Effect of Overloads on the Fatigue Performance of a Forged Microalloyed Steel

In an earlier post, the fatigue properties of a microalloyed steel, in the form of hot-rolled bar stock, were compared with the properties of the same microalloyed steel in the form of forged crankshafts. In the most recent post, the effects of overloads on the fatigue properties of the microalloyed steel hot-rolled bars were examined. In this post, the results of further studies, which were conducted to evaluate the effect of overloads on the fatigue properties of the microalloyed steel forged crankshafts, are presented. These results are also compared with those obtained from the hot-rolled bars.

The chemical compositions and mechanical properties of both the hot-rolled bars and the forged crankshafts were reported in the earlier posts. However for convenience, they are shown again here in Tables 1 and 2 below.

blog-post-27-table-1-and-2

The same steel grade was used for both product forms, and the mechanical properties are similar.

Similar to what was noted in the most recent post, to simulate the effects of overloads on the fatigue performance of the crankshafts, a fatigue testing protocol was implemented in which high-amplitude cycles are inserted between groups of low-amplitude cycles.  The test protocol is shown schematically in Figure 1.  As can be seen, the load history consists of repeated blocks, each consisting of one fully reversed overload cycle and a series of small cycles with the same maximum strain as the overload cycle.  An effective strain-life curve is determined for the small cycles, and then compared to results obtained under fully reversed constant-amplitude conditions.

no-27-fig-1-newFigure 1

The comparative strain life fatigue properties for the forged crankshafts for both constant amplitude and overload testing are shown in Figure 2.

no-27-fig-2-newFigure 2

As can be seen, the effects of inserting overloads into the loading history has the same effect of reducing fatigue life of the forged crankshafts as was observed earlier for the hot-rolled bars.  The data from the hot-rolled bars was combined with the data from the forged crankshafts shown in Figure 2 to produce the comparative graph shown below in Figure 3.

no-27-fig-3-newFigure 3

This fatigue life as a result of the overloads is quite similar for both the hot-rolled bars and the forged crankshafts, with a possible slight advantage to the hot-rolled bars.  As was noted earlier for the constant-amplitude testing, the hot-rolled bars showed a slight improvement over the forged crankshafts in the long-life region.  This was attributed to the slightly higher hardness and tensile strength for the hot-rolled bar stock.  In future posts, the effects of overloads will be examined for other combinations of steel grade and processing.

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2 Responses to Effect of Overloads on the Fatigue Performance of a Forged Microalloyed Steel

  1. Mark Veliz says:

    This is an excellent study but I do have some questions. Perhaps some detail was left off the post for the sake of brevity. From the loading chart it appears that the constant amplitude testing was conducted with a mean tensile strain. Was the testing performed with a constant R ratio, (min/max) or was it conducted with a constant max strain? Also, can you comment on the frequency of the overload cycles as related to the frequency of the standard cycles?

    • The aim in these tests is that the small cycles have no crack closure which is thought to give the most damaging cycle for a given strain amplitude. To derive the equivalent life for the small cycles the damage done by the large cycles is subtracted from unity and assigned to the small cycles in a test. The small cycles used in these test are elastic and the large cycles are elastic or nearly so. The test control condition is a maximum stress in all cycles equal to the maximum stress in the large cycle which is chosen as the constant amplitude fully reversed cycle giving a life of 10000 cycles. The number of small cycles per block is aimed at maintaining the damage done by the small cycles between 50 and 90 percent of the total damage. In this range there are sufficient large cycles to maintain a low crack opening stress level and still a large enough damage due to the small cycles to prevent undue scatter in the calculated equivalent lives. (If for example 90 percent of the damage were done by the large cycles scatter in the large cycle damage could result in negative calculated equivalent lives for the small cycles.) (Similarly very few large cycles would result in the suppression of crack closure in the small cycles not being maintained.)
      The number of small cycles per block is adjusted to maintain the above fraction of damage due to the small cycles by when necessary discarding a test which falls outside the desired fraction of small cycle damage and repeating the test with a different number of small cycles per block. After the first test meeting the criteria it is easy to increase the small cycles per block as the small cycle strain range decreases to stay within the allowable damage limits for the small cycles.

      These questions were answered by Professor Tim Topper at Waterloo University, Waterloo, Canada. Professor Topper was instrumental in developing the overload fatigue test protocol.

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