V
Preface to Handbook on Experimental Stress Analysis
Experimental science does not receive Truth from superior Sciences: she is the Mistress and the other sciences
are her servants ROGER BACON: Opus Tertium.
Stress analysis has been regarded for some time as
a distinct professional branch of engineering, the object of which is the determination and improvement of
the mechanical strength of structures and machines. Experimental stress analysis strives to achieve these aims
by experimental means. In doing so it does not remain,
however, a mere counterpart of theoretical methods of
stress analysis but encompasses those, utilizing all the
conclusions reached by theoretical considerations, and
goes far beyond them in maintaining direct contact with
the true physical characteristics of the problems under
considerations.
Many factors make the experimental approach indispensable, and often the only means of access, in
the investigation of problems of mechanical strength.
At our present state of knowledge it is remarkable
how quickly we can reach the limit of applicability
of mathematical methods of stress analysis, and there
is a multitude of comparatively simple, and in practice frequently occurring, stress problems for which no
theoretical solutions have yet been obtained. In addition to this, theoretical considerations are usually based
on simplifying assumptions which imply certain detachment from reality, and it can be decided only by
experimentation whether such idealization has not resulted in an undue distortion of the essential features
of the problem. No such doubt needs to enter experimental stress analysis, especially if it is done under
actual service conditions, where all the factors due to
the properties of the employed materials, the methods
of manufacture, and the conditions of operation are
fully represented. The advantage of the experimental
approach becomes especially obvious if we consider
that it is possible to determine experimentally the stress
distribution in a machine part in actual operation without knowing the nature of the forces acting on the part
under these circumstances, which proposition is clearly
inaccessible to any theoretical method of analysis. To
these major advantages we may add one more, from the
point of view of the average practicing engineer, whose
mathematical preparation is not likely to enable him to
deal theoretically with some of the complex strength
problems which he, nevertheless, is expected to settle
satisfactorily. To these men experimental methods constitute a recourse that is more readily accessible and
that, with proper care and perseverance, is most likely
to furnish the needed information.
Several principal methods and literally hundreds of
individual tools and artifices constitute the “arsenal” of
the experimental stress analyst. It is interesting to observe, however, that each of these devices, no matter
how peculiar it sometimes appears to be, has its characteristic feature and, with it, some unique advantage that
may render this tool most suitable for the investigation
of a particular problem. The stress analyst cannot afford, therefore, to ignore any of these possibilities. This
circumstance, together with the ever-increasing demand
on mechanical strength, will always tend to keep experimental stress analysis a distinct entity in the field of
technical sciences.
There has been a long-felt need of a comprehensive reference book of this nature, but, at the same
time, it was recognized that no one person could possibly write with authority on all the major experimental
procedures that are being used at present in the investigation of mechanical strength. It was proposed therefore
that the problem could be solved only by a concerted
effort which might be initiated most suitably under the
aegis of the Society for Experimental Stress Analysis,
and the writer was appointed as editor with complete
freedom to proceed with the organization of this undertaking. Invitations were sent to thirty eminent engineers
and scientists who were best known for their outstanding contributions in one or more of the specific branches
of experimental stress analysis. It was most impressive
to witness the readiness and understanding with which
these men, many of them not even associated with the
Society, responded to the request and joined the editor in contributing their work, without remuneration, to
the furtherance of the aims of the Society, which thus
becomes the sole recipient of all royalties from this
publication.
This being the first comprehensive publication in its
field, it may be of general interest to say a few words
about the method used in the planning and coordina-
VI
tion of the material. In inviting the contributors, I first
briefly out-lined the subject to be covered requesting, in
return, from each author a more detailed outline of what
he would propose on his respective subject. These authors’ outlines were subsequently collected in a booklet,
a copy of which was sent to each participant, thus informing him in advance of projected contents of all the
other parts of the book. This scheme proved of considerable help in assuring adequate coverage of all matters
of interest, without undue overlaps, repetition, or need
of frequent cross references. In the final plan, as seen
in the table of contents, the main body of the book was
divided into 18 chapters, each dealing with either a principal method, from mechanical gages to x-ray analysis,
or a major topic of interest, such as residual stresses,
interpretation of service fractures, or analogies. In addition to these, an appendix was devoted to the discussion
of three theoretical subjects which are of fundamental
importance in the planning and interpretation of experimental stress work. In the final outcome, not only the
book as a whole but also most of the individual chap-
ters turned out to be pioneering ventures in their own
rights, often constituting the first systematic exposition
of their respective subject matter. Another innovation
was undertaken in the treatment of bibliographical references, where an effort was made to review briefly the
contents of each entry, since it was found that the mere
titles of technical articles seldom convey a satisfactory
picture of their respective contents. Despite all precautions the book is bound to have errors and shortcomings,
and it is the sincere hope of the editor that users of
the book will not hesitate to inform him of possibilities
of improvement which may be incorporated in a later
edition.
In the course of this work the editor was greatly
aided by advice from numerous friends and colleagues,
among whom he wishes to acknowledge in particular the invaluable help received from B. F. Langer,
R. D. Mindlin, W. M. Murray, R. E. Peterson, and
G. Pickett.
Evanston, Illinois April 1950
M. Hetenyi
VII
Preface to the Handbook on Experimental Mechanics, First Edition
The Handbook on Experimental Stress Analysis, which
was published under the aegis of the Society for
Experimental Stress Analysis in 1950, has been the
comprehensive and authoritative reference in our field
for more than thirty years. Under the able editorship
of the late M. Herenyi, 31 authors contributed without compensation 18 chapters and 3 appendices to this
handbook. It received international acclaim and brought
considerable income to the Society for Experimental
Mechanics.
Since 1950, new experimental techniques, such
as holography, laser speckle interferometry, geometric
moire, moire interferometry, optical heterodyning, and
modal analysis, have emerged as practical tools in the
broader field of experimental mechanics. The emergence of new materials and new disciplines, such as
composite materials and fracture mechanics, resulted in
the evolution of traditional experimental techniques to
new fields such as orthotropic photoelasticity and experimental fracture mechanics. These new developments,
together with the explosive uses of on/off-line computers for rapid data processing and the combined use of
experimental and numerical techniques, have expanded
the capabilities of experimental mechanics far beyond
those of the 1950s.
Sensing the need to update the handbook, H. F.
Brinson initiated the lengthy process of revising the
handbook during his 1978-79 presidency of the Society.
Since M. Hetenyi could not undertake the contemplated
revision at that time, the decision was made to publish a new handbook under a new editor. Opinions
ranging from topical coverage to potential contributors
were solicited from various SEM members, and after
a short respite I was chosen as editor by the ad hoc
Handbook Committee chaired by J. B. Ligon. Despite
the enormous responsibility, our task was made easier by inheriting the legacy of the Herenyi Handbook
and the numerous suggestions that were collected by
H. Brinson.
The new handbook, appropriately entitled Handbook on Experimental Mechanics, is dedicated to
Dr. Hetenyi. Twenty-five authors have contributed 21
chapters that include, among others, the new disciplines and developments that are mentioned above. The
handbook emphasizes the principles of the experimental techniques and de-emphasizes the procedures that
evolve with time. I am grateful to the contributors,
who devoted many late afterhours in order to meet the
manuscript deadlines and to J. B. Ligon who readily
provided welcomed assistance during the trying times
associated with this editorship.
Albert S. Kobayashi 1987
VIII
Preface to the Handbook on Experimental Mechanics, Second Edition
Since the publication of the first edition, considerable
progress has been made in automated image processing, greatly reducing the heretofore laborious task of
evaluating photoelastic and moire fringe patterns. It is
therefore appropriate to add Chapter 21: “Digital Image
Processing” before the final chapter, “Statistical Analysis of Experimental Data.” Apart from the new chapter,
this second edition is essentially same as the first edition with minor corrections and updating. Exceptions to
this are the addition of a section on optical fiber sensors
in Chapter 2: “Strain Gages,” and extensive additions to
Chapter 14, which is retitled “Thermal Stress Analysis,”
and to Chapter 16: “Experimental Modal Analysis.”
To reiterate, the purpose of this handbook is to document the principles involved in experimental mechanics
rather than the procedures and hardware, which evolve
over time. To that extent, we, the twenty-seven authors,
judging from the many appreciative comments which
were received upon the publication of the first edition,
have succeeded.
Albert S. Kobayashi April 1993
IX
Preface
This handbook is a revision and expansion of the
Handbook on Experimental Mechanics published by
the Society for Experimental Mechanics in 1987 with
a second edition in 1993 – both edited by Albert
Kobayashi. All three of these trace a direct lineage to
the seminal Handbook of Experimental Stress Analysis
conceived and edited by Miklós Hetényi in 1950 and
they encapsulate the history of the field. In 1950, the capability of measuring strains on models and structures
was just becoming widely available. Engineers were
still making their own wire resistance foil gages, and
photoelasticity measurements required film processing.
Conversion of these measurements to stresses relied
on slide rules and graph paper. Now, foil resistance
gages are combined with automatic data acquisition,
and photoelasticity is just as automated. Input from both
experimental methods is combined with finite element
analysis to present stress variations in color on a computer screen. The focus then was on large structures
such as airframes; in fact, the efforts of the Society for
Experimental Stress Analysis (founded in 1938) were
crucial to the rapid development of aircraft in the 1940s.
While measurements on large structures continue to
be important, researchers today also measure the mechanical properties of specimens smaller than a human
hair.
The field is completely different now. Experimental
techniques and applications have expanded (or contracted if you prefer) from stress analysis of large
structures to include the electromechanical analysis of
micron-sized sensors and actuators. Those changes –
occurring gradually over the early years but now more
rapidly – led to a change in the society name to the Society for Experimental Mechanics. Those changes also
have led to the expansion of the current volume with
the deletion of some topics and the addition of others
in order to address these emerging topics in the “micro
world”. This volume presents experimental solid mechanics as it is practiced in the early part of the 21st
century. It is a field that is important as a technology
and rich in research opportunities.
A striking feature of this handbook is that 20
of the 36 chapters are on topics that have arisen or
matured in the 15 years since the last edition; and,
in most cases, these have been written by relatively
young researchers and practitioners. Consider microelectromechanical systems (MEMS), for example. That
technology, originated by electrical engineers only 25
years ago, now permeates our lives. It was soon learned
that designers and manufacturers needed better understanding of the mechanical properties of the new
materials involved, and experimental mechanists became involved only 15 years ago. That is just one
example; several of the chapters speak to it as well as
similar completely new topics. The reader will find in
this volume not only information on the traditional areas of experimental solid mechanics, but on new and
emerging topics as well.
This revision was initiated by the Executive Board
of the Society and managed by the very capable staff
at Springer, in particular Elaine Tham, Werner Skolaut, and Lauren Danahy. Sound advice was provided
over the course of the effort by Jim Dally and Tom
Proulx. However, the real work was done by the authors.
Each chapter was written by authors, who are not only
experts, but who volunteered to contribute to this Handbook. Although they are thoroughly familiar with the
technical details, it still required a major effort on their
part to prepare a chapter. On behalf of the Society and
Springer, I acknowledge and thank them.
Baltimore June 2008
William N. Sharpe, Jr.