Test Report of Lohr SMARTHEX™ Bolts
with Thick Washer Head

Karl H. Frank, Ph.D., P.E.
October 4, 2002

Introduction
A new bolt head has been designed by Lohr Fasteners to eliminate the need for a separate 5/16 inch hardened ASTM F436 washer which is required when A490 bolts with a diameter of 1 inch or larger are installed in connections with oversize or short slotted holes in an outer ply. The new bolt has a hex head with a thickened washer forged as part of the bolt head. The ability of the integral washer to replace a separate thick washer under the bolt head was examined in this study.

The requirement for the thick washer in the specifications is to eliminate plastic deformation of the plate material under the bolt head. The bearing area of the plate under the head is reduced when oversize or short slotted holes are used. To compensate for this reduced bearing area, thicker stiffer washers are specified to spread the load more evenly over the reduced area to reduce the bearing stress and the deformation in the plates.

A series of tests were performed using 1-1/8 inch A490 bolts to evaluate these new fasteners. Three conditions were tested:

1. Standard hex head bolt with standard F436 washer under the bolt head
2. Standard hex head bolt with 5/16 inch thick F436 washer under the bolt head
3. New Lohr SMARTHEX™ bolt with thick washer head.

The purpose of these tests was to compare the plastic deformation in the plate under the bolt head for these three conditions.

Testing Procedure
The bolts were tensioned using a Skidmore-Wilhelm bolt tension gage. A hydraulic pump with a test pressure gage attached was connected to the bolt tension gage by the port on the side of the bolt gage. The bolt along with a special test plate was inserted into the gage. The bolt was then snugged by hand to compress the hydraulic bolt tension gage. The test plate was a square plate of A36 steel with two ground surfaces and had an oversize hole drilled in the center. A tensile test of the plate material gave yield strength of 42.5 ksi and tensile strength of 64.7 S The thickness of the plate was measured in four locations adjacent to the hole prior to installation. A 0.0001 in digital micrometer was used for all the measurements.

Figure 1 Test Setup

Figure 1 shows a specimen mounted in the Skidmore-Wilhelm and the hose connected to the Skidmore-Wilhelm bolt tension device. A close up a SMARTHEX™ Bolt and the ground test plate under the washer head is shown in figure 2.

Figure 2 Close Up of Showing SMARTHEX™ Bolt In Test Setup

The bolts were then tensioned by pumping oil into the bolt tension gage until a tension of 92 kips equal to 1.15 the required minimum installed tension of 80 kips. The load was then released and the test plate removed. The thickness of the plate adjacent to the hole was then measured. The difference in the average thickness before and after testing is a measure of the plastic deformation the plate has undergone. Three samples of each condition were tested.

Before the tests were undertaken the diameter of the holes and the diameter and thickness of the washers were measured. The average hole diameter in the test plates was 1.439 in. The diameter of the integral washer of the SMARTHEX™ bolt and the thickness of the washer extension was measured. A summary of the washer measurements is given in Table 1 below.

Test Results
Figure 3 shows a test plate after a test. The deformation around the hole due to the bolt tension is evidenced by the crazed surface surrounding the bolt hole.

Figure 3 Test Plate After Test

Figure 4 summarizes the results of the tests. The standard hex bolts with a standard washer under the head exhibited the largest plastic deformation in the test plates. The measured deformation of the test plates with the standard hex bolts with the thick washer was less than the standard washer tests. The SMARTHEX™ Bolts with thick washer head exhibited the smallest deformation in 2 of the 3 tests and the lowest overall deformation.

Figure 4 Test Results

Conclusions
The SMARTHEX™ Bolts with integral thick washer forged with the head performed better than a standard bolt with a separate thick washer required when oversized holes or short slotted holes are used with A490 bolts greater than 1 inch in diameter. The plastic deformation of the plate material under the washer head of the SMARTHEX™ Bolt was less than 0.0005 in. in the A36 test plate. The SMARTHEX™ Bolt with the large integral washer head prevents excessive deformation under the bolt head and does not require a separate washer.