Rivet Shear Strength

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Shear Strength http://www.mleve13.com/BelT/Rivetlayout2.h

I vvlJevel3 V l o V i l l eAME Structures

Rivet LayoutThe concept behind any repair to an aircraft structure is that it must restore the original strength. Inorder to pass the stresses from the skin, through a patch, and back into the skin again, there must be asufficient quantity and quality of rivets. We commonly refer to a line of rivets close by on the aircraftstructure to mimic the size, type and pitch, but what do we install if there are no referencesavailable?As AME's, we should be aware of the different strengths available through the various types of jointsavailable. In order to help with this explanation, we should be familiar with the idea of "tear-out." Inthe diagram below, tear-out is shown as the material between the edge of the fastener hole, and theedge of the material. This is the amount of metal that will be lost or damaged if the stressesencountered are beyond the yeild point of the material.

lear Out. . . . ! I I

r:=::jJ r- ! ::::1.J............ ....,._"~~r..._~~~~~....-!l. . I I

The illustration below shows a double row, single shear lap joint. Other types of joints can attainhigher strength by adding rows of rivets, or layers of material.

Just above is a single shear butt joint. It employs a doubler under the shop heads to pass stressesthrough to the skins. Below, a double shear butt joint passes loads from thethrough the dark doublers. Created wlth

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This last illustration shows a double shear lap joint passing stresses from the yellow skins into theblue skin.There are a number of factors which affect the strength of a repair:

1. Thickness, temper and strength of material being repaired.2. The number of layers of material through which stresses may pass.3. The diameter, material and quantity of rivets being used in the repair.4. The quality of the installed repair.

Structural repair manuals are commonly used for reference information to obtain many of the abovedetails, but if none is available, then the AC43.13 will offer some standard repairs which can be used.For example, a chart and formula is presented to offer a guide for re-establishing the strength of theaffected area.

I Applies to 2024 T3, 2024 T36, 7075 T6 IThickness "T" Number of 2117 AD rivets per inch of Number of

in inches repair width AN3 bolts3/32" 118" 115/32" 113/16"11114"I I AN-3 I0.016" 6.5 4.9 I I I I I D

0.020" 6.9 4.9 I[}I]CDI I0.025" 8.6 4.9 3.9 I D0.032" 12.5 7.0 4.5 3.3 I[IT]0.040" 13.8 7.7 5.0 3.5 I[IT] I 3.3 I0.051" 9.8 6.4 4.5 I [ T I ] I 3.3 I0.064" 12.3 8.1 5.6 I C I I J I 3.3 I0.081" 10.2 7.1 1[]l]1 3.3 I0.091" 11.4 7.9 I C I T J I 3.3

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This illustration shows a rivet head still holding a small piece of skin material which has been tomaway from the parent skin.The strength of the rivet joint also depends upon the number of rivets. If a large number of smalldiameter rivets are used, it is possible to have the same total shear strength as a smaller number oflarge rivets. But this equation also has a dark side; if the number of small rivets is TOO great, thenthe material bearing strength will be greater than the rivet shear strength, and the rivet joint will beweaker. Conversely, if a small number of huge rivets are used, then the loads are concentrated on afear small areas, which loads up the material.If a rivet is too large for the thickness of the material, the AME will fmd the skin tends to bulgearound the rivet, particularly near the shop head. A proper balance in size, diameter, alloy, andlocation will offer the greatest strength.Rivets in double shear carry twice the load, and those in triple shear carry three times the load.

Numbers you need to knowThe standard formula for determining the correct diameter of rivet is to use three times the thicknessof the thickest sheet of metal. If the repair is required on primary structure or critical parts, the SRMwill dictate precisely what fasteners are required.Standard grip length is one-and-one-half diameter protruding from the material before rivetting.(Experienced users can properly set rivets as short as one-and-one quarter protruding) After settingthe rivet, properly set rivets should measure one-half diameter high, and one-and-one-half diameterwide.Edge distances are as follows, and apply to all solid shank rivets, nominal and oversize blind rivets,hiloks, hucks, and other special fasteners;Universal head rivets shall have an edge distance of 2 diameters minimum, to a maximum of 4 rivetdiameters.Countersunk head fasteners shall have edge distances of 2 and 1/2 diameters minimum, to amaximum of 4 rivets diameters.Rivet spacing (pitch) shall not be any closer than 3 rivet diameters, measured center-to-center. Rivetpitch is recommended at 6 - 8 times the diameter. Generally, rivet pitch does not exceed 12diameters. Pitch is generally determined by the aircraft manufacturer.Rivet gage (row to row spacing) is 75% of the pitch.Rivet hole diameters are assessed at 0.003" larger than the diameter of the rivet being installed. Asan example, install a 1/8" rivet into a 0.1285 (#30) hole.

Countersinking and DimplingCountersinking procedures are very sensitive to the thickness of the materi Created wlth



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"knife-edging" the rivet holes by countersinking the holes too deep or using material too thin. Whenupper skin thicknesses are thinner than the required amount for countersinking, one of the methodsof dimpling may be used to prepare the holes for countersunk rivets.

Rivet Top Skin Next SheetI

Method recommendedIDiameter Thickness Thickness

Countersink top sheet0.032 or more. only.

3/32" 0.025 or less 0.051" or greater Dimple top, Countersinknext.0.025 or less 0.040" or less Dimple top, Countersink

next.Countersink top sheet

0.040 or more. only.1/8" 0.032 or less 0.064" or greater Dimple top, Countersinknext.

0.032 or less 0.051" or less Dimple top, Countersinknext.Countersink top sheet

0.051 or more . only.5/32" 0.040 or less 0.072" or greater Dimple top, Countersinknext.

0.040 or less 0.064" or less Dimple top, Countersinknext.Countersink top sheet

0.064 or more. only.3/16" 0.051 or less 0.091" or greater Dimple top, Countersinknext.

0.051 or less 0.081" or less Dimple top, Countersinknext.

I I I INote that dimpling methods demand quality deburring of the holes due to the increased stretchingtendencies during forming. Without due attention, improperly deburred holes can develop cracksaround hole circumferences.

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Rivet Shear Strength