Book Review: Mechanical Testing of Composites
Advanced composites are a relatively new engineering material. As a result, reliable databases of material properties are fairly rare. Generating property databases, therefore, is often an important part of composite engineering projects.
The standard test methods used on metals cannot, in most cases, be used on composites. That is why ASTM and other groups have developed specific standards for composites. Most books on composite testing cover these standards.
Measurement and gaging techniques, again originally developed for metals, must also be modified for use on composites. Unfortunately, most testing books fail to address this subject.
This is where Manual on Experimental Methods of Mechanical Testing of Composites comes in. It purposely gives minimal coverage to ASTM and other standards, instead focusing on general techniques such as the application of strain gages and other methods of strain measurement.
The chapters on traditional strain gages have the greatest detail, and the most like a true manual. Guidelines are given for the selection of gage types, S-T-C numbers (as close to 0 as possible), and gage configurations.
The effects of gage-fiber misalignment, material anisotropy, material nonuniformity (local flaws, textile weaves, and braids), and low thermal conductivity are all explained. Gages were originally developed for isotropic, high thermal conductivity materials, and the differences in composites must be accounted for.
Noticeably absent are instructions for actually bonding the gages to laminates--the first question I always hear from technicians unfamiliar with composites. A single paragraph covers this subject in a later chapter on creep testing, but it really belongs in the gage chapters.
Of course, there are other strain measurement devices besides resistance foil gages, and this book covers most of them. Replacements for foil gages include liquid-metal strain gages (for high strain materials) and embedded fiber optics. Methods for strain field measurement include moire interferometry, shearography, and thermography.
Coverage of these other methods is rather mixed. Some of the chapters read almost like manuals, with specific instructions for applying the method. Others focus on the theory behind the method, saving the instructions for the recommended reading list. (Some of those references are entire books in themselves.) As someone who hasn't studied optics since sophomore physics, I occasionally got lost in the details of the optical methods.
Finally, the book ends with four chapters on damage evaluation, both nondestructive (NDE) and destructive. Some of the strain field techniques, such as shearography and thermography, can also be used for NDE. In fact, these latter methods have the potential to differentiate between cosmetic and critical damage, something that can be done with ultrasonics or radiography only after extensive calibration testing.
Like many books written by a collection of authors, the writing is somewhat inconsistent from chapter to chapter. In fact, poor copy editing detracts from what is an otherwise excellent reference. Hardly a page goes by without a typographical error, grammatical error, mislabeled figure, or even missing text.
Worse are the errors in equations. Before applying any formula in this book, be sure to double-check it for accuracy.
By far the worst chapters are those on basic lamination theory. Hardly an equation goes by without an error, and many are difficult to spot. In fact, I have to recommend ignoring those chapters altogether and referring to one of the many standard references on laminate analysis.
The frequency of errors diminishes as the book progresses (or maybe I just got tired of counting them). Mechanical Testing has a lot of good information not covered in other testing books. As long as you are willing to read carefully, you will find this to be a useful reference.
For a good reference on basic test methods, such as tensile and shear tests, I recommend Experimental Characterization of Advanced Composite Materials.
Details: Manual on Experimental Methods of Mechanical Testing of
Composites, 2nd edition, edited by
C.H. Jenkins,
published by The Fairmont Press, 1998, ISBN
0-88173-284-2.
I. General Theory; II. Strain Measurement; III. Fundamental Test
Techniques; IV. Optical Methods of Analysis; V. Acoustic Methods of
Analysis; VI. Thermal Methods of Analysis; VII. Methods of Damage
Evaluation; VIII. Appendices