Bolt Bearing in Composites

WICHITA STATE UNIVERSITY

MECHANICAL ENGINEERING DEPARTMENT

BOLT BEARING IN COMPOSITES

Introduction:

Background:

        Almost any apparatus or equipment we use today is made up of different parts that require some sort of assembly to put it together. Normally, this is done either with some sort of fasteners (i.e. bolts, screws, rivets, etc.), glues or just by interference fits. In this project, we will be concentrating on just fasteners, especially rivets on aircraft panels. We will also further narrow our research to just composite materials.

        Composites are the upcoming material of choice especially where weight considerations are paramount. This makes it an ideal material used in the aircraft industry. The weight savings by using composite materials far outweigh the additional cost of manufacturing them. Composites are also less susceptible to cracks due to fatigue which is the number one cost of maintenance in aircrafts today.

        As in all materials, drilling a hole in the material increases the stress at that point. Failure in the form of cracks normally occurs at this point. It is no difference in composites. One of the ways to increase the strength around the holes is to increase the thickness of the composite around the holes.

The industry standard to gauge the strength of any material that is fastened together using bolts, screws or rivets is called the bolt bearing strength. The bolt bearing strength is determined by pulling apart two like materials bolted together and determining the force required when it fails. There are also other failure modes that we will not be concentrating on as illustrated below.

[pic 1]

Problem Statement:

A typical practice in industry to increase the bolt bearing strength in composite laminate has been to add more “composite” to compensate for the stress concentrations around holes for fasteners. This is a property in cured laminates. This in turn increases the weight of the component.

Project Description:

For our project this semester, we will be looking at several different ways to reduce the weight of composite aircrafts. This will include studying the optimum arrangement of fasteners on a panel. Currently, there are two ways that two panels are attached together. They are illustrated below. We will be studying the optimum arrangement for the given loads.

In-line:

[pic 2]

Staggered:

[pic 3]

The next area that we would concentrate on would be strengthening the composite at the ends of panels where failure normally occurs. This in turn would reduce the number of fasteners needed to assemble the panels. Ideally, the weight gained by increasing the thickness of the composite around the bolt holes should be less than the weight lost by reducing the number of fasteners.