NBSIR 76-11a/

A Methodology for Testing

Life-Cycfe Performance of

Consumer Products

Julius Cohen

Institute for Applied Technology

Center for Consumer Product Technology

August 1976

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

-QC

100

.U56

76-1157

1976

UXnOaML BUREAU

or 0TANOARD3

UBKART ^^^'^^^-^^^^ MAY U 1981

A METHODOLOGY FOR TESTING ^aomo

^ LIFE-CYCLE PERFORMANCE OF , uu^y

CONSUMER PRODUCTS no.

Julius Cohen

Institute for Applied Technology

Center for Consumer Product Technology

August 1976

U.S. DEPARTMENT OF COMMERCE, Elliot L Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Acting Director

Preface

This treatise is intended prLxarily for engineers witliout background

in reliability engineering who nave an interest in life-cycle i:>erf orinance

testing of consumer products. Much of the material should be of use also

to others interested in per fordnance testing in general.

Life-cycle perlonnance testing attempts to predict a probable useful

life and to assess the performance during that time interval. This is a new endeavor in which iJBS is pioneering and requires some significant

changes in approach and execution compared to usual product testing.

Although the published literature in reliability engineering is

extensive, most deal with data handling and statistical concepts, and

tacitly assunie the data are trustworthy. Relatively little appears on

experijr,ental approach or underlying engineering concepts, and what does

appear is fragnented and scattered.

The present work is a systematic and coherent body, and an attempt is

made to rake it didactic. The emphasis first is on understanding the basic

engineering concepts, then systematically applying them.

The aim of the methodology — a body of working concepts, terminology,

rules, and procedures — is to guide in the formulation of objective tests

(tests which exclude as far as possible the subjective element), to

standardize testing, and to make testing easier and better. Predictive

testing is inherently risky. Nevertheless, it is believed that by

following a scientifically-based methodology, the risk will be minimized.

Contents

Page

Preface

Chapter 1 Introductory Remarks 1

Chapter 2 Performance 3

Chapter 3 Failures 5

general description; classification; early failure,

random failure, wear-out failure, aging failure

Chapter 4 Reliability 8

introductory remarks; mathematical formulation

Chapter 5 Maintenance and Repair 16

Chapter 6 Useful Life 18

Chapter 7 The Concept of Stress 20 an engineering tool; performance in another perspective;

stresses are synergistic; stress is varied; performance

is non-linear with stress; time is a dimension of stress

Chapter 8 Accelerated Testing 24

Chapter 9 Some Statistical Considerations 25

Chapter US The Quality of Testing 27

validity; reproducibility; correlativity

Chapter 11 Feasibility 30

Chapter 12 Procedure 31

test design & development; test execution; test

analysis & interpretation

Epilogue 36

Glossary 37

Annotated Bibliography 40

Chapter 1

Introductory ReiT^arks

Although extensive and sOiietiiiies elaborate testing of consumer

products is made in industry (e.g., for design concepts and objectives, inwarranty costing, etc.), it is not directed to determining life-cycle

perforiTiance. That is, there is no effort generally to assess product

perfornrance during a determined actual useful life. The objective of this

work is to formulate a methodology for measuring life-cycle performance of

consumer products; the resultant data are to be used for life-cycle

costing.

Test iTiethodology is a generalized coherent body of operational

concepts, terminology, rules, methods, and procedures to be used in the

solution of performance testing problems on any consumer product. Test

method, on the other hand, is the particular approach for a specific

product only, made in accordance with methodology. Thus, for example, the

tests for clothes dryers, dishwashers, and water heaters will all be

different, but all will conform to the same methodology.

Apparently, no prior standard test metliodology, as such, exists even

for initial performance testing. Instead, testing is laissez-faire:

approaches vary widely according to philosphy, funding, staff and equipment

capabilities, product and urgencies.

The present formulation makes much use of reliability engineering

concepts and methods, but reliability engineering has been concerned mainly

with the military and aerospace where operating life is sometimes very

short and reliability means close to 100%. For example, maintenance is

geared to stave off any wear-out failure, as the consequences of failure

could be dire, and parts are replaced before they can wear out. Clearly,

for consumer products such a philosophy would not be sensible. Some

existing concepts needed to be modified and new concepts needed to be

created.

Too, there is no standardized terminology for communication. r<lany

existing tenns are imprecisely defined or are understood differently by

different persons, and a suitable terminology needed to be devised; for

example: What are the meanings of useful life and durability of consumer

products? A glossary of the terms developed in the present work is given

at the back of this treatise.

Life cycle of a consumer product begins with acquisition and ends with

the end of useful life, a concept defined subsequently in this work.

Performance-related factors which determine life-cycle cost are:

energy consumption

maintenance (frequency and action)

repair (frequency and action)

tiiTie duration of useful life

Co.-ts can ce derived fron the aoove, and togetner with acquisition

cost (purcr^.ase price, delivery, installation), and disposal costs or salvage value fonn the main bases for calculation of life-cycle cost.

Chapter 2

Performance

A hijniy siynit leant characteristic of an utilitarian consumer product

is j^erf ormance: to [:)erform a needed function is tiie reason the product

exists in the first place. iMoreover, the consumer is concerned with

reliability, durability, failure, maintenance and repair. These concepts

will be developed in this study, and all will be derived or constructed

from performance. The test engineer will measure performance per se, and

from those observations will identify failures, will distinguish between

maintenance and repair actions, and will calculate reliabilities. Clearly,

performance is the plinth of the test methodology, and it is important to

understand and to define the concept.

Performance in its elemental fonn is the accomplishments of a

cariponent or product. Each consumer product of interest has a main

utilitarian function: a refrigerator to produce and maintain coldness; a

vacuuiTi cleaner to clean or remove dirt from a substrate; a clothes dryer to

dry or remove moisture from damp laundry; etc. To perform satisfactorily,

however, the refrigerator cannot merely produce and maintain coldness; the

degree of coldness in the chamber must be proper: if too cold, foods will

freeze; if not cold enough foods will spoil prematurely. Similarly, the

vacuum cleaner must remove a sufficient amount or percentage of the dirt;

the clothes dryer must remove a sufficient amount of moisture in a reasonable thve, and at suitable temperature; etc. [Incidentally, some components, like a relay and a switch, need only operate to perform

satisfactorily. J How well each function is accorrplished can be expressed

qualitatively or quantitatively in terms of performance.

So far primary performance criteria have been addressed; there are also secondary performance criteria associated with each product which have

to be considered in determining whether the overall performance of the

product is or is not satisfactory. The vacuum cleaner will be used as an

illustration. In cleaning or removing dirt from a rug, the vacuum cleaner

must not cause undue wear or damage to the rug. It must also be portable

enough to be used easily. These requirements may conflict with the primary

performance characteristic, the cleaning ability.

Cleaning ability generally improves with increased suction; the latter

can be produced by incorporating a larger, heavier motor. Using a larger,

heavier motor increases the size and weight of the vacuum cleaner, which

decreases portability and may also cause more wear or damage to the rug.

These requirements are considered by the design engineer who tries to

strike the best canpromise, within the restraint of cost. Such mutually

dependent requirements must be considered by the test designer too.

Continuing with the example of the vacuum cleaner, assume that the

cleaning ability is determined by the percentage of dirt removed from the

3

rug. The CQTiposition of tne dirt is a factor in the cleaning; does it

contain hair, sand, caked md, dry soil, dust? Some ingredients are easier

to reTove than others. Ahat type of rug will be vacuaiied? A shag, for

exaj-nple, is much inore difficult to clean than an oriental type, with its

tight weave. How is the cleaner used? A naphazard, speedy vacuLsning will

not be as effective as a systematic, repeated, or prolonged vacuuming.

Obviously, the performance of a product is not a characteristic of the

product per se, but depends also on the use conditions and enviroamental

factors.

Use conditions is defined as the method or manner in which

a product is employed by an operator, including the load.

Everything surrounding an object, exclusive of the load, is its

enviropjTtent, but only a ILnited numiDer of things in the environment can be

expected to influence performance. These are designated environmental

factors, and examples are vibration, teiiperature, humidity, dust, etc.

Line voltage also will oe considered as an environmental factor. The

environmental factors may vary from product to product.

Enviroamental factors is defined as factors exclusive of the load,

external to and LTumediately surrounding the object, which influence its

performance.

An elemental, but inadequate definition of the term performance was

given at the oeginning of this chapter; now a more accurate and useful

working definition can be stated:

Performance is defined as the accomplishments of a component or product under specified use conditions and environrr^ntal factors.

In the vacuLETi cleaner example, given above, the use conditions are the

metliod of vacuuming (haphazard or systematic; repetitiveness) ; time

duration of cleaning; type of rug (shag or tight-woven); nature and

composition of the dirt*. Enviroamental factors are the line voltage and

(possibly) ambient tenperature and humidity.

*Tne dirt and rug are the load, defined as whatever is acted on or processed

by a product to accomplish its main utilitarian purpose. For simplicity,

the load (if any) will be regarded a use condition, rather than a separate

entity.

A

Chapter 3

Failures

General Description

If satisfactory performance is one side of the coin, unsatisfactory

performance is the other: There can be no in-between; no standing on edge.

Unsatisfactory performance constitutes a failure.

Failure is defined as the state of inability to perform a function or action to test specifications.

This points up the iiiportance of the performance criteria, prescribed

in advance of testing. Under one set of criteria performance might be

judged satisfactory;" under another, a failure. Further, the clarity and

the creciseness with which the criteria are described are of importance:

if specifications are not clearly and rigorously set down, the technician

or test engineer will be given much roan in which to make subjective

decisions or misinterpretations—an altogether deplorable situation. Thus,

failure is dependent not only on the product and the prevailing use conditions and environmental factors, but also on the performance criteria

chosen, and on their expository form.

In most cases involving consumer products, failure will be the

inability of the system to meet a minimum performance level; in some cases

involving camponents such as switches and relays failure will be merely

inability to operate; in others such as motors, performance which does not

fall into the specified bounds.

Classification

Failures are classified into one of four broad categories—early,

randcxn, wear-out, and aging — which, in practice may not be easy to

identify.

Early failure

SometLTies, owing to design errors, substandard camponents, poor

assembly, or damage incurred in transportation, a product will fail

relatively early in life. The time duration over which these so-called

early failures (also called burn-in or break-in failures) occur depends

partly on the test conditions and varies from product to product according

to the nature of the constituent defects.

Early failure is defined as the failure which results early in life

vrfien substandard or weak specimens are tested.

5

Early failures are characterized by a rapidly decreasing failure rate

(percentage of failures per unit tiire), as well as its occurrence early m

life.

Rando.T failure

Saite failures arise because of latent defects in design or quality, or undetectable material weaknesses. In the event that too large a stress is

applied suddenly or the cumulative effects of stress are excessive, then

the cOiTiponent will fail. In other cases, misuse, improper iraintenance, or accidents will result in failure. Breakdown is sudden, and is not

preceeded by symptaiis of deterioration. The failures discussed iTiirediately

above are all independent of timie; i.e., when such a failure will occur is

not predictable for any individual unit, and is just as likely to occur when a product is relatively young as when old. Aptly, sucn failures are

designated random (or chance) failures.* As will be sham subsequently

(Chapter 4) , random failure is characterized by a constant failure rate .

Random failure is defined as a time-independent failure caused by (1)

sudden stresses or their cumulative effects beyond the design strength of

the component; or (2) latent quality or material defects.

'Aear-out failure

Pernaps the most iir.portant type of failure to be considered in the

present undertaking is that due to wear.

Wear is defined as the irrpairment of an object or part by use; the

utility, strength, or quality is diminished.

Aear is a naturally occurring phenomenon expected to begin once the

object is put into service. The irore severe the use, the greater the wear;

generally, the more frequent and longer the use, also the greater the wear. Products which are well designed and well fabricated will wear gradually,

and signs of wear may not be perceptible or obvious for a long time.

Wear is frequently associated with gross or macroscopic motion

(sOiTietimes called dynamic wear) , a surface of one part or object sliding,

rolling, or otherwise contacting and rubbing against another. The

mechanisms of dynamic wear are (1) friction which generates heat, causing

thermal expansion and increased internal disorder; (2) material transfer,

usually in the form of particles, from one part to another; and (3)

mechanical breaking off of rough surface elevations, the debris often

acting as an abrasive which causes rapid wear; e.g., scored surfaces.

Thus, dynamic wear is often characterized by removal or loss of constituent

matter, usually starting at a surface and proceeding inwards. Other

exajTiples of dynanic wear phenomena are cracking due to fatigue and brittle

fracture due to impact.

*P^ndam, or chance, failure is sometimes referred to in the literature as catastrophic failure, but use of this term is not advised. It is a misnoner

because failures other than random can occur suddenly, or catastrophically.

6

Because wear has been defined here as resulting fran use, it is not

necessarily conditional on motion. Exariples of wear whicli occur even in

the absence of gross or inacroscopic iTotion of the object are electronic

copponents sjcn as capacitors and IV picture tubes, and incandescent light

Dulbs. Tne rrecnanisin of static wear may involve i"notion, but it would be on

a microscopic rather than macroscopic scale.

After prolonged or severe use, the amount of wear will become

intoleraole: the affected part will becane useless and manifested by

failure. This is the condition of wear out.

Wear out is defined as becoming useless from long or excessive wear or

use. The term Lnplies failure and therefore is equivalent to the term

wear-out failure,

wear-out failure is defined as a time-dependent failure caused by use.

Aging failure

Aging failure is allied to wear-out failure, except that deterioration

is caused by environmental factors, not use. Just as for wear-out failure,

aging failure is time dependent and is usually gradual. Similarly, aging

failure will occur after long exposure to environmental factors or exposure

to excessive environmental factors. An example is corrosion.

Additionally, objects with so-called shelf life such as electrolytic

capacitors, photographic film and dry cell batteries may experience aging

failures.

Aging failure is defined as a time-dependent failure caused by

environmental factors.

7

Chapter 4

Reliability

Introductory Remarks

Reliability is an important and desirable attribute of a consumer

product. If a product is perfectly reliable it will be available always to

perform satisfactorily on demand, and it will not break down during

operation.

Nothing, however, is perfectly reliable. Reliability refers to a future event or action, and predictions cannot be made with certainty.

Ultimately all tnings fail.

Reliability, then, involves probability. In sinple terms it is tiie

probability that a product will perform satisfactorily on deirand. The

probability may depend on the age or the accumulated operating hours; for

example, wear-out failures are more likely to occur, the older the product

is; random failures on the other hand are independent of age or accumulated

operating hours. The probability depends also on a specified interval of

time during which the product is expected to perform; the longer this

interval, the more likely the occurrence of failure.

A precise, working definition of reliability should involve

probability; an arbitrarily chosen future or present point in time; and an

arbitrarily specified time interval beginning at this future or present

point in time. Two other factors should be incorporated for explicitness:

(1) That the product will have survived to this future or present point in

tim.e; (2) that reliability, like performance on which it is dependent is

not a characteristic of the product per se, but depends also on the use conditions and environmental factors.

An adequate definition of reliability can now be given:

Reliability is defined as the probability that an object which has

survived to some particular time will continue to perform satisfactorily

for an additional specified period of time under prescribed conditions of

use and environmental factors.

Mathematical Formulation

General equations

Reliability, or probability of survival, R, can be written

8

where is the number of survivors of the test at the end of an arbitrarily chosen operating period, and is the total number of units

present at the start of this same operating period.

Tne number of units which fail to survive the operating period, N^,

must be the difference between the number at the start and the number of

survivors, because units must either survive or fail. Therefore,

= N3 + (2)

* is an arbitrary constant, or parameter.

« = T- 1 - IT (3)

O O

By differentiation,

dt

dK

N

1

^

dt

o

= - dR

dt" ° 3t ^5)

N dt dt

s s

(6)

The term on the left in (6) is defined as the failure rate (per unit

number of survivors) , X , or the instantaneous probability of failure per

unit.

N is the number of units at any time t bounded by the time interval

dt. if at the start, = N^, and (6) reduces to (4).

Substituting (1) into the right side of (6) gives

R M

*The term parameter is a possible source of misunderstanding, especially

between statisticians and engineers. To the former it means an arbitrary

constant whose values characterize a member of a system, as a family of

curves. To the latter it means one of a group of related actions

contributing to a larger action; i.e., a factor. Historically and strictly,

parameter is a mathematical term and it is advisable to use it as such.

9

Rearranging terms and integrating both sides of the equation.

Xdt = - In R (8)

At t = t, , R = 1, and

R = exp Xdt] (9)

Randcxn failure

Where failures occut only by pure c±iance, every unit in an assembly of

like units has exactly the same probability of failing. This probability

does not depend on tisde, per se (age or accumulated operating time).

The number of failures dN^ in an arbitrary time interval dt will be

proportional to the nurriber of workable units (survivors) at the start of

the time interval; i.e..

where k is a constant of proportionality, and the minus sign indicates a continuously decreasing population.

Rearranging terms

1

(11)

The constant is recognizable as the failure rate X , previously

defined; cf (6). Thus, for the special case of random failure, the failure

rate per unit number of survivors is a constant ( X in general is not a constant). As the number of survivors decreases, the number of failures

per unit time will also decrease proportionately to maintain X constant.

Substituting X= constant in (9) gives (12)

dN^ = - kNg dt. (10)

R = exp [ - xt]

Equation (12) is often written in slightly modified form:

R = exp [ - t/m] (13)

10

* where m = 1/ X is the inean time between failures (1«ITBF) For the special case X = constant, m is also constant.

Mean tiie bet^^een failures is simply the arithmetic average of the

tirries to failure of a satnpling. ''any engineers use this teroi when

referrirq to repairable objects, and the term mean time to failure (MTTF)

when referring to non-repairable objects. On the other nand,

mathenaticians may use the latter term to indicate expected life of

components, as distinguished from systems.

The :TrBF does not indicate the time a product can be depended upon to

run witiiout failure; the probability of survival is given by eq. (13). The

MTBF is primarily a figure of merit for canparing one object with another

or for rapid estimation of reliabilities. Some examples to follow will

iTiake clear the significance of ("frSF, as well as some implications of

reliability tneory.

In all three probleins, below, it is assuirted that the I-ITBF is 1000 h

and that random failures only occur.

(1) What is the probability that a single unit will survive

to 1000 h?

K = e -^/^ = ^-I'^Wl^OO . ,-1 = 35.3,

Thus, it is unlikely that the unit will survive to a tijne equal to the

i-fTSF. Put another way, if 100 units were placed on test, about 63 would

have failed before reaching 1000 h.

(2) i\hat is the probability that a single unit will survive the first

1 h of operation after being put in service; that a unit viable after 1000

h will survive to 1001 h?

The chances of survival are equal for equal lengths of time throughout

the entire random failure period of time. In eq. (13) t is Uie time

duration of any arbitrarily chosen operating period, with t = 0 designating

the start of this period; t is not in general a measure of age or total

accumulated operating life since the product was new.

^ ^ g-i/1000 ^ -0.001 ^ 93^9,

Fran example (1) above, however, the unit will nave only a 36. b%

probability of surviving to 1000 h. Still, if it does survive, the

probability it will survive the next 1 h is 99.9%.

These examples snould make clear also why reliability has been defined

as it has.

*In the publisned literature the term often is tacitly restricted to random

failures. tJevertheless, it is a generalized concept, applicable to wear out

as well,

11

(3) tvhat is the probability that a single unit will survive to

130 h?

If 13C units (devices, components, etc.) are run for 1 h their chance

of survival is still 90.5%. Similarly, this saine reliability figure holds

if 500 units were run for 0.2 h; 50 units for 2 h.

Equation (13) is for a single unit. In applying this equation to an assembly of N like units, t is replaced by the product Nt.

Wear-out failure

Figure 1 is an idealized schematic diagram of the instantaneous

failure rate of a large assembly of virtually identical new components or systems vs accumulated operating time since first put on test, T. (The

designation T is used to distinguish such accumulated tijne from t, any

aribtrarily chosen interval of time.) Components or systems are neither

replaced nor repaired on failure; simply removed.

Figure 1. Component or system failure rate as a function of

operating life.

Initially a few early failures assumedly appear, and as the number of

these substandard units dies out, the failure rate rapidly decays. Then

the random failure period of time sets in, characterized by the constant

failure rate. This is the period of highest reliability. This exclusively

random failure period of time eventually terminates with the onset of wearout failures, and the failure rate then increases rapidly. Random failures

can continue to occur, of course, but wear-out failures are expected to

predominate.

12

The MT3F, in general, has already been defined. The mean time between

wear-out failures (sometimes called mean vv-ear-out-life) is designated M to

distinguish it frcn mean time between randan failures, where m was used.

The time dependence of wear-out failures is usually approxiiTiate<J well

by a noriral, or Gaussian, distribution, and the failures tend to cluster

around M. Half the population, exclusive of any early or random failures,

will have failed by the time T = M is reached. In contrast, for the

exclusively random failure period 63% of the population will have failed by

the time T = m. Generally, however, m >> M, so only a small percentage of

the population fails randomly up to the onset of wear-out failures, and

approximately half of the population fails subsequently in the tLme period

bound by T = M.

Wear-out reliability is characterized rratliematically by two

parameters, or arbitrary constants, M and a , the standard deviation, and

it is convenient to use standard units in which a is used as a unit of

measure-nent of deviation. Thus, multiplication of a function by o standardizes it.

The failure density function or (failure) distribution f (t) is defined

as the failure frequency per unit (or component). Thus, fron (4)

^(^^ - N" dt" - 3t

ITie standardized failure density function or standardized (failure)

distribution 0 (t) is defined as

(J) (t) = of (t)

Similarly, the standardized failure rate curve r (t) is defined as

r (t) = a X (16)

From (7) and (14)

X=q^ (17)

a A = (18)

From (15) and (16)

r (t) = (19)

Usually wear-out failures display a normal, or Gaussian, distribution

which is given by

13

f (T) = exp - [ (T-M) V2 o^J (20)

where T is the cumulative tirae measured from T = 0 when the product was

first put on test.

Fran (14) the wear-out reliability (cumulative probability of survival

from T = 0) , R^, is given by

00 J

exp - [(T-M) V2 a^J dT (21)

Figure 2 shows a standardized failure rate curve for the Gaussian

distribution of failures, together with curves of and (T)

.

1

0.5

<f{T): Standardized wearout failure density function

i?jv(2'): Probability of surviving wearoul r(7^ Standardized failure rale curve

Standardized failure raler .

z <f{T)

-3.5 .0009 .9998 0.0009

-3.0 .OOAA .9987 0.0044

-2.5 .0175 .9938 0.0176

-2.0 .0540 .9772 0.0553

-1.5 .1295 .9332 0.1388

-1.0 .2420 .8413 0.2877

-0.5 .3521 .6915 0.5092

0 3989 .5000 0.7978

+0.5 3521 .3085 1.1413

+1.0 .2420 .1587 1.5249

+1.5 .1295 .0668 1.9386

+2.0 .0540 .0228 23684

+2.5 .0175 .0062 2.8226

+3.0 .0044 .0013 33846

+3.5 .0009 .0002 4.5000

-5<r— 4(r — 3» —2a —\c

Per hour failure rale:

Wearoul density function: f(t) -

M +la +2(T +3(7 +4(r +5ff

v*(T) " " Standard deviation in hours

Age r (hours)

Figure 2. The Gaussian failure rate (after Bazovsky, loc. cit.)

Notice that the failure rate increases steeply at T > (M-1,5 a) . The

probability of survival (reliability) to time T = (M-1.5 a) is 93%; to T =

M it is 50%.

The values of (T) together with R (T) can be obtained directly from

normal probability tables.*

*For example. Tables of Normal Probability Functions , NBS, Applied

Mathematical Series No. 23, (U.S. Government Printing Office,

Washington, D. C, 1953).

14

If random failures are not negligible with respect to wear-out

failures, the coribined cuTiulative probability of a product to survive both,

for the life period froQ T = 3 when the product is new, to an age T is

R = exp [ - XT] (22)

vdiere R is defined by (21). The shape of the curves in figure 2 would be

altered.

15

Chapter 5

Maintenance and Repair

.'•iaintenance is defined as an action performed on a satisfactorily

operative equipment to keep it performing satisfactorily for a longer time

than would otherwise be the case; i.e., maintenance postpones wear-out

failure of the equipment.

Maintenance embraces systematic inspection; prevention of incipient

failures, including replacement of parts; lubricating; and cleaning.

Effective maintenance results in a lower failure rate; hence, greater

reliability.

Repair is defined as an action performed on an equipment which does

not perform satisfactorily, to restore it to a satisfactory level of

performance by fixing or replacing parts which caused the malfunction.

Replacement and repair are not contradictory. Tne definition of

repair is to be applied to the unit in toto; i.e., the entire assembly,

rather than to its individual components. Thus, a component replaced does

not constitute a repair of the conponent, but the assembly, equipment, or machine, of which the component is a part rray be repaired by such action.

Although tiie military have done much work in maintenance engineering

and definitions of their terminology are often quoted in the published

literature, they are not suitable for the present work. For example,

according to English usage maintenance is upke^, and repair is fix.

Notwithstanding, the military do not distinguish between the two, and the

latter is considered corrective maintenance. That time has to be spent on

the equipment, thus reducing its availability, is important; not

distinguishing the type of action.

The situation for consumer products is not equivalent: consumers are

not indoctrinated in the jargon, and would be misled by omission of the

term repair. It is advantageous, for this work, to distinguish between

maintenance and repair.

Another source of possible misunderstanding is the militarily-inspired

term, m.aintainbility. It is not interchangeable with the term maintenance.

The former is a characteristic of design and installation relating to the

ease of performing a maintenance action. A similar distinction applies to

repairability and repair.

If maintenance has been prescribed, the design engineer has considered

it necessary to meet the design objectives of the product, and assumes it

will be carried out as specified. In laboratory testing (as well as controlled field testing), this is generally a valid assumption; in field

or home use, however, maintenance may not be done, or done properly.

16

Although naintenance actions and repair actions are sometimes

identical in nature (e.g., replacing a battery in an automobile before or after it has failed), the latter generally is more complex, more difficult

to perform, and costlier than the former. If repairs have not been irade

proficiently, subsequent failures will result prematurely [see also.

Chapters 3 and 10 (Correlativity) ]

.

17

Chapter 6

Usefid Life

Useful life, like performance, is a product characteristic required to

be measurable in the laboratory. As for performance testing, engineering

skills are necessary for objective determination. That consaT>ers may

dispose of still-functional products does not alter this fact.

A Definition of the term useful life hinges on the concept of

usefulness since the word life simply denotes a period of time.

Usefulness of a product is intimately bound up with functionality in

the sense of both intended performance and service expected as due by

virtue of the nature, structure, and condition of the product.

Specifically, if a product generally perform-s satisfactorily, does not

break do-wn in use, and will perform satisfactorily on demand, the product

is useful. ^These criteria can be sum.marized with a single term —

reliability , a concept in which pr<±ability of survival depends on an arbitrarily chosen time duration (see Chapter 4, Reliability).

Reliability itself is an engineering concept, but as the ultirrate aim

of the present work is to assist in the obtaimrient of product value, a suitable reliability, or one expected as due, must include economic

considerations. (Fulfillment of expectations considered due, and

acceptability are related.) Although determination of a suitably high

reliability rray involve consideration of the type, age, and initial or replacement cost of the product, the most important factors are repair cost

(relative to replacement cost) and reliability, per se. Usually

reliability is perceived as the time to or between failures.

If the cost of a repair is considered prchibitively high, the useful

life of an individual unit is effectively terminated at failure, regardless

of reliability. On the other hand, if the failure rate becorries excessive — i.e., the product becomes unreliable — the useful life is terminated

independently of any repair costs. In the case of non-repairable objects,

useful life of an individual unit terminates when failure occurs.

Useful life is defined as the time span over which the reliability of

the product is suitably high, or to a time of failure when it is considered

uneconomical to repair regardless of reliability, or until an irrepairable

failure occurs.

Satisfactory performance is inherent in reliability; if a product fails, it

is no longer reliable, unless repaired.

18

Note on the term durability

Maybe it's because the word is so old and familiar — it can be traced

back to the Middle Ages — that durability crops up so often in both

consuTier - and product testing vocabularies. This is why the term is being

treated here. To the consumer it means tlie lasting quality of a product —

the ability to endure; and although not articulated, it probably connotes

something akin to the technical term, useful life. As used by engineers,

however, it is perhaps the term most pluralistic in meaning and the most

vague. Sometimes it is supposed to mean useful life; sometimes, resistance

to wear out; sometimes it alludes to reliability; other times it refers to

ability to endure an abnormally high stress, including that caused by

misuse or abuse.

Tne following qualitative definition of the term is suggested:

Durability is the ability to endure prescribed operational and

enviromrtental conditions without failure due to wear out; i.e., durability

is the lasting quality over the useful life.

In any event, it is advisable to circumvent terminological confusion

where possible; for purposes of life-cycle performance the term useful life

is preferable.

19

Chapter 7

The Concept of Stress

An engineering tool

The concept of stress affords a valuable analytical tool for test

design and interpretation. Put simply, performance is regarded as reaction

to stress; stress being the ccncomitant combination of use conditions and

enviroorental factors. As performance permeates the iristhodology , however,

the consequences of stress are far reaching, affecting failure,

reliability, and useful life, as well.

To oe of value, testing aLus to relate laboratory results to normal

field, or horne, results. Differences between laboratory - and field

stresses can lead to disproportionately large discrepancies; even to

spurious results. Application of the concept should further testing

effectiveness. The concept will be elaborated and applied below.

Performance in another prespective

How a product is used and factors in its enviromient can sensitively

affect resultant performance. Analogous to the stress - strain

relationship of n-echanics, both use conditions and evnironmental factors

are considered as stresses acting on the product, and performance as the

reaction, or strain. Generally, larger the stress, and the greater the

strain, the greater the toll. Thus, increased performance is usually

obtained at sacrifice of useful life. Converse relations also hold.

Stresses are synergistic

Use conditions and enviroaTtental factors act concatiitantly on a product; they may occur independently of one another. Nevertheless, the

ccrnbined effects of each as stresses may be synergistic. Several examples

follow:

a) A soldered connection will rexiain intact if temperature is

elevated and there is little vibration; or if there is much vibration, but

temperature is low. If both temperature and vibration beca-ne excessive,

however, the connection will sever.

b) A part may corrode gradually, even if surrounded by high moisture,

provided temperature is law; or if terrperature is high, provided there is

little moisture. If both moisture and temperature are high, however,

corrosion will be rapid.

c) A battery whose use conditions are fixed will wear out more

rapidly, the higher the ambient teiperature.

20

Sometimes, the combined actions of use and environment result in a decrease of stress: A switch or relay not used may fail owing to dust

acajmulation or corrosion at the contacts. Use staves off aging failures

by .Tecn^nically removing dust particles or destroying the thin oxide film.

Stress is varied

Different persons may use the same, or nearly identical, products

differently according to their needs and habits; and even an individual

user may not always use a product identically each time. Environmental

factors may vary sLTiilarly; for example, because of temporal changes or climatic differences. Nevertheless, statistical metliods can be used to

construe so-called normal use conditions and environmental factors in the sense of being typical or average; in other words, normal stress. Testing

for every stress would not be feasible.

In testing for performance or life, however, to propose only the

normal stress for imitation generally would be unrealistic: Many products

customarily are overstressed at some time during their useful lives; for

example, the occasional extra-heavy load of laundry in the washer; the

current surge, or transient, in a speaker which burns out the coil; a brown-out in which reduced voltage overtaxes running electric motors (an

example in which low magnitude of a factor is more damaging than high).

However infrequently overstress occurs, its effect on the product's

deterioration can be much more severe than that of normal stress acting

more frequently, or for longer duration.

Stress pattern — the model proposed as the example to be imitated in

the laboratory — should not be based merely on a constant normal stress,

however much it may simplify testing. It should include the occasional

customary overstress.

Performance is non-linear with stress

Small changes or differences in stress generally make large

differences in performance and useful life. The relationships usually are

not linear because the laws of physics and chemistry which govern

performance and life are usually exponential. For example, for

incandescent light bulbs, a 10% increase over rated voltage increases light

output 40*, but decreases life over 60%,

Small discrepancies in assumed normal stress and overstress, vrfiether

by engineering judgement, based on experience and intuition, or statistical

analysis of human factor data, can lead to large discrepancies between

results in the laboratory and those actually occurring in the home.

Further, sufficient overstress can precipitate failures in the

laboratory which normally would not occur in actual home use, thus making

laboratory results invalid.

21

Two different kinds of tirne need to be distinguished for product

testing: clock and operating. The former refers to time of day or month;

the latter to the time a product is energized or used. In both cases time

generally signifies a duration. Further, time may be expressed as number

of cycles (of use)

.

Consur.ers usually conceive of product life in clock time without

regard for actual time operated; for example, 15 years for a vacuum

cleaner, 7 years for a color television receiver, 10 years for a water

heater. In the laboratory, operating tme or number of cycles is recorded.

Host products are not run or used continuously, so the total operating

time during its useful life is less than the corresponding clock time. For

example, a vacuum cleaner with an assumed useful life of 15 years (130,000

hours), typically will have accumulated 500 hours operating time.

As a matter of practical necessity, almost all life testing

"compresses" clock time in the sense of being carried out in less time than

the actual useful life of the product in the home. This is accomplished by

any one of tlie following methods: (1) increasing stress per se to

accelerate failures; (2) increasing frequency of use; (3) running the

product continuously.

The effects of tirve (duration - and frequency of use) on stress need

to be considered, as products may be over - or under stressed as a result.

Some examples follow:

(1) An environmental factor acts gradually and causes an aging

failure to occur in the home after a long time; e.g., corrosion, insulation

on motor windings, hoses in dishwashers and washing machines.

"Compressing" time will understress the product, and the failure will not

show up.

(2) An internal combustion engine is run nearly continuously or frequently on test instead of being cycled as in normal use. This

unders tresses the engine and retards wear, as moving parts are always

lubricated.

(3) An internal combustion engine run for very short times increases

stress through corrosive actions.

(4) A light duty motor run too long will be overstressed and fail;

e.g., some coffee mills and blenders.

"Compressing" time in the laboratory does not always understress or overstress. For some products, time can be "compressed", yet normal stress

maintained; vacuim cleaners and hair blower-dryers, are considered

examples

.

22

In a product such as a clothes dryer run continuously, the nx^tor may

oe put under stress, but the timer unders tressed.

"Compression" of clock tL-ne itself is not of engineering

significance; how it affects product stress, is. If understressed, product

useful life will oe prolonged; if overstressed, useful life will De

snortened, and non-typical failure modes also may result. If stress is

normal, numoer of cycles or operating tixte in the laboratory may be

equivalent to its home counterpart.

To refer to life testing as accelerated testing in the sense of

"compressing" time would convey little useful information. Further,

failures may be decelerated; not accelerated. In the next chapter,

accelerated and normal testing will be defined in terms of the significant

factor, stress.

23

Chapter 8

Accelerated Testing

Life testing in the laboratory can be done in one of two general ways:

(1) normal testing, and (2) accelerated testing. In the former, the use

conditions and environmental factors maintained during the testing are made

to approxi-nate those prevalent during actual normal hone use of the

product; in the latter, a use condition or environmental factor is made

more severe than normal in order to bring on failures more rapidly, and the

data are extrapolated to normal stress conditions. The advantages of

accelerated testing are (1) test results may be obtained with less testing

and in shorter time and (2) lower cost.

Notwithstanding, accelerated testing, in the absence of historical

data, is inherently risky — by making a use condition or environmental

factor more severe than normal, failure mechanisms different fron those

which would occur normally miay be brought on; further, it is difficult to

predict the performance at the normal stress level. There should be

justification for predicting what the characteristic would be at normal use conditions and enviroamental factors. Otherwise, predictions are little

more than guess work. Justification can take the form of historical data

or a quantitatively calculable knowledge of the processes governing the

predominant failure mechanisms.

Because of the risk involved in accelerated testing it is advisable to

do noriTial testing when practical; except if historical data are available.

If the product class were rapidly evolving, or the time to complete testing

were excessively long, or the cost were prohibitive, the normal test would

be considered Lmpractical. A way to reduce clock time while still doing a normal stress test may be to increase the frequency of use (an exajnple

where this should work is the vacuum cleaner).

In sumirary, where historical data are not available, because of the

risk in making accelerated tests, normal tests are advisable wherever

practical. Where normal tests are not practical, and this is often the

case, then accelerated tests should be considered, but accelerated tests

will not always be feasible and the specific product class must be

considered and evaluated. A necessary condition for making credible

accelerated tests is (1) historical data or (2) a calculable knowledge of

the main failure mechanians.

*See, for example, Rabinowicz, E., et alii, Transactions of the ASME, Series

B, Journal of Engineering for Industry 92, No. 3, 706-713 (Aug. 1970).

24

Criapter 9

Some Statistical Considerations

Tnis monograph deals with engineering test methodology. Additionally,

there exists well-developed statistical methodology which should be used

conjunctively. Treatment of that field is neither within the scope of the

present work, nor is it necessary; the reader is referred to some esteemed

publications on the subject. It is the intent of this chapter only to

indicate the importance of statistical methods in test planning and in

analyzing results, particularly with regard to validity.

Tne number of units which make up the total population of a given

product generally is very large, but it is feasible to test only a limited

number of units. The sampling is supposed to be representative of the

population, but unless properly chosen it may not be, and the results of

the test may be invalid — that is, a (mathematical) solution obtained for

the saiTipling, may not be a solution of the population.

Suitable sampling involves two factors: (1) sampling size; (2)

sampling selection. (1) The size is required to be sufficiently large to

be statistically significant. How large depends on the shape of the

failure distribution curve of the population. This may already be known

from prior testing or might be estimated initially; (2) the sampling

should be randanly selected.

Random does not irean the same as haphazard; the former is a deliberate

process and there are means for random selection. Randa-n Luplies that the

probability of selecting one unit is the same as any other (available)

unit. Lotteries, dice throwing, roulette are assumed tj^be random

processes; tables of random numbers may also be useful. On the other

hand, if all units are bought frcni the same vendor, for example, these may

have been consecutively manufactured within a short space of time. Lot

sampling — taking samples at spaced out intervals of tLne — is a means

for avoid inij such a problem.

*MIL-STL}-781B; I>^atriella, M. G., Experimental Statistics , NBS Handbook

91 (U.S. Government Printing Office, 1966)

**See, for example. Fisher, R. A. and Yates, F. , Statistical Tables for

Biological, Agricultural, and Medical Research , 6th ed.. Table XXXlll

(Hafner Publishing Co., New York, 1963).

25

statistical metlxx3s are available which enable an estiirat^ to be irade

of the confidence attached to a given set of statistical data. Confidence

coefficient (or confidence level) expresses the probability of

truthfulness; that is, it gives the probability that an interval of values

will include the parar.eter value. The data reported should include a statement of both level and limits.

*See, for exaiiple, Amstadter, B. L., Reliability Mathematics , cited in

Bibliography.

26
j

I

I
Chapter 10

The Quality of Testing

Life-cycle perforir.ance testing seeks to assess the qualities of a product with prolonged use. That it does so satisfactorily should not be

assumed; it is conditional on the quality of the testing meeting standards.

Tnus, the quality of testing itself needs to be assessed and the criteria

are (1) validity, (2) reproducibility, and (3) correlativity.

Validity

Validity is the truthfulness of the data; it relates to whether what

is clar-.ed to oe neasured is in fact r^easured. Causes of invalidness

include faulty instrUiTientation; human errors in measurement, judgment, and

reasoning; errors or negligence in sai?.pling selection; failure to recognize

or distinguish primary and extraneous factors.

Reproducibility

Reproducibility measurements establish the consistency of a test. It

is not sufficient that tests be reproducible within an individual

laboratory; to have utility there must also be inter-laboratory

reproducibility. Too, failure to obtain reasonable reproducibility may

cast serious doubt on the test's validity. Reproducibility depends in

general on (1) saiipling constancy or invariance; (2) measurement apparatus,

techniques, and skills; (3) following of instructions and carrying out of

test procedures.

To abet attainr.ent of reproducibility — particularly inter-laboratory — different samplings should be chosen judiciously to ensure they are virtually equivalent. For intra-laboratory testing to establish precision,

the time between successive tests should be kept short so that constancy of

the product may be reasonably assumed. Also, test procedures and

descriptions should be written clearly and precisely, to minimize the

possibility of misunderstanding or suojectiveness by test engineers or technicians.

Correlativity

The usefulness of life-cycle performance testing in the laboratory is

contingent on the relevancy of the results to tnose which actually occur in

the field, and the quality of the relationship is the correlativity.

Correlativity is defined as the quality or state of exhibiting

correlation between results of performance testing in the laooratory and

corresponding results obtained from field or home use.

I

27

Obtaining or demonstrating correlation, however, is difficult and may

be full of pitfalls: (1) it is difficult to deterTiine or predict what

actual noriTal use conditions and environmental factors in the field are;

(2) use and enviroa':>ent generally cover a much broader spectrum in the

field than is feasible to srnulate in the laboratory; (3) it is often

difficult to siTiUlate changing environiT,ental factors accurately over a prolonged period of tijiie; (4) product evolveiiient, manufacturing or material

changes, and variations in conformance of manufactured parts to design

specifications make a statistically significant and random sampling

difficult; (5) the requirement of reproducibility in laboratory testing may

impose conditions which are not representative of actual field use.

Laooratory data are obtained from controlled testing; field data may

be frotn (partially) controlled testing or uncontrolled activities, such as surveys.

Consider first uncontrolled field data. Information is obtained from

the consurrer on the frequency and nature of repairs, retention life, and

possibly tne reason for disposal. There are several pitfalls: (1)

iraintenance actions, which are iiTiportant factors in life, performance, and

reliability, may not have oeen carried out as specified; (2) products rray have been misused or abused; (3) the consumer may not have perceived

failures accurately or in conformity with tlie test designer's definition;

(4) repair actions may not have been made proficiently; e.g., primary

failure may not have oeen recognized and corrected; replacemient parts may

have been of substandard quality; vsorKmanship may have been poor. If

repairs are not proper, failures will occur prematurely; (5) tne retention

life is not necessarily the same as the useful life, and indeed products

are often disposed of before the end of their useful life; e.g., because of

obsolescence or for aestnetic reasons. Thus, uncontrolled field aata pose

a serious problem in validity: Do the data represent what are supposed to

be measured or represented, free of extraneous, unaccountable or unrecognized factors which cause deleterious results? Because very little

performance data on consum.er products over an extended life exists at the

present time, the problem is perhaps academic, but it does point up the

pitfalls inherent in any approach which attemipts to correlate laboratory

results with uncontrolled field results. It miay be possible to indicate

validity through use of statistical tools, but the problems are awesome.

It is desirable, perhaps even necessary, to resort to controlled field

tests instead. Here, a sufficiently large number of new units, randomly

chosen together with those for laboratory testing, are placed in the field

and periodic inspections are made by qualified technical personnel with

appropriate test equipment and parts, to ensure that prescribed maintenance

is done, to confirm the accuracy of the consumer's perception of failure

relative to the test specifications, and to carry out repairs proficiently,

when necessary. Only the stress pattern (use conditions and environmiental

factors) is not controlled. On the other hand, in the laboratory, the

stress pattern is controlled, and this is presumed to constitute the only

difference between laboratory and field tests.

28

Controlled laboratory and controlled field tests together constitute

an experi.T:ent in which there are presiirr^bly only two variables: laboratory

stress pattern and field stress pattern. The chances of deTonstrating

correlation are considered good and the analysis of results is much

SLT^lif ied.

29

Chapter 11

Feasibility

Although not an engineering concept, the term feasibility is treated

here oecause it is an important consideration in test planning.

A coTTaTon meaning of feasiole is: capable of being done or carried

out, and tnis implies being given the proper conditions. Just as performance is not an attribute of an equipment or object per se, but

depends also on the stress (use and enviroaTrental factors) , so feasibility—the quality of being feasible—is not a sole function of the

plan, test, or action being considered, but depends also on constraints and

conditions. ExaiTiples are: funding, manpovs'er, equipment and alloted time.

Given the same plan or test, it may be feasible under certain specified

conditions; not feasible under others.

iNhether or not a plan or test is feasible thus cannot be answered with

any assurance unless the conditions are specified. Because of its

Lrportance, the conditional aspect should be made explicit, rather than

remain LTplicit. The term feasible therefore is redefined:

Feasible means capable of being carried out under specified conditions

such as funding, manpower, alloted time, and equipment.

30

Chapter 12

Procedure

Tnis chapter sets forth in outline form a suggested procedure for use

in formulating tests. An understanding of the concepts discussed aoove is

prerequisite to intelligent application.

1. State objectives

exaiTple: To test life-cycle performance of a widget.

2. List data to be presented

exaniple: a) proi^able useful life, together with confidence level and

limits;

b) number and type of maintenance actions during useful life;

c) number and causes of failures; repair actions (components,

materials, time to repair) during useful life;

d) energy consumed during useful life.

3. Search & familiarization

a) sampling characterization

1) hanogeneous or heterogeneous? (If heterogeneous,

different tests for different units m.ay be required.)

2) repairable or non-repairable?

3) product evolution slow or rapid? (If slow, historical

data will be of great value; in its absence, tests may

still be worthwhile. If rapid, historical data may be

of little or no value: the product may be obsoleted

by the time tests are comipleted.)

D) historical data , particularly as regards failure modes,

mean tijues to - or between failures, and stresses. (For

slowly evolving products, historical data, if available,

will not only greatly facilitate testing, they may also permit

accelerated, or overs tress, testing to be made.) In-war ranty

data, service records, laboratory results are all useful

information.

c) existent tests — if available, review for applicability.

31

d) published literature — service manuals, trade journals

market research reports, etc.

e) discussions — with design and test engineers.

f) inspection ; design; construction; materials; operation.

Identify major working parts and likely wear-out contenders

(low MTTF)

.

4. Test design & development

a) human factors input

1) determine use conditions; load; use-environment,

all as a population distribution — (these may not

be independent variables);

2) frequency and duration of use;

3) how was information obtained? (e.g., questionnaires,

observations)

;

4) number in sampling;

5) is sampling random? How chosen?

6) is sampling representative of user population?

7) what is the confidence level of these data?

b) identification of stresses

which use- (including load) and use-environment conditions are known to or believed to affect performance significantly?

Consider use and environment singly and in combination.

Examples of possible environmental factors: ambient temperature;

humidity; dust; mechanical shock; vibration; temperature and

humidity; temperature and vibration.

c) selection of stress pattern

1) if historical data are available, consider accelerated testing

and how this test might be integrated with the previous; if no historical data are available, avoid overs tress (except for that

customarily expected) and attempt to use normal stress testing

where possible.

2) restrict testing to priine stresses only (side exper iui^nts may

be desirable to help in determination).

32

3) prescrioe use, load, and environmental factors; specify

allowable ranges; include customarily expected over-stresses

prescribe operating tLiie schedule.

d) stress analysis

1) list frajor components of product, especially those known

to or believed to fail typically in use.

2) indicate whether these conponents will be over-, underor normally-stressed by stress pattern.

3) consider desirability of altering stress pattern,

above [4.c)3)].

e) performance and failure specifications

1) state rain utilitarian function (primary performance

attribute) ; specify (usually miinimum) level of

acceptable performance;

2) specify salient secondary performance criteria.

Examples to consider are degradation of the load

Dy the product; excessive noise; excessive vibration;

leaking of liquid or gaseous fluids.

f ) specification of useful life criteria

1) specify minimum acceptable reliability;

2) si:>ecify maximum acceptable repair cost;

3) for allowable repairs, specify reliabilities which

will justify costs.

It is advisable to obtain human factor data to assist in determination

of the preceding three criteria.

For repairable products useful life terminates with failure to meet

criterion 1), 2) or 3) above. For non-repairable products useful life

terminates with failure.

For products where average retention life is less than probable

useful life, it m.ay be desirable to terminate testing at a time equivalent

to the former.

g) sampling

1) estimate number of units required for a desired confidence

level and limits; or

33

2) estimate confidence level and limits for the nuiriber of

samples obtainable. Are they satisfactory?

3) arrange to obtain random sampling, if possible.

h) repair readiness

1) are replacement parts readily available? If not,

stock selected parts.

2) are qualified repairirten on site? If not, secure or have personnel trained.

i) controlled field tests

1) how many units (or households) are desirable or are accessible?

2) how are units (households) to be selected? Consider, for

example, geographic location; income level; size of family.

3) how are field data to be obtained? Scxne possibilities

are: manufacturer's in-warranty records; other service records.

Can arrangements be made with consumer to provide free or discounted repairs beyond warranty period? Can inonitoring

instrumentation be installed on products?

j) feasibility

1) consider funding, qualified personnel, equipfnent,

facilities, time duration of project, etc.

2) estimate what may be feasible under the above constraints.

3) modify statement of olpjectives or data to be presented, if

indicated by 2), above.

5. Test execution

ALL PERFORMA^^JCE CRITERIA AND USEFUL LIFE CRITERIA MUST BE SPECIFIED IN

ADVT^NCE OF TESTIt>)G!

a) simulate prescribed use conditions and environmental

factors;

b) monitor performance frequently;

c) note, inspect, and identify all failures.

1) classification? (Early; random; wear out.) Don't guess — if

unable to identify, pass up.

34

I

2) failure irtechanism?

d) jidKe qualified repair, if appropriate (see 4.f), tliis

chapter); return unit to test.

e) maintenance

1) carry out all maintenance actions prescribed by rranufacturer

(exception: if maintainability is so poor as to preclude a maintenance action by consumer, this may be omitted at

discretion of test designer, but the omission should be noted).

2) correction of incipient failures are to be included as maintenance actions.

f) keep complete records of data.

g) testing of a unit terminates in accordance with useful life

criteria (or in special circumstances, earlier — see Chapter 6)

.

Test analysis & interpretation

a) data handling: no omissions; no rationalized "laundering";

no unjustified extrapolations. Early failures are not counted

in reliability and life testing.

b) is test acceptable?

1) validity?

2) reproducibility?

3) correlativity? (Do failure modes and distributions of

laboratory - and field tests agree?)

c) re-appraise assa^rptions in light of field data.

d) repeat stress analysis in light of field data.

e) revise for future tests, as required.

35

Epilogue

Life-cycle performance testing, like rriany other kinds of testing, is

not an exact science. It attempts to predict a probable life, judgments

freauently are required, and many of tne interpretations and conclusions

follow from a course of action prescribea, presuinably, by an authority.

Nevertneless, testing properly practiced is a science, albeit a difficult

one. Unfortunately, in practice the difficulties are often aggravated by

pressures of time, facilities, and funding.

Testing is inperfect, but that is no reason not to test. The question

is whether the testing likely will provide new or better needed information

compared to what presently is available, and whether it justifies the cost.

Testing is car'plex problem solving. Regarded and practiced as a cumulative and continuous science, it will becone increasingly upgraded,

increasingly valuable.

36

Glossary

accelerated testing ; testing made ^'nile .uaintaining a supernoriTval or excessive stress pattern — i.e., the laooratory use conditions or environmental factors are more severe than those prevalent during normal

field or ho.-ne use.

aging failure : a time-dependent failure caused by enviroamental factors.

burn in ; the process of testing previously unused devices, objects, or products to weed out substandard or weaK specimens.

burn-in failure ; see early failure.

break in ; see burn ia.

chance failure ; see randoin failure.

correlativity ; the quality or state of exhibiting correlation between

results of performance testing in the laboratory and corresponding results

obtained from field or heme use.

duraoility ; the lasting quality of a product; the ability to endure

prescribed use conditions and environmentals factors over the useful life.

early failure ; the failure which results early in life when substandard or weak specimens are tested for the first tLne; break-in failure; burn-in

failure; "infant" mortality.

enviroarental factor ; factors exclusive of the load, external to and

ijTuTCdiately surrounding the object, which influence its performance.

failure ; the state of inability to perform a function or action to

specifications; for finished products (systems) minimum levels of

performance are usually specified.

feasible ; capable of being carried out under specified conditions such as funding, manpower, alloted time, and equipment.

feasibility ; the quality of being feasible.

load ; whatever is acted on or processed by a product to accomplish its

rrvain utilitarian purpose.

m.a inta inabil i ty ; a characteristic of design and installation relating to

the ease of performiing a maintenance action.

37

fTiaintenance : an action performed on a satisfactorily operative equipment

to keep it performing satisfactorily for a longer time than would otherwise

oe tne case.

.ifeG/i-tL-^e-between-f allures ; the arithnetic average of the times to failure

of a sa'qpling of repairable objects.

mean-tiire-to-f allure ; the arithmetic average of the times to failure of a

saTipling of non-repairable objects.

normal testing : testing made while maintaining a normal stress pattern —

i.e., the laboratory use conditions and environmental factors are assumed

to approximate those prevalent during field or hame use.

parameter ; statistical — an arbitrary constant whose value characterizes

a member of a system, as a family of curves. Engineering — one of a group

of related actions contributing to a larger action.

performance ; the accomplishments of a device or equi^xiient under specified

use conditions and environiTtental factors.

prL-nary failure ; failure of a component as a direct result of some deterioration of the camponent itself.

random failure ; a time-independent failure caused by (1) sudden stresses

or their cumulative effects beyond the lower limits of the design strength

of the component; (2) latent quality defects.

reliability ; the probability that an object which has survived to some particular time will continue to perform satisfactorily for an additional

specified period of time under prescribed conditions of use and

envircximental factors.

repair ; an action performed on an equipment which does not perform

satisfactorily, to restore it to a satisfactory level of performance by

fixing or replacing parts which caused the mialfunction.

repairability ; a characteristic of design and installation relating to the

ease of performing a repair action.

retention life ; the time span from inception of operation to disposal

(which riiay be before or after the end of useful life).

secondary failure ; failure of a component as a result of some deterioration in some other component.

stress ; the concc«nitant cof±»ination of use conditions and environmental

factors.

38

test method ; a particular approach or procedure for testing of a specific

attr i^:>ute{s) of a consumer product.

t fjT. r. t rrx>o i ogy ; a generalized , corierent body of Tiethods, procedures,

^f.irti concrrpts, rules, ard ^x/stulates, to rje used in tne solution of

testing probleirts on any class of consurrter product.

use conditions ; the method or manner in which an equipment is employed as determined by an operator; including the load.

useful life ; the tme span over which the reliability of the product is

suitably high or to a time when it is considered uneconomical to repair

regardless of reliability; or until an irrepairable failure occurs.

wear : the impairnent of an object or part by use ; the utility, strength or quality is dLninished.

wear-out ; to become useless from long or excessive wear or use . The term

implies failure and therefore is equivalent to the term wear-out failure.

wear-out failure : a tiine dependent failure caused by use; see wear out.

wear-out life ; the tLiie it takes one-half the original units, exclusive of

any early or random failures, to fail because of wear out.

«

39

.\nnotated Jihliography

Pooerts, Nocuan .-i. , ••'iatheTiatical '-'.ethoas in Reliability Engineer inc| , (.-IcGraw-Hill Book Co., New York, 1964).

This book treats tnose mathematical ideas and techniques which the author

had found to be useful in tne field of reliability engineering. Sane of

the statistical fundaTentals of the theory, as well as mathematical ways of

handling data are presented. Although statistical factors to be considered

in setting up tests are discussed, no guidance to making these tests

experimentally meaningful is provided. In short, the book title is

appropriate, as the emphasis is on irathariatics, but one wishes tliat the

author, a physicist, had seen fit to include more discussion of the

engineering aspects of the subject.

Haviland, R. P. , Engineering Reliability and Long Life Design , (Van

Nostrand, New York, 1954).

The intent of this volurie is to provide concepts and techniques essential

to attaining engineering solutions to reliability problems. The intent is

noble, the execution inadequate. Nevertneless, this is a useful book,

particularly for its coverage of performance and life testing.

Goldman, A. S., and Slattery, T. B., Maintainability ; A Major Element of

System Effectiveness , (John Wiley and Sons, Inc., New York, 1974).

This systems-oriented text deals with fundamentals that interrelate

maintainability to systari effectiveness and cost, and is heavily slanted to

military considerations and applications.

Barovsky, I., Reliability Theory and Practice , (Prentice-Hall, Inc.,

Englewood Cliffs, New Hersey, 1961).

This book can be recorrLrended to engineers who desire to learn how

quantitative theory may be applied to reliability testing of components and

systems. Published fifteen years ago, it is still in print and it is still

tirely, which — considering that a great deal of work has since been done

in this field — is quite commendable. The level is intermediate, and

knowledge of elementary calculus will suffice to follow mathematical

derivations. Tne author is a design engineer with a good feel for the

practice of reliability. A word of caution; because the author writes

from experience in the aerospace industry where reliability means virtually

no failures and parts are replaced well before they can wear out, some statements and remarks may appear unrealistic or even absurd, if thought of

in relation to consum^er products.

Amstadter, Bertram L., Reliability J'lathematics , (McGraw-Hill Book Company,

New York, 1971)

.

40

This readable book might well oe titled Reliability Mathematics for

Engineers . It emphasizes the application of statistics to reliability

engineering, ratlier than the rigorous derivation of formulas, and it meets

a need. Ample use of graphs, tables, and illustrations facilitate its

application. It is nighly recomi-nended to engineers who aspire to do their own statistical planning and analysis — which is probably desirable and

certainly camniendable — and are willing to make the effort to teach

themselves.

Kivenson, Gilbert, Durability and Reliability in Engineering Design , (Hayden Book Co., Inc., New York, 1971).

The author's aim is to provide the knowledge essential to designing better

and more reliable functional units by examining various breakdown

mechanisms causing degradation, and by providing techniques for mininiizing

deterioration. He does not succeed: One does not become a design engineer

by reading a book; the subject is much too extensive, complex, and esoteric

for that. On the other hand, the reader can expect to get some appreciation of the difficulties facing the design engineer, his

limitations, and the need for testing and redesign. Tnat "Durability"

appears in the title is puzzling: the term hardly appears in the text and

the subject is oarely treated; this is essentially a book on reliability

and engineering design for reliability. Nevertheless, this slim volume is

useful, especially for its treatment of the engineering as well as the

elementary methematical aspects of reliaoility analysis,

Gilmore, H. L. and Schwartz, H. C, Integrated Product Testing and

Evaluation , (John uiley & Sons, New York, 1969).

The authors have produced a dichotomy: a part deals with the managerial

aspects of product testing; the balance with the engineering. That each

should appreciate the other's problems is certainly a valid point; that

each will make the effort to do so is debatable. In any event, this book

should prove useful to engineers, particularly for its detailed treatment

of environm.ental testing such as temperature, humidity, mechanical shock

and vibration and for its treatment of statistical techniques.

Finney, D. J. Experim.ental Design and its Statistical Basis , (The Univ., of

Chicago Press, Chicago, 1955).

This simple and elegant little book was written by a statistician for

biologists, but it may be read by others as well with profit. Fearing that

his intended reader is not only unknowledgeable in mathematics, but would

be frightened off by it, the author adopts a sympathetic manner, offers

encouragement and rewards for labor, and keeps the mathematics to a bare

easy-to-swallow minimum. Yet, he does explain a few basic ideas on

statistical analysis and shows its importance in formulating and analyzing

experiments, specifically those in biology. Although its application to

consuiTier product testing is limited, it is almost delightful reading and is a good introduction to statistical appreciation.

41

NBS-1 14A IR E V 7- 7i) U S. DEPT . or COMM. BIBLIOGRAPHIC DATA

SHEET

1. I'lHI I' A 1 ION OK Kl I'OK I NO. NBSIR 76-11572. (lov't Accession

No.

3. Recipient's Accession No.

4. rm.i: and sum in. i A Methodology for Testing Life-Cycle

Performance of Consuaer Products

5. Publication Date August 1976

6. Performing Organization Code

7. AUTHOR(S) Julius Cohen

8. Performing Organ. Report No.

9. PKKFORMING ORC.AN'IZATION NAMK AND ADDRKSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/«'ork Unit No. 4460123

11. Contract/Grant No.

12. Sponsoring Organization Name and ( omplctc Addrcs.s (Street, City, State, ZIP)

Same as No. 9«

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLHMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificanl information. If document includes a significant

bibliography or literature survey, mention it here.)

Life-cycle performance testing of consumer products attempts to

predict a probable useful life and to assess the performance

during that time duration. A me tho do lo gy --wo rking concepts,

terminology, rules, and p ro ce d ure s -- to help accomplish this aim

is presented. A glossary and an annotated bibliography are

included.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Consumer products; life-cycle performance; methodology; reliability;

terminology; testing; useful life.

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