Fastener engineers and mechanical designers have much to learn from the art of "torque-turn signature analysis". Experienced practitioners of the art of fastener signature analysis are capable of stripping away much of the mystery and uncertainty which often is associated with the strength and reliability of threaded fasteners and bolted joints.

Illustrated in this short introduction are examples of typical signatures which the reader can readily identify. The basic concepts of "torque-tension" combined with a fundamental understanding of strength of materials and basic stress analysis provide the engineer with the knowledge needed for understanding signature analysis of threaded fasteners.

The following examples are found on the LabMaster 9404/9504 Demo v.67 disk for the laboratory test systems and portable transient recorders manufactured by R S Technologies Ltd of Farmington Hills, Michigan, USA. The plots are identified with the ".RST" data file record names associated with each example.

The torque angle signatures can be analyzed to determine installation torque, thread strip, underhead embedment, bolt yield, and most important, fastener tension. The M-Alpha plots with reference to the "elastic origin" are directly linked to the SR1 Bolted Joint Design software for Windows 95.

Example 1: TIGHTEN.RST

In this example the M12x1.75 fastener is tightened to 60 lb-ft. The signature has been recorded with a "record threshold" of 20 lb-ft. The plot below shows both torque and tension vs. angle of turn, with "0" angle located at the threshold.

The LabMaster software can automatically locate the "elastic origin" on the M-Alpha Diagram by projecting a tangent line from the final point on the torque-angle curve to the "0" torque or, if present , the prevailing torque level.

The M-Alpha Diagram for the TIGHTEN.RST shows that the installation torque resulted in a projected elastic tightening angle of approximately 85 degrees, resulting in a clamp force of about 7,500 lb .

The corresponding F-Alpha curve confirms the relationship between torque and angle with the concept of the "elastic origin". Note that the 85 degree elastic tightening angle for the bolt results in approximately 7,500 lb. clamp force.

The next example illustrates a "breakaway" torque audit on the bolted joint tightened for the example above.

Example 2: BRAKAWAY.RST

Breakaway hand torque audits are often used to attempt to correlate dynamic installation torque with the measured "breakaway point. In this example the fastener was torqued in the tightening direction until an additional angle of turn of about 10 degrees was attained.

Note that the initial breakaway torque level is slightly above the 60 lb-ft installation torque. This breakpoint , not the peak torque of about 70 lb-ft is the proper hand torque audit point.

Locating the "Elastic origin using the M-Alpha plot is a powerful tool , standard with the LabMaster instruments. When applied to the breakaway audit curve , the M-Alpha Diagram can locate the elastic origin for the original tightening process, providing the means to directly audit the fastener tension . This is a most remarkable plot, as "TENSION" not torque is the desired result.

BRAKAWAY.RST (M-Alpha Diagram)

The M-Alpha Diagram clearly shows the torque breakaway point related to the installation torque , and also shows the 85 degree initial tightening angle which correlates with a pre-load of 7,500 lbs. clamp force.

Example 3: RELEASE.RST

If the torque-angle signature is recorded when a fastener is loosened the elastic tightening angle can be estimated, and thus the approximate fastener tension released can be directly estimated F-Alpha slope for the joint has been established.

M-Alpha plots and release-angle plots can be used to directly estimate bolt tension, or pre-load, which is the ultimate goal of the fastener tightening process.