WO2012100780A1

WO2012100780A1 - Shear and tension or compression biaxial material testing fixture

Info

Publication number: WO2012100780A1
Application number: PCT/DK2012/050030
Authority: WO
Inventors: Siavash Talebi TAHAR; Ole Thybo Thomsen
Original assignee: Aalborg Universitet
Priority date: 2011-01-25
Filing date: 2012-01-24
Publication date: 2012-08-02

Abstract

The present invention relates to an S-shape shear and tension or compression biaxial material testing fixture which can cover the full envelope of shear-axial loading conditions. The biaxial testing fixture consists of two tensile load applying fork-lugs, two boomerang-shape arms and a couple of specimen holders connected to a centralized test specimen. It can be used for testing mechanical properties of wide range of brittle or ductile materials. The novel testing fixture is a simple and inexpensive fixture that may be simply attached to a universal test machine capable of only uniaxial tensile loading in order to provide biaxial deformation at different shear to axial ratios. The different ratios are provided by selecting different attachment points on the boomerang-shape arms.

Description

SHEAR AND TENSION OR COMPRESSION BIAXIAL MATERIAL TESTING

FIXTURE

FIELD OF INVENTION

This present invention relates to testing equipment and, more specifically, to a uniaxially tensile driven fixture for applying axial tensile, axial compressive or shear loading to a test specimen, or combinations of axial tensile or axial compressive and shear loadings to a test specimen.

BACKGROUND TO THE INVENTION Recent developments in alloys, ceramics, composites and low density core materials used in sandwich panels have substantially advanced the state of the art and have resulted in the development of interest in such materials for use in structural applications. Currently, there is no simple fixture for use in a universal test machine to test these materials in a multi-axial stress state in a full range of biaxial shear and axial loading envelope.

Considerable effort has been made in an attempt to satisfy a long-felt need for determining properties of brittle composites, alloys and soft core materials, such as low density foams. Most efforts have focused on compressive, torsional and tensile testing. A need exists for expanding the capabilities to include biaxial testing, that is shear, axial compressive and/or tensile testing along orthogonal axes for a full range of the biaxial loading envelope.

Biaxial load testing machines are known in the art which apply a monoaxial load to a test piece from orthogonal directions. U.S. Pat. No. 3,797,303 describes such a device which applies monoaxial compression load to a test specimen. In this invention, the device provides two orthogonal compression assemblies carried independently on guides. However, with this type of device some amount of undesirable moments to the test specimen can occur during compression loading, resulting in inaccurate load bearing data of the specimen.

Also, a triaxial compression test apparatus is shown in U.S. Pat. No. 4,615,221 which tests cylindrically-shaped samples by imparting a compressive force along the longitudinal axis of the sample. This compressive force then creates a second force along the axial axis of the cylindrical sample through the use of hydraulic fluid surrounding the object. However, the application of this device is limited to compressive testing of cylindrical samples.

A hydrostatic self-aligning axial/torsional mechanism is described in U.S. Pat. No. 4,928,532 for testing specimens without introducing bending moments induced by other testing means. The test specimen can be tested by this invention for uniaxial strength, torsional strength, or a combination of the two. However, this invention does not test for compressive and/or tensile axial strength from orthogonal directions and accordingly does not consider the inducement of bending moments in the test specimen during the loading operation. It should be noted that the term "biaxial" as used in U.S. Pat. No. 4,928,532 refers to a combined axial and torsional loading, whereas in the present invention the term "biaxial" is understood to mean axial and shear loading along orthogonal axes. Another example of biaxial testing apparatus is described in U.S. Pat. No. US 2009/0282929 for use in a universal machine. However this system is complex and friction in the gearing system should be considered for soft material specimens.

Hence, an improved fixture for biaxial material testing would be advantageous, and in particular a more efficient fixture with which a wider range of loading conditions can be tested would be advantageous.

OBEJCT OF THE INVENTION

Accordingly, it is an object of the present invention to provide a fixture that can be attached to a universal test machine capable of only uniaxial deformation in order to provide biaxial deformation at different shear to axial ratios. It is thus an object of this invention to provide a means to test the shear- compressive or shear-tensile biaxial strength of a specimen.

It is another object of this invention to provide a means to test the pure shear, compression or tensile uniaxial strength of a specimen.

Another object of this invention is to provide a means for applying axial and shear stress to a specimen without inducing significant moments in the specimen.

Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, an S-shape shear and tension or compression biaxial material testing fixture which covers the full envelope of shear-axial loading conditions is provided. The biaxial loading apparatus consists of two tensile load applying fork-lugs, two boomerang-shape arms and two specimen holders connected to a centralized test specimen.

The first discovery of the present invention is that a uniaxial tensile load can be converted to shear and/or axial load components on a test specimen by use of one of the loading holes on each of the load transferring boomerang-shape arms attached to the tensile load transfer fork-lugs. An inner end of the each arm is connected to a specimen holder and the test specimen is located in the centre of the fixture assembly and is fixed on specimen holders.

Another aspect of the present invention is that the full envelope of shear-axial including both tension and compression loading conditions can be provided. A biaxial load will be applied to a test specimen depending on the selection of attachment points on the boomerang-shape arms, such as shear-compressive or shear-tensile loading conditions. By selecting certain connecting points on the arms, a uniaxial loading condition will be obtained, i.e. compressive, tensile or shear loading. By varying the attachment locations of the load transfer fork-lugs on the boomerang-shaped arms, different ratios between the shearing and axial load components can be achieved.

The pattern of attachment points on the boomerang-shape arm follows a desired curve relative to the centroid of the test specimen. The distribution pattern of the attachment points may be followed by numerous curves such as, but not limited to, spiral, ellipse or off-centric circular curve.

The test specimen will preferably be bonded to the specimen holders or be bolted to the arms directly. Soft materials, like low density foams, will be bonded to the specimen holders and hard materials, like composites or metals, will typically be bolted to arms or specimen holders. Depending on the specimen material and loading condition, different specimen shapes including but not limited to butterfly shape, bone shape, block shape may be used.

Accordingly, the present invention relates in a first aspect to a biaxial material testing fixture for applying a biaxial load to a test specimen by using a conventional material testing machine to apply a unidirectional tensile load to the testing fixture, the testing fixture comprising:

- a first fork-lug movable along a longitudinal axis of said test apparatus during testing, said first fork-lug having a first connecting bolt and first spacers capable of rotation about an axis of said first connecting bolt, the first fork-lug being connectable to a first boomerang shape arm by the first connecting bolt and having another fixed or articulated connection to movable means of the load applying testing machine;

- a second fork-lug fixed along a longitudinal axis of said fixture, said second fork-lug having a second connecting bolt and second spacers capable of rotation about an axis of said second connecting bolt, the second fork-lug being connectable to a second boomerang shape arm via the second connecting bolt and having another fixed or articulated connection to a fixed side of the load applying test apparatus;

- a first boomerang shape arm having a set of curved distributed first attachment holes adapted to be connected to said first fork-lug at one of said first attachment holes by said first connecting bolt and having a first internal side connected to at least one first specimen holder;

- a second boomerang shape arm having a set of curved distributed second attachment holes adapted to be connected to said second fork-lug at one of said second attachment holes by said second connecting bolt and having a second internal side connected to at least one second specimen holder;

- at least one first specimen holder connectable to said first boomerang shape arm;

- at least one second specimen holder connectable to said second boomerang shape arm;

so that a test specimen is connected to the at least one first specimen holder at one connection side and to the at least one second specimen holder at another connection side during use of the testing fixture.

The first and second attachment holes may be provided along a quasi-spiral curve. An example of such a quasi-spiral curve is shown in the figures.

The at least one first specimen holder may comprise first double fixing bolts and first double fixing holes connectable to said first boomerang shape arm by said first double fixing bolts, and the at least one second specimen holder may comprise second double fixing bolts and second fixing holes connectable to said second boomerang shape arm by said second double fixing bolts.

The first and second specimen holders may be dovetail shaped.

The first and second specimen holders may alternatively be a plurality of specimen holder bolts adapted to fasten the test specimen to be tested to the boomerang shape arms via attachment holes provided in the test specimen.

The testing fixture may form an S-shape setup.

In preferred embodiments of the invention, connection via predetermined first and second attachment holes on said first and second boomerang shape arms imply a uniaxial tensile load upon said test specimen.

Connection via predetermined first and second attachment holes on said first and second boomerang shape arms may imply a uniaxial compressive load upon said test specimen. Connection via predetermined first and second attachment holes on said first and second boomerang shape arms may imply a pure shear load upon said test specimen.

Connection via predetermined first and second attachment holes on said first and second boomerang shape arms may imply a biaxial shear-tensile load upon said test specimen.

Connection via predetermined first and second attachment holes on said first and second boomerang shape arms may imply a biaxial shear-compressive load upon said test specimen.

A testing fixture according to the present invention may further comprise support means connectable to the first and second boomerang shape arms to ensure in- plane mutual motion of the arms and specimen holders during testing.

Another aspect of the present invention realtes to a method of testing a test specimen by use of a testing fixture according to any of the preceding claims. The method may further comprise visually recording the deformation of the specimen during testing.

The first and second aspects of the invention may be combined.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be hereinafter described with reference to the accompanying drawings which illustrate preferred embodiments of the invention. The drawings however are merely illustrative of how the invention might be put into effect, so that the specific form and arrangement of the features shown is not to be understood as limiting on the invention.

Fig. 1 is a perspective view of an embodiment of the S-shape biaxial material testing fixture comprising two tensile load applying fork-lugs, two boomerang- shape arms, and a couple of specimen holders connected to a centralized test specimen; Fig. 2 depicts a schematic plane view of the central S-shape assembly of the fixture comprising boomerang-shape arms, and a butterfly shape specimen at the centre; Fig. 3 is a perspective view of another embodiment of the central S-shape section of the fixture involving boomerang-shape arms, specimen holders and a 3D butterfly shape specimen;

Fig. 4 shows a perspective view of the boomerang-shape arm;

Fig. 5 illustrates a perspective view of an alternative embodiment of the S-shape biaxial material testing fixture for hard material specimen testing;

Fig. 6.A through Fig. 6.F present different specimen shapes for testing with the biaxial material testing apparatus. Possible shapes include but are not limited to the six shown configurations;

Fig. 7 shows a specimen holder for bonding to soft material specimen; Fig. 8 depicts a load applying fork-lug with rigid cylindrical end connection;

Fig. 9 presents an alternative loading fork-lug with hinged end connection;

Fig. 10 shows a perspective view of a testing fixture comprising support means connectable to the first and second boomerang shape arms to ensure in-plane mutual motion of the arms and specimen holders during testing;

Fig. 1 1 shows a schematic plane view of the testing fixture in Fig. 10; and Fig. 12 shows a schematic bottom or top view of the testing fixture in Fig. 10 and 1 1 . DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings in which like numerals represent like elements throughout the several views, the preferred embodiment of the present invention will be described. Fig. 1 depicts a perspective view of an embodiment of the S- shape biaxial material testing fixture comprising two tensile load applying fork- lugs 10 and two boomerang-shape arms 1 and a couple of specimen holders 30 connected to centralized test specimen 20. Load applying fork-lugs 10 will be connected to the cross heads of a universal test machine or another pulling system. Tensile load will be applied to fork-lugs 10 through the direction 100 and the load is transferred to the boomerang-shape arms 1 by connecting bolt and nut 11 & 12. Spacers 50 fill gaps between the fork-lug 10 and the arm 1. The boomerang-shape arm 1 transfers load to the specimen holder 30 by bolts 31, and the specimen 20 experiences the loads via bonding or bolting connection to the specimen holder 30.

Fig. 2 presents a schematic plan view of the central S-shape assembly of the fixture comprising boomerang-shape arms 1 , and a butterfly shape specimen 20 at the centre. By appropriate selection of attachment holes 2 on the arms 1 (shown in Fig. 3), the uniaxial load will be converted to shear and axial load components relative to specimen 20. The exact positions of the holes and which holes to use for a given desired loading can be determined e.g. by computer simulations and/or experimentation.

In accordance with the principles of the present invention, the fixture having boomerang shape arms 1 with a series of attachment holes 2 provided on a curve 7 operatively connected to specimen 20 at the centre of the S-shape system. The curve shown in the figures is quasi-spiral, but other curves are also covered by the scope of the present invention. By varying the choice of connection holes 2 on the curve 7, different unidirectional and bidirectional loading conditions on the specimen 20 will be achieved, i.e., loads 8, 8a, 8b, 8c and 8d generate pure tensile, tensile-shear, pure shear, shear-compressive and pure compressive loadings, respectively.

In the embodiment shown in Fig. 2, the attachment holes 2 are located at a radius 5 and an angle 6 relative to the centre of the specimen 20. An imaginary path of the holes 2 follows a suitable curve 7 which in the figure is shown as (but not limited to) a quasi-spiral distribution curve.

Referring now to the perspective view of Fig. 3, an example of the central S- shape assembly for soft material testing is shown. The S-shape assembly comprises two l-section boomerang-shape arms 1 , specimen holders 30 and a soft 3D butterfly shape specimen 20. Use of l-section of boomerang-shape arms 1 minimizes unwanted load and coupling on the specimen 20 due to arm weight which is crucial for soft material testing, such as for low density foams. The specimen holders 30 are connected to the arms 1 by fitting into a dovetail socket 3 on the arm 1 as shown in Fig. 4 and will be fixed by bolts 31 passing through bolting holes 4 to prevent sliding out and any unwanted backlash. The specimen holders 30 can alternatively be directly connected to the arms 1 by strong bolts without dovetail sockets 3.

Fig. 5 is an alternative example of an S-shape assembly for hard material testing comprising strong boomerang shape arms 1a, hard material specimen 20a and specimen holder bolts 31a. The solid cross section of the arms 1a provides rigid and strong arms for hard material testing. The strong bolts 31a connect the hard material specimen 20a to the arms 1a directly, and the large size attachment holes 2a can carry high loads for hard material testing. Specimen bolting holes 21 are distributed on the connecting edges of the specimen 20a as shown in Fig. 6.C. Fig. 6.A shows a 3D double curved bone-butterfly shape specimen, and a simpler 2D butterfly shape sample is depicted in Fig. 6.B. Some other bonding contact specimen types for tensile or compressive loading are shown in Fig. 6.D through Fig. 6.F. A soft material specimen 20 is preferably bonded to bonding surface 35 (shown in Fig. 7) of specimen holder 30, and the dovetail 33 is fitted into the dovetail socket 3 (shown in Fig. 4). Connecting bolts 31 (depicted in Fig. 3) are passed through holes 4 (shown in Fig. 4) and hole 32 (illustrated in Fig. 7) to fix it.

Finally, the assembled S-shape fixture will be connected to rigid end load applying fork-lugs 10 (shown in Fig. 1 ) by connecting bolt and nut 11 & 12 and spacers 50 through one of the connecting holes 13 on the rigid end fork-lug 10 and attachment holes 2 on the boomerang shape arm 1. The rigid end fork-lugs 10 will be connected to a load applying machine through cylindrical hole 16 (shown in Fig. 8) and pining hole 14. Fig. 9 depicts an alternative fork-lug 10a with articulated end to provide more flexibility of the fixture. A combined setup of the system, i.e. one rigid end fork-lug 10 on one side and another articulated end fork-lug 10a on the other side, provides average level of system stability and flexibility.

Preliminary tests have shown that for some types of materials, some loading conditions, such as compression loading, may result in undesired buckling of the test specimen 20. This leads to undesired loading and deformation of the test specimen 20. It can be prevented by providing the testing fixure comprising support means connectable to the first and second boomerang shape arms 1 to ensure in-plane mutual motion of the arms 1 and specimen holders 30 during testing. An example of such support means is shown in Figs. 10 through 12. Fig. 10 is a perspective view, Fig. 1 1 is a plane view of the testing fixture in Fig. 10; and Fig. 12 shows a schematic bottom or top view of the testing fixture in Figs. 10 and 1 1. The support means comprises transverse bolts 34 arranged in holes in the boomerang shapes arms 1 perpendicular to the longitudinal axis of the testing fixture. Connection elements 35 are arranged at both ends of the transverse bolts 34, the connection elements 35 each comprising holes in two perpendicular orientations. Each connection element 35 is located between two thrust bearings 37 to obtain frictionless rotational movement of the connection elements 35. Longitudinal bolts 36 are arranged parallel to the longitudinal axis of the testing fixture to fixate the transverse bolts 34 and thereby also the boomerang shape arms 1 relative to each other. A person skilled in the art will understand how to design support means designed in alternative ways to fulfil the same purpose.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. E.g. the testing fixture can also be used for testing of other materials than those specifically mentioned. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

ITEMS

The following numbered items provide in term of conceptual statements further disclosure of the present subject matter. 1. A biaxial material testing fixture for applying a biaxial load to a test specimen, comprising:

(a) a first fork-lug movable along a longitudinal axis of said apparatus, said first fork-lug having a first jointing bolt and first double spacers capable of rotation about the said first bolt axis connecting to first boomerang shape arm and having another fix or articulated connection to movable mean of load applying universal machine;

(b) a second fork-lug fixed along a longitudinal axis of said fixture, said second fork-lug having a second jointing bolt and second double spacers capable of rotation about the said second bolt axis connecting to second boomerang shape arm and having another fix or articulated connection to fix side of load applying universal machine;

(c) a first boomerang shape arm having first set of quasi-spiral distributed attachment holes connected to the said first fork-lug at one of the said first attachment holes by said first jointing bolt and having first internal side connected to a first dovetail specimen holder having first double fixing bolts;

(d) a second boomerang shape arm having second set of quasi-spiral distributed attachment holes connected to the said second fork-lug at one of the said second attachment holes by said second jointing bolt and having second internal side connected to a second dovetail specimen holder having second double fixing bolts;

(e) a first dovetail specimen holder having a first double fixing holes connected to the said first boomerang shape arm by the said first double fixing bolts; (f) a second dovetail specimen holder having a second double fixing holes connected to the said second boomerang shape arm by the said second double fixing bolts;

(g) a butterfly shape specimen connected to the first specimen holder at one connection side and to the second specimen holder at another connection side.

The test apparatus according to item 1 , wherein the assembly forms a S- shape setup.

The fixture of item 1 wherein said axial force is tensile force along said longitudinal axis.

The device of item 1 wherein selected said first and second attachment holes on said first and second boomerang shape arms imply a uniaxial tensile load upon said test specimen.

The apparatus of item 1 wherein selected said first and second attachment holes on said first and second boomerang shape arms imply a uniaxial compressive load upon said test specimen.

The fixture of item 1 wherein selected said first and second attachment holes on said first and second boomerang shape arms imply a pure shear load upon said test specimen.

The device of item 1 wherein selected said first and second attachment holes on said first and second boomerang shape arms imply a biaxial shear-tensile load upon said test specimen.

The apparatus of item 1 wherein selected said first and second attachment holes on said first and second boomerang shape arms imply a biaxial shear- compressive load upon said test specimen.

The apparatus in accordance with item 1 said apparatus further is capable to use means for visually recording the deformation of the specimen during said uniaxial or combined compression, tensile and shear load applications.

Claims

1. A biaxial material testing fixture for applying a biaxial load to a test specimen by using a conventional material testing machine to apply a unidirectional tensile load to the testing fixture, the testing fixture comprising:

- a first fork-lug movable along a longitudinal axis of said fixture during testing, said first fork-lug having a first connecting bolt and first spacers capable of rotation about an axis of said first connecting bolt, the first fork-lug being connectable to a first boomerang shape arm by the first connecting bolt and having another fixed or articulated connection to movable means of the load applying testing machine;

- at least one second specimen holder connectable to said second boomerang shape arm; so that a test specimen is connected to the at least one first specimen holder at one connection side and to the at least one second specimen holder at another connection side during use of the testing fixture.

2. Testing fixture according to claim 1 , wherein the first and second attachment holes are provided along a quasi-spiral curve.

3. Testing fixture according to claim 1 or 2, wherein the at least one first specimen holder comprises first double fixing bolts and first double fixing holes connectable to said first boomerang shape arm by said first double fixing bolts, and wherein the at least one second specimen holder comprises second double fixing bolts and second fixing holes connectable to said second boomerang shape arm by said second double fixing bolts.

4. Testing fixture according to any of the preceding claims, wherein the first and second specimen holders are dovetail shaped.

5. Testing fixture according to claim 1 or 2, wherein first and second specimen holders are a plurality of specimen holder bolts adapted to fasten the test specimen to be tested to the boomerang shape arms via attachment holes provided in the test specimen.

6. The testing fixture according to any of the preceding claims, wherein the testing fixture forms an S-shape setup.

7. The testing fixture according to any of claims 1 to 6, wherein connection via predetermined first and second attachment holes on said first and second boomerang shape arms imply a uniaxial tensile load upon said test specimen.

8. The testing fixture according to any of claims 1 to 6, wherein connection via predetermined first and second attachment holes on said first and second boomerang shape arms imply a uniaxial compressive load upon said test specimen.

9. The testing fixture according to any of claims 1 to 6, wherein connection via predetermined first and second attachment holes on said first and second boomerang shape arms imply a pure shear load upon said test specimen.

10. The testing fixture according to any of claims 1 to 6, wherein connection via predetermined first and second attachment holes on said first and second boomerang shape arms imply a biaxial shear-tensile load upon said test specimen.

1 1 . The testing fixture according to any of claims 1 to 6, wherein connection via predetermined first and second attachment holes on said first and second boomerang shape arms imply a biaxial shear-compressive load upon said test specimen.

12. The testing fixture according to any of the preceding claims further comprising support means connectable to the first and second boomerang shape arms to ensure in-plane mutual motion of the arms and specimen holders during testing.

13. A method of testing a test specimen by use of a testing fixture according to any of the preceding claims.

14. A method according to claim 13, further comprising visually recording the deformation of the specimen during testing.