Experimental And Analysis Of The Effect Of Welded Engineering Essay

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Abstract

This study examined the fatigue behavior of dynamic load applying to cruciform fillet-welded joints made with mild steel plates of different thickness. Fatigue tests were carried out on two types of welded joint, made from two 6, 12, or 15mm thickness main plates and one 9mm thickness cross plate. The fatigue strength was also theoretical calculate on 18 parametric models welded joints with different plate thickness. From the experimental and theoretical results, the effects of the thickness ratio of the main plates on fatigue strength will evaluated.

Keyword

Fatigue strength, Dynamic load, Fillet-welded joints, Plate thickness

Introduction

1.1 Overview

Fatigue occurs when a material is subjected to repeat loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure.

Shielded metal arc welding (SMAW) is a process that melts and joint metals by heating them with an arc established between a sticklike covered electrode and the metals. It is often called stick welding. The electrode holder is connected through a welding cable to one terminal of the power source and the work piece is connected through a second cable to the other terminal of the power source [1].

The core of the covered electrode, the core wire, conducts the electric current to the arc and provides filler metal for the joint. For electrical contact, the top 1.5cm of the core wire is bare and held by the electrode holder. The electrode holder is essentially a metal clamp with an electrically insulated outside shell for the welder to hold safely.

The heat of the arc causes both the core wire and the flux covering at the electrode tip to melt off as droplets. The molten metal collects in the weld pool and solidifies into the weld metal. The lighter molten flux, on the other hand, floats on the pool surface and solidifies into a slag layer at the top of the weld.

These studies have examined dynamic load applying to cruciform welded joints with cross and main plates of the different thickness. We considered welded joint with steel plates of different thickness, such as the joints in steel bridges, using an experimental and theoretical analysis of two types of test specimen and 18 parametric models, respectively. We discuss the effect of the different thickness of the main plates and cross plate on weld root crack propagation and fatigue strength.

1.2 Objective

This experiment is to analyze the crack length and propagation. Besides that also analyze the fatigue strength & fatigue life on mild steel material and analyze the welded effect of fatigue strength on mild steel specimen.

1.3 Problem Statement

The experiments of welded effect on the mild steel material of fatigue strength have been conducted. However, most of the available experiment was conducting under the old model of fatigue testing machine. No investigation and analysis for the fatigue strength under the new fatigue testing machine of Instron model 8801.

Literature Review

2.1 Weld Defect

The local weld geometry, toe angle, toe radius, undercuts and crack strongly influence the fatigue strength. The local geometry affects the local stress concentration and together with defects of different types fatigue cracks may form during cycle loading and lead to large scatter in fatigue life. Cold laps, sharp toe radius and large design root cracks are typical fatigue starting points. Toe radius is the geometrical parameter that most significantly affects the stress concentration and, hence, fatigue life for fatigue failure from the toe. Below shows the four most common types of weld defect [7].

Figure 1: Cold lap at weld toe

Figure 2: Crack at weld toe undercut

Figure 3: Root defects and initial crack

Figure 4: Interbead crack

Fatigue Life

The relationship between stress range and fatigue life (S-N diagram) is shown in below [3]:-

Figure 5: S-N diagram

The stress range was calculated from the weld throat area, A, which was calculated as follow:-

(1)

Where s is the weld size, Pw is the weld penetration depth, g is the root gap size, and W is the specimen width, as shown in below [3]:-

Figure 6: Definition of the weld throat

Effect of main and cross plate thickness on fatigue strength

The fatigue strength of welded joints can be predicted using the equation based on the known fatigue strength for joints made of plates of the same thickness, irrespective of whether plates of different thickness are combined. From the analytical model can express as:-

(2)

Where tm,l is the plate thickness on left hand side, tc is the cross plates thickness and tm,r is the left hand side plates thickness [3].

Methodology

There have three sections to analysis the welded effect to mild steel material. That is mechanical properties analysis, FEA simulation analysis and fatigue strength analysis.

3.1 Mechanical Properties Analysis

The mechanical properties of the specimen is analyze from the static force applying on a sample material until a point of the specimen fracture. This experiment is to obtain the stress-strain curve of the material. Below is the dimension of the specimen.

Figure 7: Specimen

This experiment is analyzed with Universal Testing Machine (UTM). Before begin of experiment, warm-up the machine approximately 15 minutes. Then the calibration is needed to perform by the laboratory assistance. Draw two lines measuring 50mm apart from the center of the specimen. These two lines is the gauge length to determine the final elongation of the specimen after fracture. After that place the specimen to the grip of the UTM machine and tighten it. After final confirmation, press the 'START' button to running experiment.

After collecting the data, plot the S-N curves by using MS EXCEL. From the graph can obtain the yield stress, maximum stress and fracture stress. Then obtain the hardness of specimen with rockwell expriment.

3.2 FEA Simulation Analysis

Normally the failure occurs in the area of high stress concentration. To perform this analysis, first develop the geometry of the specimen using CAD software (solidworks or AutoCAD). Then the file of specimen drawing saved in a format 'igs' or 'step' for import the design into the FEA analysis software. Next meshing the specimen model to enable the calculation is done by the software. Various parameters are determined to enable the software to generate feasible data. After configured the required variables, get into the solver to perform calculation and generated visual result. The results that get from the software are shown as color codes where red is the high stress concentration and the blue is the low stress concentration.

3.3 Fatigue Strength Analysis

Fatigue analysis is very similar to the mechanical properties analysis. The specimens on the experiment are 8 specimens for each parameter, 6M12M9C and 6M15M9C. The detail parameters of specimen are shown in below:-

Figure 8: Parameter 6M12M9C

Figure 9: Parameter 6M15M9C

This experiment is used the instron-8801 machine. Before begin for this experiment, warm-up the machine. The machine is program to run dynamic loading without specimen for a few minutes. Then the calibration is needed to perform by the laboratory assistance. Next load the specimen to the machine and determine the wave pattern of the dynamic loading using the machine's control interface. After the final confirmation, start the experiment. When the experiment is running need to observe the machine and atop the machine immediately when the specimen is fail.

After finish all the experiment, collect all the data the plot the graph using Mathlab software. The graph that plot with Mathlab software is called S-N curve. The curve is increase until a point the curve become horizontal and the horizontal line is called endurance limit.

Project Progress

Based on the Gantt chart, in the end of October 2010 the progress needs to achieve until experiment stage.

4.1 Parameter Analysis

To evaluate the effects of the ratio of the thicknesses of the two main plates and the change in cross-plate thickness on the fatigue strength of load applying to cruciform fillet-welded joints, parametric analyses examined 18 parameters with main plates that were 6, 9, 12, or 15 mm thickness and a cross-plate 9 mm thick. The 18 parameters to be calculated is show in the below table:-

Table 1: Parametric Models

No

Parameter

Main Plate Thickness (mm)

Cross Plate Thickness (mm)

tm,l

tm.r

tc

1

M6M6C6

6

6

6

2

M6M6C9

6

6

9

3

M6M9C6

6

9

6

4

M6M9C6

6

9

9

5

M6M12C6

6

12

6

6

M6M12C9

6

12

9

7

M6M15C6

6

15

6

8

M6M15C9

6

15

9

9

M9M9C6

9

9

6

10

M9M9C9

9

9

9

11

M9M12C6

9

12

6

12

M9M12C9

9

12

9

13

M9M15C6

9

15

6

14

M9M15C9

9

15

9

15

M12M12C6

12

12

6

16

M12M12C9

12

12

9

17

M12M15C6

12

15

6

18

M12M15C9

12

15

9

The equation that used for calculate the fatigue strength on cycle 2x106 for all parameters is shown below:-

From the data of calculation, plot the graph fatigue strength vs /.

Figure 10: Fatigue Strength for all Parameters

4.2 Mechanical Properties Analysis

The mechanical properties of the mild steel specimen is analyze with using UTM machine. From this experiment obtain the stress-strain and the load-displacement curve. Below is the graph:-

Figure 11: Stress-Strain Diagram

Figure 12: Graph Load Vs Displacement

From the stress-strain diagram obtain the maximum stress and the yield stress of this mild steel plate. From the mechanical properties that get in this experiment will continue for the FEA simulation analysis. Besides that, obtain the hardness of specimen with Rockwell experiment. Below is the table for the material composition and mechanical properties:-

Table 2: Mechanical Properties

Material

Ultimate Stress

Yield Stress

Strain

Hardness

Fe-410WA

410Mpa

271Mpa

31.9%

70.5HRB

Table 3: Material Composition

Material

C%

Si%

Mn%

P%

S%

Fe-410WA

0.23

0.4

1.5

0.05

0.5

Conclusion

In conclusion, based on the Gantt chart progress already achieved the mechanical properties analysis. Follow on will continues with FEA simulation analysis and fatigue strength analysis.

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