Construction and Design Questions

Question 1

EXPLAIN ELASTIC AND PLASTIC BEHAVIOUR OF MATERIALS?

Answer 1

(ELASTIC) - THE RECOVERY OF ORIGINAL DIMENSIONS IS KNOWN AS ELASTIC BEHAVIOUR
(PLASTIC) - DEFORMATION OF THE SOLID IS PERMANENT AFTER THE LOAD IS REMOVED

Question 2

Name five basic types of loads that may be applied to Materials?

Answer 2

Tension
Compression
Shear
Bending
Torsion

Question 3

EXPLAIN HOW A DUCTILE FRACTURE OCCURS IN METALS. [6]

Answer 3

• DUCTILE FRACTURE IS CHARACTERISED BY APPRECIABLE AMOUNTS OF PLASTIC DEFORMATION BEFORE AND DURING CRACK INITIATION.I
  *IN THE CASE OF A TENSILE TEST, A SHEAR STRESS IS DEVELOPED AS A RESULT OF THE APPLICATION OF A NORMAL STRESS ON THE SPECIMEN.
• IT IS OFTEN FOUND THAT THE FINAL FRACTURE OF A TENSILE TEST MANUFACTURED FROM A DUCTILE MATERIAL OFTEN SHOWS A “CUP - CONE” EFFECT CHARACTERISED BY A SHEAR LIP AT AN ANGLE OF 45º TO THE LENGTH OF THE TEST PIECE.

Question 4

EXPLAIN HOW A BRITTLE FRACTURE OCCURS IN METALS.?

Answer 4

*BRITTLE FRACTURE IS A COMMON NAME FOR FAST, UNSTABLE FRACTURE THAT CAN OCCUR IN MANY MATERIAL TYPES.
*IT OFTEN OCCURS IN STRUCTURAL STEELS AT LOW TEMPERATURES OR UNDER APPLIED STRESSES DEPENDING UPON THE SEVERITY OF RESIDUAL STRESSES AND EFFECT OF STRESS CONCENTRATIONS.
*THE ONSET OF BRITTLE FRACTURE IS ALSO INFLUENCED BY TEMPERATURE.
* BRITTLE FRACTURES ARE OFTEN CHARACTERISED BY A CHEVRON APPEARANCE IN WHICH THE CHEVRONS INDICATE THE ORIGIN OF THE FRACTURE.

Question 5

EXPLAIN HOW FATIGUE OCCURS IN METALS.?

Answer 5

*FATIGUE CRACKS TEND TO START AT A DISCRETE POINT OR , LINE WHERE STRESS IS CONCENTRATED.
*THIS MAY BE A WELD TOE ON A FILLET WELD, WELD ROOT, OR A WELD DEFECT.
*THE FRACTURE SURFACE IS USUALLY MARKED IN BANDS CALLED “BEACH MARKS” THAT INDICATE THE PROGRESS OF THE CRACK FROM THE CRACK INITIATION POINT OR LINE.
* FINAL FRACTURE IS CHARACTERISED BY FAST FRACTURE DUE TO OVERLOADING ON THE REMAINING CROSS SECTION OF SOUND MATERIAL.

Question 6

Name three types of stages that Creep occurs in and briefly describe each one.

Answer 6

a.) Primary Creep – Rapid increase in length known as primary creep where the creep rate decreases as the metal work hardens.
b.) Secondary Creep – Constant creep rate, the (steady state) secondary creep & it is this period that forms the bulk or the creep life of a component.
c.) Tertiary Creep – occurs when the creep life is almost exhausted, voids have formed in the material and the effective cross section has been reduced. The creep rate accelerates as the stress per unit increases until the specimen fails.

Question 7

Describe the principals of a stable and unstable structure. ?

Answer 7

a.) Internal stresses that acts together and therefore causes a structure to be stable or unstable.
b.) A stable structure remains standing but an unstable structure would yield and even fracture. This is the principle of equilibrium

Question 8

Describe the term equilibrium?

Answer 8

*A structure is in equilibrium when all forces or moments acting upon it are balanced.
*This means that each and every force acting upon a body or part of the body is resisted by either another equal and opposite force or set of forces whose net result is zero.

Question 9

What is a Moment?

Answer 9

For a moment to develop, the force act upon the body in such manner that the body bend to bend and rotate

Question 10

How are moments calculated?

Answer 10

Moment = Vector Force x Vector Distance.

Question 11

Name the type of stresses that will be produced in a longitudinal weld and a circumferential weld.?

Answer 11

a.) Longitudinal – circumferential stress (is Hoop Stress)
b.) Circumferential – Longitudinal stress (is Axial Stress)

Question 12

THE DESIGNER OF A WELD SHOULD HAVE KNOWLEDGE AND EXPERIENCE FOR WELDED STRUCTURES, LIST FIVE ITEMS WHICH SHOULD BE CONSIDERED.

Answer 12

CUTTING AND SHAPING OF MATERIAL
ASSEMBLY METHODS
PREPARATION AND FABRICATION OF WELDED JOINTS
WELDING PROCESSES
APPLICABLE CODES OF CONSTRUCTION

Question 13

COMPARED TO I - BEAM GIRDERS, BOX GIRDERS HAVE A NUMBER OF KEY ADVANTAGES AND DISADVANTAGES, NAME TWO ADVANTAGES AND TWO DISADVANTAGES.

Answer 13

ADVANTAGES - BETTER RESISTANCE TO TORSION
LARGER GIRDERS CAN BE CONSTRUCTED, BECAUSE THE PRESENCE OF TWO WEBS ALLOWS WIDER AND HENCE STRONGER FLANGES TO BE USED

DISADVANTAGES - MORE EXPENSIVE TO FABRICATE MORE DIFFICULT TO MAINTAIN, BECAUSE OF THE NEED FOR
ACCESS TO A CONFINED SPACE INSIDE THE BOX

Question 14

DEFINE THE TERM LEFM. ?

Answer 14

LINEAR ELASTIC FRACTURE MECHANICS

Question 14

DEFINE THE TERM - STRESSED SKIN.?

Answer 14

A LOAD APPLIED BY WIND ON A STRUCTURE IS CARRIED BY BOTH THE FRAME AND THE STRESSED SKIN, EXAMPLE OF A STRESSED SKIN ARE PRESSURE VESSELS WHERE THE WALLS CARRY THE EFFECT OF THE PRESSURISED CONTENTS OF THE VESSEL

Question 15

EXPLAIN THE TERM STRESS INTENSITY FACTOR. ?

Answer 15

IT IS EXPRESSED IN TERMS OF THE APPLIED STRESSES AT THE CRACK TIP

Question 16

DEFINE THE TERM EPFM.?

Answer 16

ELASTIC PLASTIC FRACTURE MECHANICS

Question 17

EXPLAIN HOW A CTOD TEST IS CARRIED OUT.?

Answer 17

THE TEST IS PERFORMED BY PLACING THE SPECIMEN INTO THREE POINT BENDING & MEASURING THE AMOUNT OF CRACK OPENING. THIS IS DONE BY MEANS OF A STRAIN GAUGE ATTACHED TO A CLIP PLACED BETWEEN TWO ACCURATELY POSITIONED KNIFE EDGES AT THE MOUTH OF THE MACHINED NOTCH.

Question 18

EXPLAIN THE TERM FITNESS FOR SERVICE.?

Answer 18

IT CAN BE USED TO DEMONSTRATE THAT A GIVEN FLAW CAN BE LEFT AS IS TO AVOID UNNECESSARY REPAIRS

Question 19

WHEN APPLYING THE FITNESS FOR PURPOSE APPROACH CERTAIN FAILURE MODES SHOULD BE IDENTIFIED, NAME THREE.?

Answer 19

FRACTURE
FATIGUE
LEAKAGE
CORROSION
STRESS CORROSION
CREEP

Question 20

EXPLAIN HOW FATIGUE CAN OCCUR IN VARIOUS STRUCTURES?

Answer 20

FATIGUE IS A FAILURE MECHANISM THAT INVOLVES THE PROPAGATION OF A CRACK UNDER REPEATED OR CYCLIC LOADING

Question 21

CAN FATIGUE OCCUR WHEN THE MATERIAL IS BELOW THE YIELD POINT OF A MATERIAL?

Answer 21

Yes

Question 22

WILL THE USE OF A HIGHER STRENGTH MATERIAL BE OF BENEFIT IN INCREASING FATIGUE LIFE.?

Answer 22

No

Question 23

DESCRIBE THE PHENOMENON OF CREEP. STATE THREE ELEMENTS THAT CONTRIBUTE TO THE RATE OF CREEP DAMAGE

Answer 23

CREEP IS THE TERM GIVEN TO THE MATERIAL DEFORMATION THAT OCCURS AS A RESULT OF LONG TERM EXPOSURE TO LEVELS OF STRESS THAT ARE BELOW THE YIELD OR ULTIMATE STRENGTH. RATHER THAN FAILING SUDDENLY WITH A FRACTURE, THE MATERIAL PERMANENTLY STRAINS OVER A LONGER PERIOD OF TIME UNTIL IT FINALLY FAILS.
CREEP DOES NOT HAPPEN UPON SUDDEN LOADING BUT THE ACCUMULATION OF CREEP STRAINS IN LONGER PERIODS CAUSES FAILURE OF THE MATERIAL. THIS MAKES CREEP DEFORMATION A TIME - DEPENDENT DEFORMATION OF THE MATERIAL.
THE RATE OF THIS DAMAGE IS A FUNCTION OF THE MATERIAL PROPERTIES AND THE EXPOSURE TIME, EXPOSURE TEMPERATURE AND THE APPLIED LOAD (STRESS).
DEPENDING ON THE MAGNITUDE OF THE APPLIED STRESS AND ITS DURATION, THE DEFORMATION MAY BECOME SO LARGE THAT A COMPONENT CAN NO LONGER PERFORM ITS FUNCTION.

Question 24

Three basic modes of crack tip deformation ?

Answer 24

Mode I – opening
Mode II - sliding
Mode III - tearing

Question 25

If stress relieving is carried out would it be of any benefit to the fatigue life of a structure?

Answer 25

This is only true where the applied stress range is partly or wholly compressive, if the stress range is all tensile only a small improvement will be made

Question 26

List three methods to reduce the possibility of fatigue

Answer 26

*Grind the toes of a weld, make sure undercut has been removed
*Peening the weld, this will introduce compressive stresses at the surface of a weld
*Remove any excessive welding, possibly grind or machine flush with the parent material to avoid stress raisers at the toes of a weld.