6 1. INTRODUCTION TO RELIABILITY IN MECHANICAL DESIGN
Nth Generation Design
Project
Team
Phase Five:
Implementation
Phase Four:
Detailed Design
Phase Two:
Design Specifications
Phase ree:
Conceptual Design
Phase One:
Needs Assessment
Figure 1.1: e iterative-interactive five-phase design process.
happen between any phase. For example, during Phase Four the project team could modify final
design option based on virtual numerical simulation and the physical testing on the prototype.
For another example, during Phase ree, the project team could back to work on Phase Two
to make some modifications. Description and procedure of the engineering design process such
as the five-phase engineering design process is the summary of past successful and unsuccessful
design experience. It is a piece of important and critical knowledge and skill. e project team
should follow it. It does not mean that following the five-phase design process will guarantee
to have a successful design result for a design project. It only suggests that there will be a very
high possibility that design project will end with a failure or high cost or long period of time if
the procedure of engineering design process does not follow.
1.2 FAILURES IN ENGINEERING DESIGN
Mechanical components are designed to execute required performances/functions under the
specified working environment and loading conditions (design specifications) within the spec-
ified life of service. However, due to the practical and economic limitations, a perfect design
does not exist. So, some of the mechanical components will certainly fail. Failure is defined as
a phenomenon that mechanical components cannot satisfy the design specifications or not pro-
1.2. FAILURES IN ENGINEERING DESIGN 7
vide the required performance. For example, if a bar under the rated loading fractures, the bar
is said to be a failure. If a shaft under the specified working environment and loading condition
has excessive deflection, which might affect the proper gear engagement on the shaft, the design
of the shaft is said a failure. For another example, if a camera can take a photo, but the image
of the photo is not clear, the camera is said to be a failure.
e well-known failure-rate curve [3], known as the bathtub curve, is widely utilized to
describe and explain the failure of electronic, mechanical, and electro-mechanic components.
e schematic of a typical bathtub cure of the failure rate vs. time is shown in Figure 1.2. e
horizontal axis is the time of the component in service. e vertical axis is the failure rate, which
is defined as the frequency failures per unit of time. For example, if a design component with
20,000 units in service for 5,000 hr have 254 failures, the failure rate of this design component
will be: (254/20000)/(5000), that is, 2:54 10
6
failure per hour or 2:54 failure per million hour.
e typical bathtub curve consists of three different stages. e first stage is the early stage of
the product life known as the infant mortality stage, where there is a rapidly decreasing failure
rate. e failures in the first stage are mainly due to manufacturing defects and poor-quality
control procedures. ese failures can be prevented and eliminated if the careful manufacturing
and proper quality controls are applied during the production. As these defective components are
replaced/repaired, the failure rate decreases as time progresses in the first stage. e second stage
has an almost constant failure rate, which is known as the useful life stage. e constant failure
rate of the product indicates that there is no dominant failure mechanism to induce a failure;
that is, the failure is mainly due to random causes. For example, a mechanical component failed
due to accidental overload when the material strength of this component was in the lower end of
such material’s normal strength range. Products’ life should be designed to be in the second stage.
e failure rate of some mechanical products in the second stage are listed in Table 1.1 [3, 4].
e failure rates listed in the table represent the current industrial product design level with both
practical and economic considerations. e third stage is known as the wear-out stage, where
there is a rapidly increasing failure rate. e dominant failure mechanism of the products in this
stage is “wear-out,” such as the cumulative irreversible fatigue damage due to continuous cyclic
Figure 1.2: e bathtub curve.
8 1. INTRODUCTION TO RELIABILITY IN MECHANICAL DESIGN
Table 1.1: Failure rate of some mechanical products [3, 4]
Mechanical
Component
Failures Per Million
Hours
Mechanical
Component
Failures Per Million
Hours
Accelerometer 35.1 Gear 0.17
Actuator 50.5 Gear shaft 6.7
Air compressor 6.0 Gyroscope 513.9
Air pressure gauge 2.6 Heat exchanger 1.1
Ball-bearing 1.1 Hydraulic valve 9.3
Boiler feed pump 0.42 O-ring 2.4
Brake 4.3 Roller bearing 0.28
Clutch 0.6 Shock absorber 0.81
Diff erential 15.0 Spring 5.0
Fan 2.8 Storage tank 1.6
Gasket and seal 1.3 thermostat 17.4
loads. It is strongly recommended that the designed products’ life would not be extended into
the wear-out stage because the failure rate would be very high.
Mechanical components have many different failure modes such as static failure, fatigue
failure, creep failure, corrosion failure, wear failure, instability failure, and excessive deflection
failure [5–7]. However, static failure, and excessive deflection failure, and fatigue failure will be
focused on in this book.
• e static failure. When a component’s maximum stress due to working load exceeds ma-
terial strengths such as yield strength and ultimate strength, the component is defined as
a static failure. For example, a component of brittle material will fracture when the com-
ponent’s maximum stress exceeds the material’s ultimate strength. For another example, a
component of ductile material will have excessive deflection and lose the capability of car-
rying out working load when the component’s stress exceeds the material’s yield strength.
• Excessive deflection failure. Mechanical systems typically have at least one moving com-
ponent. When the excessive deflection of a component causes the mechanical system to
fail to satisfy the required performance and design specifications, this is defined as an ex-
cessive deflection failure. For example, excessive deflection of a shaft might cause big noise
during the gear engagement, which might exceed the permissible sound level.
• e fatigue failure. When a component is subjected to cyclic load, the component will be
gradually degraded due to fatigue damage. e fatigue damage is irreversible and is accu-
mulated during the service. After the accumulated fatigue damage reaches a critical value,
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