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Stress concentration Experiment مختبر التصميم

 


Experiment No. 4

Name: Stress concentration.

Aim: To study the effect of notch and welding joint through comparing the tensile and fracture strength of these processed pieces with non deformed standard tensile specimen of same dimensions.

Introduction: In order to understand the effect of machining process on the pieces like making a notch or scratch occurred during a process, or welding two pieces together it is required to make a tensile test for these pieces to compare the mechanical properties under different conditions. From this comparison a clear idea was taken for the effect of any process on the original mechanical properties.

Theory:  

                 σ max = Kt * σnom   ,   σnom = F / Area

          The stress concentration factor is found from the appendix tables of design book by Shigely. While the area (A) = d t , where t is the sample thickness.

 

                                            Notched Rectangular bar in Tension

 


 

    Procedure & Calculations:

1-  Fix the first sample [non deformed sample] in the tensile test machine and record the value of the yield, ultimate & fracture load.

2-  Measure the new dimensions of tested sample after fracture [Gauge length & width].

3-  Calculate the yield, ultimate & fracture strength and the percentage of elongation & area reduction of the first sample after fracture.

4-   Repeat the steps 1, 2 & 3 for sample 2 (with notch) & sample 3 (with welding).

5-  Compute the percentage of strengths change, elongation change between the three samples.        

 

            The samples test that to be used in the experiment [as per the standard] are:



Discussions:

1-    What is the importance of getting the stress concentration factor on design process?  What the factors that should be taken in consideration in the elements design?

2-    Compare the results of sample 2 & 3 with sample 1 independently by getting the percentage stresses increasing or reduction?  Also repeat the calculation for the percentage area & elongation increasing or reduction?

3-    Is the fracture of the notched sample occurred at the notched area? How can you reduce the effect of the notches on stress concentrating?

4-     Is the failure occurred in the welding area or away? What indications you got from both cases? How can you improve the welding?

 


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