Brittle Fracture and Thermal Stress

Question 1

Explain and differentiate between stress and strain.

Answer 1

A stress is a force intensity or a force per unit area.

Strain is the degree of deformation a material undergoes as the result of an applied stress. Strain in a material is a ratio of the amount of elongation to the original length of the material.

Question 2

Explain Yield Stress

Answer 2

Yield stress is the stress at which the transition from elastic to plastic behavior (plastic deformation) occurs.

Question 3

Explain ultimate tensile strength

Answer 3

The maximum stress supported by a material
before it fractures.

Question 4

Explain brittleness

Answer 4

Brittleness is the ability of a material to sustain
little or no plastic deformation prior to
fracturing. On a stress-strain curve, a brittle
material would have a yield point and fracture
point very close together.

Question 5

Explain ductility

Answer 5

Ductility is the ability of a material to undergo
plastic deformation prior to fracture. A stressstrain
curve for a ductile material would show
the yield point and the fracture point far
removed.

Question 6

Explain what is meant by fatigue failure.

Answer 6

When a material undergoes a periodic
loading and unloading, micro cracks develop in
its structure. The micro cracks grow in time and
failure may occur as the material is stressed even
well below its normal fracture stress. This is fatigue failure.

Question 7

Describe the terms stress intensity factor and fracture toughness.

Answer 7

The stress intensity factor (K) measures how likely propagation of a crack or flaw is. It defines the amount that existing stress is intensified in the region of a flaw. It is a
function of the flaw size and the stress acting in the region of the flaw.

If K exceeds some critical stress intensity value, denoted by K_c, the crack will
propagate. If it doesn’t, the crack will remain stationary. K_c is sometimes called the fracture toughness.

Question 8

Describe the brittle fracture mode of failure.

Answer 8

There is little or no plastic deformation prior to brittle fracture. Brittle fracture in metal is characterized by a rapid rate of crack propagation, with no gross deformation
and very little micro-deformation.

Question 9

Explain three conditions necessary for brittle fracture to occur.

Answer 9

1. A flaw such as a crack
2. A stress (usually above 5,000 to 8,000 psi) of sufficient intensity to develop a small deformation at the crack tip
3. A temperature low enough to promote brittle fracture

Question 10

Explain the effect of fast neutron irradiation on reactor vessel materials.

Answer 10

Experimental evidence discloses that neutrons, whose energy is in excess of 1 MeV, damage reactor vessels and structural materials via direct collision with the atoms of that material. The atoms are knocked out of their lattice positions and are called “knocked on” atoms. This results in vacancies in the crystalline material of the
vessel.

The reference temperature (RTNDT) shifts from an initial temperature to a higher one with an increase in the fast neutron dose on the vessel wall. This shift causes the material to be more susceptible to brittle fracture at higher temperatures. The RTNDT must be adjusted as the vessel ages to account for neutron irradiation effects.

Question 11

Define and describe Nil-Ductility transition temperature (NDTT) and reference temperature for nil-ductility transition (RT_NDT)

Answer 11

The temperature below which a metal fails by brittle fracture is called the nil-ductility transition temperature. Another name often used is the reference temperature for nil-ductility transition (RT_NDT). This is more commonly referred to as simply the reference temperature (RT_NDT).

Question 12

Describe the drop weight test and describe how the data is used.

Answer 12

The Drop Weight Test establishes a value for the temperature where brittle fracture becomes highly probable. The weight is dropped from various heights to
obtain different values of impact energy. RT_NDT is then determined by:

• Determining the temperature at which two samples did not break. This will typically be about 0°F.
• Lowering the specimen temperature by 10°F and performing the drop weight test again. If the samples break at this temperature, this temperature is the drop weight RT_NDT.

Question 13

Describe the Charpy V-notch test and explain how the obtained data is used.

Answer 13

To conduct the Charpy V-notch test, a specimen is notched and placed between two supports. A pendulum is then caused to strike the specimen causing it to fracture or deform. The test is performed at a temperature no greater than (NDT + 60°F)..

This is done until a minimum 35 mils lateral expansion is observed and a crack develops due to 50 ft-lbf impact. The temperature at which these criteria are met is recorded as the Charpy V-notch Temperature (Tcv). Using the value of Tcv obtained in this manner, the reference temperature (RTNDT) can then be
determined by the equation (RT_NDT = T_cv - 60F). If the RT_NDT from this test is less than the RT_NDT obtained from the drop weight test, the larger value serves as the reference.

Question 14

Describe and differentiate between the stresses induced in a reactor vessel wall during heatup and cooldown.

Answer 14

Both the inner and outer walls of the vessel are subject to tensile stress, with the inner wall experiencing the greatest stress. Raising or lowering the temperature of the reactor vessel results in either tensile or compressive stress. During a heatup, the inner wall obviously heats up faster than the outer wall. The resulting stress changes from compressive to tensile throughout the vessel wall. During a cooldown, the inner wall also cools down faster than the outer wall. The resulting stress changes from tensile to compressive throughout the vessel wall.

Question 15

Explain the methods used to minimize the possibility of brittle fracture by using operating limitations.

Answer 15

To limit the possibilities of brittle fracture,
limitations have been placed on reactor
operations. Technical Specifications and
Operating Procedures limit the rate of change of
reactor coolant temperature during a heatup or
cooldown. This provides for safe plant operation
and ensures thermal stress is below the ASME
Boiler and Pressure Vessel Code.

Question 16

Describe pressurized thermal shock (PTS) and the associated safety concerns.

Answer 16

During abnormal or accident conditions, the
RCS may be subjected to an over-cooling event
where RCS temperature decreases and pressure
remains near normal, or temperature and
pressure decrease followed by repressurization of
the RCS. This is PTS.

Question 17

Explain the operational concerns of uncontrolled cooldown.

Answer 17

The concern is that rapid cooling of the reactor vessel internal surface will cause a temperature gradient across the vessel wall. The temperature distribution causes thermal stress, with a maximum tensile stress at the inside surface of the vessel and a compressive stress at the outside surface.

Residual, embrittlement and pressure stresses are also tensile on the inner wall of the pressure vessel. Combining these existing tensile stresses with a rapid cooldown exposes the inner wall to more tensile stress than any other operating condition, reducing the margin between K_I and K_Ic.