Usually a ship consists of thousand tons of steel structures. These structures need to be designed precisely and built with materials of required properties. These structures normally work under different working conditions. As a result it is necessary to check whether it will be in a good service condition and safe during its lifetime. Creep behavior is an important criterion to be considered on this purpose.

Pressure distribution over a propeller blade is normally non-uniform. But here for simplicity we assumed it as uniform. The intention of this analysis was to show a procedure and compare the results to show how it can be used in real cases and for more complicated blade shapes. These two blade designs were designed on AUTOCAD. So it will not be difficult to try for other designs and import to ABAQUS CAE for analysis.

Propulsion systems for surface ships and underwater vehicles have followed the standard propulsion mechanism for many years. Propellers have to be designed in a way to reduce noise and vibrations and hence cavitation to the lowest possible level in order to achieve propeller efficiency. To ensure the safety and durability of the propeller creep analysis of the design given the material properties is very important. Under a probable working condition the locations can be detected where stress concentration will occur by static analysis. Then creep analysis can show the probable locations where permanent deformation means creep can occur. Prediction of  time after which the creep strain value crosses the danger limit is also an important one. Now it will be easier to take necessary precautions at the right time to avoid the initiation of crack propagation or sudden breakdown. The creep analysis will help a designer to modify his blade design considering other criteria to design the best one. For example two simple CAD designs of propeller blade are compared under same working conditions.

See the full analysis here PDF

First we have to test  the method ,we are working with , with experimental data before applying on ship structures.

Time dependent strain graphs are fitting into power equation in order to define the expression for uniaxial creep strain in terms of uniaxial stress σ , time t , and temperature T

The power law equation is modified to Bailey-Norton law in order to define the expression for unaxial creep strain in terms of unaxial stress , time and temperature

A sample creep test data was extracted for Grades of Stainless Steel –Grade 310

Stress to develop a creep rate of 1% at the indicated time at the indicated temperature

Then applied the stress and temperature for two separate time duration to see the results matching with the experimental data or not. We observed two analyses to see creep rate 1% is verified or not.

Read the full verification test here PDF

                               

Deformed model shape

Contour plot of Mises stress

Contour plot of equivalent creep strain

Displacement history plot

XSYMM boundary condition region

ZSYMM boundary condition region

U2 boundary condition region

Surface to which internal pressure will be applied

Surface to which end cap pressure will be applied

Linear elastic properties

Young’s modulus (Pa)	Poisson’s ratio	Temperature (ºC)
2.05E+11	0.304	100
2.00E+11	0.310	200
1.90E+11	0.316	300
1.85E+11	0.322	400
1.75E+11	0.323	500
1.60E+11	0.319	600

Thermal expansion properties.

Coefficient	Temperature (ºC)
1.20E-5	100
1.27E-5	200
1.33E-5	300
1.38E-5	400
1.42E-5	500
1.46E-5	600

Power law creep properties

Coefficient	Value
A (power law multiplier)	2.5003E-59
n (equivalent stress order)	6.62
m (time order)	0.0

A model of the intersection of a pipe with a cylindrical pressure vessel

Creep is the permanent deformation of a component under a static load maintained for a period of time. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures . Boiler tube, turbine blade, pump rotor, concrete etc. structures are needed to be creep tested before going into service. Creep test ensures the material used in the structure fulfills the requirements of strength and stability. In the recent times alloys are preferred by engineers than pure materials because of their special properties. These materials should also be creep tested before replacing the previous material. Creep analysis is mostly significant in the ship structures which are subjected to high temperature and stress. Structures like engine foundation, boiler tube, propeller blade, plates welded with one another etc. normally work under extensive working conditions. It is shown in this thesis, is a procedure, by following it one can get results of creep analysis different of structures at various conditions. At first a standard problem of a model of the intersection of a pipe with a cylindrical pressure vessel has been worked out and the properties of the model has been observed while the system operates at elevated temperature and carries internal pressure. In the first step a static analysis is performed, during which the internal pressure is applied. In the second step a quasi-static transient analysis is carried out to determine the creep behavior of the pressurized vessel and pipe. Then a verification test was performed to show whether the procedure exhibit similarity with collected experimental data. To show stress and temperature dependence of creep with time several curves were generated at different variables. These curves are helpful to understand the creep behavior more precisely. Then two propeller blade designs were analyzed to compare their creep behavior. This can be used to decide which design is better having less creep deformation. A plate welded with another forming a T-joint was worked out in another analysis. Static analysis showed the safety limit of load it can withstand for the given boundary condition and temperature. Creep analysis at loads lower than safety load showed how creep spread over the structure.