ASTM C1819 Test for Hoop Tensile Strength of Advanced Ceramic Composite Tubes Using Elastomeric Inserts

Scope:

ASTM C1819 is used to determine the Hoop stress and strength of a CFCC tube. Hoop stress is the stress that occurs along the pipe’s circumference when pressure is applied. Hoop stress acts perpendicular to the axial direction. Hoop stresses are tensile and are generated to resist the bursting effect that results when pressure is applied. 

This test method is used for material development, material comparison, material screening, material down selection, and quality assurance. This test method is not used for material characterization, design data generation, and material model verification.  

ASTM C1819 test method is used primarily to test CFCC tubes. CFCCs are essentially ceramic matrices reinforced by fibers. The ceramic matrix and the fiber reinforcement each can be made from a wide range of materials: oxide, graphite, carbide, nitride, and other compositions. 

Although this test method is not intended for discontinuous fiber-reinforced, whisker-reinforced or particulate-reinforced ceramics, it may be equally applicable to these composites. This test method does not apply to flat plates, and it only applies to tubes because composite tubes have different flaw populations and fiber architecture as compared to flat plates.

Test Procedure:

In ASTM C1819, the dimensions of the gauge section of each test specimen are measured. The test mode and rate to be used are determined. This type of test configuration is sometimes referred to as an overhung tube. 

The elastomeric insert, pushrods, and bore of the tubular test specimen are cleaned and greased. The elastomeric insert is slid into the tube. One pushrod is slid into each end of the tube, sandwiching the insert between the pushrods inside the tube.

There are two free ends of the pushrods sticking out from the test specimen. These free ends are inserted into the upper and lower grips of the test machine. 

The test mode and test rate are selected in the test machine as per ASTM C1819 testing requirement. The test specimen is temporarily supported such that the insert is centered in the test specimen between the two pushrods. The insert is preloaded to remove the “slack” from the load train and to take up the clearance between the insert and tube wall, such that the temporary supports are not necessary and can be removed. The amount of preload will depend on the insert material and clearance between the insert and tube wall and therefore must be determined for each situation. The extensometer is mounted on the test specimen gauge section and the output is zeroed. 

The data acquisition and the test mode are initiated. Internal pressure is exerted on the tube by the expanding insert. After the specimen is fractured, the test machine and the data collection of the data acquisition system are disabled. 

For ASTM C1819 testing, the breaking load is recorded with an accuracy of 61.0% of the load range. The specimen is removed from the grip interfaces. The specimen along with any fragments from the gauge section is placed into a suitable, non-metallic container for later analysis.

The resulting maximum pressure and the pressure at fracture are used to determine the hoop tensile strength and the hoop fracture strength. The stress-strain data is used to determine the hoop tensile strains, the hoop proportional limit stress, and the modulus of elasticity in the Hoop directions. 

Specimen Size: 

For ASTM C1819 testing, the geometry of the tubular test specimen depends on the ultimate use of the hoop tensile strength data. A minimum of five test specimens are required for estimating a mean. More test specimens may be necessary if estimates regarding the form of the strength distribution is required. Fewer tests can be conducted for an indication of material properties if material cost or test specimen availability limits the number of possible tests.

Data:

The following data is calculated in ASTM C1819 testing:

Internal Pressure:

p = internal pressure
F = axial force required by the tubular test specimen
r_i^tube = internal diameter of tube units of mm

Hoop Tensile Stress:

h= hoop tensile stress
P = internal pressure
η _m = maximum stress factor
r_i^tube = inner radius of the tube
r₀^tube = outer radius of the tube

Hoop Tensile Strain:

ɛ _h = hoop tensile strain
∆r = change in radius
r₀^tube = outer radius of the tube

Hoop Tensile Strength:

S_hu = hoop tensile strength
p_max = maximum internal pressure
η _m = maximum stress factor
r_i^tube = inner radius of the tube
r₀^tube = outer radius of the tube

Hoop Tensile Fracture Strength:

S_hf = hoop tensile fracture
p_max = internal pressure at fracture
η _m = maximum stress factor
r_i^tube = inner radius of the tube
r₀^tube = outer radius of the tube

Modulus of Elasticity:

E=∆σ_h/ ∆ɛ_h

E = Poisson’s ratio
∆σ_h/ ∆ɛ_h = the slope of the( σ_h– ɛ_h ) linear region of the plot of longitudinal strain versus hoop strain

Poisson’s Ratio:

v=- ɛ_l/ ɛ_h

ν = Poisson’s ratio
ɛ_l/ ɛ_h = the slope of the linear region of the plot of longitudinal strain versus hoop strain

Conclusion:

ASTM C1819 is used to determine hoop tensile strength of continuous fiber-reinforced advanced ceramic (CFCC) tubes.