BIAS EXTENSION TEST STANDARD Xiongqi Peng and Jian Cao Advanced Materials Processing Laboratory Department of Mechanical Engineering Northwestern University Evanston, IL August 2003 For Internal Use 1. Purpose To measure the in-plane combined shearing and tensile response of textile composite materials to testing displacement and velocity. 2. Experimental Set-up and Sample Preparation A sample of material, with the dimension of the test sample in the loading direction relatively greater than the width, and the yarns initially oriented at ± 45-degrees to the loading direction, is gripped at two ends (see Fig. 1). A tensile force is applied at the gripper. The force required to deform the material is recorded at the gripper as a function of gripper displacement. Figure 1. Bias extension set-up Great care should be given to make sure that fiber yarns are oriented ± 45-degrees to the edges. Any small misalignment will lead to tensile or compressive forces in the fiber directions, resulting in large scatter in measured force readings. 1
K H D C I E B L F A G J Figure 2. Bias extension sample A special recommendation is made for the specimen preparing of balanced woven composites. Here, we provide a general procedure for preparing bias extension test specimen for balanced woven composites (refer to Fig. 2), based on our experience, by following this procedure, the sample will have an aspect ratio of two and it will greatly improve the repeatability of testing results. 1) On a composite fabric, draw a line along the weft direction from point A to point B. Count the yarn number in this direction, take 16 yarns as an example. 2) Extend line AB to point C along the weft yarn direction by another 16 yarns. 3) Count 16 yarns from point B in the warp direction to obtain point D and point G. 4) Follow the same procedure to draw lines DEF and AEH. 5) Draw lines connecting points C and H, H and F, F and G, and G and C. 6) Extend line HC in two ends to points K and I, respectively, and extend line FG in two ends to point J and L, respectively, to generate two grip regions for the grippers. 7) Connect KL and IJ. 8) Use tape to cover the grip regions before cutting the sample. This can prevent the fiber yarn loose of the sample. 9) Cut sample along lines KI, LJ, JL and LK. For dry textiles, pre-conditioning (pulling the specimen in the gripper several times prior to collecting data) is not recommended as this pre-conditioning will incur fiber yarn slippage along corners of the gripper regions (around H, F, C and G) and thus damage the testing specimen. 2
As a rule, the sample should be gripped to ensure no slippage between the tows of the sample and the gripper during the testing. No slackness of the sample before testing should be allowed. 3. Definitions For the purposes of this standard, the following definitions apply: The material shear angle is defined as (see Fig. 3), π θ = 2Φ (1) 2 where Φ is the central region (pure shear) angle and can be calculated from the gripper displacement using, W + D Φ = arccos (2) 2W where D is the gripper displacement and W is the width of the sample. D 2Ф H=2W W Figure 3. Deformation of bias samples 3
The angular shear rate in the material is defined as, & θ = 2D (3) 2 2 W + D W 2 where D & is the gripper displacement rate. It must be noted that the above derivation for the shear angle is based on some ideal assumptions. 4. Required Report Items Geometry: o W: sample width o H: sample height o N 1 and N 2 : Numbers of fiber yarns in the center shear zone in two directions Testing conditions: o Diagram and description of test device o Method of clamping o Special treatment applied to the sample if any o Test speed Test Results: o Raw data: Load versus head displacement of every test. CAUTION: Assuming that at the beginning of a test, every detail is taken care of, report all the test results. Do not delete any one of test results afterwards simply because it does not match the other similar tests. You may provide a justification and discard the result from a particular test when reporting processed data, but not when reporting the original raw data. o Processed data: Load versus shear angle o Temperature profile o Any slippage 4
o Measured shear angle if applicable. o Evolution of dimension AD (Fig.2) with respect to the head displacement Processing Method: o Measurement techniques, especially if strain is measured directly. o Definition of quantities to be measured and procedure for obtaining these from raw data 5. Apparatus Testing machine: any suitable tensile testing instrument. If the testing machine cannot provide grippers wide enough for the specimen, custom-made grippers are necessary. Environmental chamber or oven for heating of thermoplastic or thermoset prepreg samples if required. Computer to log measured displacement and force. 6. Test Procedure The alignment of the testing apparatus is crucial to the prevention of buckling of the testing specimen. Always do a good alignment before any tests. Specifically, make sure that surfaces X and Y are in the same plane (see Fig. 4) and coincide with the testing plane of the testing machine. Surface X Surface Y Figure 4. Alignment of grippers in bias extension 5
In the absence of an obvious preferred testing speed, a normalized gripper 1 displacement rate of 1 s is recommended, i.e. D & 1 1 (4) W = s For prepregs the material temperature during testing should be measured. As the oven temperature can often lag behind the material temperature, use of a temperature probe is recommended, i.e. a thermocouple embedded in the test material. Note that for thin materials, embedding the thermocouple can be difficult due to the narrow thickness of the material test sheet. In this case, sandwiching the thermocouple between two sheets of the material is recommended (the sheets can be held together using large staples). The probe should be positioned in the oven at the initial mid-way height of the sample. Data collection: monitor the force and crosshead displacement throughout the test. Test termination: usually the normalized gripper displacement should be greater than 0.35, i.e. W = 0. 35 before ending the test. This corresponds to a shear angle o of approximately55. Record the gripper displacement at the point of sample buckling if it happens. Measure the shrinkage between points A and D in Fig. 2. At least 5 repeats should be conducted under identical conditions. Ideally results should be presented for all tests; if not, the median curve should be presented along with error bars representing minimum and maximum force readings at 5 equally spaced displacements (or shear angles). 6