UNIT-I (Module -6) Shock and Impact Consideration in Design - Copy.pptx

Design of Machine Elements
Module 6 Shock and Impact Loading

1. M-703 Theory and Design of Machine-I By Ashutosh Singh Raghubanshi DEPERTMENT OF MECHANICAL ENGINEERING UNIVERSITY INSTITUTE OF TECHNOLOGY, BARKATULLAH UNIVERSITY, BHOPAL

2. TOPIC: Shock and Impact Consideration in Design DEPERTMENT OF MECHANICAL ENGINEERING UNIVERSITY INSTITUTE OF TECHNOLOGY, BARKATULLAH UNIVERSITY, BHOPAL

3. Shock and impact consideration in Design A mechanical or physical shock is a sudden acceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation . Shock describes matter subject to extreme rates of force with respect to time. Shock is a vector that has units of an acceleration (rate of change of velocity). The unit g (or g) represents multiples of the acceleration of gravity and is conventionally used . A shock pulse can be characterised by its peak acceleration, the duration, and the shape of the shock pulse (half sine, triangular, trapezoidal, etc.). The Shock response spectrum is a method for further evaluating a mechanical shock. Shock loading presents an interesting set of problems to the design engineer. In the engineering community, design for static loading traditionally has been the most commonly used design procedure. Designing for shock loading, however, requires a change of thinking in several areas. The objectives of this discussion are to introduce the basic principles of shock loading and to consider their effect upon the integrity of structures To understand shock loading, one must first establish the various loading scheme definitions .

4. When choosing a material for shock loading applications, the designer should also remember to consider other characteristics of the material such as machineability , weldability, corrosion resistance, etc . For example, very high strength steels are appropriate for shock loading applications; however, they are not very weldable . There are four loading modes that are a function of the strain rate and the number of loading cycles experienced by the member. Static loading: Static loading occurs when a force is slowly applied to a member. Fatigue loading: Fatigue loading occurs when the member experiences alternating, repeated, or fluctuating stresses. Shock loading: In shock loading, an impact-type force is applied over a short instant of time. Shock/Fatigue combination loading: Shock/fatigue combination loading is equivalent to a shock load applied many times.

5. Effects of shock Mechanical shock has the potential for damaging an item (e.g., an entire light bulb) or an element of the item (e.g. a filament in an Incandescent light bulb ): A brittle or fragile item can fracture. For example, two crystal wine glasses may shatter when impacted against each other. A shear pin in an engine is designed to fracture with a specific magnitude of shock. Note that a soft ductile material may sometimes exhibit brittle failure during shock due to time-temperature superposition. A malleable item can be bent by a shock. For example, a copper pitcher may bend when dropped on the floor. Some items may appear to be not damaged by a single shock but will experience fatigue failure with numerous repeated low-level shocks. A shock may result in only minor damage which may not be critical for use. However, cumulative minor damage from several shocks will eventually result in the item being unusable. A shock may not produce immediate apparent damage but might cause the service life of the product to be shortened: the reliability is reduced. A shock may cause an item to become out of adjustment. For example, when a precision scientific instrument is subjected to a moderate shock, good metrology practice may be to have it recalibrated before further use. Some materials such as primary high explosives may detonate with mechanical shock or impact. When glass bottles of liquid are dropped or subjected to shock, the water hammer effect may cause hydrodynamic glass breakage.

6. Shock loading refers to a sudden and drastic increase of load, similar to a “hammering” effect. The most common occurrence is when a load is dropped onto a ball transfer unit from a height or when ball units travel over an uneven surface, causing an uneven distribution of load. What Is Shock Loading? Why Do You Need to Consider Shock Loading? When shock loading occurs, although the load itself may be within the load-bearing capabilities of the ball units, the forces generated by the shock loading can momentarily increase the load beyond the ratings listed in our catalogue and on our website. This can damage certain units and permanently affect their performance and durability . It’s a good idea to regularly check your equipment to ensure that shock loading damage has not occurred. Damaged units may result in reduced performance and unsafe working conditions.

7. Impact Stress Sometimes, machine members are subjected to the load with impact. The stress produced in the member due to the falling load is known as impact stress. Consider a bar carrying a load W at a height h and falling on the collar provided at the lower end, as shown in Fig. Let A = Cross-sectional area of the bar, E = Young's modulus of the material of the bar, l = Length of the bar, δl = Deformation of the bar, P = Force at which the deflection δl is produced, σi = Stress induced in the bar due to the application of impact load, and h = Height through which the load falls.

8. We know that energy gained by the system in the form of strain energy.

9. Problem: An unknown weight falls through 10 mm on a collar rigidly attached to the lower end of a vertical bar 3 m long and 600 mm2 in section. If the maximum instantaneous extension is known to be 2 mm, what is the corresponding stress and the value of unknown weight? Take E = 200 kN /mm2 .

10. Resilience When a body is loaded within elastic limit, it changes its dimensions and on the removal of the load, it regains its original dimensions. So long as it remains loaded, it has stored energy in itself. On removing the load, the energy stored is given off as in the case of a spring. This energy, which is absorbed in a body when strained within elastic limit, is known as Strain energy . The strain energy is always capable of doing some work. The strain energy stored in a body due to external loading, within elastic limit, is known as resilience and the maximum energy which can be stored in a body up to the elastic limit is called proof resilience . The proof resilience per unit volume of a material is known as modulus of resilience . It is an important property of a material and gives capacity of the material to bear impact or shocks.

11. Mathematically, strain energy stored in a body due to tensile or compressive load or resilience