Fatigue modelling in tooth root of surface-hardened gears

doctoral thesis

• Title (croatian): Modeliranje zamora u korijenu zuba površinski otvrdnutih zupčanika
• Author: Ivan Čular
• Mentor: Krešimir Vučković (mentor)
• Committee member: Zdenko Tonković (predsjednik povjerenstva)
• Committee member: Ivica Galić (član povjerenstva)
• Committee member: Srečko Glodež https://orcid.org/0000-0003-4273-7265 (član povjerenstva)
• Granter: University of Zagreb; Faculty of Mechanical Engineering and Naval Architecture; Zagreb
• Defense date and country: 2022-12-09, Croatia
• Scientific / art field, discipline and subdiscipline: TECHNICAL SCIENCES; Mechanical Engineering
• Universal decimal classification (UDC): 621 - Mechanical engineering. Nuclear technology. Machinery

Abstract

Spur gears are one of the most commonly employed machine elements for power transmission.
During their operating life, they are subjected to variable and cyclic loading. Consequently,
material fatigue may occur in the tooth root region, more commonly referred to as bending
fatigue. Steel gears are typically subjected to heat treatment processes such as carburizing to
reduce wear. Additionally, the gears may be shot peened to induce a beneficial, fatigue-resistant
surface layer of compressive residual stresses. However, this may result in bending fatigue
crack initiation below the surface, i.e., subsurface bending failure. This type of crack is hardly
detectable during regular service intervals. Hence, it may often go unnoticed, grow, and
propagate rapidly through the brittle carburized layer, resulting in tooth breakage and gear
failure.
This doctoral thesis establishes a computational model for predicting the bending fatigue crack
location (surface vs. subsurface) and the required number of cycles for surface-hardened spur
gears. Due to the relative complexity of the gear’s geometry and loading conditions, the finite
element method is employed to obtain load-induced stresses and strains. The multilayer method
is employed to account for inhomogeneous material, while the strain life approach (ε – N) is
used to predict the bending fatigue life.
The research is divided into multiple stages. During the initial stages, bending fatigue with
surface/subsurface crack initiation is investigated on relatively simple geometry, such as
surface-hardened gear steel specimens. Then, the model is gradually upgraded to account for
more complex loading conditions, residual stress distributions, and inhomogeneous material
until the final model applicable to the running gear pair is acquired.
According to the obtained numerical results and confirmed by the experimental investigations
from the available literature, subsurface bending fatigue crack initiation tends to occur only in
carburized and additionally shot-peened gears. The critical region for subsurface failure is
located in the proximity of sharp loss of compressive residual stresses and still relatively high
load-induced stresses. The probability of subsurface crack initiation can be reduced by
modifying the gear’s geometry (choosing a smaller normal module) or increasing the beneficial
compressive residual stresses by prolonging the carburization time.

Abstract (croatian)

Zupčanici su jedni od najčešće korištenih strojnih elemenata u prijenosnicima snage i gibanja.
Vođeni strogim proračunskim kriterijima prema relevantnim standardima kao što je ISO 6336,
čelični zupčanici s ravnim zubima u pravilu zadovoljavaju minimalni očekivani radni vijek.
Međutim, postojeći standardi ne uzimaju u obzir zajednički utjecaj dodatnih postupaka
površinskog otvrdnjavanja, kao što su cementiranje i sačmarenje, na pojavu zamora materijala
u korijenu zuba. U takvim slučajevima, javlja se potreba za računskim modelom koji može
procijeniti ne samo broj ciklusa do pojave zamorne pukotine, već i mjesto nastanka spomenute
pukotine (površinski naspram potpovršinski nastanak). Mjesto nastanka zamorne pukotine od
posebne je važnosti zbog njene otežane detekcije tijekom redovnih servisnih intervala.
U okviru rada razvijen je računski model temeljen na metodi slojeva (eng: multilayer method)
te metodi konačnih elemenata (eng: finite element method – FEM) koji predviđa mjesto te broj
ciklusa do nastanka zamorne pukotine u korijenu zuba zupčanika. Zbog relativne kompleksnosti
modela, istraživanje je započeto na uzorcima izrađenim od materijala tipično korištenim za
izradu čeličnih zupčanika. Predloženi model postepeno je unaprijeđen kako bi se simulirala
stvarna geometrija te opterećenje zupčanog para.

• Keywords: spur gear; bending fatigue; finite element method; subsurface crack initiation; multilayer method; residual stress
• Keywords (english): zupčanici s ravnim zubima; zamor u korijenu zuba; metoda konačnih elemenata; potpovršinski nastanak pukotine; metoda slojeva; zaostala naprezanja
• Language: english
• URN:NBN: urn:nbn:hr:235:093779
• Study programme: Title: Mechanical Engineering and Naval Architecture; Study programme type: university; Study level: postgraduate; Academic / professional title: doktor/doktorica znanosti, područje tehničkih znanosti (doktor/doktorica znanosti, područje tehničkih znanosti)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Created on: 2022-12-13 14:02:07