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2.8 Fundamentals of thermo-mechanical failures .1 Overview of thermo-mechanical structural
2.8.4 Thermo-mechanical failure modes
fatigue life is calculated by a strain-based approach. The Chaboche method is a stress-based approach. In fact, it is generally believed that a strain-based method is more suitable than a stress-based method for LCF problems because LCF is controlled mainly by the plastic strain rather than the elastic stress as encountered in the case of HCF. As an example, in the Chaboche’s model, the number of cycles to failure for each decomposed simple fatigue cycle due to pure fatigue, Nfi , is calculated based on stresses as below (for example, used by Morin et al. (2005) for a diesel cylinder head):
N s s
s s
s
fi ssuu ssii C
l
= ssssuu – ssssii, i
, C1
, l C1
, l 1
, 1
s, s 1
s s
s – s
s, s 1
s – s
s s
max – max ma
si mi m s si m,, s 11
s, s 1
si m s
s, s 1
s,, ax s 11
s, s 1
s ax s
s, s 1
s,,,, sflflffllfll 1111 Ê
ËÁ ÊÁ Ê ËÁË
ˆ
¯
, ¯ 1
, ˜ˆ 1
˜ˆ
¯˜¯
, ¯ 1
, ˜ 1
, ¯ 1
, 1
ÊD Ë
, Ë 1
, ÁÊ 1
ÁÊ ËÁË
, Ë 1
, Á 1
, Ë 1
, 1
ˆ
¯˜
ˆ˜ ˆ
¯˜¯
C C C2
2.4 where i indicates the ith decomposed simple fatigue cycle, su is the ultimate stress, si,max is the maximum stress, sfl is the fatigue limit stress, Dsi is the stress amplitude, and C1 and C2 are material constants.
These three prevailing models of life prediction, along with their corresponding material testing procedures, were elaborated by Ogarevic et al. (2001). It should be noted that the three methods are not interchangeable.
Therefore, once a method is chosen, it is not easy to change to another one because the empirical database is usually built with one method.
temperature LCF failures, while the coolant-side of the cylinder head may have high temperature HCF failures. Which failure occurs first depends on particular engine applications. There are interactions between the mechanical and thermal failure modes, also between the various more detailed failure modes such as fatigue, creep, and oxidation. Material strength decreases due to the accelerated aging of the mechanical properties under elevated temperatures. The interactions affect the lifetime of the component. For a detailed introduction to engine thermal loading, the reader is referred to Heywood (1988) and French (1999).
Thermal failures
Thermal failures are caused by the thermal load which is essentially the metal temperatures, temperature gradients and thermal stresses in the component. Thermal load in engine components is related to gas temperature, heat flux, component design, and material properties. The components exposed to excessively high temperatures may have failure due to ablation, distortion, corrosion, creep, relaxation, and thermal fatigue. Material property degradation at high temperatures reduces the material strength to resist the failures. Excessive thermal deformation may induce the problems of scuffing and clearance interference, and aggravate the wear of tribological components. Alternating thermal stresses and strains cause thermal fatigue, especially under the effects of creep and relaxation. Thermal stresses may also aggravate mechanical fracture failures when the total of mechanical and thermal stresses exceeds the ultimate tensile strength of the material.
As engine power density increases, thermal failures have become more difficult to control than mechanical failures. Thermal failures have become the primary limiting factor for engine reliability.
In-cylinder gas temperature and exhaust manifold gas temperature are two most important indicators of the thermal load for engine components.
In-cylinder gas temperature affects the metal temperatures of the cylinder head, the injector tip, and the piston. Exhaust gas temperature affects the metal temperatures of the exhaust valve, the valve seat, and the exhaust manifold. It should be noted that the peak in-cylinder gas temperature and the exhaust manifold gas temperature are not always coherent. At a fixed engine speed, a higher peak in-cylinder temperature resulting from higher power or lower air–fuel ratio corresponds to a higher exhaust temperature.
However, a lower peak in-cylinder temperature resulting from retarded fuel injection timing causes an increase in the exhaust temperature. The heat transferred to the component at different engine speeds is dependent on the timescale rather than the crank angle scale. Moreover, using the maximum temperature in a thermo-mechanical cycle to predict isothermal fatigue life may not be the safe or conservative method (Ogarevic et al., 2001).
Ablation and excessive thermal deformation
Examples of ablation include unusual burn-out due to over-fueling for power cylinder components, backfire, or uncontrolled burning of soot in the DPF. Excessive thermal deformation usually occurs in the piston, the cylinder head and the exhaust manifold, and is caused by high in-cylinder gas temperatures or exhaust manifold gas temperatures. For instance, the piston temperature at the top ring position should not become excessively high, otherwise the lubrication of the ring may fail, resulting in ring scuffing or sticking in the ring groove.
Fracture and rupture
Fracture refers to the separation of a material into two or more pieces under the loading of stress. Fracture can be an instant or acute rupture of a component caused by mechanical overload in a single event, or as a final result of the chronic crack initiated and propagated due to long-term fatigue.
Mechanical overload occurs when the applied load is greater than the ultimate strength (tensile, compressive, or shear) of the component. It can result in a tension failure, compression failure, shear failure, or bending failure (with both tensile and compressive forces). Ductile rupture is the ultimate failure of ductile materials as a consequence of extensive plastic deformation due to tension. In brittle rupture, no plastic deformation occurs before rupture.
Rupture under tension occurs after the following steps: plastic deformation, necking, void nucleation, crack formation, crack propagation, and surface separation. The mechanism of cracking due to fatigue is different. At high temperatures the ultimate strength of the metal material decreases so that fracture occurs more easily. Fracture is a common structural durability problem in engine components.
Fatigue
According to the source of stress, fatigue can be classified into mechanical fatigue and thermal fatigue. For engine components the effects are usually combined as thermo-mechanical fatigue. According to the number of cycles to failure, fatigue includes high cycle fatigue and low cycle fatigue. HCF produces failures after a high number of cycles (e.g., greater than 104 cycles) with low stresses and elastic deformation. LCF produces failures after only a low number of cycles (e.g., smaller than 104 cycles) with high stresses and plastic deformation. Note that both mechanical and thermal fatigue can be either HCF or LCF. The timescale of one cycle can vary greatly from one engine cycle (e.g., for the loading cycle in HCF due to cylinder pressures) to several hours (e.g., for the loading cycle in LCF such as slow thermal
cycles). Although the damage per cycle of LCF is greater than that of HCF, if the occurrence frequency of LCF is much lower than that of HCF, the component may reach a HCF failure earlier than a LCF failure. Different failure modes (e.g., different crack locations) may have different fatigue mechanisms (i.e., LCF or HCF, thermal stress induced or mechanical stress induced). The discussions on the mechanism and modeling of fatigue are detailed later.
Creep
Creep is a slow time-dependent irreversible process of plastic deformation for a metal material under the influence of stresses which are lower than the yield strength of the material. Creep results from long-term stresses and generates chronic strain accumulation or stress relaxation. Creep is generally damaging and related to inter-granular cracking and void growth over time.
The strain increase in creep is usually nonlinear with time. Both temperature and mechanical stress can generate creep, with temperature as the primary factor. Creep is especially significant at in-phase-loading conditions and less significant in the situation of out-of-phase loading. The creep under alternating mechanical or thermal stress is larger than the static creep.
Creep increases with temperature. The rate of creep in terms of strain rate increases exponentially with metal temperature at high temperatures. The effect of creep becomes noticeable at approximately 30% of the melting temperature of metals. Large creep strain may cause cracks and fracture.
When the temperature is sufficiently high, even if the stress is designed much lower than the yield strength, creep may occur as a plastic deformation to cause failure. Creep and plastic deformation occur at elevated temperatures in thermal cycles. This produces tensile stresses after the thermal load is removed.
Creep needs to be considered in the thermal failures encountered in high temperature operations of the engine. The plastic strain due to creep can be modeled as a function of time, temperature, and stress. The rate- dependent visco-plasticity theory needs to be included in the stress analysis for creep.
Corrosion
Chemical corrosion in the engine occurs due to the corrosive combustion products (e.g., sulfur in the fuel) in the exhaust gas in the combustion chamber, the exhaust and intake systems, the EGR system, and in the lubricant oil.
Corrosion aggravates fatigue and wear of the component.
Oxidation
Oxidation damage is caused by repeated formation of an oxidation layer at the crack tip and its rupture. The rate of growth of oxidation layer thickness is proportional to the square root of time in the absence of cyclic loading;
and the rate of growth is much higher in cyclic loading conditions where the oxidation layer repeatedly breaks and the fresh surface is exposed to the environment (Ogarevic et al., 2001). The details of the modeling of oxidation damage and creep damage were provided by Su et al. (2002) in their investigation of cylinder head failures.