SPRING 2011

8 THE DISTRIBUTOR’S LINK
Bengt Blendulf
Bengt Blendulf is president/principal lecturer of EduPro US, Inc., a company specializing in fastener engineering
education and consulting. In 1966, Bengt joined a leading European fastener manufacturer. Educated in Sweden, he
moved to the United States in 1974 to start a subsidiary for a Swedish fastener manufacturer. Bengt also served for
eight years on the faculty in the College of Engineering and Science at Clemson University. Since 1997 he (EduPro)
teaches highly rated courses in Fastener Technology and Bolted/Screwed Joint Design in the U.S., Canada, Mexico,
Europe, Asia and India, for engineers and other fastener professionals. Bengt was the chairman of ASTM F16.96
Bolting Technology from 1996 to 2006. In 2006 he received the Fred F. Weingruber award from ASTM for “his efforts
to promote and develop standards for the fastener industry.” He is the author of an extensive lecture book as well as
over 100 technical papers and articles. His business address is: EduPro US, Inc., PO Box 232, Alameda, CA 94501;
phone 510-316-3234; email: bengt@edupro.us; web: www.edupro.us.
FASTENER STRENGTH - REAL OR NOT?
When a bolt or a screw is manufactured, it is
subjected to a number of tests to determine the fastener
meets the specification to which it is made. For inch
fasteners (bolts, screws, nuts, washers and rivets)
ASTM F606 is the standard for testing most often
specified, and for metric fasteners ISO 898/1 (bolts,
screws and studs) or ISO 898/2 (nuts) are the normal
specifications. The ISO 898 standards contain all
relevant data for the fasteners, including materials,
strength classes, test programs, types of tests and
marking requirements. That is also the case for inch
standards according to SAE J429. But when we use
ASME and ASTM fastener standards for types and
physical sizes, we also need to consult ASTM F606 for
the testing procedures. That way, SAE and ISO are a lot
easier to use since all is covered in one single
document. The most obvious parts of any fastener
testing program include proofing load, proof
stress/Rp0.2 (yield in older terms), tensile strength,
wedge test, hardness and various checks for flaws.
For the designer of a bolted/screwed joint, the most
important issue is, of course, the available strength or
load bearing capacity of the fasteners. But, if he/she
only takes the table values for proofing loads as bench
marks for the joint loads there will be problems coming.
All types of tensile testing, including proofing load and
Rp0.2 (yield) are performed by applying a straight and
slow load to the fastener in a test apparatus.
The minimum proofing loads and ultimate tensile
loads are listed in tables in ISO 898/1, SAE J1199 and
ASTM F568M for metric sizes and property classes. For
inch fasteners we find the corresponding data in SAE
J429 for the commonly used grades (Grade 5, 8, etc.).
There are also a number of ASTM specifications that, in
some cases, are almost duplicates of the SAE grades
that most of us use, but also some that are unique to
ASTM and not so often known or used in the mechanical
industries.
Aside from finding load data, sometimes easy,
sometimes not, a well educated designer will likely base
fastener loads on a percentage of the proofing load. That
is the load level where no fasteners in a lot show any
permanent elongation after testing. In early text books
(unfortunately still used in many technical colleges) a
percentage, usually 75 % of yield strength (now Rp0.2
for heat treated fasteners) was suggested, but proofing
load data are a much more reliable starting point.
Proofing loads for high strength fasteners (8.8, Grade 5
and higher) usually are about 90 % of Rp0.2.
How much of the proofing load should we subject
thee fastener to?
There are several factors to consider, including the
most important as follows:
1. Tightening torque (real facts or charts).
2. Friction coefficients (threads and bearing areas).
3. Quality of assembly tools (scatter).
4. Torsional stresses
Tightening torque
I have over the years cautioned my readers about
using torque charts. Although well intended, they are
based on assumptions that may not reflect the
conditions of a particular safety critical joint. Some of
these torque charts just indicate values for “plain” or
please turn to page 170