Objective

• To measure the average and concentrated strains on a mild steel specimen under tension due to the effect of a notch by placing two strain guages one close to and one farther away from the notch.

• By estimating the ratio of concentrated stress and average stress, compare the results with the given model graph.

Introduction

The notch experiment is for finding the stress concentration around a horizontal notch (slit). As per Peterson's stress concentration (refer stress distribution in the previous section) factor study, the stress concentration around a horizontal notch which has notch length less than half of its total specimen width can be obtained from the following formula:

 

where K_t is the stress concentration, P is the load applied on the specimen, t is the thickness of the specimen, W is the width of the specimen, a is the length of the notch.

Referring to the following graph,  (showing the variation of Y with respect to the geometric ratio a/W)

Fig 1: (Y) vs (a/W) graph

From this we can observe the manner in which the stress concentration at the notch on the specimen varies with increase in loads.

The strain gages are bonded on to the specimen in the following manner:

Fig 2: Strain gages bonded on to the notch specimen fixed on the holding chuck of UTM

Figure : Stress distribution around a Notch 

Specimen details:

Material: Mild steel

Specimen test section width: 31.5 mm

Specimen gage length: 163 mm

Specimen thickness: 3.4 mm

Notch width: 1 mm

Notch length: 8mm