical composition analysis using a GNR device. For microstructural investigation, samples were extracted from the damaged water cooling pump. The specimens underwent a series of sanding steps with grit sizes 60, 120, 240, 400, 600, 800, 1000, and 1200, utilizing a Forcipol brand sanding machine. Following the sanding process, the specimens were polished with an alumina-water cooling solution on a polishing cloth. Post-polishing, images were captured at 100x and 200x magnifications using an Olympus optical microscope.
The samples were analyzed with a scanning electron microscope (SEM) (LEO 1430VP, Carl Zeiss, Jena, Germany) equipped with an energy-dispersive X-ray analysis (EDS, Oxford Instrument Link ISIS, Oxford, UK) operating at 10–20 kV with a working distance of 10 mm. A SHIMADZU device was employed for the hardness testing, and the average hardness value was determined using the Vickers hardness measurement method under a 1.961 N load, resulting in HV0,2.
3. RESULTS AND DISCUSSION
3.1. Optical emission spectroscopy
Samples taken from the water cooling pump shaft were prepared for spectral analysis. The chemical composition of the shaft was determined using a GNR brand device. The results of the analysis, along with the chemical properties of the water cooling pump shaft material, are presented in Table 1. Nowadays, the components of a rolling bearing are typically made from martensitic hardened and low-tempered AISI 52100 bearing steel. Certain grades of this material exhibit tensile strengths (Rm) exceeding 2000 MPa and hardness values above 62 HRC [38, 39]. This type of steel is extensively utilized in various applications, including bearings in rotating machinery. It is also commonly used in valve bodies, pumps, fittings, high-load wheels, bolts, double head bolts, gears, and internal combustion engines. The composition includes elements such as carbon, chromium, iron, manganese, silicon, phosphorus, and sulfur, with particularly high levels of carbon and chromium. This grade of steel is known for its corrosion resistance, exceptional hardenability, and good machinability. When Table 1 was examined, no compounds that could cause damage were detected in the chemical analysis of the material.
The chemical analysis with the surface was performed to qualitatively identify the damaged water cooling pump axle alloy and confirm the presence of any other associated components. No components that could cause damage were detected from the chemical analysis.
3.2. Hardness analysis
The hardening takes place in the outermost part of the part and it reaches the appropriate hardness towards the center of the axis. It was determined that the hardness was the highest on the outer surface with a value of 830 HV, and it decreased towards the center of the axis and reached a core hardness of 414 HV (Figure 2). If the outer surface becomes too hard, it makes the material brittle. The inner surface of the material is softer but comparatively harder than most similar materials. This makes the material extremely brittle.
3.3. Macroscopic examination
In the fracture surface examination, macro photos were taken and examined. Figure 3a shows the macro photo of the fracture surface. The damage started due to the intense corrosion deposit at the crack initiation indicated by the red line in Figure 3a.
Two different morphologies were detected in the fracture surface examination. A smooth plateau region about 180 degrees from the rough area and fibrous morphology spreading from the central area to the rough area were observed. The vertical geometry of the fracture indicates the fatigue propagation.
The presence of a flat plateau characteristic of fatigue propagation favours fatigue fracture. A fibrous structure was observed as the crack progressed along the section. Finally, the small rough region is the region where it can no longer carry the load, and sudden breakage is observed. It is possible to determine the initiation and propagation of the damage and the type of stress by examining the visual characteristics of the fracture damage. These types of damage correspond to rotational bending fracture at low load (Figure 3b) [40].
3.4. Micro-structure analysis
Micro-structure images taken from the middle regions of the material are given in Figure 4a. Spherical cementite phases are seen here. In addition to the properties of the phases, their shape also affects the mechanical properties. When the hard and brittle cementite structure is spherical, the phase boundary area per unit volume decreases. This results in a soft and low-strength structure. As seen in Figure 4b, decarburization and carbon segregation are observed in certain regions from the surface to the centre at the processing temperature.
a) images taken from the middle regions of the material and b) microstructure images of a soft and low strength structure.
In this case, ferrite-rich regions are often observed in the agglomeration regions. In these regions, it is also possible to observe plastic deformation regions similar to the dense layered structure stacked on top of each other [41]. During hot working, there may also be scale submersion or crack propagation in such agglomeration zones at a certain depth inward from the surface in the section. Both formations can essentially reduce the fatigue strength of the workpiece and cause the workpiece to be damaged in a shorter time, especially under cyclic loads.
3.5. SEM (scanning electron microscope) and EDX analysis
Figure 5a shows the intergranular morphology in the final part of a fatigue failure. In the literature review, it was seen that DAS et al. [42] reached a similar conclusion. In their study, it was stated that the rupture of the intergranular structure plays an important role in controlling the initiation and propagation of corrosion fatigue cracks (Figure 5b).
a) SEM image of cracked surface with crack filling showing intergranular fracture b) SEM image of crack initiation and propagation.
While SEM images were taken from the damaged circulating sample surface, spatial EDS analysis was also performed. The elemental analysis of the fractured surface is shown in Figure 6. As it can be understood from the EDX analysis, oxidations occurred in the fracture regions. The data obtained from the EDX analysis gave similar elementary results to the chemical analysis composition.
3.6. Modelling and finite elements analysis
The Solidworks program was used to model the damaged part of the water cooling pump. The drawing was created as a solid by taking measurements from the damaged part with the help of a digital caliper. The design and detailed dimensioned view are given in Figure 7.
In Figure 8, the distribution of the von-Mises stress values of the loading and boundary conditions, the finite element network, and the water cooling pump shaft is shown for the original design condition. The variable element mesh model was applied to the mathematical model. Thus, critical regions were divided into more frequent elements. Tetrahedral structure was used as mesh. The total number of nodes and elements was 23707 and 93669, respectively. In the analysis, 52100 steel was selected as the material for the boundary conditions. The pump part of the shaft was fixed and a force was applied to the gear part to create a similar force on the motor. Thus, rotational movement is provided.
Boundary conditions and Von Mises stress distributions of the circulation pump damaged by fracture.
Figure 8 shows the distribution of the Von Mises stress values of the water cooling pump shaft for a damaged design condition. The blade side (pump side) of the water cooling pump shaft was fixed in the analysis. A force was applied to the gear part, which was moved from the engine by the belt, in such a way as to make the tooth surfaces turn (Figure 8). In addition, rotational motion was given over the entire shaft. As shown in Figure 8, it has been determined that the maximum Von Mises stress at the critical fracture zone of the water cooling pump shaft for the original design condition and the damaged design condition is σ = 210 MPa.
Consequently, the water cooling pump shaft should have an infinite life at this loading for the original design condition. However, from the above observations, the fact that the fracture-damaged area and the high-stress concentration region of the water cooling pump shaft occur in the same parts as the damaged crack origin proves to affect the reason for the failure. The damaged fracture region acts as a stress enhancer and may be an additional factor in the present situation. Fatigue fracture almost always occurs in parts such as notches, cracks, or other stress concentrations. In the analysis, it was seen that the maximum stresses occur in the critical section transition region. It should be considered that this condition causes high-stress concentration and may lead to the initiation of fatigue cracking during operation.
4. CONCLUSIONS
In this study, it was observed that intergranular morphology coalescence occurred due to fatigue, but dense corrosion deposit was observed at the crack initiation. Corrosion is one of the main causes of damage to mechanical parts [43]. Spherical cementites were observed in the optical microscope images of the material. This makes the outer surface of the shaft hard. However, the internal structure remained softer due to the surface hardening process. Ferrite-rich regions have been detected towards the inner parts of the material. It is possible to observe plastic deformations in these regions. Both formations can essentially reduce the fatigue strength of the workpiece and cause shorter damage to the workpiece, especially under cyclic loads. Additionally, decarburization and carbon separation [44] have been observed in some regions due to the hardening process in the material.
When the SEM images of the material are examined, the presence of intergranular fracture supports the fact that the production is defective. It has been determined that the pump shaft material is AISI e 52100 alloy steel. However, the cyclically varying stresses cause some wear on the material’. Thus, the rupture event occurs well below the static limits. At variable stress, rupture begins at a point of discontinuity in the inner structure or outer surface. Around this point, the material fatigues, and a crack occurs. Over time, this crack deepens, eventually exceeding the tensile strength limit in the region outside the crack and causing the element to break suddenly.
It has been concluded that the damaged circulation pump reduces the fatigue strength of the pump shaft due to problems in the material’s internal structure during production. In the stress analysis, the critical region of the material and the fracture region overlap. It has been concluded that the fatigue strength of the material decreases due to microstructure problems during production, and the shaft breaks due to fatigue in a short time at lower loads due to the corrosion effect and varying cyclic stresses.
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