The previous blog examined the performance of SAE 8615 simulated-carburized steel in monotonic tensile tests and compared its performance in axial and bending fatigue conditions.
This blog will explore the performance of SAE 8615 steel with a shallow-carburized case depth (defined as 10 percent of the test coupon’s radius, which is approximately 2.5 mm). The monotonic tensile properties will be compared to the simulated-carburized core condition. The material’s axial fatigue and bending fatigue performance (4-point bending condition) will also be examined, in which an additional pulse of periodic strain overload will be applied along with the nominal amplitude cycles. The periodic overload fatigue behavior will be useful to assess the performance of the steel in applications involving shock loads.
The coupons were prepared as per standard testing protocol and tested for monotonic tensile and fatigue performance. A periodic overload strain cycle was added to the axial and 4-point bending cycles, as shown in Figure 1.
Figure 1: Periodic Overload Fatigue Cycle
The load history in these tests consists of repeated load blocks with one fully-reversed overload cycle, followed by a group of smaller constant amplitude cycles having the same maximum tensile stress as the overload cycle. The overload cycles are applied at frequent intervals to maintain a low crack opening stress resulting in the subsequent cycles being fully open.
NOL denotes the number of overload cycles. The overload-strain amplitude is twice the amplitude of the subsequent small cycles. This high load simulates the real-life loading conditions of the material. εa,SC denotes the strain amplitude for small cycles and εa,OL denotes the strain amplitude for the overload cycles.
Monotonic Tension Tests:
Two specimens, machined to standard dimensions and subsequently polished, are tensile tested to determine the mechanical properties listed in Table 1. The stress-strain curve obtained from these tension tests are as shown in Figures 2 and 3.
Figure 2: Engineering Stress-Strain Curve
Figure 3: True Stress-Strain Curve
Table 1: Monotonic Tensile Properties
Table 1 also shows the values from simulated carburized core coupons. Comparing these values, carburization of 2.5mm case depth has increased the material’s yield strength by 22 percent, the ultimate strength by 14 percent and the strength co-efficient value by a staggering 114 percent. The true fracture strength has decreased 40 percent.
When the SAE 8615 carburized steel is subjected to cyclic strains, it first progresses through a transient response stage before stabilizing to exhibit a constant response (with time), a property called cyclic stabilization. Figure 4 shows superimposed monotonic and cyclic stress-strain curves. Note the cyclic stress-strain curve is obtained after the stabilization occurred.
Figure 4: Composite Plot of Monotonic and Cyclic Stress-Strain Curves
From Figure 4, strength increases as the material is subjected to cyclic stresses, indicating a cyclically hardening property. In the previous blog, SAE 8615 without carburization was observed to cyclically soften. The added 2.5 mm deep carburized layer changed the material’s cyclic response.
Figure 5 shows the true stress versus reversals to failure and Figure 6 shows true strain versus reversals to failure. The parameters σf’, b, εf’ and c, which are calculated from these graphs, are used to characterize the cyclic properties of the material tabulated in Table 2.
Figure 5: True Stress Amplitude vs. Reversals to Failure
Figure 6: True Plastic Strain Amplitude vs. Reversals to Failure
Table 2: Cyclic Properties of SAE 8615 in Shallow- and Simulated-Carburized Condition
Comparing these values with the SAE 8615 simulated-carburized case, carburization of 2.5mm case depth has reduced the fatigue strength co-efficient, , by 12 percent. Whereas significant increases are observed in the values of cyclic strength co-efficient, cyclic yield strength and the fatigue limit values, which have gone up 53 percent, 38 percent and 29 percent respectively.
A response called the “effective” strain-life curve is obtained when the material is subjected to periodic overload strain-controlled fatigue tests. Figure 7 below shows the effective strain-life curve, along with the periodic overload data curve.
Figure 7: True Strain Amplitude vs. Reversals to Failure
The strain amplitude decreases with the increasing number of reversals to failure as commonly observed in all materials, shown in Figure 7. The overload data curve, having a different slope than the total fatigue curve, shows this trend is magnified with strain amplitude decreasing more rapidly with increase in the reversals to failure. It should be noted this curve is for the axial fatigue test of the material.
Bending fatigue with overload tests:
The true strain amplitude versus reversals to failure data when the shallow-carburized specimen was tested in 4-point bending is shown in Figure 8.
Figure 8: True Strain Amplitude vs. Reversals to Failure
A comparison between the curves from equivalent axial and constant-amplitude 4-point bending tests is shown in Table 3 and from overload axial and overload 4-point bending tests is shown in Table 4.
Table 3: Equivalent axial vs. CA 4-point bending values at different reversals to failure (lives)
Table 4: Overload Axial vs. Overload 4-point Bending Values at Different Reversals to Failure (lives)
While there is no significant change in strain-amplitude levels in the equivalent axial and constant amplitude 4-point bending tests, there is a big difference in the overload case. In the overload case, for 1 x 104 reversals, the strain amplitude the materials can sustain is the same in axial and bending cases. However, for 1 x 105, 2 x 106 and 1 x 106 reversals, a decrease of 29 percent, 70 percent and 75 percent in strain amplitude is seen, with the overload axial specimen being able to withstand significantly higher strain amplitudes compared to the bending samples.