This article focuses on the fracture failure of engine connecting rod bolts. Through a series of experimental methods such as macro and micro observation of fracture surfaces, metallographic structure inspection, hardness and tensile property testing, thread size measurement, and chemical composition analysis, the fracture causes of connecting rod bolts are deeply analyzed. The results show that the fracture of the bolts is mainly due to fatigue and shear overload, and the root cause is bolt loosening, which is related to insufficient pre – tightening force during assembly and small thread diameters. Based on this, corresponding preventive measures are proposed to improve the reliability and safety of engine connecting rod bolts.
1. Introduction
Threaded fasteners are indispensable mechanical parts in engines. Engine components are connected in an orderly manner through threaded connections, accounting for approximately 70% of all connections in engines. In the operation of engines, connecting rod bolts are crucial components. They are subjected to complex alternating loads, including the inertial force of the piston – connecting rod reciprocating motion and the centrifugal force of the connecting rod rotation. In addition, during the compression and explosion strokes of the cylinder, they are also impacted by thousands of alternating stresses per minute. Therefore, the failure analysis and prevention of connecting rod bolts are of great significance for ensuring the normal operation of engines and the safety of mechanical equipment.
A supercharged gasoline engine underwent a bench test. During the test, when the engine speed reached approximately 4 kr/min, white smoke suddenly ejected from both sides of the engine block, followed by an open fire on the exhaust side of the engine, and the bench was urgently stopped. Inspection found that there were damaged openings on the intake and exhaust sides of the fourth cylinder of the engine, and the connecting rod caps and bolts were fractured, while other components remained intact. The oil temperature during the test did not exceed 100 °C. The material of the connecting rod bolts is 40Cr, with a surface oxidation treatment and a strength level of 10.9. The technical requirements include a hardness of 35 – 40 HRC, a tensile strength MPa, and a yield strength of 940 – 1070 MPa. This study aims to determine the fracture nature of the bolts, analyze the fracture causes, and propose preventive and improvement measures through a comprehensive experimental and analytical approach.
During operation, the connecting rod is mainly affected by four forces: the gas force acting on the piston, the inertial force of the piston assembly, the inertial force of the connecting rod, and the pre – load (including the pre – tightening force of the connecting rod bolt during assembly and the pre – tightening force of the piston pin due to interference fit). Connecting rod bolts mainly bear the pre – tightening force during assembly, which is a static load with constant magnitude and direction. In a four – stroke engine, they also bear the alternating loads generated by the reciprocating inertial force of the piston – connecting rod and the centrifugal force of the connecting rod rotation. For example, when the piston is at the top dead center during the expansion process, if the gas pressure is greater than the sum of the reciprocating inertial force and the centrifugal force, the connecting rod is compressed, and the effect of the reciprocating inertial force and the centrifugal force on the connecting rod bolt is zero. However, when the piston is at the top dead center during the intake process, these two forces act to stretch the connecting rod bolt. Moreover, when the wear gaps of the bearings at the large and small ends of the connecting rod increase, the connecting rod bolts also bear impact loads. Due to the change in the position of the connecting rod, the force on the bolts also changes, making the force on the connecting rod bolts complex. The fracture of connecting rod bolts during engine operation can lead to serious accidents, damaging the crankcase, crankshaft, and connecting rod, and even endangering personal safety. Therefore, connecting rod bolts not only need to have an appropriate pre – tightening force to ensure a tight fit between the connecting rod body and the connecting rod cap but also need to operate reliably under high – stress conditions to avoid fatigue damage.
3. Experimental Process and Results
3.1 Macroscopic Observation
The appearance of the damaged connecting rod is shown in Figure 2a. Both the connecting rod and the connecting rod bolts are fractured, and severe collision damage marks can be seen around the connecting rod cap and the joint surface of the connecting rod. During the experiment, the two fractured bolts were named 1# and 2# bolts respectively. The appearance of the 1# bolt damage is shown in Figure 2b. The bolt fractured at the smooth rod part at the joint surface of the connecting rod. The fracture surface size is significantly smaller than the bolt hole diameter, indicating that the bolt has necked. According to the differences in the macroscopic characteristics of the fracture surface, the fracture surface is divided into four regions: Region I is located outside the large – end hole of the connecting rod. This region has a small area, and the fracture surface is perpendicular to the bolt axis, flat, smooth, and crescent – shaped, with arc – shaped characteristics. Adjacent to Region I and inward is Region II. This region is also basically perpendicular to the axis and is rougher than Region I. The regions on both sides of Region II at a 45° angle to the axis are Region III, which is a typical shear lip. Region IV is located on the opposite side of Region I, approximately at a 45° angle to the axis. The fracture surface of this region has severe friction and visible high – temperature oxidation color, as shown in Figure 2c. The fracture surface was taken out from the bolt hole by wire cutting, and its morphology is shown in Figure 2d. Significant plastic deformation occurred near the fracture surface, with obvious necking.
The 2# bolt did not fracture at the joint surface of the connecting rod but at the 4th and 5th threads. The fracture surface is approximately at a 45° angle to the axis, and visible bending deformation from the inside to the outside can be seen on the smooth rod part, as shown in Figure 3a. The macroscopic morphology of the fracture surface is shown in Figure 3b. The fracture surface is severely worn, and a metal extrusion morphology can be seen on the inner side of the large – end hole of the connecting rod. The nut of the bolt has come off, and both the deformation and collision damage are severe, as shown in Figure 3c, which is presumably caused by the collision with the cylinder block after coming off.
3.2 Microscopic Observation
A JSM – 5600LV scanning electron microscope was used to observe the fracture surfaces of the bolts microscopically.
- 1# Bolt Fracture Surface: The low – magnification morphology of Region I is shown in Figure 4a. It is crescent – shaped, with a maximum crack propagation depth of approximately 0.5 mm and obvious arc – shaped characteristics. The low – magnification morphology of the source area is shown in Figure 4b. The crack originated from the bolt surface, showing a linear source characteristic, and no obvious metallurgical or processing defects were observed in the source area. The high – magnification morphology of Region I shows fatigue characteristics, as shown in Figure 5a. The fracture surface of Region II is rough, and the high – magnification morphology shows equiaxed dimple fracture characteristics, as shown in Figure 5b. The fracture morphology of Region III shows shear dimple characteristics, and the fracture surface of Region IV shows friction morphology, with shear dimple fracture characteristics under high magnification.
- 2# Bolt Fracture Surface: The area near the inner side of the large – end hole of the connecting rod on the 2# bolt fracture surface shows a metal extrusion morphology, as shown in Figure 6a. Under high magnification, friction marks can be seen, and the direction of the marks is consistent with the extrusion direction, as shown in Figure 6b. The unworn areas of the fracture surface all show shear dimple fracture characteristics, as shown in Figure 6c.
3.3 Thread Size Measurement
An unused bolt and a used bolt (from the same engine as the faulty bolt but from other cylinders) were selected for thread size measurement. The results are shown in Table 1. It can be seen that the major diameter, pitch diameter, and minor diameter of the external threads of both bolts are smaller than the standard requirements, and the external thread size of the used bolt is slightly larger than that of the unused bolt. The pitch diameter and minor diameter of the internal threads are larger than the standard requirements, with the minor diameter exceeding the standard value by a relatively large margin.
| Bolt | Large diameter | Medium diameter | Small diameter |
|---|---|---|---|
| Outside screw – Unused units | 8.886 – 8.892 | 8.277 – 8.288 | 7.840 |
| Outside screw – Used units | 8.923 – 8.936 | 8.298 – 8.307 | 7.860 |
| Technical requirement | 8.996 – 9.000 | 8.346 – 8.350 | 7.913 – 7.917 |
| Inside screw – Unused units | – | 8.426 – 8.528 | 8.356 – 8.408 |
| Inside screw – Used units | – | 8.456 – 8.473 | 8.166 – 8.477 |
| Technical requirement | 9.000 – 9.004 | 8.350 – 8.354 | 7.917 – 7.921 |
| Table 1: Thread Size Measurement Results |
3.4 Metallographic Inspection
Metallographic specimens were cut perpendicular to the bolt axis near the fracture surfaces of the 1# and 2# bolts and from an intact bolt (from the same engine as the faulty bolts but from other cylinders). After grinding, polishing, and etching, they were observed under a metallographic microscope. The metallographic structures of the fractured bolts and the intact bolt were uniform, all being tempered sorbite, as shown in Figure 7. No abnormal structures were observed in the fractured bolts.
3.5 Hardness Testing
The fractured bolts and the intact bolt were tested for Vickers hardness. The results are shown in Table 2. It can be seen that the hardness values of the 1# and 2# bolts are close to that of the intact bolt, all meeting the technical requirements (30 – 40 HRC).
| Position | 1 | 2 | 3 | Average | HRC |
|---|---|---|---|---|---|
| 1#bolt | 385.75 | 377.24 | 374.48 | 379.16 | 38.7 |
| 2# bolt | 377.24 | 366.35 | 365.01 | 369.53 | 37.7 |
| Whole bolt | 365.00 | 362.36 | 380.06 | 369.14 | 37.5 |
| Table 2: Hardness Testing Results of Bolts |
3.6 Tensile Property Testing
Two non – standard tensile specimens were cut from two bolts on other cylinders of the same engine as the faulty bolts and were tested for tensile properties at room temperature on a 4507 – type tensile machine. The results are shown in Table 3. It can be seen that the tensile strength of the bolts meets the technical requirements ( MPa), and the yield strength is higher than the technical requirements (940 – 1070 MPa).
| Sample | /MPa | /MPa | 1% | /% |
|---|---|---|---|---|
| 1# | 1213 | 1105 | 12.7 | 51.6 |
| 2# | 1212 | 1116 | 12.8 | 51.2 |
| Table 3: Tensile Testing Results of Bolts |
4. Analysis and Discussion
The fracture surface observation results show that there is a small “crescent – shaped” area on the outside of the large – end hole of the 1# bolt fracture surface. This area is flat, smooth, and shows arc – shaped characteristics, with fatigue characteristics microscopically. The fracture surface of the 2# bolt is approximately at a 45° angle to the axis, and visible bending deformation from the inside to the outside can be seen on the smooth rod part. The overall microscopic morphology of the fracture surface is shear dimple. These characteristics indicate that the fracture nature of the 1# bolt is fatigue fracture, and the fracture nature of the 2# bolt is shear overload fracture. The performance tests and metallographic structure analysis of the bolts show that the structures of the two connecting rod bolts are the same as that of the intact bolt, all being tempered sorbite, and the mechanical properties meet the standard requirements. Therefore, the fracture of the bolts is not related to the material.
From the macroscopic observation, it can be seen that the nut of the 2# bolt has fallen off, and the fracture position of the bolt is not at the joint surface of the connecting rod but at the 4th and 5th threads. This indicates that the 2# bolt became loose during operation. Once the 2# bolt became loose, the tensile load borne by the 1# bolt increased accordingly. When it exceeded the yield strength of the bolt, the bolt would elongate axially, which is confirmed by the obvious necking around the 1# bolt. After the 1# bolt elongated, the force on the connecting rod was no longer in the vertical direction, that is, it was deflected, causing the bolt to be subjected to an additional bending load. The direction of the load was from the outside of the large – end hole of the connecting rod to the inside (for the 1# bolt). As a result, fatigue cracks originated on the outside of the 1# bolt and propagated towards the inside of the large – end hole. Judging from the fracture surface characteristics of the 1# bolt, most of the fracture surface is the instantaneous fracture area, and the fatigue area is small, indicating that the fatigue stress is large and the fatigue propagation life is short. That is, after the 1# bolt originated cracks, it fractured in a short time. At the moment of the 1# bolt fracture, the 2# bolt was subjected to a large bending load from the inside to the outside, resulting in shear fracture of the 2# bolt. Therefore, the high – magnification morphology of the 2# bolt fracture surface is shear dimple, belonging to overload fracture.
In summary, the fracture process of the connecting rod bolts is as follows: the 2# bolt and nut became loose → the 1# bolt elongated → the 1# bolt originated fatigue cracks → the 1# bolt fractured → the 2# bolt sheared and fractured.
The loosening of the 2# bolt may be related to insufficient pre – tightening force during assembly. Insufficient pre – tightening force during assembly leads to loose fixation, causing the nut to loosen, and making the connecting rod bolt bear excessive impact force. At the same time, the contact between the heads of the connecting rod bolts, the nuts, and the supporting surfaces of the connecting rod is uneven, causing the connecting rod bolts to be subjected to a deflected force. Insufficient pre – tightening force of the 2# bolt during assembly leads to uneven tightening of the two bolts on one connecting rod, resulting in uneven stress on both sides. The increased stress on the 1# bolt causes it to originate fatigue cracks.
On the other hand, the thread size measurement results show that the major diameter, pitch diameter, and minor diameter of the external threads of the bolts on other cylinders of the same engine as the faulty bolts and the unused bolts are all small, and the pitch diameter and minor diameter of the internal threads are large. This makes the nut more likely to loosen, which is also related to the loosening of the bolts.
