Determination of block punch strength index.

SCOPE:-

• This test is used to determine the shear strength of rock. So, from this test we can estimate the strength of a material or component against the type of yield or structural failure where the material or component fails in shear.
• Shear strength testing is used to determine the load at which a plastic or film will yield when sheared between two metal edges. Shear strength results are important to designers of film and sheet products that tend to be subjected to shear loads, or in applications where applied crushing loads are a risk.
• The block punch strength index test is intended as an index test for the strength classification of rock materials. It is also be used to predict other strength parameters with which it is correlated, for example uniaxial compressive and tensile strength.

APPARATUS:-

• Universal testing machine
• Vernier caliper
• Steel fixture/assembly
• Rock disc

THEORY

Shear Force:-

A force which is applied parallel to the sections is known as Shear force. The shear force is simply calculated as the maximum force applied divided by the shear area (punch circumference x specimen thickness).

Shear Strain:-

The distortion produced by Shear Stress on an element or Rectangular Block is shown in the diagram. The Shear Strain or "Slide" can be defined as the change in the right angle.It is measured in Radians and is dimensionless.

Shear Stress:-

The intensity of internal resistance when the applied force is parallel to the section being sheared is called shear stress. OR If the applied load consists of two equal and opposite parallel Forces which do not share the same line of action, then there will be a tendency for one part of the body to slide over or shear from the other part. If the section L M is parallel to the forces and has an area “A” then the average Shear Stress.

Shear Strength:-

Shear strength in engineering is a term used to describe the strength of a material or component against the type of yield or structural failure where the material or

component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force. When a paper is cut with scissors, the paper fails in shear.

In structural and mechanical engineering the shear strength of a component is important for designing the dimensions and materials to be used for the manufacture/construction of the component (e.g. beams, plates, or bolts) In a reinforced concrete beam, the main purpose of stirrups is to increase the shear strength.

Punch Shear Test:-

This test method is intended as a comparative test, and not as a quantitative measure of the shear strength of the material.  As a materials screening test it does have the advantages of requiring a simple specimen and utilizing a simple test procedure.

Rock specimens in the form of thin cylindrical discs prepared from cores or blocks are placed into an apparatus which is designed to fit the point load device, and are broken by the application of load by a rectangular rigid punching block.

PROCEDURE:-

• Measure the diameter of steel bar and find its cross sectional area.
• Measure the dia and length of sample and also find its radius.
• Fix the lower jig and upper jig in the machine.
• Fix the zero error of the machine.
• Place the steel sample Sover the lower jig.
• Apply the shear load until the bar gets sheared.
• Apply the load gradually and note the reading when the bar gets sheared.
• Calculate the shear strength by using the relationship

The load is then gradually applied to the specimen at a constant rate such that failure occurs within 10–60 s as suggested by ISRM for point load strength. Fracturing is thus forced to take place along two parallel planes on which the normal stress is considered to be zero while the tensile stresses caused by bending are reduced. The load Ft D which is the load required for the failure of a specimen of any diameter and any thickness is recorded. After failure, theoretically, the specimen is broken into three parts, the two ends which are fixed in the apparatus and the middle part of the specimen which is punched out. The test should be rejected as invalid if the parallel fracture planes are either absent or not fully developed (irregular failure) or cross joints develop.

COMMENTS:

• For engineering purposes the Block Punch Index test seems to be as good as other index tests in indirectly assessing strength, especially if only little rock material is available.
• BPI test was not an accurate device for directly determining shear strength of the rock specimen and should only be used as a strength index.