Predicting the durability of piston engine crankshafts

A method for testing crankshafts of automobile and tractor engines under the action of variable bending moment is proposed. The test machine consists of a frame on which the crankshaft is installed through elastic springs on the extreme root necks. On the extreme necks, pendulums are installed, one end of which is rigidly fixed, and at the opposite ends of the pendulums, two inertial (unbalanced) vibrators are installed, creating two equal forces in the horizontal plane, directed in opposite directions and changing their magnitude. This pair of forces creates a variable bending moment relative to the axis of the crankshaft, acting on all its elements, and causes fatigue failure in the weakest sections. Based on the test results, a graph is plotted in the coordinates of maximum stress – the number of loading cycles before failure, which allows you to determine the parameters of the crankshaft fatigue life model. According to the corrected linear hypothesis of fatigue damage accumulation, shaft failure occurs when the sum of accumulated damage is equal to a certain value, which is determined by the results of fatigue tests.


Introduction
The main assumption, which is usually accepted in all theories for calculating the durability of a part under the action of a load spectrum, is that the effect of cyclic stresses of a given amplitude leads to fatigue damage, the value of which is determined by the number of cycles of stress of this amplitude, as well as the total number of such cycles before the destruction of the sample. As applied to crankshafts, it can be assumed that in the most loaded sections (usually this is the transition zone from the connecting rod neck to the cheek along the overlap line), the crystal lattice distortions occur, leading to the appearance of tears in some direction in which the greatest stresses act, which gradually pass under the influence of variable stresses into the crack. Under the influence of alternating stresses, the crack grows. When the total accumulated damage reaches a certain critical value, and the crack significantly weakens the cross-section, fatigue failure occurs.

Purpose of research
Development of a method for predicting the durability of piston engine crankshafts based on fatigue tests of full-scale samples

Materials and methods of research
In [1,3,4], the possibility of predicting the cyclic durability of crankshafts based on the results of fatigue tests is shown. Figure 1 shows an approximate graph of fatigue life for details [5,6]. Stress V-1No -the value of the endurance limit at the point of fracture A-for steel at N = 2•10 6 ...5•10 7 cycles. Its presence is associated with a IOP Publishing doi:10.1088/1757-899X/1086/1/012032 2 change in the mechanism of predominant development of fatigue. At a high stress level, fatigue failure occurs at N < 10 5 cycles as a result of the accumulation of plastic deformations along the shear planes. This is lowcycle fatigue, where the patterns of static destruction are more pronounced. If the destruction occurs when the number of cycles N = 2•10 5 ...5•10 6 , then in this case, the regularities of fatigue destruction appear [5,7,8]. At a low stress level (along the straight line AD), diffusion processes of dislocation movement develop. However, to obtain a section of the fatigue curve, time-consuming and lengthy tests are required [7,8,9].
The diagram of the installation for fatigue testing of crankshafts, developed under the guidance of the author, is shown in Figure 2. the establishing consists of a frame 1, on which the crankshaft 3 is installed on the extreme root necks through elastic springs 2.
On the same necks, pendulums 4 are installed, one end of which is rigidly fixed to the necks, and at opposite ends of the pendulums, two inertial (unbalanced) vibrators 5 are installed to create cyclic alternating loads, rotating with the same angular velocity Z 2 and creating two equal forces P = mrZ 2 in the horizontal plane.
On the same necks, pendulums 4 are installed, one end of which is rigidly fixed to the necks, and at opposite ends of the pendulums, two inertial (unbalanced) vibrators 5 are installed to create cyclic alternating loads, rotating with the same angular velocity Z and creating two equal forces Р = mrZ 2 in the horizontal plane [6].
Here m is the mass of the unbalanced balance weight of the vibrator located at the radius r; Z is the frequency of its rotation. The components of forces on the horizontal x-axis are equal to Рg = mrZ 2 sinM, and on the vertical y -axis -Рv = mrZ 2 cosM (M = Zt is the angle of rotation when the unbalanced mass m rotates around the vertical y axis, t is the current time).
In this case, the vertical component of the inertia force from the one balance weight load ( Figure 2) Рv= РcosM it is balanced by a similar component from the second balance weight, and the forces from the vibrators 5 on the left and right create equal and oppositely directed forces 2РsinM. Thus, the crankshaft is loaded with a pure bend in a symmetrical cycle with a moment , where l is the length of the pendulum 4.for the same type of fatigue testing of crankshafts, executive document RD 23.3.62-89 "Methodology for accelerated fatigue testing of crankshafts of tractor and combine engines" was developed [5,6]. To control the loading on the crankshaft rod journal in the section plane of the first crank pasted strain gauges 6, which are connected to the measuring instrument 7. Before the beginning of tests conducted calibration of the strain gages mounted on the crankshaft, which for both ends of the pendulum 4 (instead of system loading 5 - Figure 2) a static load was applied. The stand for calibration of strain gauges consisted of a rigid frame, supports in the form of prisms, a hydraulic Jack and a compression dynamometer DOSM-3 [4]. The appearance and growth of a fatigue crack in the test knee of the crankshaft is recorded by increasing the value of the signal received from the strain gages to the control and measuring equipment. The appearance of a crack in the cross section of the crankshaft reduces the frequency of its natural vibrations Zs, and since the tests were carried out in the resonant mode, the vibration amplitude increased. Tests are terminated if the signal value increases by 15% from the original value [10,11]. The crankshaft can be installed in the test machine so that all the cranks are loaded simultaneously or each crank separately (Figure 3). the amount of bending moment acting in the n-th crank under test is monitored continuously throughout the entire test time. The control system consists of strain gages pasted on the connecting rod neck of the tested knee, a strain amplifier, and two oscilloscopes: electronic (for visual monitoring of the signal value from the strain gage) and loop (for periodic monitoring of the signal value after calibration under static loading). The drive of the vibrator 5 was carried out from a balancing machine.
The error in determining the controlled parameters during testing did not exceed the values indicated in table 1. At loading of each crank pure bending in its plane with time Mesh destruction must occur in the cross section of the least stiffness from the fillets interfaces connecting rod neck of crankshaft with a cheek with access to a fillet of the pairing crank journal and cheeks [12][13][14]. An example of the destruction of the crankshaft in this section is shown in Figure 4.
When testing, it is necessary to obtain at least three values of cycles . Since the value 0 N is selected in advance (usually taken 0 N = 2•10 6 ...5•10 7 cycles), such tests actually determine not only the maximum bending moment, but also the effect of the stress concentration in the fillet. In this case, it is quite simple to determine the effect of structural or technological measures on increasing the fatigue strength of the crankshaft [15][16][17].
4. The results of the study and their discussion As deterministic models for the fatigue life of the crankshaft at the loading point (uniaxial stress state) can take it [7,15] C N m a where Vа -the amplitude of the alternating voltage; N -the average number of cycles to failure; C and m -parameters of the test to the crankshaft, depending on the material, manufacturing technology and other factors. It is known that fatigue tests with the constant amplitude of variable stresses are characterized by a large variation in the number of cycles before failure. Therefore, in equation (1), we always mean the (2) The obtained values C and m allow us to find the endurance limit for the base number of cycles Nb = 210 6 or the value ) /( . Since the rated voltage can be calculated , the value of the parameter ratio ) /( is determined based on experimental research data, which is taken into account when calculating at the design stage [18,19,20] To determine the durability of the crankshaft in hours, assume that during the time ti of the engine in operation in one of the modes characterized by the value of the torque Мi and the angular speed of rotation of the shaft Zi during the entire service life (until destruction) T, it will be produced ni loading cycles, which for four-stroke engines is equal to -the total number of loading cycles. ai Q -the number of cycles of repeating amplitudes in the loading block. When using a limited endurance limit, you should assume: