Examining TIFF through a series of case studies that intentionally push gearsets beyond their functional limits ­– studies that demonstrate how targeted validation, microgeometry control, and heat-treatment optimization can be used to diagnose, mitigate, and ultimately prevent subsurface fatigue failures.

This article presents a comprehensive framework for identifying and mitigating tooth interior fatigue fracture (TIFF) in straight-toothed, net-forged bevel gears used in automotive differentials. As modern drivetrains demand higher torque in increasingly compact and reliable packages, traditional gear design methodologies are being pushed to their limits. While surface- and near-surface-initiated failures are well understood and routinely addressed through established practices, subsurface failure modes such as TIFF require a more advanced understanding of material behavior and internal stress distributions.

Leveraging on real-world case studies, current industry practices, and academic research, this work defines effective strategies for diagnosing and addressing TIFF. Techniques including visual inspection, magnetic particle inspection (MPI), scanning electron microscopy (SEM), microgeometry optimization, and heat-treatment modifications are critically evaluated for their diagnostic and preventative effectiveness.

By consolidating proven techniques and engineering insights, this article serves as a practical guide for professionals encountering TIFF for the first time, as well as a technical reference for experienced engineers seeking to enhance their expertise. Advancing the understanding of complex failure modes like TIFF is essential to improving drivetrain reliability and supporting the continued evolution of high-performance gear systems.

1 Introduction

Modern vehicle drivetrains demand higher torque in smaller, more reliable packages, pushing traditional gear design methodologies to their limits. This challenge affects all drivetrain subsystems, particularly the automotive differential, which plays a critical role in distributing torque to the wheels. Within this context, straight-toothed, net-forged bevel gears are increasingly susceptible to a specific and less-understood failure mode: tooth interior fatigue fracture (TIFF).

Table 1: TIFF vs TFF according to AGMA 1010-F14[3].

Historically, gear design has focused on preventing surface-initiated failures such as root breakage and pitting. These issues are now routinely mitigated through advanced modeling techniques, optimized material selection, and surface treatments including case hardening, shot peening, and finish refinement. Sophisticated simulation tools further support these efforts by modeling elastic 3D, multi-body gear behavior to ensure stress levels remain within material limits.

As surface durability has improved, subsurface failure modes like TIFF have become more prominent. These failures are inherently more difficult to detect and often occur in designs that meet conventional performance criteria. TIFF is particularly challenging not because its solutions are complex, but because the failure mode is unfamiliar and can be easily misinterpreted, especially by those encountering it for the first time. This article provides a practical guide for identifying and addressing TIFF. It consolidates findings from literature, case studies, and industry experience to present proven diagnostic and mitigation strategies. By sharing these insights, the goal is to accelerate recognition and resolution of this critical failure mode, supporting both emerging and experienced engineers in advancing drivetrain reliability.

2 Identification

This section outlines the characteristics of tooth interior fatigue fracture (TIFF), how it can be identified, and where it most commonly occurs. TIFF is also commonly referred to as tooth flank fracture (TFF), or tooth flank breakage (TFB). These terms all describe a failure mode in which a crack initiates at below the active flank at approximately mid-tooth height.

The key distinction, according to most available literature, between TIFF and TFF/TFB lies in the appearance of the final fracture. TIFF typically produces a flat fracture plateau due to fully reversed loading conditions. This occurs in idler gears, where torque is transmitted in both directions, creating two potential crack initiation sites [1,2]. In the context of this article, the focus is on the pinion gear within the automotive differential, which experiences similar loading conditions and is particularly susceptible to this failure mode. In contrast, TFF/TFB is associated with single-flank loading and presents a different fracture morphology. The primary crack in TFF/TFB typically propagates at an angle of approximately 40-50 degrees relative to the tooth flank surface. These differences are illustrated in Figure 1.

Figure 1: TIFF vs TFF [10].

While TIFF is not a new failure mode, its occurrence has increased in recent decades as the industry continues to push for higher torque densities and as traditional surface failure modes are more effectively mitigated. Among the most widely used preventive measures is case hardening — a heat-treatment process that produces a hard, wear-resistant surface while maintaining a ductile core. This hardened layer introduces beneficial compressive residual stresses at the surface, enhancing resistance to bending fatigue, contact fatigue, and wear. However, these surface compressive stresses are counterbalanced by tensile stresses deeper in the tooth core. This stress gradient creates a transitional zone between compressive and tensile regions, which can become a critical site for subsurface crack initiation. TIFF most frequently occurs in carburized idler gears and often manifests below the allowable stress thresholds associated with traditional failure modes.

2.1 Visual Identification

TIFF can be visually distinguished from other gear failure modes by its characteristic final fracture, which typically occurs at mid-tooth height and presents as a distinct, flat plateau across the fracture surface. This plateau is a hallmark of fully reversed loading conditions, such as those experienced by the pinion gear in a differential.

While visual inspection can be somewhat subjective, the fracture morphology is often sufficient for preliminary identification, especially when supported by additional indicators. One such indicator is the condition of the fractured portion: the upper half of the tooth is frequently dislodged as a single, intact fragment, rather than breaking apart irregularly.

Figure 2: Real-World Example of TIFF.

Figure 2 presents a full view of the gear, clearly showing the distinct mid-height fracture pattern typical of TIFF.

Figure 3: Top half of tooth dislodged.

Figure 3 shows the dislodged fragment, often the entire addendum of the tooth, separated cleanly from the remaining structure.

 

2.2 Magnetic Particle Inspection (MPI)

Magnetic particle inspection (MPI) is a non-destructive technique that has proven effective in identifying fatigue cracks associated with TIFF. When one tooth exhibits signs of TIFF, it is common for adjacent teeth on the same gear to be affected to varying degrees. MPI is particularly useful after sectioning, as it highlights existing subsurface cracks with high contrast, as shown in Figure 4.

While traditional microscopy can also reveal these cracks, MPI offers a more efficient and scalable method for detection. Figure 4 illustrates this advantage, showing a tooth that has fully fractured due to TIFF alongside an adjacent tooth nearing failure. Notably, the crack appears slightly above mid-tooth height.

Figure 4: MPI after cross-sectioning.

This variation is attributed to the direction of crack propagation and the specific location selected for sectioning relative to the gear’s face width. A more advanced application of MPI involves performing the inspection prior to sectioning. This approach enables detection of fatigue cracks as they reach the tooth surface before the fractured segment becomes fully dislodged. The primary advantage of this method is that it does not require destructive sectioning, preserving the integrity of the gear for further analysis or continued use. Although this technique is more challenging and less commonly applied, it has shown potential for early-stage detection. A specific example of this application is presented in section 4.3.

2.3 Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) is a powerful tool for detailed fracture analysis and can be highly effective in identifying micro-cracks associated with TIFF. Unlike conventional optical microscopy, SEM offers significantly higher resolution and magnification, enabling the observation of crack initiation sites and propagation paths at the microstructural level.

SEM is particularly valuable when precise characterization of the failure mode is required. It can confirm the presence of fatigue striations, secondary cracking, and other microstructural features that support a TIFF diagnosis, making it an essential tool for validating complex or ambiguous failure cases.

2.4 Testing Attributes

In testing environments, TIFF-related failures are often difficult to detect prior to complete gear failure. Figure 5 illustrates this behavior, showing a torque trace with normal operation followed by a sudden torque overload that immediately terminates the test. This abrupt and binary failure pattern, characterized by normal function followed by catastrophic failure, has been observed consistently across multiple tests and gear designs.

Figure 5: Sudden Failure due to TIFF.

This behavior contrasts sharply with traditional failure modes, which typically exhibit a gradual degradation in performance. In those cases, torque capacity often declines progressively, providing early warning signs before ultimate failure occurs.

3 Literature Review

In recent years, TIFF has received increasing attention within both academic and industrial communities, resulting in a growing body of published work. This section reviews the most relevant literature, beginning with key academic and technical contributions that have advanced the field, followed by a summary of formalized industry standards and classification systems currently in use.

3.1 Research

Academic investigations into TIFF have significantly advanced the understanding of its initiation and propagation, as well as the material and geometric factors that influence subsurface crack development. These contributions have shaped the diagnostic and preventative strategies currently employed in both research and practice. The following review highlights the most impactful findings and their implications for gear design and failure mitigation.

Figure 6: MackAldener’s CIRF Single Stage {TFF} (a) vs Idler usage {TIFF} (b) [13].

3.1.1 MackAldener

M. MackAldener is widely credited with introducing the term tooth interior fatigue fracture (TIFF) in the publication “Tooth Interior Fatigue Fracture & Robustness of Gears” [13]. In this work, TIFF was identified as a distinct failure mode that did not align with existing classifications at the time. The defining characteristics were described as mid-tooth height crack initiation, a distinct fracture plateau, and a tendency to occur in case-hardened idler gears operating under moderate loads.

In addition to characterizing the failure mode, MackAldener proposed the first predictive framework for assessing TIFF risk. This approach used the Findley criterion [8] to define a crack initiation risk factor (CIRF), calculated as the ratio of Findley stress to the material’s critical shear stress. The CIRF methodology has since gained broad acceptance, with notable implementations such as a modified version integrated into widely accepted gear software [2], which incorporates specialized loaded tooth contact analysis (LTCA) to enhance prediction accuracy.

3.1.2 Tobie/FZG

The work by Tobie/FZG [15] represents a culmination of multiple publications [17], resulting in the development of what is known as the material-physically based FZG model for predicting TFF. While the terminology differs, TFF rather than TIFF, the underlying failure mode remains the same, with the distinction primarily reflecting a focus on single-stage gear applications.

Figure 7: Influencing parameters on stress [10].

The material-physically based model is notable for its complexity, requiring a wide range of detailed input parameters, followed by iterative integration steps. Despite this, it offers a comprehensive approach by accounting for numerous contributing factors, as illustrated in Figure 7. The model enables fatigue assessment not only at the surface but also within the subsurface regions of the gear flank, making it a powerful tool for evaluating internal fatigue mechanisms with high fidelity.

3.1.3 WITZIG/FZG

The work by Witzig/FZG [20] is among the most frequently cited in the study of TFF. Their objective was to develop a more practical and accessible approach to the material-physically based FZG model.

Focusing specifically on subsurface fatigue cracking in case-carburized gears, their method streamlined the complex input requirements and iterative calculations of the original model, resulting in a user-friendly calculation framework.

Referring back to Figure 7, Witzig concentrated on two primary influences: Hertzian contact stress and compressive residual stress. This simplification enables the calculation of local subsurface shear stresses (τeff ) at any point on the gear tooth, which can then be compared to the local material strength(τper ), as shown in Equation 1 (material exposure [14]):

Equation 1

The commonly accepted threshold for this exposure factor is Aff,max ≤ 0.8 [16], beyond which the risk of tooth flank fracture becomes significant. This work formed the premise for ISO/TS 6336-4 [12] on cylindrical gearing and also serves as the basis for ISO/TR 10300-4 (currently in draft) which is the bevel gear counterpart as stated by Pellkofer et al. [14].

3.1.4 Summary of Research Contributions

The preceding studies represent some of the most influential contributions to the understanding and prediction of TIFF and TFF. While each approach varies in complexity, scope, and application, they collectively advance the field by addressing critical aspects of internal fatigue behavior. Beyond these foundational works, a broader body of research continues to explore related topics such as fatigue criterion selection, hardness gradient modeling, residual stress prediction, crack propagation, and the application of finite element method (FEM) and computer-aided engineering (CAE) tools across different gearset types and sizes.

Despite these variations, a common objective unites all efforts: the accurate prediction of local material stress and local material strength. When the applied stress exceeds the material’s capacity, crack initiation, propagation, and eventual failure become inevitable. This shared goal underscores the importance of continued refinement in both modeling techniques and material characterization to improve gear durability and performance.

3.2 Industry Standards

As of the time of writing, there are no known industry standards that specifically address the prediction of tooth interior fatigue fracture (TIFF). However, ISO/TS 6336-4:2019 incorporates the work of Witzig/FZG to provide standardized calculations for the tooth flank fracture (TFF) load-carrying capacity of spur and helical gears. It is also notable that the content previously developed under ISO/DTS 19042 has since been integrated into ISO/TS 6336-4.

In terms of failure identification, two standards are currently available: ANSI/AGMA 1010-F14 [3], and ISO/TR 10825-2:2022 [11]. Both documents clearly draw a firm distinction between TIFF and TFF (subsurface initiated bending fatigue), with ISO/TR 10825-2 explicitly stating “TIFF and TFF are unrelated”. According to these standards, the primary distinction lies in the nature of the failure mechanism: TIFF is characterized as a true material limitation, whereas TFF is typically associated with material defects such as inclusions.

4 Case Studies and Experimental Results

This section presents real-world observations and test results derived from the author’s experience as a gear engineer for Eaton’s Vehicle Group, specifically in the design of straight-toothed, net-forged bevel gears for automotive applications. All data has been normalized and stripped of proprietary or non-relevant content to focus exclusively on aspects related to TIFF. The case studies presented are based on complete axle bench tests, as illustrated in Figure 8.

Figure 8: Test Cell Setup.

4.1 Case Study 1

This case study highlights the influence of contact stress on the onset of TIFF in a legacy gearset adapted for a new application.

During routine testing, the gearset, which had previously been produced and fielded for many years without issue, experienced premature failure. The test technician initially reported the issue as a “pinion tip fracture” (Figure 9), based on the visible dislodging of the gear tooth addendum. While this description accurately captured the external symptom, further investigation using the diagnostic techniques outlined in the identification section confirmed the failure to be consistent with TIFF. In this case, visual inspection was supported by magnetic particle inspection (MPI), which revealed a fully fractured tooth alongside an adjacent tooth exhibiting a subsurface crack nearing failure. This finding, shown in Figure 5, provided clear evidence of a fatigue-driven failure mode consistent with TIFF.

Figure 9: “Pinion Tip Fracture.”

With the failure mode established, the focus shifted to understanding the underlying cause. Given the gearset’s successful production history, the presence of TIFF in the new application was unexpected. A structured RedX® problem-solving approach was employed, leveraging B vs. W™, a comparison between “Best of Best” (BoB) units, those with no history of TIFF, and “Worst of Worst” (WoW) units that had failed. This methodology, rooted in data-driven engineering analysis, enabled the team to isolate key differences in performance.

The investigation revealed a subtle deviation in microgeometry that, while within acceptable tolerances, resulted in elevated contact stresses under the new application conditions. This variation, though not previously problematic, became significant due to the increased performance demands. This finding aligned with internal design guidelines and was further supported by insights from the literature, which emphasize the role of Hertzian contact stress in subsurface fatigue. By refining the geometry to better align with the updated load profile, contact stress levels were reduced, and subsequent testing confirmed the effectiveness of the corrective action in eliminating TIFF.

4.2 Case Study 2

This case study illustrates the critical role of heat treatment in mitigating TIFF by examining its influence on a gearset intentionally pushed beyond its functional limits. The design process followed established best practices: macro geometry was defined by packaging constraints, and microgeometry was optimized using allowable stress guidelines for a premium carburized steel. Final analysis was conducted at the system level under flexible boundary conditions (FEM Case & Cross Shaft) using Ansol’s Transmission3D (T3D).

The objective of this study was to strengthen predictive models, validate simulation tools, and refine internal engineering guidelines. By intentionally exceeding the gearset’s operational limits and inducing premature failure, the team was able to gain deeper insight into TIFF behavior under extreme conditions. The two primary variables under investigation were microgeometry (Hertzian stress) and heat treatment (subsurface strength). The initial configuration, Microgeometry A with Heat Treatment A, resulted in a clear TIFF failure, as shown in Figure 10.

Figure 10: TIFF Failure #2.

Under elevated loading, system deflections exceeded the microgeometry’s design capabilities, leading to localized stress concentrations and subsurface fatigue. The microgeometry was subsequently revised (Microgeometry B) to re-center the contact pattern under the increased deflection conditions. The gearset was remanufactured and re-tested using the same heat treatment (Heat Treatment A). However, the microgeometry correction alone had minimal impact on fatigue life, indicating that the gearset had reached its functional stress limits. Further improvement required increasing the material’s local strength.Material strength can be enhanced either by selecting a different alloy or by modifying the heat treatment process. This study focused on the latter, with future work potentially exploring material alternatives.

A common question in the gear design community is: “Change it to what?” Internal guidelines at the time emphasized the importance of case-hardening depth (CHD), surface hardness, and core hardness balance in resisting subsurface fatigue. These principles are reinforced by the work of Boiajiev et al. [7],who observed that increasing CHD reduces peak material exposure and shifts it deeper below the surface. Similarly, Bai et al. [4] demonstrated that a combination of high surface hardness, low core hardness, and deep effective case depth significantly inhibits crack initiation. Bai’s model further quantified CHD as the most influential parameter in subsurface fatigue resistance. These findings validate and align with the engineering practices previously established within the organization.

Figure 11: Normalized Fatigue Life for all Configurations.

Production furnaces were adjusted to achieve targeted heat-treatment profile. While minor manufacturing modifications implemented concurrently, which may introduce confounding effects, does not diminish the core findings. The revised heat treatment resulted in a 380 percent average increase in fatigue life, with all failures transitioning to surface-initiated contact fatigue. This outcome confirms the effectiveness of heat treatment optimization in mitigating TIFF risk.

4.3 Case Study 3

The third and final case study involved the revitalization of a legacy gearset, this time manufactured using cold successive forging rather than hot forging. The objective was to evaluate whether the heat-treatment mitigation strategies highlighted in Case Study 2 would translate successfully to this alternate manufacturing method. As with Case Study 1, the gearset had a long production history without prior issues. And similar to Case Study 2, the test strategy intentionally pushed the design beyond its functional limits to provoke failure and deepen understanding of subsurface fatigue behavior. The elevated load testing successfully produced a TIFF-like failure, shown in Figure 12.

Figure 12: Case Study 3 – TIFF Failure.

Upon closer inspection, the fracture exhibited characteristics consistent with TIFF, such as a plateaued fracture plane at mid-tooth height on a fully reverse-loaded gear. However, an unusual feature was observed: a vertical crack at mid-face width, propagating in the profile direction, illustrated in Figure 13. This behavior deviated from typical crack propagations across all failure modes, which generally propagate along the face width.

Figure 13: “Vertical Crack” (A) and Vertical Crack with MPI (B).

The vertical crack appeared consistently across nearly every tooth of both pinion gears in multiple test samples. To rule out test cell anomalies, the gearset was re-tested at a separate facility. The results were consistent, confirming the repeatability of the failure mode. To further investigate, the pinion was sectioned along the face width to observe crack propagation. The cross-sectional view confirmed that the crack initiated near the tip and propagated in the profile direction (Figure 14). While the fracture did not align perfectly with established TIFF classifications, it shared several key indicators.

Figure 14: Cross-Sectional View of “Vertical Crack.”

Given the overlap in characteristics and the known sensitivity of TIFF to subsurface stress conditions, the failure was treated as a variant of TIFF. The gear geometry remained unchanged, and the heat-treatment process was adjusted based on internal guidelines, which again, are further supported by literature recommendations.

Although the improvement was not as dramatic as in Case Study 2, the results were still highly favorable. The revised heat treatment led to a 106 percent increase in fatigue life (Figure 15), and the unusual vertical cracking was no longer observed. The final failure mode transitioned to surface-initiated contact fatigue, indicating a successful mitigation of the subsurface cracking behavior.

Figure 15: Gear Fatigue Results – Case Study 3.

While the exact classification of the original failure remains open to interpretation, the fracture behavior aligned closely with known TIFF characteristics and responded predictably to heat-treatment adjustments. These findings further reinforce the critical role of heat treatment in managing subsurface fatigue.

However, the root cause of the atypical vertical crack observed in this case remains uncertain. It is not yet clear whether this behavior is a byproduct of the cold forging process, a result of application-specific loading conditions, or influenced by another factor entirely. Future work should aim to isolate and investigate these variables to better understand the origin of this anomaly and determine whether additional design or process controls are necessary.

4.4 Case Study Summary

The three case studies presented in this section demonstrate a structured, data-driven approach to understanding and mitigating tooth interior fatigue fracture (TIFF) in straight-toothed, net-forged bevel gears. Each case leveraged intentional overloading during validation testing to provoke failure, enabling deeper insight into subsurface fatigue mechanisms under real-world conditions.

Case Study 1 highlighted the sensitivity of TIFF to subtle deviations in microgeometry, even in legacy designs with proven field performance. Case Study 2 expanded on this by demonstrating that while microgeometry optimization is necessary, it is not always sufficient — heat treatment emerged as a critical factor in extending fatigue life and shifting the failure mode. Case Study 3 further validated the effectiveness of heat-treatment strategies across different manufacturing methods, while also revealing an atypical fracture behavior that warrants further investigation.

Collectively, these studies reinforce the importance of robust validation, precise control of microgeometry, and tailored heat-treatment processes in managing TIFF risk. They demonstrate how intentional, well-structured testing can be used not only to diagnose failure modes but also to refine predictive models and strengthen engineering practices. The methodologies and insights presented here are intended to serve as a practical guide for engineers facing similar challenges.

5 Conclusion and Future Work

Modern vehicle drivetrains demand higher torque in increasingly compact and reliable packages, placing significant pressure on traditional gear design methodologies. This article has focused on the growing relevance of tooth interior fatigue fracture (TIFF) in straight-toothed, net-forged bevel gears, a sub-surface failure mode that challenges conventional surface-focused design strategies.

The goal of the gear designer is to ensure optimal performance through advanced modeling techniques, tailoring gear geometry and material properties to specific applications. While the industry has made significant progress in predicting and preventing surface-initiated failures, the emergence of subsurface fatigue mechanisms like TIFF underscores the need for deeper understanding and more sophisticated predictive tools.

Drawing from the author’s experience across multiple global programs, this article examined TIFF through a series of case studies that intentionally pushed gearsets beyond their functional limits. These studies demonstrated how targeted validation, microgeometry control, and heat-treatment optimization can be used to diagnose, mitigate, and ultimately prevent subsurface fatigue failures. The insights presented here are intended to serve as a practical guide for engineers facing similar challenges in high-performance drivetrain applications. Looking ahead, several opportunities for continued advancement have been identified. Future work may include applying current predictive industry standards to the three case studies to assess their effectiveness in forecasting TIFF. Additionally, exploring alternative materials and hardness profiles could yield further improvements in fatigue resistance. Finally, confirming the exact nature of the atypical fracture observed in Case Study 3, and understanding the cause of its unusual vertical crack propagation, remains an important area for investigation.

By addressing these challenges, the engineering community can continue to evolve its understanding of subsurface fatigue, enabling more robust, reliable, and efficient gear systems for the next generation of automotive drivetrains. 

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