Apparatus for measuring shear stress and strain characteristics of adhesives

An apparatus and method of measuring the stress and strain characteristics of adhesives under uniform shear conditions. An Iosipescu sample (10) is used, in which at the shear line, the sample is joined by the adhesive to be tested. Pure shear is applied to the sample at the bond line and the distortion of the adhesive bond is measured. The apparatus (20) is attached to the specimen (10) by clamp means (30). The apparatus (20) comprises a fork means (22) and means to transmit the displacement (21) of the two parts of specimen (10) to the fork means (22) and means to measure the displacement, for example strain gauges attached to a Wheatstone bridge circuit. The specimen (10) with apparatus (20) attached is placed into a tensile testing machine especially adapted to receive the sample and apply shear to the adhesive bond line.

This invention relates to a novel apparatus and method for measuring the 
stress and strain characters of adhesives under uniform shear conditions. 
The invention makes use of an Iosipescu sample wherein the adhesive to be 
tested is placed at the shear line of the sample as a join. 
Recently significant development have resulted in adhesives and their use 
in adhesive bonding so that adhesive bonding is now widely used as a 
joining method for critical structural components, especially in aerospace 
applications. 
To achieve the required reliability and safety margins, designers need data 
on the shear stress-strain behaviour of adhesives, particularly on 
properties such as shear modulus and elastic and plastic strain to 
failure. Such data are not easily obtained, partly because of the 
difficulties in measuring strain in a bond line which may be only 0.1 mm 
in thickness. Although a number of test configurations are in existence 
all have some disadvantages, ranging from cost and problems in specimen 
preparation to a questionable shear field in the region of interest. 
Prior-art methods of testing the characteristics of adhesives include the 
"thick-adherend lap shear-type" method which is fully disclosed in "Stress 
analysis concepts for adhesive bonding of aircraft primary structure" R. 
B. KRIEGER, from the International Conference on Structural Adhesives in 
Engineering, Bristol, UK, 1986, p. 1. The method uses essentially a lap 
shear specimen constructed with thick adherends to minimize shear stress 
non-uniformity. To measure shear distortion of the adhesive bond line two 
sensors (i.e. LVDT coils) are used for which separate modules provide 
power excitation, signal averaging and recording. Also, since the adhesive 
shear stress is not entirely uniform across the test area, there are some 
doubts as to whether a maximum distortion of adhesive layer is obtained, 
and thus measured, for a given shear stress as a result of inherent 
adherends bending. 
The ASTM method (Designation: E229-70 (Reapproved 1981)) measures the pure 
shear of adhesives by applying torsional shear forces to the adhesive 
through a circular specimen which produces a uniform stress distribution. 
The torsional shear forces are applied by a torsional shear jig without 
inducing bending, peeling, or transverse shear stresses in the bond line. 
The shear strain is measured with an ASTM Class A or Class B-1 
extensometer, or alternatively an optical lever system is used. However, 
even though this method is reasonably accurate and gives direct results, 
specimen preparation is very time consuming and costly. 
A much simpler method involves using strain gauges directly on the specimen 
per se, but this method is also costly, since the gauge, once attached to 
the specimen, can be used only once. Then after use is disposed of with 
the sample. 
Therefore, in the light of the apparent problems with the prior art, the 
applicants have invented an apparatus which is reusable and inexpensive, 
and also simpler to use, producing more accurate and direct measurements 
of strain. 
The present invention is predicated upon selection of the "Iosipescu" 
method (see N. Iosipescu, Journal of Materials, Vol. 2, No. 3, 1967, p 
537). The test was originally developed for measuring the shear 
characteristics of metals and has in more recent years been extensively 
studied in relation to testing fibre-reinforced composites. These studies 
have amply demonstrated the uniformity of the shear stress across the 
sample. The Iosipescu method involves choosing a specimen which at the 
shear line, when a shearing force is applied thereto, exhibits shear in 
that direction only, thus giving a true value of shear. 
The present invention adapts the Iosipescu method to measuring the shear 
characteristics of adhesives by dividing the normal Iosipescu specimen 
into two at the shear line and rejoining the two sections with the 
adhesive to be tested. 
The invention provides an apparatus for measuring the extent of the shear 
distortion in adhesive bonds comprising means for applying a shear force 
to an Iosipescu-type specimen, having an adhesive bond line as the shear 
line and means for measuring the resultant distortion of the adhesive 
bond. 
Preferably, the invention may provide an apparatus for measuring shear in 
adhesive bonds wherein the apparatus may comprise an elastic fork with two 
strips wherein said strips deflect as a result of a shearing force applied 
to the specimen, means for attaching said fork to an Iosipescu-type 
specimen having an adhesive bond line as the shear line and said strips 
having means to measure the strain attached thereto. More preferably, 
strain gauges may be attached to the elastic strips to measure the strain 
resulting from any shear force applied to the specimen. Further, the 
strain gauges may be attached to a Wheatstone bridge circuit to measure 
the strain. Circuit bridge excitation power, signal conditioning and 
recording may be provided by an existing internal circuitry of the tensile 
testing machine. 
Preferably said means for applying a shear force to an Iosipescu-type 
specimen comprises a clamp means adapted to retain the Iosipescu-type 
specimen and a force applying means. The clamp means thus aids in 
retaining the sample in position and allow the shear force to be 
accurately applied to the adhesive bond line of Iosipescu sample. 
Preferably the clamp means comprises at least two clamps which are adapted 
to fit onto either side of the bond line of the Iosipescu-type specimen 
and further, each of the clamps may be connected to the force applying 
means. 
The force applying means is preferably a tensile testing machine and the 
clamps means may be adapted to fit into the existing recesses of the 
tensile testing machine. 
The invention further provides a method of measuring the shear distortion 
in adhesive bonds comprising the steps of: 
(a) preparing an Iosipescu-type specimen having an adhesive bond line of 
the adhesive to be tested as the shear line; 
(b) applying a shear force to the Iosipescu-type specimen on its bond line; 
and 
(c) measuring the resultant distortion of the adhesive bond. 
The method of the invention generally uses the apparatus as previous 
described. 
Preferably, the resultant distortion of the adhesive bond is measured by an 
elastic fork with two strips wherein said strips deflect as a result of a 
shearing force applied to the specimen, means for attaching said fork to 
an Iosipescu-type specimen having an adhesive bond line as the shear line 
and said strips having means to measure the strain attached thereto. More 
preferably, strain gauges may be attached to the elastic strips to measure 
the strain resulting from any shear force applied to the specimen. 
Further, the strain gauges may be attached to a Wheatstone bridge circuit 
to measure the strain. Circuit bridge excitation power, signal 
conditioning and recording may be provided by an existing internal 
circuitry of the tensile testing machine. 
Preferably said shear force is applied to an Iosipescu-type specimen 
comprises a clamp means adapted to retain the Iosipescu-type specimen and 
a force applying means. The clamp means thus aids in retaining the sample 
in position and allow the shear force to be accurately applied to the 
adhesive bond line of Iosipescu sample. 
Preferably the clamp means comprises at least two clamps which are adapted 
to fit onto either side of the bond line of the Iosipescu-type specimen 
and further, each of the clamps may be connected to the force applying 
means. 
The force applying means is preferably a tensile testing machine and the 
clamps means may be adapted to fit into the existing recesses of the 
tensile testing machine.

FIG. 1 discloses a schematic drawing of adhesively bonded Iosipescu-type 
specimen [10], in which two adherends [11] of the material to be joined 
are connected with the adhesive [12] on the bond area ([13] on FIG. 1a). 
FIG. 1a discloses an adherend [11] wherein .alpha. is 45.degree. to the 
bond area [13]. 
FIG. 2a illustrates a preferred embodiment (expanded view) of the apparatus 
of the invention [20]. 
The embodiment discloses three parts; means [21] to transmit the relative 
displacement of the adherends to the elastic strips of a two prong elastic 
fork [22] with sensing devices thereon [24] preferably strain gauges 
wherein each prong fits into the means for transmitting the relative 
displacement of the adherends [21] and means to support the elastic fork 
[23]. 
FIG. 2b shows the apparatus (expanded view) in the displacement mode. The 
adherends shear deflection is reproduced by the means [21] which transmit 
the relative deplacement of the adherends [10] to the elastic strips [22] 
thus for a given individual displacement (d) each strip experiences end 
applied moment (M.sub.A). As a result of the opposing applied moments the 
elastic strips of the fork are subjected to bending giving rise to surface 
compressive and tensile strains (i.e. two-.epsilon. and two+.epsilon. 
respectively) away from the plane of inflection (x-x). 
The slotted bar [23] interference fitted onto the other end of the fork 
[22] assists in balancing counteracting reactive moments (M.sub.R) of 
opposing direction. 
Preferably, to each side of the two elastic strips of the fork a small 
uniaxial strain gage is adhesively bonded (close to the sample end of the 
strips) and wired to form a Wheatstone bridge circuit. This forms the 
sensing means [24], as illustrated, from which the strain values are 
recorded. 
The problem of determining the actual site of strain gauge attachment has 
been resolved by a combination of a theoretical and experimental approach. 
Theoretically, it could be shown that the magnitude of the surface 
compressive or tensile strain along the strip's free length and its 
dependence on that length can be related to a single beam theory. The 
illustration, (FIG. 3) showing one elastic strip (side view), is analogous 
to a beam member constrained at both ends, for which it is known that the 
case is governed by 
##EQU1## 
where y is displacement and x is distance along the beam from either 
constrained end. Also from FIG. 3 
EQU y=0 for x=0 
EQU y=d for x=L and 
##EQU2## 
where d is the total end displacement and L is the free length. Solving 
equation (i) with these conditions gives 
##EQU3## 
However, surface strain (tensile) is given by 
##EQU4## 
where t is beam thickness and 
##EQU5## 
is the curvature at any point along the beam. Therefore using equation 
(ii) and solving gives 
##EQU6## 
which indicates that strain at the half-length of the beam (x=L/2, point 
of inflection) vanishes and increases along the beam reaching a maximum at 
either end (i.e. x=0 or x=L) thus reducing equation (iii) to 
##EQU7## 
Note also, from equation (iv), that for a given displacement (d) strain is 
increased by decreasing beam length (L). 
From the above it is shown that if the strain gauges are to be usefully 
employed, their position should be close to an end of an appropriately 
short elastic strip. Their equally distant positions from the chosen end 
will also ensure that the circuit bridge is temperature compensated and 
that the magnitude of the compressive and/or tensile strains, seen by the 
strain gauges attached to these sites, are comparable. 
The experimental part of this approach was concerned with implementing the 
above findings. This entails optimizing the strips' length and positioning 
the strain gauges so that the circuit bridge produced largest output 
signal (i.e. highest strain) for a minimal displacement with signal 
linearity over the entire measuring range. 
FIG. 4 illustrates the preferred embodiment of the apparatus [20] as 
attached to the specimen [10]. The apparatus [20] is attached to the 
sample in this preferred embodiment with G-type clamps [30]. A positioning 
device (not shown) can be used to align the apparatus symmetrically over 
the adhesive bond line. 
FIG. 5 illustrates the specimen [10] with an attached apparatus [20] in 
grips [40] suitable for tensile loading and adapted to fit into a tensile 
testing machine. The specimen is kept in place by securing screws [41] 
being previously aligned by a positioning device (not shown) to ensure 
that the bond line is properly aligned. To this fixed arrangement the 
shearing force is applied in the direction of the arrows shown by the 
tensile testing machine. 
In practice, the specimen final dimensions correspond to the Iosipescu data 
set out in the journal article by N. Iosipescu. A preferred adherend may 
have the following dimensions (depending on the tensile strength testing 
machine and grips etc.), FIG. 1A 
1=50 mm 
t=10 mm 
b=20 mm 
a=4 mm 
c=12 mm 
.alpha.=45.degree. 
No lateral notches are considered necessary as suggested in the Iosipescu 
journal. Adherends can be reused several times by remachining after each 
test if they cannot be cleaned by simple chemical means. This remachining 
process by which a fraction of a millimeter is taken off the bonding 
surface and notch faces is considered acceptable since there is no 
evidence of permanent set due to repeated application of the loads reached 
during the testing of the adhesive. 
The bonding step is the most critical of the testing procedure. Adherends 
must be axially aligned before, during and after bonding to ensure bond 
line thickness uniformity and to allow the accurate measurement of the 
bond line thickness. The bonding pressure to the adherends must be 
accurate and constant while the adherends are being bonded. Ideally, a 
calibrated screw-operated spring clamp which encloses the specimen can be 
used to retain the specimen axially aligned and place a constant pressure 
to the specimen. 
The bonding surface of the adherend is pretreated in a conventional way, 
for example, when epoxy film adhesives are being tested, the bonding 
surface of the aluminium adherend is first subjected to a standard surface 
pretreatment (i.e. vapour degrease and chromic acid etch). The cure is 
performed in a temperature controlled, air circulating oven. Specimens 
temperature near the bond line is monitored by a thermocouple inserted 
through the wall of the bonding assembly. 
The bond line thickness must be measured in order to obtain meaningful and 
reliable shear data from the shear stress-strain curve. The selected 
technique involves measurement of the end-to-end distance of the adherend 
pair before and after bonding, thus giving the bond line thickness by 
difference. These measurements are performed by the use of a commercially 
available comparator equipped with a high resolution Mikrokator. Using the 
Mirkokator, reliable and reproducible bond line thickness measurements 
with an accuracy of 2.5.times.10.sup.-3 mm can easily be achieved. In 
practice, handling of aluminium adherends should be minimized to maintain 
dimensional stability. 
Prior to testing, the apparatus of the invention must be calibrated so in 
order to determine the relationship between the displacement of the strips 
and the Wheatstone bridge output. 
Thus the strips are displaced and the distance measured and the 
corresponding Wheatstone bridge output is recorded. 
For calibration purposes the Wheatstone bridge was operated at 1.03 V 
supplied by a Strain Gauge Meter model RD-203, manufactured by Applied 
Measurements, Australia and the output read from a Keithley 177 uV, 
digital multimeter. The resulting calibration curve, representative of any 
part of shearometer's measuring range of about 0.5 mm, is shown in FIG. 6. 
The typical sensitivity is 2.3 uV/um. 
The apparatus is then attached to the specimen as illustrated in FIG. 4 and 
aligned onto the specimen to ensure that it lies symmetrically over the 
adhesive bond line. 
The specimen with the apparatus attached thereto is then placed in the test 
grips suitable for tensile loading. Once again the specimen is aligned to 
ensure that the bond line is in the line of shearing forces. 
The specimen is now ready for shear testing. 
During actual shear testing the adhesive bond is continuously deformed as 
one adherend is moved with respect to the other at a preselected speed. 
One half of the attached apparatus reproduces this movement and as it does 
so the Wheatstone bridge becomes unbalanced due to the change in 
resistance of a stretched strain gauge foil. The recorded bridge output 
voltage is, hence, taken as being equivalent to a combined strain on 
adhesive and adherends. The contribution of the adherends is subtracted 
from above by evaluating their elastic deformation under identical test 
conditions using an identically shaped non-bonded specimen. 
An example of the data and its reproducibility obtainable from this method 
is shown in FIG. 7. This gives the shear stress-strain curve of the 
epoxy-based film adhesive FM1000, which has been extensively used in the 
aircraft industry for many years, especially in such applications as 
helicopter rotor blades. From the initial, linear part of the curve the 
shear modulus can be determined: the rest of the curve provides 
information on the adhesive's elastic limit and strain to failure as well 
as the shear failure stress. 
The application of the Iosipescu shear test as adapted for the 
determination of properties of structural adhesives under conditions of 
uniform shear has been described. This invention is an inexpensive, 
reusable and highly sensitive instrument that gives reproducible results. 
As a whole, it has been shown that the apparatus and method of using 
thereof enables determination of the complete adhesive stress-strain 
behaviour and thus evaluation of shear modulus, elastic and plastic strain 
to failure and the ultimate shear stress.