A crack with a depth of 2 cm is found in a cast iron rod of an old steam engine. Every weekend the machine is used for 8 hours for demonstration purposes, during which it is run at 15 rpm and the load in the rod varies from +6.4Ã—104 N to -6.4Ã—104 N. It may be assumed that the crack is completely closed under compressive load but that under tensile load crack closure is negligible. The Paris relation is sufficiently accurate until failure.
a) Is it safe to use this machine for demonstrations and for how long?
b) Answer these questions assuming a load variation from +0.92 MN to -0.92 MN.
Given: Kc = 16 MPa m0.5
âˆ†Kth = 5 MPa m0.5
cross section rod = 0.04 m2
factor in the Paris relation: C = 4.3Ã—10-8 m(MPam0.5)-4
KI = 1.12 ï³ï€ ï€¨ï°a)0.5
The cutter bars of two combine harvesters from different manufactures develop different failure types with time. Cutter bar A loses the knife sharpening after 10 seasons, while cutter bar B does not lose the knife sharpening but some blades have to be replaced time to time due to local fracture of the edge. Through measuring the volume and the weight the density of both blades are around 7.8 â€“ 7.9 Kg/dm3. The hardness of the blades from A is 2.2 GPa while the hardness of the blades from B is 8 GPa. Discuss two possible manufacturing differences that would lead to this situation. Use suitable references to support your discussion. Suggest two possible technologies to improve the life for bar A. Suggest a way to reduce the brittleness of bar B. Justify your suggestions and compare with real examples if possible.
[Hint] Use heat treatment graphs, composition-mechanical properties, material data, surface engineeringâ€¦
The mechanical loading of a component for the purpose of analysing fracture is defined by the stressâ€“intensity factor, KI, which depends on the applied stress as well as on the flaw size as illustrated by the following sketch.
A common way to measure the flexural strength of materials is by the threeâ€“point bending method. The sample, in the shape of a slide is supported on two roller pins. The force on a pin place above the slide to produce fracture in the slide is measured. If this force is F, the spacing between the roller supports at the bottom is L, the width of the glass side is w, and its thickness is b, it is possible to derive flexural strength equation.
The flexural strength of the materials is taken from CES, and length = 10 cm, Width = 2cm and b= 0.1cm.
Calculate the force required to break a glass soda lime 0080, a fused alumina (99.9) and silica (fused) slides with the three-point bending method. Which slide is least likely to break? What will be the force for breaking if the sample slide is twice as thick?
(b) Assuming that the fracture toughness data are taken from CES, explain using suitable examples or references what is meant by maximum admissible flaw (MAF) and calculate the MAF size a for glass soda lime 0080, alumina (99.9) and silica (fused) slides.
(c) Draw using excel the evolution of flaw size in the glass slide if the flexural strength varies from 10 MPa to 100 MPa.
You have been given CES Edupack to select the best material for a high performance connecting rod out of titanium alloys, aluminium alloys and cast irons. In this application, high performance connecting rod is likely to fail by fatigue or buckling. The square connecting rod (Length 0.15m) is subjected to a 40kN load.
Using Material selection methodology shown in class, translate the design requirements into a material specification using the performance equation. Discuss how you have considered conflicting objectives in your selection process. Support your choice with suitable examples or references.
Express the material performance index for each of the possible failures
Express and show the coupling line.
Select the best material for the above conditions.
Discuss your selection with some real case.
Show every step in the calculations.
The buckling load is given by:
The section is square.
Where Î² takes into account the mass of the bearing houses.
these are 4 questions