Test for Hoop Tensile Strength of Advanced Ceramic Composite Tubes Using Elastomeric Inserts ASTM C1819
ASTM C1819 is used to determine hoop tensile strength of continuous fiber-reinforced advanced ceramic (CFCC) tubes. The CFCC tubes are subjected to internal pressure, produced by the expansion of an elastomeric insert. This test method is used for material development, material comparison, material screening, material down selection, and quality assurance.
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 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.
This test method is used primarily to test CFCC tubes with continuous fiber reinforcement. 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, it only applies to tubes because composite tubes have different flaw populations, fiber architecture as compared to flat plates.
The dimensions of the gage section of each test specimen are measured. 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 on the test machine. 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 therefore must be determined for each situation. The extensometer is mounted on the test specimen gage section and 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.
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 gage 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 hoop tensile strains, the hoop proportional limit stress, and the modulus of elasticity in the Hoop directions.
The geometry of the tubular test specimen depends on the ultimate use of the hoop tensile strength data. Minimum five test specimens are required for estimating a mean. More may be necessary if estimates regarding the form of the strength distribution are 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.
p = F/π(ritube)2
p = internal pressure F = axial force required by the tubular test specimen ritube = internal diameter of tube units of mm
Hoop Tensile Stress:
σh= η mpX2(rtitube)2/(r0tube)2 – ( ritube)2
h= hoop tensile stress P = internal pressure η m = maximum stress factor ritube = inner radius of the tube r0tube = outer radius of the tube
Hoop Tensile Strain:
εh= 2 ∆r/2r0tube
ɛ h = hoop tensile strain ∆r = change in radius r0tube = outer radius of the tube
Hoop Tensile Strength:
Shu= η mpmaxX2(rtitube)2/[(r0tube)2 – (rtitube)2]
Shu = hoop tensile strength pmax = maximum internal pressure η m = maximum stress factor ritube = inner radius of the tube r0tube = outer radius of the tube
Hoop Tensile Fracture Strength:
Shf= η mpfX 2(rtitube)2/[(r0tube)2 – (rtitube)2]
Shf = hoop tensile fracture pmax = internal pressure at fracture η m = maximum stress factor ritube = inner radius of the tube r0tube = outer radius of the tube
Modulus of Elasticity:
E = Poisson’s ratio ∆σh/ ∆ɛh = the slope of the( σh– ɛh ) linear region of the plot of longitudinal strain versus hoop strain
v=- ɛl/ ɛh
ν = Poisson’s ratio ɛl/ ɛh = the slope of the linear region of the plot of longitudinal strain versus hoop strain
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