13 How and why products fail 131 Failure mechanisms

We have already established that variability, or the lack of control and understanding of variability, is a large determinant of the quality of a product in production and service and, therefore, its success in avoiding failure. In addition, understanding the potential failure mechanisms and how these interact with design decisions is necessary to develop capable and reliable products (Dasgupta and Pecht, 1991). It is helpful next to investigate the link between the causes and modes of failure and variability throughout the life-cycle of a mechanical product.

Mechanical failure is any change or any design or manufacturing error that renders a component, assembly or system incapable of performing its intended function (Ullman, 1992). However, it is also possible to suggest several key aspects of failure (Bignell and Fortune, 1992):

• A failure is said to occur when disappointment arises as a result of an assessment of an outcome of an activity.

• Failure can be a shortfall of performance below a standard, the generation of undesirable effects or the neglect of an opportunity.

• Failure can occur in a variety of forms, namely: catastrophic or minor, overwhelming or only partial, sudden or slow.

• Failure can arise in the past, present or future.

• Failure will be found to be multi-causal, and to have multiple effects.

For the product to fail there must be some failure mechanism caused by lack of control of one or more of the engineering variables involved. Most mechanisms of mechanical failure can be categorized by one of the following failure processes (O'Connor, 1995; Rao, 1992; Sadlon, 1993):

• Overload - static failure, distortion, instability, fracture

• Strength degradation - creep failure, fatigue, wear, corrosion.

Figure 1.15(a) shows the results of an investigation to find the frequency of failure mechanisms in typical engineering components and aircraft components. By far the most common failure mechanism is fatigue. It has been suggested that around 80% of mechanical failures can, in fact, be attributable to fatigue (Carter, 1986). Failures caused by corrosion and overload are also common. Although the actual stress rupture mode of failure is cited as being uncommon, overload and brittle fracture

Percentage of failures

Mechanism

Engineering

Aircrafl

components

components

Corrosion

29

3

Fatigue

25

61

Brittle fracture

16

-

Overload

11

IS

High temperature

7

2

corrosion

S CCI

6

e

corrosion fatigue'

HE

Creep

3

-

Stress rupture

-

1

Wear/abrasion/

3

7

erosion

(a) Frequency of failure mechanisms

Cause

Percentage of failures

Engineering components

Aircraft components

Improper material selection

38

-

Fabrication imperfections

15

17

Faulty HT

15

-

Design errors

11

16

Unanticipated service conditions

e

10

Uncontrolled environmental conditions

6

Inadequate inspection/QC

5

-

Material mix

2

-

Inadequate maintenance

-

44

Defective material

-

7

Unknown

-

6

(b) Frequency of causes of failure

(b) Frequency of causes of failure

Key SCC - Stress currijiiDri Lmikinj, HE ■ Hydrogen embnlllement. HT ■ He.31 treatment, QG ■ Quality control

Figure 1.15 The frequency and causes of mechanical failure (Davies, 1985)

failures may also be categorized as being rupture mechanisms, distinct from strength degradation.

Figure 1.15(b) provides some insight into the reasons for the mechanical failures experienced. Some root causes of failure are found to be improper material selection, fabrication imperfections and design errors. Other causes of failure are ultimately

DESIGN ERRORS

PROCESSING AND MATERIALS

MAINTENANCE

UNPREDICTABLE LOADS

UNSPECIFIED

Figure 1.16 Designer's responsibility for mechanical failures (designer's share is shaded) (Larsson etal., 1971)

related to variability in production and service conditions. In general, several primary root causes of failure can be suggested (Ireson et al., 1996; Villemeur, 1992):

• Design errors

• Production errors (manufacturing, assembly)

• Handling/transit damage

• Misuse or operating errors

• Adverse environmental conditions.

From the above, there is a strong indication that the design errors are responsible for many mechanical failures found in the field. This is supported by another study, the results of which are shown in Figure 1.16. Through making poor or uneducated design decisions, the designer alone accounted for around 35% of the failures of those investigated.

Historically, the failure of products over their life-cycles, sometimes termed the 'bath-tub curve', can be classified into three distinct regions as shown in Figure 1.17. A detailed breakdown of the attributable factors in each region are also given below (Kececioglu, 1991).

Infant mortality period - Quality failures dominate and occur early in the life of the product. In detail, these can be described as:

• Poor manufacturing techniques including processes, handling and assembly practices

• Poor quality control

• Poor workmanship

• Substandard materials and parts

• Parts that failed in storage or transit

• Contamination

• Improper installation.

Useful life period - Stress related failures dominate and occur at random over the total system lifetime - caused by the application of stresses that exceed the design's

Quality failures

Stress-related failures

Wear-out failures

Quality failures

infant

mortality period

Useful life period

Wear-out period

Figure 1.17 'Bath-tub' curve showing typical life characteristic of a product (Priest, 1988)

strength (most significant period as far as reliability prediction activities are concerned). In detail, these can be described as:

• Interference or overlap of designed-in strength and experienced stress during operation

• Occurrence of higher than expected random loads

• Occurrence of lower than expected random strengths

• Misapplication or abuse

Wearout period - Failure occurs when the product reaches the end of its effective life and begins to degenerate and wear out. In detail, these can be described as:

• Poor servicing or maintenance.

1.3.2 The link between variability and failure

There exists a relationship between the failure characteristics of a product over its life-cycle, as described by the three periods of the bath-tub curve above, and the phenomenon of variability. It has already been established that the potential for variation in design parameters is a real aspect of product engineering. Subsequently, three major sources of undesirable variations in products can be classified, these being (Clausing, 1994):

• Production variations

• Variations in conditions of use

• Deterioration (variation with time and use).

The above fits in with the overall pattern of failure as described by Figure 1.17. The first two and sometimes even all three parts of the bath-tub curve are closely connected to variations.

The manufacturing process has a strong impact on component behaviour with respect to failure, and production variabilities arising from lack of precision or deficiencies in manufacturing processes lead to failures concentrated early in the product's life (Klit et al., 1993; Lewis, 1996). A common occurrence is that correctly designed items may fail as a result of defects introduced during production or simply because the specified dimensions or materials are not complied with (Nicholson et al., 1993). Production defects are second only to those product deficiencies created by inadequate design as causes of accidents and improper production techniques can actually create hazardous characteristics in products (Hammer, 1980).

Modern equipment is frequently composed of thousands of components, all of which interact within various tolerances. Failures often arise from a combination of drift conditions rather than the failure of a specific component (Smith, 1993). For example, typically an assembly tolerance exists only to limit the degradation of the assembly performance. Being 'off target' may involve later warranty costs because the product is more likely to break down than one which has a performance closer to the target value (Vasseur et al., 1992). This again is related to manufacturing variation problems, and is more difficult to predict, and therefore less likely to be foreseen by the designer (Smith, 1993).

Variations in a product's material properties, service loads, environment and use typically lead to random failures over the most protracted period of the product's expected life-cycle. During the conditions of use, environmental and service variations give rise to temporary overloads or transients causing failures, although some failures are also caused by human related events such as installation and operation errors rather than by any intrinsic property of the product's components (Klit et al., 1993). Variability, therefore, is also the source of unreliability in a product (Carter, 1997). However, it is evident that if product reliability is determined during the design process, subsequent manufacturing, assembly and delivery of the system will certainly not improve upon this inherent reliability level (Kapur and Lamberson, 1977).

Wearout attracts little attention among designers because it is considered less relevant to product reliability than the other two regions, although degradation phenomena are clearly important for designs involving substantial operating periods (Bury, 1975; Pitts and Lewis, 1993).

Many of the kinds of failures described above may be reduced by either decreasing variations or by making the product robust against these variations (Bergman, 1992). For example, the smaller the variability associated with the critical design parameters, the greater will be the reliability of the design to deal with unforeseen events later in the product's life-cycle (Suh, 1990). As stated earlier, with reference to an individual manufacturing process, variation is an obvious measure for quality performance and the link between product failure, capability and reliability is to a large degree embedded within the prediction of variation at the design stage. However, two factors influence failure: the robustness of the product to variability, and the severity of the service conditions (Edwards and McKee, 1991). To this end, it has been cited that the quality control of the environment is much more important than quality control of the manufacturing processes in achieving high reliability (Carter, 1986).

In order to quantify the sometimes intangible elements of variability associated with the product design and the safety aspects in service requires an understanding of 'risk'. The assessment of risk in terms of general engineering practice will be discussed next. This will lead to a better understanding of designing for quality and reliability, which is the main focus of the book.

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