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Chapter 5

The nature of engineering design

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Chapter 5: Goal and Objectives

The goal of this chapter is to give an overview of the parameters that

influence the engineering design process in industry.

The main objectives are to:

• Discuss the various issues that have to be considered in design

• Review the major phases of the design process

• Explain the use of codes and standards in design

• Discuss the effect of component geometry on design

• Rationalize the use of the factor of safety in design

• Calculate the probability of failure of a component at the design

stage

Levels of engineering design

• Development of existing products or designs, redesign, by

introducing minor modifications in size, shape, or materials to

improve performance. This type represents a large proportion of

the design effort in industry and may be accompanied by failure

analysis to reduce the likelihood of further failures.

• Adaptation of an existing product or design to operate in a new

environment or to perform a different function.

• Creation of a totally new design that has no precedent. This type

often requires the solution of problems which may not have been

encountered before and could require a considerable effort in

research and development.

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Parameters influencing engineering design

General considerations in engineering design

Design involves tradeoffs among the many, and often conflicting,

conditions that it has to satisfy. These include:

• Human factors: adapting the product to make convenient for

human use.

• Industrial design, aesthetic and marketing considerations

• Environmental considerations: comply with guidelines, e.g.

Environmental Protection Agency (EPA), International

Organization for Standardization (ISO)

• Functional requirements: define the minimum level of

performance that an acceptable design must have in addition to

safety, marketability, and cost.

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Major phases of design I

Preliminary and conceptual design

1. Identification of the need, evaluating the product feasibility,

selecting the most promising concept and defining the objective

of the design.

2. Functional requirements and operational limitations are directly

related to the required characteristics of the product.

3. System definition, concept formulation, and preliminary layout

are usually completed, in this order, before evaluating the

operating loads and determining the form of the different

components or structural members.

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Major phases of design II

Configuration (Embodiment) design

4. Preliminary materials selection, preliminary design calculations,

and rough estimation of manufacturing requirements. Preliminary

design begins by expanding the conceptual design into a detailed

structure of subsystems and sub-subsystems.

5. The evaluation phase involves a comparison of the expected

performance of the design with the performance requirements.

Evaluation of the different solutions and selection of the optimum

alternative can be performed using decision making techniques,

modeling techniques, experimental work, and/or prototypes.

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Major phases of design IIIDetail (Parametric) design

6. Detail design results in drawings that are suitable for use in

manufacturing.

7. The next step is detailing, where the material is selected and

specified by reference to standard codes. The temper condition of

the stock material, the necessary heat treatment, and the expected

hardness may also be specified for quality control purposes.

8. The bill of materials, is a listing of every thing that goes into the

final product including fasteners and purchased parts. It is also used

by purchasing, marketing, and accounting.

9. When the product gets into use, its performance in service gives the

feedback for future design modifications.

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Effect of component geometry

Stress concentration factor under static loading relates the maximum

stress at the discontinuity to the average or nominal stress:

Kt, = Smax, /Sav, (5.1)

The value of Kt depends only on the geometry of the part as given in

Table 5.1

Stress concentration in fatigue

Fatigue stress concentration factor, or fatigue strength reduction

factor, Kf, is usually defined as:

(Endurance limit of notch free part)/(endurance limit of notched part

Kf, = 1 when the material is not at all sensitive to notches

Kf, = Kt, when the material is fully sensitive to notches

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Guidelines for design to avoid stress

concentration

1. Abrupt changes in cross section should be avoided. If they are

necessary, generous fillet radii or stress relieving grooves should

be provided, Fig. 5.3.a.

2. Slots and grooves should be provided with generous run-out radii

and with fillet radii in all corners, Fig. 5.3.b.

3. Stress relieving grooves or undercuts should be provided at the

end of threads and splines, Fig. 5.3.c.

4. Sharp internal corners and external edges should be avoided.

5. Oil holes and similar features should be chamfered.

6. Weakening features should be staggered to avoid the addition of

their stress concentration effects, Fig. 5.3.d.

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Design guidelines for shafts subjected to fatigue loading I

Fig. 5.3 (a, b)

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Design guidelines for shafts subjected to fatigue loading II

Factor of safety I

The main parameters that affect the value of the factor of safety,

which is always greater than unity, can be grouped into:

1. Uncertainties associated with material properties due to variations

in composition, heat treatment and processing conditions as well

as environmental variables such as temperature, time, humidity,

and ambient chemicals.

2. Parameters related to manufacturing processes also contribute to

the uncertainties of component performance. These include

variations in surface roughness, internal stresses, sharp corners,

identifying marks, and other stress raisers.

3. Uncertainties in loading and service conditions.

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Factor of safety II

ns accounts for uncertainties in material properties:

ns = S/Sa (5.4)

where : S = nominal strength, Sa = allowable stress

n1 allows for uncertainties in loading conditions:

Nl = L / La (5.5)

Where: L = the maximum load, La = normal load.

The total or overall factor of safety (n) combines the uncertainties in

material properties and manufacturing processes as well as the

uncertainties in external loading conditions can be calculated as:

n = ns nl (5.6)

Common values of factors of safety range from 1.5 to 10.

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Reliability of componentsL = externally applied load, S = load carrying capacity

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From Fig. 5.4, the value of z at which failure occurs is:

2/1222/122//0 LSLS LSLSz (5.8)

Table 5.2 Values of z and corresponding levels of reliability and probability of failure

z Reliability Probability of failure

- 1.00

- 1.28

- 2.33

- 3.09

- 3.72

- 4.26

- 4.75

0.8413

0.9000

0.9900

0.9990

0.9999

0.99999

0.999999

0.1587

0.1000

0.0100

0.0010

0.0001

0.00001

0.000001

Design example 5.1 – Estimate the probability of

failure of a structural member I

A structural element is made of a material with

average tensile strength of 2100 Mpa

subjected to a static tensile stress of an average 1600 MPa.

If the strength and stress to vary according to normal distributions

with standard deviations of = 400 and = 300 respectively,

What is the probability of failure of the element?

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Design example 5.1 – Estimate the probability of

failure of a structural member II

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From Fig. 5.4, LS = 2100 - 1600 = 500 MPa,

Standard deviation of the curve 2/122

LSLS = [(400)2 + (300)

2]

1/2 = 500.

From Eq. 5.9, z = - 500/500 = -1

From Table 5.2, the probability of failure of the structural element is 0.1587 i.e. 15.87%,

which is too high for many practical applications.

Design example 5.1 – Estimate the probability

of failure of a structural member IIISolution to reduce the probability of failure:

• Impose better quality measures on the material to reduce the

standard deviation of the strength.

• Increase the cross-section of the element to reduce the stress.

If the standard deviation of the strength is reduced to = 200, the

standard deviation of the curve will be [(200)2 + (300)2]1/2 = 360,

z = - 500/360 = - 1.4, which gives a probability of failure value of

0.08 i.e. 8%.

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Alternatively, if the average stress is reduced to 1400 MPa, LS = 700 MPa,

z = - 700/500 = - 1.4, with a similar probability of failure as the first solution.

Design example 5.2 - Estimating the coefficient of

variation in material strength

Problem

If the range of strength of an alloy is given as 800 to 1,200 MPa.

What is the mean strength, the standard deviation and coefficient of

variation?

Solution:

• The mean strength can be taken as 1,000 MPa.

• The standard deviation σ can be estimated as:

σ = (1200 - 800)/ 6 = 66.67 MPa

• The coefficient of variation is then: = 66.67 / 1000 = 0.0667.

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Chapter 5: Summary I

1. Engineering design is an interdisciplinary process that transforms consumer needs into instructions that allow successful manufacture of the product.

2. A good design should result in an attractive and user friendly product that performs its function efficiently and economically within the prevailing legal, social, safety, and reliability requirements.

3. Major phases of design are:

• conceptual design,

• configuration (embodiment) design, and

• detail design.

Chapter 5: Summary II

4. A design code is a set of specifications for the analysis, design,

manufacture, and construction of a structure or a product.

A standard specification describes the characteristics of a part,

material, or process and contains both technical and commercial

requirements.

5. The factor of safety is used in design to ensure satisfactory

performance.

This factor is normally in the range of 1.5 to 10 and is used to

divide into the strength of the material to obtain the allowable

stress and/or the load to obtain the allowable load.

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Chapter 5: Summary III

6. The lack of homogeneity of a material property or variations in the externally applied load can be statistically described by:

• mean value,

• standard deviation, and

• coefficient of variation.

These parameters can be used to estimate a factor of safety and to calculate the probability of failure of a component and its reliability in service.

When the available material properties are not available in a statistical form, approximate methods may be used.