Welded Joints MCQ Quiz - Objective Question with Answer for Welded Joints - Download Free PDF

Explanation:

Welding symbol:

• The arrow of the welding symbol indicates the point at which the weld is to be made and the line connecting the arrow to the reference line is always at an angle.
• Whenever the basic weld symbol is placed below the reference line, the weld is made on the side where the arrow points (referred to as the arrow side).
• Whenever the basic symbol is placed above the reference line, the weld is to be made on the other side of the joint.
• The symbol for fillet weld is . 

• Figure: (a) Fillet weld symbols shown on the bottom side of the reference line indicates that the weld is located in the same position where the arrow points.
• Figure: (b) Fillet weld symbols shown on the upper side of the reference line indicates that the weld is on the opposite side where the arrow points.

∵ there is nothing mention about the arrow side or other side, ∴ single fillet weld will be the required answer.

Additional Information

Some of the common welding symbols is given below.

Concept:

In Parallel Fillet Weld failure occurs because of shear and the Strength of Parallel Fillet Weld is given by

\(\tau = \frac{P}{tl} = \frac{P}{0.707nhl}\)

where \(\tau\) = Shear Strength of Joint, P = Force Acting on the Plate, h = leg of the weld, t = throat of the weld, l = length of the weld, n = no of weld

Calculation:

Given:

h = 10 mm, b = 100 mm, P = 75 kN, \(\tau\) = 60 MPa, \(\sigma\) = 80 MPa, n = 2 (Double Parallel Fillet Weld)

In a double parallel fillet weld, two lines of welding are to be provided. Hence each line shares a load of 

\(F = \frac{P}{2 } = \frac{75}{2}\)

F = 37.5 kN = 37500 N

\(F = 0.707\tau nhl\)

37500 = 0.707 × 60 × 10⁶  × 10 × 10^-3 × l

l = 88.4 mm

Concept:

The weld strength formula for V-groove or double butt joint is:

\(P = (t_1+t_2)\times l\times σ_t\)

where t₁ and t₂ are the thickness of the plate, l is the length of the plate, P is the tensile force, and σ_t is the normal stress.

Calculation:

Given:

t₁ = t₂ = 6 mm, l = 60 mm, P = 36 kN

\(\sigma_t = \frac{P}{2t\times l}\)

\(\sigma_t = \frac{36000}{2\times 0.6\times 6} = 50 \ MPa\)

Explanation:

Welding defects:

• Cracks
• Cavities
• Solid inclusions
• Incomplete fusion
• Imperfect shape or unacceptable contour

Welding cracks:

Fracture-type interruptions either in the weld or in base metal adjacent to the weld.

Serious defect because it is a discontinuity in the metal that significantly reduces strength.

Caused by embrittlement or low ductility of weld and/or base metal combined with high restraint during contraction.

In general, this defect must be repaired.

Cavities:

Two defect types, similar to defects found in castings:

1. Porosity - Small voids in weld metal formed by gases entrapped during solidification. Caused by the inclusion of atmospheric gases, sulfur in the weld metal, or surface contaminants.

2. Shrinkage voids - Cavities formed by shrinkage during solidification.

Solid inclusion:

Nonmetallic material is entrapped in the weld metal. A most common form is slag inclusions generated during AW processes that use flux. Instead of floating on top of the weld pool, globules of slag become encased during solidification.

Incomplete fusion:

A weld bead in which fusion has not occurred throughout the entire cross-section of the joint.

Explanation:

Weld Time:

• It is the time during which welding current is applied to the work in making a weld.
• It is measured in cycles of line voltage as are all timing functions.
• One cycle is 1/60 of a second in a 60 Hz power system.

Squeeze Time :

• It is the time interval between the initial application of the electrode force on the work and the first application of current.
• Note that this is the process definition.
• The control definition is the time interval between sequence initiation and beginning of weld current.
• Squeeze time is necessary to delay the weld current until electrode force has built up to the desired level.

Hold Time:

• It is the time during which electrode force is maintained on the work after the last impulse of welding current ceases.
• Hold time is necessary to allow the weld nugget to solidify before releasing the welded parts.

Off Time:

• It is the time during which the electrodes are off the work.
• The term is only applicable where the weld cycle is repetitive (control set on “REPEAT”).

Additional Information 

Duty cycle:

• It is the amount of time during which the transformer will be used for welding under normal loading conditions.
• It is the time span in successive 10 min interval during which current can be drawn from the transformer, after that there should be a compulsory break otherwise the temperature of its essential components will increase.
• Usually, a 60% duty cycle is the standard industrial rating.
• A power source with such a rating can supply its rated output for 6 minutes in every 10-minute interval of its operation.
• I2D = Constant, where D = Duty cycle and I = Current

Explanation:

Concept:

Heat generated (Q) in the spot welding is given by, Q = I² × R × t

Where I is the flow of current, R is the resistance and the t is the duration for which the current flows

Calculation:

Given, Current flow time t = 0.2 s, Heat generated = 1000 J and, Resistance = 200 μΩ

1000 = I² × 200 × 10^-6 × 0.2

Explanation:

Welding defects:

• Cracks
• Cavities
• Solid inclusions
• Incomplete fusion
• Imperfect shape or unacceptable contour

Welding cracks:

Fracture-type interruptions either in the weld or in base metal adjacent to the weld.

Serious defect because it is a discontinuity in the metal that significantly reduces strength.

Caused by embrittlement or low ductility of weld and/or base metal combined with high restraint during contraction.

In general, this defect must be repaired.

Cavities:

Two defect types, similar to defects found in castings:

1. Porosity - Small voids in weld metal formed by gases entrapped during solidification. Caused by the inclusion of atmospheric gases, sulfur in the weld metal, or surface contaminants.

2. Shrinkage voids - Cavities formed by shrinkage during solidification.

Solid inclusion:

Nonmetallic material is entrapped in the weld metal. A most common form is slag inclusions generated during AW processes that use flux. Instead of floating on top of the weld pool, globules of slag become encased during solidification.

Incomplete fusion:

A weld bead in which fusion has not occurred throughout the entire cross-section of the joint.

Explanation:

Metal deposit rate (MDR) = Area (A) x Weld Speed (V) = Constant

A₁ V₁ = A₂V₂; V₂ = 2V₁ (given)

\({A_1}{V_1} = {A_2}\left( {2{V_1}} \right) \Rightarrow {A_2} = \frac{{{A_1}}}{2}\)

We know that,  \(V = {V_o} - \left( {\frac{{{V_o}}}{{{I_{sc}}}}} \right) \times I\) ……….. (1)

Here,

V = (100 + 40 L)

\(\left. {\begin{array}{*{20}{c}} {\begin{array}{*{20}{c}} {{V_1} = 100 + \left( {40 \times 1} \right) = 140V,}\\ {{V_2} = 100 + \left( {40 \times 2} \right) = 180V,} \end{array}}&{\begin{array}{*{20}{c}} {{I_1} = 250A}\\ {{I_2} = 200A} \end{array}} \end{array}} \right\}\begin{array}{*{20}{c}} {Voltage\;\& \;current\;are\;inversely\;proportional\;} \end{array}\)

Putting the value of V₁, V₂, I & I₂ in the equation

\(\begin{array}{*{20}{c}} { \Rightarrow 180 = {V_o} - \left( {\frac{{{V_o}}}{{{I_{SC}}}}} \right) \times 200 - - - - - \left( 2 \right)\;}\\ { \Rightarrow 140 = {V_o} - \left( {\frac{{{V_o}}}{{{I_{SC}}}}} \right) \times 250 - - - - - \left( 3 \right)}\\ {\overline { \Rightarrow 40 = \left( {\frac{{{V_o}}}{{{I_{SC}}}}} \right) \times \left( {250 - 200} \right)} } \end{array}\)  

We got the above relation by subtracting equation 2 and 3

By solving \(\left( {\frac{{{V_o}}}{{{I_{sc}}}}} \right) = \frac{4}{5}\)       ----(4)

Using (4) in (2) we have

\(\Rightarrow 180 = {V_o} - \left( {\frac{4}{5}} \right) \times 200\)

⇒ V_o = 180 + 160 ⇒ V_o = 340V      ----(5)

using (5) in (4) we have

Concept:

In Parallel Fillet Weld failure occurs because of shear and the Strength of Parallel Fillet Weld is given by

\(\tau = \frac{P}{tl} = \frac{P}{0.707nhl}\)

where \(\tau\) = Shear Strength of Joint, P = Force Acting on the Plate, h = leg of the weld, t = throat of the weld, l = length of the weld, n = no of weld

Calculation:

Given:

h = 10 mm, b = 100 mm, P = 75 kN, \(\tau\) = 60 MPa, \(\sigma\) = 80 MPa, n = 2 (Double Parallel Fillet Weld)

In a double parallel fillet weld, two lines of welding are to be provided. Hence each line shares a load of 

\(F = \frac{P}{2 } = \frac{75}{2}\)

F = 37.5 kN = 37500 N

\(F = 0.707\tau nhl\)

37500 = 0.707 × 60 × 10⁶  × 10 × 10^-3 × l

l = 88.4 mm

Concept:

The weld strength formula for V-groove or double butt joint is:

\(P = (t_1+t_2)\times l\times σ_t\)

where t₁ and t₂ are the thickness of the plate, l is the length of the plate, P is the tensile force, and σ_t is the normal stress.

Calculation:

Given:

t₁ = t₂ = 6 mm, l = 60 mm, P = 36 kN

\(\sigma_t = \frac{P}{2t\times l}\)

\(\sigma_t = \frac{36000}{2\times 0.6\times 6} = 50 \ MPa\)

Concept:

Calculate the heat supplied for melting and heat required for melting.

And then equating both of them will give you the thickness of nugget.

Calculation:

Given:

thickness of steel sheets = 2 mm.

I = 4 kA, time (t) = 0.2 seconds.

Diameter of Nugget = 5 mm

Assuming cylindrical shape of Nugget.

Volume \(= \frac{\pi }{4}{D^2}{t_n}\), where t_n → Thickness of Nugget

Now,

Heat required to melt the above volume of Nugget = Heat required to melt 1 kg × Total mass (m)

∵ density (ρ) = mass/Volume

∴ m = ρ × volume

∴ Heat required to melt volume of nugget (Q)= Heat required to melt 1 kg × ρ × volume

\(\therefore {\rm{Q}} = \left( {1400} \right) \times \frac{{\rm{\pi }}}{4}{\rm{\;}}{{\rm{D}}^2}{{\rm{t}}_{\rm{n}}} \times \left( {8000} \right){\rm{\;kJ}}\)

And

Heat supplied = I²Rt= (4× 10³)² × (200 × 10^-6) × 0.2

∴ Heat supplied = 640 J.

Equating the heat calculated by two methods:

\(640 = \left( {1400} \right)\frac{\pi }{4}{\left( {0.005} \right)^2}{t_n}\left( {8000} \right) \times {10^3}\)

640 = (219.911 × 10³) t_n

t_n = 2.91 × 10^-3 m

Explanation:

Welding symbol:

• The arrow of the welding symbol indicates the point at which the weld is to be made and the line connecting the arrow to the reference line is always at an angle.
• Whenever the basic weld symbol is placed below the reference line, the weld is made on the side where the arrow points (referred to as the arrow side).
• Whenever the basic symbol is placed above the reference line, the weld is to be made on the other side of the joint.
• The symbol for fillet weld is . 

• Figure: (a) Fillet weld symbols shown on the bottom side of the reference line indicates that the weld is located in the same position where the arrow points.
• Figure: (b) Fillet weld symbols shown on the upper side of the reference line indicates that the weld is on the opposite side where the arrow points.

∵ there is nothing mention about the arrow side or other side, ∴ single fillet weld will be the required answer.

Additional Information

Some of the common welding symbols is given below.

Concept:

• Joggle joints are used where a strong joint on the flat surface is needed to join two pieces of sheet metal.
• Generally, there are hand tools available to put the joggle into the sheet metal.
• Joggled welded joints are used where severe loading is encountered, and the upper surface of both pieces must be in the same plane.