February 27

# Examination of Stress and Stress Types in Engineering: Everything You Need to Know

## Introduction

Stress is simply defined as force per unit area. It always tends to change the shape of the body. Stress is actually the measure of internal forces developed between the molecules of the body. These internal forces are the reaction of the external forces applied on the body that cause it to deform.

The formula for normal stress is given as

Where σ is stress, F is applied load and A is the surface area on which load is applied

The SI unit of stress is same as pressure i.e. pascal or N/m2. It is also measured in pound per square inch or “psi”.

Stresses induced in the component are caused by internal resistance provided by it to external forces acting on it. These stresses depend upon the nature of material from which a specific component under consideration is made. There can be various stresses produced in material; depending upon the environment to which the component is exposed.

In continuum mechanics, the body behaves as a continuum. So, these internal loads are distributed continually within the volume of the body. These forces cause deformation in the body. The deformation can cause a permanent change in the shape of the body if the material is not strong enough.

Based upon the direction of forces on the component, stresses can be:

• Tensile
• Compressive
• Shear
• Fatigue

## Tensile Stress

A tensile stress is generated in a body when load is applied in such a way that tends to elongate the material along the axis of applied force. In simple words we can say that tensile stress is due to pulling the body.

## Compressive Stress

When the applied load acts in such a way that it reduces the length of that material, then the stress generates due to this load is called compressive stress. This type of stress is usually due to opposite forces. Compressive stresses are generally higher than tensile stresses in a body.

## Shear Stress

Shear stress in a body is produced when forces act parallel to the surface area. These forces are called shear forces. It occurs when force tends to cause deformation in a body by slipping along a plane or planes parallel to the imposed stress.

## Fatigue Stress

Fatigue occurs when a material is kept on loading and unloading. If the loads are above a specific value, microscopic cracks will begin to form at the stress. Finally a crack reaches a critical size, the crack will propagate suddenly, and the fracture is happened. The shape of the structure significantly affects the fatigue life; square holes or sharp corners leads to elevated local stresses where fatigue cracks can initiate.

Also depending upon ambient conditions, stresses can be:

• Mechanical
• Thermal

## Mechanical Stress

Simple stress is also called mechanical. These stresses are induced when physically some load is applied to an object. The load can be compressive, tensile or shear. The stresses produced by such type of loads are called mechanical stresses. They are explained above in detail.

## Thermal Stress

Thermal stress is created when a change in size or volume is observed due to a change in temperature. Thermal expansion or compression due to change is surrounding temperature can result in thermal stresses. Usually these are observed in a material when expansion or compression is prevented i.e. the ends are fixed. Thermal stress can also be either compressive or tensile. It depends upon the heat flow direction and coefficient of thermal expansion. Generally when a body is heated it produces compressive stress and when it is cooled down, tensile stresses are produced

In this experiment, we have to analyze stresses on cylinder head of Honda Fit engine, having specific engine parameters, with a suitable material. We will analyze mechanical stresses by subjecting it to pressure forces of some magnitude, as well as thermal stresses using engineering, on both compression ignition and spark ignition engines, using software CATIA. In our problem mechanical stresses are of compressive nature. So by analyzing stresses using software, we will calculate some results

In internal combustion engines they lie on the top of cylinders, thus forming a combustion chamber. It is fixed using nuts and bolts.

Its operating behavior determines following properties of engine:

• Power output
• Torque
• Exhaust gas emission behavior
• Fuel consumption

It encloses following:

## Spark Plug for SI and Fuel Injector for CI Engines Opening

Spark plug is inserted through it which ignites air-fuel mixture.

## Intake and Exhaust Valves

These valves are inserted through valve bridge. Valve bridge is metal piece inserted through during casting which is a path through which valves are inserted. Hoop and axial stresses are produced in it, when pressure is generated in combustion chamber. Valve bridge experience maximum stresses due to less thickness of material and high temperature caused by improper cooling.

## Cams Opening and Bolt Hole

During stress analysis, bot holes serve as boundary conditions(constraints), where stress concentration is higher due to restriction of movement at these parts.

## Manufacturing and Mechanical Behavior

For IC engines, cylinder heads demand high requirements for mechanical properties. Different materials are used for its manufacturing. The engine parameters for which cylinder head is being manufactured determine which material would be suitable for it. After material selection, a suitable method is adopted to manufacture it.

Some methods to manufacture cylinder heads are:

## Sandcasting

It is also known as sand molded casting. It is a metal casting process in which sand is used as a mold material. Over 70% of all metal castings are produced by this process.

## Pressure-die Casting

This process is a quick, reliable and cost effective for the production of high volume metal components. It consists of injecting a molten metal alloy under high pressure into a steel mold.

## Permanent Mold Casting

This process employs reusable molds also called permanent molds. Molds are filled either by gravity or gas pressure or vacuum.

Desirable mechanical properties for cylinder head are as follows:

• High tensile and fatigue strength
• High creep resistance at temperatures between room temperature and 250 degree C
• Low porosity
• High ductility and elasticity
• High resistance to thermal shock
• Light weight

## Geometry and Working Environment

It acts as a head of combustion chamber as the covers the opening of a cylinder in which piston reciprocates and maintains the pressure inside the chamber in order to execute proper power cycle of engine. The shape of combustion chamber and location of intake and exhaust valves determine overall geometry of engine head. The cylinder bore and distance between cylinders greatly affect it. It also allows proper path for intake of fresh air in cylinder and exhausting hot gases out of the cylinder.  During power cycle, when the piston reaches top dead center and compresses air-fuel mixture, it is burnt and as a result high pressure is produced in the combustion chamber. These pressure forces exert load on cylinder head due to which stresses are produced in it. When high temperature is produced during combustion, for appropriate cooling, coolant is passed into cylinder head through crankcase and multiple openings. Due to temperature change during combustion, thermal stresses are induced in cylinder head. So, it is required to make a suitable design of cylinder head with specific material that can bear such stresses and it may not affect the performance of engine, as the passage of intake and exhaust gases will be affected, if cylinder head damages.

These figures show cylinder head damage when mechanical and thermal stresses surpass the material strength due to which design of cylinder head is disturbed. So it is a very difficult task to design a cylinder head from specific material for a specific type of engine.

While observing stress analysis of cylinder head, we should keep in mind following engine specifications:

• Stroke
• Bore
• Maximum power and Torque produced by engine
• Maximum pressure developed during combustion
• Temperature attained by engine in combustion chamber

## Stresses in a Cylinder Head

With respect to its working environment, stresses generated in it are caused by following:

## Residual Stresses

It refers to stress that is present in structure or component, while there is no external load applied. These stresses are produced during manufacturing of cylinder head in casting process. When a material is being cooled, then due to thermal gradients, different parts of an object expand or contract by different amounts. Due to this differential temperature, such type of stresses are produced. At some point, this stress can increase strength of material, then a crack will form.

It has been observed that residual stresses are less when component is quenched with forced air and slower cooling rather than water quenching.

## Stress Test

A neutron diffraction experiment was performed on two semi‐permanent mold castings of a cylinder head.  One casting was quenched using water and the other was quenched using fresh air. This experiment compared the strains and stress profiles along the web area between the intake and exhaust valve ports indicating the main differences in strain and stress distribution for the two alternative quenching tests.

The aim of this test to differentiate residual stresses pattern in both castings which were quenched with different methods. These stresses were categorized as:

## Hoop Stress

It is the value of stress that acts circumferentially (in direction of tangent) on every particle in cylinder wall. It is also called tangential stress.

Ϭt =p(ri2-ro2)/ro2-ri2 + pri2ro2/r2(ro2-ri2)

## Axial Stress

It is a normal stress that is parallel to axis of axis of symmetry of cylindrical surface. It is present when cylinder is closed.

Mathematically,

Ϭ=pri2/ro2-ri2

This stress is normal to cylindrical surface, at each point. It is coplanar but perpendicular to axis of symmetry.

Ϭr =p(ri2-ro2)/ro2-ri2 + pri2ro2/r2(ro2-ri2)

The axis system chosen is shown in figure:

The material used was Aluminum A356 in both castings and the graph for both can be shown as:

Figure 1 (water quenched)

Figure 2 (air quenched)

These residual stresses induced in valve bridges during both types of quenching are compared by changing depth from its surface.

## Stress Due to Mean Contact Pressure

They are generated in cylinder during combustion of air-fuel mixture. When spark plug ignites air-fuel mixture, then the gases formed during combustion produce high amount of pressure in combustion chamber. Due to its cylindrical design, hoop stresses induced in its inner and outer surface.

The theoretical formula to calculate these stresses is given as:

For inner surface:

Ϭ = Pc R2 +ri2/R2-ri2

For outer surface:

Ϭ = -PC ro2 +R2/ro2-R2

where,

Pc is contact pressure

ri is inner radius valve guide.

## Thermal Stresses

These are caused by fluctuations in temperature field of engine which lead to fatigue in cylinder heads. Temperature differences between different parts of cylinder head causes these stresses. The regions around valves experience thermal loading from the burning gases present in cylinder during combustion cycle and also during burned gases when they flow through exhaust valves. Thermal stress also depends upon heat flux through different parts, their thermal conductivity and cooling, when cooling passes through passages in cylinder head. Cracks due to fatigue stresses can be first seen at hottest parts of cylinder head.

## Stress Analysis for SI Engine

In these engine, spark plug is used. They are lighter than compression ignition engines. In such types of engine, pressure generated in combustion chamber is less due to low compression ratio. Therefore, an appropriate material should be used for cylinder head for these engines which is lighter in weight and can bear maximum stresses generated in it due to pressure forces.

The cylinder head of Honda Fit engine is shown:

Engine parameters of Honda Fit engine are as follows

 Bore 73mm Max Torque 155Nm Max Power 97kW Stroke 89mm

Operating speed= 4600rpm

Through this data we can calculate we can calculate mean effective pressure developed in combustion chamber which the cause of mechanical stresses in cylinder head.

Pme = 2π×Tnc/Vd

For 4 stroke

nc = 2

So       Pme = 2611483Pa

Mechanical Stress Analysis:

For analysis software, catia software is used. The material Aluminum is applied on the model . After it, the supports and specific value of pressure 2611483 Pa  are applied on specific parts of model. After it model is meshed into “linear” type elements.

Properties of Material used:

The mechanical properties of aluminum are shown as:

 Young’s Modulus 7e+010N_m2 Poisson Ratio 0.346 Density 2710kg_m3 Yield Strength 9.5e+007N_m2 Thermal Expansion 2.36e-005_Kdeg Cost per mass unit (US\$lb) 0.94 USD/lb

## Results and Discussion

After applying constraints and pressure, we get values of stresses produced in cylinder head. It can be seen that the magnitude of stress at each point is different. The color band in the figure shows the magnitude of stresses. The red color shows the region where magnitude of stress is largest. The highest stress produced in it is 2.67e+007 N-m2  which is less than yield strength of aluminum. So we can use cylinder head made of aluminum in Honda Fit engine.

## Thermal Stress Analysis

In order to calculate stress distribution due to temperature, boundary conditions(constraints) are defined and then temperature field(thermal load) is applied on the meshed model.

The boundary conditions are defined by differential equation as follows:

kx ðT/ðx nx + kyðT/ðy ny + kzðT/ðz nz =α(T2-T1)

Where:

K: heat conduction coefficient

T2: fluid temperature

α :convection coefficient

T: temperature of part surface

After applying boundary conditions and temperature field, we get some mathematical results which show values of stresses, resulting due to temperature and constraints, at each point of cylinder head which can be described by figure below:

As we see maximum thermal stress is 8.66e+009 which is less than yield strength of aluminum. So aluminum material is suitable for cylinder head.

## Stress Analysis For CI Engine

Diesel engines differ from gasoline engines due to following:

• Compression ratio is much higher than that of gasoline engines
• The torque generated by these engines are relatively higher
• They are heavier due to thick walls of combustion chamber and heavy material used for cylinder head, in order to withstand very high pressure caused by large compression ratio.

Based upon such reasons, material selected for cylinder head in CI engines is different from material used for it in SI engines.

## Engine Parameters

Stress test was applied on cylinder head of PERKINS Phaser 210Ti diesel engine haiving cylinder head as shown:

The specifications of engines are:

 Stroke 127mm Bore 100mm Torque 737Nm Power 123.485kW

Operating Speed = 1600rpm

Pme = 2π×Tnc/Vd

After putting values we get,

Pme = 4.64Mpa

## Mechanical Stress Test

For analysis software, catia software is used. The material Cast Iron. After it, the constraints were applied on bolt holes and specific value of pressure 4.64 MPa are applied on specific parts of model. After it model is meshed into “linear” type elements.

## Properties of Material Used

The properties of cast iron are shown as:

 Young Modulus 2e+011N_m2 Poisson Ratio 0.266 Density 7860Kg_m3 Thermal Expansion 1.17e-005_Kdeg Yield Strength 2.5e+008N_m2 Cost per unit mass (US\$lb) 0.61

## Results

After applying constraints and pressure, we get values of stresses produced in cylinder head. It can be seen that the magnitude of stress at each point is different. The color band in the figure shows the magnitude of stresses. The red color shows the region where magnitude of stress is largest. The highest stress produced in it is 1.54e+007 N-m2  which is less than yield strength of cast iron. So we can use cylinder head made of cast iron in Perkins Diesel Engine.

## Thermal Stress Analysis

In order to calculate stress distribution due to temperature, boundary conditions(constraints) are defined and then temperature field(thermal load) is applied on the meshed model. After applying boundary conditions and temperature field, we get some mathematical results which show values of stresses, resulting due to temperature and constraints, at each point of cylinder head which can be described by figure below:

## Economic Analysis

### SI Engine

Aluminum was selected as suitable material for cylinder head of SI Engine. The reason was that, aluminum is lighter than the other competitive materials and it can withstand the thermal and structural stresses. Though its unit cost is large as compared to cast iron but due to its less strength-to-weight ratio, it suits to the working environment of SI engine.

### CI Engine

Cast Iron was selected as suitable material for the cylinder head of CI Engine. We did not use aluminum for this purpose because it cannot bear the high pressure forces that are produced during the combustion process in CI Engine. Though its density is higher as compared to aluminum but due to its relatively lesser cost and suitability to working environment, it was chosen for cylinder head.

Haseeb Shahzad is an experienced Mechanical Engineer with a demonstrated history of working in Industry. He has good Skills in Research work, AutoCAD, Solidworks CATIA and other engineering software.