Fatigue Performance of Hot-Formed Automotive Antiroll Bars ^†

Abstract

This study investigates the fatigue life of heat-treated and shot-peened antiroll bars used in heavy-duty truck suspensions. These components enhance vehicle stability by increasing torsional stiffness. Experimental tests, including microstructure inspection, hardness, and roughness measurements, assess material properties and manufacturing effects such as decarburization and shot peening. Subjected to multiaxial proportional loading, the bars present complex fatigue behavior. The study focuses on fatigue life influencing factors and the determination of S-N curves for the fatigue-based design of full-scale components.

1. Introduction

The automotive industry’s efforts to reduce manufacturing costs and vehicle emissions have led to the adoption of lightweight, high-strength spring steels and advanced manufacturing processes for components, such as anti-roll bars. These bars typically undergo a comprehensive manufacturing process consisting of hot forming, quenching, tempering, and shot peening to enhance their fatigue life [1]. However, heat treatment can cause surface decarburization, degrading local mechanical properties [2,3], while shot peening improves fatigue life through residual stresses but may also promote crack initiation due to increased surface roughness [4,5].

The tested antiroll bar, used in heavy trucks, is a hot-formed cylindrical U-shaped component. It connects the left and right sides of the vehicle frame and mounts to the axle via elastomer bushings, as it is schematically presented in Figure 1. Its purpose is to reduce axle roll, especially during cornering or zigzag maneuvers, by increasing suspension torsional stiffness—hence the name “antiroll bar”. Due to its function, the bar is subjected to fully reversed loading (R = –1) [6,7]. Given its U-shape and loading conditions, it experiences multiaxial, proportional stresses from both cyclic bending and cyclic torsion.

Since modern literature still lacks studies on full-scale industrial components in conjunction with high-strength materials under multiaxial (here proportional) loading, this paper aims to present a way of implementing these loading conditions in a single load-cycle to failure curve appropriate for the design of corresponding full-scale components. To address this, a thorough characterization of the material state and properties in the core and the decarburized surface area has been performed to reveal the effect of material degradation. Fatigue tests under fully reversed loading at various levels were conducted and evaluated to determine fundamental experimentally verified S-N curves that can be used by design engineers for a practical and cost-effective, fatigue-based design approach applicable to stabilizer bars and similar components [8,9,10].

2. Material and Manufacturing

To meet strict requirements for weight, stiffness, and fatigue life, the antiroll bar is made from alloyed (Cr, V) high-strength spring steel and manufactured through the following process:

• Hot forming of the bar, decreasing the initial diameter from 49 mm to 48 mm
• Hot forming to a U-shape and cooling in oil at 60 degrees C.
• Tempering at 600 degrees C.
• Shot peening to increase endurance by introducing compressive residual stress up to a depth of approximately 300 μm.

Notice that the heat treatment process yields the desired martensitic structure but also induces decarburization on the top layers of the surface, and especially intense on individual spots. At first, samples were taken from the bar curvature, the most highly stressed area, to characterize the microstructure at both the core and surface regions. Figure 2 presents images taken by an optical microscope, indicating also a decarburized spot on the surface. (marked with black dashed line).

The image on the left-hand side of Figure 2 reveals a martensitic structure in the core, induced by the heat treatment, and thus, the Ultimate Tensile Strength (UTS) of the material is expected to be high, 1200 MPa or above [11]. Also, from the surface microstructure image on the right-hand side of Figure 2, a typical, intense decarburized spot can be identified (washed-out, white area), which leads to the conclusion that near the surface, the mechanical properties are degraded. Further investigations followed to quantify the effect of the heat treatment process on specific mechanical properties.

3. Surface Properties

3.1. Hardness

Vickers micro-hardness measurements were conducted to evaluate the hardness distribution from the surface towards the core, in small intervals, applying a load of 2.94 N (or 0.3 kgf) for 15 s. The results from micro-hardness measurements evaluate the above-mentioned statements, the martensitic structure in the core, and the degraded surface properties due to decarburization. In Figure 3, the hardness distribution is presented.

The low surface hardness of 375 HV confirms the presence of decarburized zones near the surface. At a depth of ~80 μm, hardness peaks at ~535 HV due to shot peening-induced work hardening. Deeper into the core (beyond ~200 μm), values stabilize around 500 HV, consistent with the martensitic structure’s properties [2,12], as shown in Figure 2. The decarburized depth can also be estimated based on methods in [13,14].

3.2. Roughness

Surface characterization was completed with roughness measurements, performed according to ISO 4287 [15] using a Taylor Hobson contact instrument with a 200 μm gauge range. Roughness values were calculated per DIN 4768 [16], focusing on the average roughness depth R_z. For this, the evaluation length was divided into five sections (i = 1–5), each 2 mm long, and R_z was calculated as the average of the total heights R_t,i from each section.

Across multiple specimens, the average value of R_z was 17.4 μm—typical for shot-peened, high-strength steel components used in suspension systems [8,17].

4. Fatigue Testing and Results

4.1. Fatigue Testing Principle

Fully reversed fatigue tests (R = –1) were performed on full-scale components. Loading was applied at the bar’s left and right tips, while mounting was achieved via two brackets with inner elastomer bushings positioned along the straight section, following the principle shown in Figure 1.

Finite Element (FE) analysis identified the curved sections as failure-critical due to peak major principal stresses. To validate these results, strain gauges were installed in the direction of the first principal stress at those locations, as shown in Figure 4.

The gauges were aligned ~2–3° from the vertical axis to match the estimated direction of the maximum principal stress. Due to the complex geometry and potential manufacturing variations, precise placement at the stress peak was technically challenging.

Moreover, the relation between the major principal stress on the failure critical area and the applied force was calculated and compared with the corresponding relation from FE calculations with negligible deviations. This relation was found to be linear for the range of the examined forces. This relation is visually presented in Figure 5 below, and the corresponding calculated slope values are mentioned.

4.2. Failure Behavior

Fifteen antiroll bars were tested under fully reversed force-controlled loading (minimum to maximum force ratio R = −1). Hereby, three different force levels were selected to cover the whole fatigue life range of technical interest, i.e., from approximately 10⁴ to 10⁶ cycles-to-failure. All specimens failed in one of the curvature areas, on or very close to the calculated highly stressed positions. Figure 6 shows a typical fracture, exemplarily.

On the front view of the fracture area (right image), the two separate areas (crack propagation area and brittle fracture area) can be identified. Crack initiates on the upper surface of the curvature area and then moves toward the core.

4.3. Fatigue Life Results and Load-Cycles Curves

The fatigue life results of the tested bars determined at three force levels are presented in Figure 7 below by means of circle symbols.

The measured data corresponds to the specimen fracture. However, it should be noticed that the crack propagation phase is small, relative to the number of cycles until the development of a small fatigue crack of ~1 mm length on the surface.

Typically, an S-N curve in the area of middle and high cycle fatigue (10⁴ < N < 10⁶) is expressed by the well-known Basquin’s law for a constant stress ratio [18]. Substituting the stress value with the corresponding applied force, the equation becomes

$${\mathrm{F}}_{\mathrm{a}}=\mathrm{A}{\left(2{\mathrm{N}}_{\mathrm{f}}\right)}^{\mathrm{k}}$$

(1)

where F_a stands for the force amplitude, A is the fatigue strength coefficient, and k is the fatigue strength exponent. Linear regression of the test data yields the median Force-Cycles to failure curve equipped with 50% survival probability (see the solid line in Figure 7), assuming a normal distribution. Lower-bound force-cycles to failure curves are used to reflect higher survival probabilities (e.g., 90% or 97.5%), derived by shifting the median curve using

$${\mathrm{Y}}_{\mathrm{P}}={\mathrm{Y}}_{50\%}-\mathrm{a}·\mathrm{s},$$

(2)

where Y_P is the target Load-Cycles to failure curve, Y_50% the median curve, a stands for the shift coefficient, and s for the standard deviation of the fatigue life. There are many suggestions for choosing the shift coefficient, such as the Owen tolerance limit [19] or according to the ASTM standard [20], but these are not covered here. Instead, a factor of a = 1.28155 for probability of survival P_S = 90%, and a =1.9755 for P_S = 97.5% according to the FKM guideline [6], were used for this study. Notice that the curve for P_S = 97.5% (dotted curve in Figure 7) is recommended in the FKM guideline [6] for fatigue design of metallic components, before applying any additional safety factor.

Furthermore, the width of the scatter band T_N is introduced, expressing the ratio of the cycles for 90% to 10% probability of survival (see the two dashed lines in Figure 7). The width of the scatter band amounts to T_N = 1:3.5, which is considered relatively high, since typical scatter values for suspension components made of high-strength steels usually lie between 1:2 and 2.5 [8].

5. Conclusions

The present study demonstrates (a) the extraction and processing of the design fatigue properties of hot-formed antiroll bars and (b) the characterization of the material core and surface of the component.

The characterization of the material using optical microscopy and micro- and macro-hardness measurement provided a valuable insight into material quality and an assessment of the mechanical properties without performing conventional tensile tests, which are not possible in the case of the very thin surface state. Surface roughness information may be used as a macroscopic factor of surface quality, since high roughness values have a negative impact on fatigue life [6,7].

Fatigue tests at various load levels have been performed and assessed to determine fundamental S-N curves, covering various probabilities of survival and the whole fatigue life range of practical interest. These curves can be directly used by design engineers for fatigue-based design of antiroll bars.

This study may be expanded by investigating analytical or computational fatigue life assessment, following existing guidelines and theory, such as the Nominal Stress Concept, Local Stress Concept, or the FKM guideline, as previous studies suggest [6,17,21,22].