Predicting Fatigue Performance for Fiber Reinforced Plastic Gears

Predicting the mechanical performance of gears is essential to reducing the time and cost associated with developing new gear applications. At Envalior, we have developed a framework based on experimental case studies, local fiber orientation and intrinsic material behavior that accurately predicts the fatigue lifetime of gears.

Developing successful gear applications takes a solid product with accurate failure predictions—and being the first ready for market. Yet, predicting the short- and long-term mechanical performance of gears requires looking at static strength due to misuse or stall torque, as well as considering durability performance with root fracture as the failure mode.

Testing root stress fatigue.

To significantly reduce the time and cost of developing gear applications, we can use a framework validated by experiments to accurately predict lifetime fatigue performance in gears using simplified tests, such as standard tensile bar fatigue testing or simplified gear tests. The framework includes a calculation for the stiffness added by the glass fiber content, as well as for applications with different load conditions, including part geometry, gating locations, and temperature.

With dedicated application testing during each design phase, we can drive to a successful final design in an iterative way. And if we combine simplified test results with expertise in gear applications and fundamental material knowledge, including fiber orientation and intrinsic material behavior, we can predict the gear failure mode and its corresponding torque level.

Durability during large amplitude and number of cycles

Root stress fatigue in gear applications suffer from two main failure mechanisms: plasticity-controlled failure and crack-growth failure. Since glass fiber reinforced gears are mainly used in applications with cyclic loading that is large in both amplitude and number of cycles, the lifetime of the part will largely be determined by crack-growth controlled failure. 

This failure is caused by small initial flaws from processing, handling or the addition of fillers that result in stress concentrations under load, and eventually propagate a craze or crack that leads to failure. Using a set of equations developed and validated using an experimental case study, we can accurately predict fatigue lifetime, including temperature dependence. 

Materials and Methods for Testing

We used Stanyl® TW271F6 across the tests, with no moisture at the start of the test. Solid lubricant (PTFE) was added to the samples for durability tests. The test gears were injection molded in a two-cavity mold via six gates and reinforced with 30% glass fiber by weight, offering good compensation for shrinkage during molding. After molding, the quality of the gears is a 12, according to ISO 1328.

Using a custom, environment-controlled experimental setup with two motors, where one rotates the gear at a given speed while the other brakes to ensure a constant torque output for the driven gear. The local tooth temperature increases due to friction until a steady-state temperature is reached, which is measured via infrared sensor.

At weld lines, fiber orientation is not parallel to the root of the teeth. This results in a difference in stiffness and strength at these locations in the part. We assumed an average uniform fiber orientation in the tooth of the gear, to determine a factor that represents the stresses of the stress-strain curve, though the factor will be strongly dependent on gear geometry and gate locations.

Differences in fiber orientation at weld lines cause differences in local stiffness and strength.

Accurately Predicting Gear Durability

The durability of the gears was tested under different torque levels with various environmental temperatures, and the lifetime was recorded. The experiments demonstrate the lifetime is severely affected by the applied torque level, and by environmental conditions.

At Envalior, we have used this data and testing methodology to develop an approach that accurately predicts root failure based on non-isothermal durability measurements on a gear tester. We can predict isothermal root stress fatigue curves using a temperature dependent pre-factor. The predictions have demonstrated accuracy within a factor of 3 on lifetime, except for applications with very high temperatures or very long testing times, where wear becomes a potential failure mechanism. 




Benjamin van Wissen

Benjamin van Wissen

CAE Expert/Scientist for Envalior

Benjamin van Wissen is a CAE Expert/Scientist for Envalior. He performs FEA analysis to polymer applications, including strength, stiffness, fatigue, creep, NVH and thermal performance assessments. He is specialized in polymer gear strength calculations and design. Besides that he is responsible for fundamental material research in order to characterize complex FEA material cards. Before joining Envalior, Benjamin worked in the automotive and chemicals industries. He is a strong engineering professional skilled in FEA using Abaqus, Altair Hyperworks (including Optistruct and CFD), Digimat and KISSsoft for gear design and calculations.

Published on

08 July 2024


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