CE290E Composites Lab

Lab #2: Tensile Testing

Lab Instructor: John F. Halterman and Dr. Ever Barbero

March 4, 1999 - 10:45-12:30 - G-07 ERB

Purpose: To introduce testing of composites and collect data to compare to theory.

Summary: Three longitudinal and three transverse fiberglass samples from the plate made in Lab-1 will be placed in a tensile testing machine. Strain gages in the longitudinal and transverse direction are on each sample. Loads will be applied, and stress and strain will be determined. This will allow the calculation of various laminate properties, such as Ex, Ey, nxy, nyx, etc.

Pre-lab Comments: Most of the testing will be done by the lab instructors. Some wiring connections will be done by students.

Brief Concept of Tensile Testing: Strain gages are resistors, which vary in resistance with respect to the strain placed upon it. The stress is obtained by dividing the load by the cross-sectional area. A computer is used to record various data, which is recorded to a file. The testing machine applies a stroke at a constant rate (0.5 in/min). This induces a load on the specimen, which is measured using a load cell. Strain gages are placed on the specimen, and using a Whetstone Bridge (a balancing electrical circuit, strain indicator), the strain is calculated. The data (time, stroke, load, strain_1, strain_2) is recorded on a computer. The modulus of elasticity is calculated using Hooke’s law. Poisson’s ratio is determined from the longitudinal and transverse strain measurements. The figure below is an illustration of the testing apparatus.

Tensile testing.

Each group will be provided with two tabbed and gaged samples. The samples have been cut from the plate manufactured in Lab 1. On sample was cut along the roving direction (L-sample), the other transverse to it (T-sample). Both samples have a longitudinal and a transverse gages. The test results file for each sample will be provided on disk at the end of the Lab.

Before testing:

  1. Measure and record thickness and width of samples at midpoint.

 

Use the lab results to compute:

  1. The modulus Ex of the laminate by the chord method (ASTM D3039). The chord method consists of drawing a chord line between the data points at 1000 and 6000 microstrain (10-6 in/in). You do not need to do it graphically, just use the values in the data file. If no data is available for exactly 1000 and 6000, use the nearest values.

  2. The Poisson's ratio nxy= - ey/ex of the laminate at 1000 microstrain, form the L-sample.

  3. The Poisson's ratio nyx= - ex/ey of the laminate at 1000 microstrain form the T-sample.

  4. Compute the longitudinal tensile strength Fxt of the laminate as the maximum stress.

  5. Compute the transverse tensile strength Fyt of the laminate as the maximum stress.

Shear testing:

An ASTM D5379 was performed on sample w4s-2. The load was transformed into stress by dividing by the sample cross-section, which was measured prior to testing.

The stress-strain data is displayed in the figure along with the result of using Eq. 4.66. Since Eq. 4.66 was written for a unidirectional composite, the parameters used are G12 and F6. For laminates, we just replace Gxy for G12 and Fxy for F6.

 

You are asked to:

  1. Compute the shear stiffness of the laminate Gxy by the chord method. The chord method consists of drawing a chord line between the data points at 1000 and 6000 microstrain (10-6 in/in).
  2. Estimate the shear strength Fxy of the laminate.