EN-GJS-500-7 nodular cast iron reached a completely reversed axial fatigue strength of 204 MPa, establishing a measured endurance ratio of 0.41 against its 500 MPa nominal tensile-strength grade [S1].
Fatigue design must therefore distinguish gray, compacted graphite, and spheroidal graphite iron, account for mean stress and notches, and avoid treating the endurance ratio as an invariant material constant [S1][S3][S4].
What the Fatigue-to-Tensile Strength Ratio Represents
The endurance ratio is the long-life fatigue strength divided by the ultimate tensile strength, and it is useful only when both values refer to comparable material, geometry, loading, and test conditions [S4]. A ratio above or below another iron does not by itself establish superior fatigue resistance, because graphite morphology, matrix structure, casting defects, residual stress, and surface condition can dominate the result [S3][S6].
For EN-GJS-500-7 specimens cut from an off-highway axle, the completely reversed stress ratio R = -1 produced a mean high-cycle fatigue strength of 204 MPa with slope parameter k = 8.87 [S1]. Dividing 204 MPa by the grade designation of 500 MPa gives an indicative endurance ratio of 0.41, although the designation is not a substitute for the material's measured ultimate tensile strength. The same investigation recorded 124 MPa under a different mean-stress condition, demonstrating why a single unrestricted fatigue number is inadequate [S1].
Comparison Across Cast Iron Structures
Gray, compacted graphite, and spheroidal graphite irons occupy different fatigue regimes because graphite shape controls crack initiation and propagation: lamellar graphite promotes early crack formation, while spheroidal graphite generally supports greater ductility and fatigue capability [S3][S6]. Compacted graphite occupies an intermediate design position in terms of graphite morphology, but its fatigue strength must still be tied to matrix condition and applied loading [S3].
Across the three main structures, the key comparison is not tensile strength alone. Gray iron generally favors casting, damping, and compressive or moderate-load applications but is less tolerant of severe cyclic loading; compacted graphite iron is suited to more demanding cyclic service with greater strength and ductility than gray iron; spheroidal graphite iron offers the broadest fatigue-capable range and is used where shock, bending, or high-cycle loading dominates [S3]. For the component designer, the decisive criteria are fatigue strength at the actual stress ratio, notch sensitivity, defect tolerance, and consistency between laboratory and production properties [S1][S6].
Heat treatment further separates the grades. In FCD450 spheroidal graphite iron, quenching and tempering increased pearlite fraction, yield strength, tensile strength, and hardness while reducing ferrite, elongation, and toughness [S5]. That trade-off is important: a higher tensile strength after heat treatment does not guarantee a proportionally higher endurance ratio, because fatigue is sensitive to microstructural defects and mean stress [S5][S6].
Selecting a Ratio for Axles and Structural Components
For an axled application, use the measured fatigue curve rather than a generic tabulated ratio when the component contains geometric discontinuities. EN-GJS-500-7 axle research covered plain, blunt-notch, and sharp V-notch specimens under constant-amplitude axial loading at R = -1 and pulsating tension [S1]. The study covered prior work on EN-GJS-400, EN-GJS-450, EN-GJS-600, EN-GJS-700, and EN-GJS-900, but those grades are not interchangeable merely because their fatigue behavior was studied in the same literature [S1].
The local strain-energy-density method used in the axle study correlated both geometrical notch effects and mean-stress effects, producing a master curve for EN-GJS-500-7 [S1]. That result supports a practical selection rule: use an endurance ratio as a preliminary material screen, then replace it with strain-life data, a local damage method, or component-specific fatigue testing when geometry and load history control the result. Off-highway axles face variable amplitude, mean stress, overload, and several stress concentrations, conditions under which a smooth-specimen ratio can be badly misleading [S1].
The same reasoning applies to the material families described in the cast iron reference. Gray, compacted graphite, and spheroidal graphite grades can share similar nominal strength classes while exhibiting materially different fatigue behavior because their crack-initiation mechanisms differ [S3].
Mean Stress Can Change the Apparent Ratio
Tensile mean stress reduces fatigue life, while compressive mean stress increases it, so the stress ratio must accompany any fatigue-to-tensile ratio [S5]. In FCD450 iron, modified Goodman, Gerber parabola, Morrow, Smith-Watson-Topper, and Walker equations were compared against a baseline distribution. Smith-Watson-Topper showed the closest agreement, followed by modified Goodman, Morrow, Gerber parabola, and Walker [S5].
For example, the 204 MPa value at R = -1 cannot be multiplied by a tensile-strength ratio and treated as valid for R = 0 or R > 0 [S1]. The EN-GJS-500-7 study found a strong mean-stress effect, with fatigue strength falling to 124 MPa under another tested loading condition [S1]. An engineer selecting a design allowable should therefore state the load ratio, notch geometry, specimen orientation, and surface condition alongside the numerical ratio.
This matters for rotating or oscillating components where tensile and compressive phases are unequal, and for structures exposed to overload during service. The appropriate fatigue metric is not an unqualified fatigue limit but the allowable alternating stress after correcting the actual mean stress with a suitable model [S5].
Heat Treatment, Defects, and Production Variability
Fatigue strength in ductile iron is influenced by small defects such as graphite nodules and microshrinkage cavities, so internal soundness can override a nominal endurance ratio [S6]. The FCD450 heat-treatment study confirms that changing ferrite and pearlite fractions shifts tensile properties, elongation, and toughness, not just static strength [S5]. A gray iron study also found that increasing austempering temperature increased fatigue strength while reducing tensile strength, demonstrating that tensile strength and fatigue strength do not always move in the same direction [S2].
For procurement and design, request fatigue data tied to the production condition: grade, graphite morphology, heat treatment, specimen location, machining or casting surface, stress ratio, and cycle count. Do not transfer a ratio from a polished laboratory specimen to a rough casting or machined component without validation. This is especially important for alloy steel comparisons, where changing material family changes the defect population, notch response, and heat-treatment response rather than merely changing the numerator of a ratio.
For production acceptance, fatigue scatter should be treated as a quality variable, not hidden inside one nominal ratio. The truck-component study evaluated fatigue properties in as-cast spheroidal graphite irons, confirming that component geometry and as-cast condition are part of the fatigue assessment [S7].
Where the Ratio Is Useful and Where It Fails
The ratio is useful for first-pass material selection, comparing like-for-like fatigue data, and detecting a casting or heat-treatment process that has changed fatigue behavior disproportionately. It is not a substitute for ultimate tensile strength, a fatigue curve, or a notch factor, and it should not be applied across different graphite structures without supporting data [S3][S4].
A defensible workflow has four stages. First, define the relevant grade and microstructure, whether gray, compacted graphite, or spheroidal graphite iron. Second, obtain tensile strength, fatigue strength, stress ratio, surface condition, and cycle count from the same material condition. Third, adjust for notch severity and mean stress, using component-specific local methods where geometry controls initiation. Fourth, validate the selected allowable with production evidence, including defect sensitivity and scatter [S1][S5][S6].
For engineers making a coarse preliminary estimate, the EN-GJS-500-7 result provides a concrete anchor of about 0.41 at R = -1, not a universal design ratio [S1]. For gray and compacted graphite iron, use the cited fatigue-property comparison as the starting point, then confirm the selected value with representative testing [S3].
The next trackable engineering signal is the fatigue result under the actual mean stress, including the notch and surface condition; a shift from the 204 MPa fully reversed baseline toward 124 MPa under the tested mean-stress condition shows how quickly a nominal endurance ratio can lose engineering meaning [S1].