Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1998-10-26
2002-01-01
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06334362
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a device for measuring the shearing action in the core of a sandwich structure. This may comprise sheets or shells of homogeneous or porous materials, composites or the like. The sandwich structure may be used e.g. in a boathull and then usually comprises two hard outer layers and at least one intermediate spacing layer, a core usually made of balsa wood or foamed plastic. The measuring device can be used for static and dynamic measurement of the shearing action in the core material of this hull in order to verify dimensioning while testing or to monitor loads during operation. It can also be used for measurements in laboratory test loads of structural parts such as panels for aeroplanes or vehicles comprising a honeycomb layer as the core. The structures need of course not be panel-shaped. They may be structural parts of an optional form such as frames, reinforcing elements etc.
BACKGROUND ART
For measuring the shearing action in the core material of hulls of sandwich structure, use is in most cases made according to prior art of cylindrical plugs with a &phgr;=35 mm of the core material of the test object and with a length of the thickness of the core layer. In the plugs, foil-type strain gauges are fixed, inclined at an angle of 45 to the axial direction of the plugs. The plugs are arranged in holes bored perpendicularly through one of the outer layers and the core layer. They are attached by gluing in these holes, accurately aligned such that the strain gauges are arranged in a direction in which maximum strain arises. Planes perpendicular to this direction appear when subjecting the core material to shearing load, the planes being inclined at an angle of 45 to the central layer of the core layer and, in for instance rectangular sandwich panels, along a ring located at a varying distance from the panel edge. When the core material cracks owing to too great shear stress, this mostly takes place in such a plane.
The use of such plugs is disadvantageous. The premanufacture and installation of plugs that are not reused is costly, time-consuming and requires especially trained staff. In order that the relatively large holes that are made in the outer layer for the installation should not reduce the strength of the hull in an impermissible manner, the strength of the layer must be reestablished. One way of reestablishing the strength of the layer is by making a cover of the same character as the layer over the plug. It is difficult to ascertain in advance the direction of planes having maximum shear stress, such that the plugs obtain an attachment with a reliable alignment of the strain gauge. The use requires costly calibration and yields poor accuracy. For some core materials, e.g. honeycomb, it cannot be used at all.
DESCRIPTION OF THE INVENTION
Technical Problem
The object of the invention is to provide a device for measuring the shearing action in the core of sandwich structures without the above-mentioned drawbacks. It should be possible to use the device without any extensive preparatory work. Reliable measurements should be possible independently of the alignment of attachment of the device to the test object.
Solution
According to the invention, a device is provided, comprising a sensing means which is adapted to be inserted into a cavity in the core through an opening in the surface of the sandwich structure and abut against at least part of the wall surface that defines the cavity. Moreover, the device comprises an instrument for measuring the angle between the sensing means and a reference plane of the object or the change of this angle. By the measuring operation, a measure of the shearing load in the material adjoining the cavity can be obtained.
FIG. 1
illustrates a core layer from a sandwich panel
1
, a longitudinal section being indicated. The panel is assumed to consist of a number of partial layers
2
of the same material and thickness, attached to each other. Through this panel and thus all the partial layers, a cylindrical hole
3
is bored in the longitudinal section, preferably perpendicular to the directions of extent of the panel. A sensing means
4
is inserted in the hole. The means is designed as a cylindrical rod having the same diameter as the bored hole and having such a length as to extend a distance above the top face
5
of the panel.
When the material is subjected to shearing action by, for instance, the lowermost layer of the panel being affected by a force which is parallel with the layer and which is assumed to be directed in the longitudinal direction of the panel to the left in the Figure, while the uppermost layer is kept in position, the central plane of each layer is displaced by the same distance to the left in relation to the closest layer above. Since the hole in each partial layer is displaced together with the partial layer, the rod will follow the movements of the partial layers and thus be turned through an angle
7
, which in the ideal case is equal to the shear angle of the shearing action and proportional to the shearing load. On the top face of the uppermost partial layer, i.e. the upper side
5
of the panel, which is here used as a reference plane, an instrument
8
for measuring angular changes of the rod is attached with its symmetry plane in a plane in common with the longitudinal section. The instrument can operate according to some mechanical, optical or electrical principle. In the latter case, it may comprise two electrically operated transducers
9
of prior-art design for contactless measurement at two different levels of the distance from the transducers to the rod
4
. By measurements with the aid of these transducers and knowing the vertical distance between the transducers, the angle
7
and consequently the shearing load can be calculated. If at least one more measuring instrument
10
is used, positioned in a direction different from the longitudinal direction of the panel, it is possible, besides measuring an angular change in an optional direction, also to calculate this direction by means of the relationship between the signals of the two measuring instruments.
In case of other shapes of the test object, other reference planes may be used. These need not be located on the outer surface of the object. It may pass, for instance, through three points lowered into the material, the shearing action of which is to be measured. The shaped of the sensing means as well as that of the cavity are not restricted to the shape in the above example. If the means is mounted for turning in e.g. a device connected to the measuring instrument, the means would need to be in contact with the boundary surface of the cavity in a single point only. The contact may then by maintained with a spring assembly on the sensing means. Such resilience is appropriate also for the purpose of taking up lost motion for better accuracy.
Advantages
According to the invention, an inexpensive and easily applicable device is provided, which without any extensive preparatory work yields good values of the shearing action in the core of a sandwich structure and the direction of the shearing action. The hole for the rod and small fixing screws, if any, for the device is much smaller than the holes which are normally made in the hull for cable lead-ins, screws for fixing various kinds of accessories and the like. Therefore, much less damage is caused to the test objects in these measurements compared with measurements according to prior art. By the measuring instrument not being sealed in the test object, it is readily accessible for service and check-up, which is important in long-term measurements in which the measuring accuracy of the plugs may be reduced. Moreover, this results in the device being reusable, which reduces the measuring costs.
REFERENCES:
patent: 3975685 (1976-08-01), Bielsten et al.
patent: 4328621 (1982-05-01), Benjamin
patent: 4481902 (1984-11-01), Meyer et al.
patent: 4854175 (1989-08-01), Budhu
patent: 4958522 (1990-09-01), McKinlay
patent: 5036709 (1991-08-01), McRae
Davis Octavia
Fuller Benjamin R.
Jacobson & Holman PLLC
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