Shear Test

 

To measure shear strength there are several shear test methods in existence for composite materials. They can be grouped by their way of force application (compressive force via rim of specimen, friction force via clamping system) or their required specimen geometry (with or without notch, miniaturised). Furthermore the tensile test can be modified to a shear test using fibre orientation of +/-45° and also the flexural test can be used to determin shear properties. All of the established methods have in common that testing provides reliable results for small shear deformations (<5%) i.e. small shear forces only.

Dependable characterisation of high performance fibre reinforced composite materials gains increasing importance. We specialized in this field and additionally offer the innovative shear frame test described in DIN SPEC 4885. It was developed in collaboration with German Federal Institute for Materials Research and Testing (BAM). This method is tolerant to high deformation and thus compatible to measure high shear strength composite laminates.

Shear Frame Test of flat specimens using DIN SPEC 4885:2014-01

  • Reference testing procedure for determination of shear properties of fiber reinforced composite materials
  • Short, long and continuous fiber reinforced composites
  • Determination of shear modulus and shear strength even for shear deformation of more than 5%
  • Fiber orientations: 0°, 90°, 0°/90°
  • Geometry of specimen: 165mm x 165mm x 2…4mm
  • Doublers: not necessary
  • Strain gauges: 2 pc per specimen in 45°-orientation

The shear test using DIN SPEC 4885 is a picture-frame shear test. Laminates with thermosetting or thermoplastic matrices reinforced with fibres in 0°- or 0°/90°- direction can be tested. The square shaped specimen is built symmetrically and balanced around the midplane. Typically the thickness of the specimen is 2 … 4mm.

It requires expensive test fixtures and significant amounts of test material, as opposed to the established tests. But it provides great advantages:

  • Shear strenght can be monitored in the linear as well as non-linear range of load-deformation-behaviour for shear deformations (slidings) larger than 5%. For the first time, maximum shear strenght even in this range can be determined.
  • The all-side clampings prevent free rims and thus prevent load shifting effects.
  • The results thereby show small variance usually less than 3%, which makes the method suitable for parameter studies or quality check.
  • The single layers of the specimen exclusively see pure shear stress. It is distributed sufficiently homogenous over the specimen. The maximum shear stress is located in the center which typically is the location of failure.

For the shear test using DIN SPEC 4885 we developed the picture-frame testing system GZ-S80. Hydraulic clampings provide quick and easy change of specimen. The testing routine takes only few minutes, so it can be integrated into industrial environments for shear tests series.

Iosipescu Shear Test using ASTM D 5379-12

  • Standard testing method for determination of shear properties of fiber reinforced composite materials
  • Laminates made of a thermosetting or thermoplastic matrix and unidirectional fibers
  • Testing of multidirectionally oriented fibers possible
  • Fiber orientations: 0°, 90°, 0°/90°
  • Geometry of specimen: 76mm x 20mm x 2…10mm
  • Doublers: not necessary
  • Strain gauges: 2 or 4 pc per specimen in +/-45°-orientation

Following ASTM D 5379 the Iosipescu shear test induces a state of pure shear at the specimen mid-section by applying two counteracting moments. The two opposing sections of the apparatus can be moved in opposite directions. A composite specimen is gripped at the rim of each side and the force is induced via four points. Ninety-degree notches are machined in the specimen and determin the area of failure. The central region is assumed to be in pure shear.For determining shear strength using Iosipescu, there are some uncertainties in the test results:

  • The method is limited to small deformations only. Large deformations lead to load shift for horizontal fibre orientation.
  • Shear stress decreases at the central region of the specimen (by 20% for longitudinal and by 10% for transversal orientation). There is no homogenious stress distribution in the cross section.
  • The machining of the notch produces varying stress situations in the notch area which makes the specimen fail at different loads.
  • The test applies concentrated forces to the specimen which can cause local crushing on the edges of the specimen.

On the other hand, the Iosipescu method provides an inexpensive and flexible way of testing shear strength. Varying laminate orientations can be tested easily using only one testing method.

V-Notched Rail Shear using ASTM D 7078-12

  • Standard testing method for determination of shear properties of fiber reinforced composite materials
  • Laminates made of a thermosetting or thermoplastic matrix and unidirectional fibers
  • Testing of multidirectionally oriented fibers possible
  • Fiber orientations: 0°, 90°, 0°/90°
  • Geometry of specimen: 76mm x 56mm x 2…10mm
  • Doublers: not necessary
  • Strain gauges: 2 or 4 pc per specimen in +/-45°-orientation

In the Rail Shear test following ASTM D 7078 and ASTM D 4255, a composite specimen is gripped along each side of a long narrow central region by clampings and the two opposing sections of the apparatus are loaded, in opposite directions. The central region of the specimen is assumed to be in pure shear.Among the benefits, the rail shear test is limited by the following:

  • The method is limited to small deformations only. Large deformations lead to load shift for horizontal fibre orientation.
  • There is no homogenious but parabolic stress distribution in the cross section.
  • Specimen of multiaxial or parallel to load direction oriented fibres include fibres not connected to the clampings, which leads to unwanted effects.
  • ASTM D 7078 uses a notched specimen. The stress distribution is strongly influenced by the manufacturing of the specimen.
  • For using ASTM D 4255, several holes must be drilled into the specimen in the clamping region, which leads to stress concentration.

On the other hand, the rail shear test method is a system of low complexity which uses simple and flat specimen.

Shear Frame Test of flat specimen using DIN EN ISO 14129:1998-02

  • Standard testing method for determination of shear properties of fiber reinforced composite materials
  • Laminates made of a thermosetting or thermoplastic matrix and unidirectional fibers
  • Fiber orientation: +/-45°
  • Geometry of specimen: 250mm x 25mm x 2mm
  • Doublers: 50mm x 25mm x 2mm (with +/-45°-orientation)
  • Strain gauges: 2 pc per specimen in 0°/90°-orientation

The shear test by DIN EN ISO 14129 or ASTM D 3518 measures characteristic shear values on flat samples with a ±45° fibre orientation. The specimen’s laminate structure is symmetrical and balanced. The testing routine is a static tensile test.The preface of the norm already quotes limits of this method:

  • The method is limited to small deformations only (<5%). Large deformations lead to load shift for horizontal fibre orientation.
  • The resulting stress condition in the specimen is multiaxial. The single layer see normal stresses as well as shear stresses. The stresses vertical to fibre orientation additionally cause break between fibre loads which influence the characteristic shear values. Owing to the principle, determining shear strength can’t be done error free.
  • The free rims of the specimen cause load shifting effects.
  • The load distribution must be assumed to be inhomogenious over the entire specimen width.
On the other hand, the testing routine is impressivly simple. It basically modifies the established tensile test using a smart fibre orientation to create shear stress.

Short Beam Shear Test using DIN EN 2563 and 2377, DIN EN ISO 14130 and ASTM D 2344

  • Standard testing method for determination of shear properties of fiber reinforced composite materials
  • Laminates made of a thermosetting or thermoplastic matrix and unidirectional fibers
  • Geometry of specimen: 20mm x 10mm x 2mm
  • Radius pressure pin: 5mm
  • Radius support: 2mm
  • Temperature range: 20…280°C

The Short Beam Shear test (SBS) described in DIN EN 2563 /2377, DIN EN ISO 14130 and ASTM D 2344 for the determination of apparent interlaminar shear strength is a modified 3-Point flexural test. The span length of the fixture is small as compared to the specimen’s thickness which induces shear stress to the specimen. The geometry of the specimen is a short beam of thermoplastics or thermosettings continuously reinforced by fibres in 0° or 0°/90°-direction. The norm defines apparent interlaminar shear strength as “maximum shear stress calculated at half thickness of specimen at the moment of first failure.”[1]The norm also quotes limits of this method:

  • Depending on the material configuration, “the results can be influenced by interfering strain/flexural stresses or flexural/torsion moments etc.” [2]
  • The specimen may possibly fail due to flexural or plastic deformation, which makes it impossible to determin the real shear stress at the moment of failure.
  • The method may not be used to determin characteristic values for construction purposes. It may only be used for material selection or quality check.
  • The results are no quantitative (absolute) values. They may only be used for comparison within one test series.
On the other hand, the SBS testing method works with only very small and geometrically simple specimen. Using a small amount of material, qualitative information about the resin-fibre relation can be obtained.

[1]: DIN EN 2563, “Aerospace series, Carbon fibre reinforced plastics, Unidirectional laminates, Determination of the apparent interlaminar shear strength”, CEN, 1996
[2]: DIN ISO 14130, “Fibre reinforced plastic composites, Determination of apparent interlaminar shear strength using short-beam method”, CEN, 1997

Dynamic testing of materials

Measuring the fatigue behaviour of fibre reinforced plastics (FRP) is the key to predict service lifetime of parts and structures, whereat the anisotropic FRP show a very different behaviour from isotropic materials like metals. For FRP, fatigue is essentially created by initiation, growth and propagation of a multitude of cracks. The evolution of damage is influenced not only by the materials of matrix and fibres but also by the composition of the laminate. Although it is highly specific to the individual material, it usually follows the stages below:

  • Initiation of lateral cracks in laminate layers out of loading direction
  • Initiation of longitudinal cracks alongside load-bearing fibres
  • Delaminationen alongside rim of specimen
  • Internal delaminationen due to lateral and longitudinal cracks
  • Fracture of load-bearing fibres due to buckling
  • Delamination due to shear stress
  • Complete failure of the part/ specimen

We use a traction device for alternation loading (R=-1), tensile pulsation loading (R=0) or compression pulsation loading (R=-∞). The fibre direction configuration specifies the type of stress: longitudinal or lateral for tension and compression, +/-45° for shear tests. In order to prevent early fibre buckling, we use the Anti-Buckling Test Fixture GZ-BS32.

For characterizing the dynamical flexural material properties we use our 3-/4-PointFlexural test fixture GZ B-50. Here, too, tensile and compression properties can be investigated using fibre orientation.

We monitor the fatigue behaviour of the material of your choice, e.g. thermoplastics or thermosets, reinforced with carbon or glass long or continuous fibres, performing endurance stress/ operational reliability tests and generating “Wöhler” curves of your specific material configuration.

Adhesion Test for Release Agents

The plastics industry relies on release agents to ensure smooth production processes. Release agents form the critical barrier between mold wall and plastic melt in the manufacture of molded articles. To ensure flawless demoulding the release agent must be applied carefully all over the inner surface of the mold walls, which is unfeasible for automatisation. For this purpose integrated release agent polymer systems are developed. To quantify their residual adhesion force we developed the testing system GZ RA-20.

The tested mixture is applied between two heated parallel surfaces of tool steel. Temperature and duration are set according to process configuration or can be determined using GZ RA-20, respectively. Subsequently one surface will be lifted by a set speed monitoring the residual adhesion force induced by the polymer mixture.