strength consideration in product design, shiva,deepak

Department of Mechanical Engineering, Manipal

STRENGTH CONSIDERATIONS IN PRODUCT

DESIGN

BY: SHIVAPRASAD.P (080922004)

DEEPAK KUMAR SHETTY.K (080922006)

CLASS: 2ND

SEMESTER M.Tech CAMDA.

DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING

MANIPAL INSTITUTE OF TECHNOLOGY, MANIPAL

DATE OF SUBMISSION: 30-01-2009


CONTENTS

1. Introduction.

2. Principal stress trajectories (Force flow lines).

3. Balanced design.

4. Criteria and objectives of design.

5. Strength based design.

6. Rigidity based design.

7. Impact based design.

8. Design for Tension and Compression Strength

9. Designing for Stiffness

10. Conclusion.

11. Reference.


INTRODUCTION:

Product design deals with the human activity of converting new ideas into reality for the

fulfillment of the mankind. Safety is a very important consideration in product design. Whether

the people paying for the accidents are the consumers, distributers, the manufactures or the

insurance companies, it is the consumer who suffers. Hence the machine must possess the

essential strength that will not lead to accidents, in case of functional failures.

Strength is the characteristic of the machine component defining the maximum force it can

bear. The load is the actual force imparted to the component, when the machine operates. The

strength of the components may vary, because of uncertainties in the material properties and

finishing processes. Sometimes the load even exceeds the estimated load, which may lead to

functional failure of the machine or accidents. This can be avoided by providing extra strength to

the component as a margin of overloading which is termed as factor of safety. Factor of safety is

the ratio of strength to load. If a component is given a very large factor of safety, the excess

strength leads to more material wastage. So a better engineering practice is to obtain an accurate

evaluation of the strength and the load, and to use small factor of safety. To support a load with

the least amount of material, lines of force should be allowed to flow directly between points of

external forces. Geometry of the component is also altered to keep the strength uniform

throughout, in order to get a balanced design.

PRINCIPAL STRESS TRAJECTORIES (FORCE-FLOW LINES):

Machines are designed to transmit forces such that effective work can be done. The ability of

machine element to carry external load is its strength. The line of forces can be imagined to flow

from the point of application of force to the end of the product.

In pure compression and pure tension applications, the forces flow smoothly and straight

through the part. Components carrying pure compressive force or pure tensile force are quite

efficient in utilizing the strength of the material. If the force flows in curves, there is usually

bending applied to the member. Maximum shear stresses exist in planes at 45° to the lines of

force. In Strength of Materials terminology, force flow lines are termed principal stress

trajectory.

Example: 1) Flow of lines of force from the hammer to the punch. They pass

along the length of the punch and come out at the tip. Since the stress is inversely

proportional to cross sectional area, there is maximum stress at the tip of the punch

where area approaches to zero. Therefore to avoid failure of the punch the maximum

stress should be less than the yield stress.


2) The curved section of the C-clamp is subjected to

bending and axial tension, whereas at the jaws are subjected to

only compressive load. The force flows from the curved beam

to the screw. The failure of C-clamp may be due to

compressive yielding of jaws, bending failure in curved

section, shear failure of screw threads. These sections are

designed for enough strength to prevent each mode of failure.

3) To support a load with the least amount of material, lines of force should be allowed to

flow directly between points of external forces. Direct lines of force do not cause any bending

and result in the shortest physical length of a load-carrying member. It is clear by the illustration

of two types of chairs as shown in the diagrams below. Fig.(a) depicts the chair design and

Fig.(b) illustrates force flow. The chair with direct force-flow is more efficient than the chair

with curved force-flow. Force-flow visualization is a good technique for understanding how

forces are transmitted through components and assemblies.

Bending can be decomposed into tension and compression and shear must occur between tension

and compression on a flow line as in case of bolted assemblies.

Fig. (a) Fig. (b)


BALANCED DESIGN:

In order to prevent failure, the strength of a member has to be greater than the induced stress

in the member. However, a memb

possible, the safety factor should be kept uniform throughout each machine element for a

balanced design. The component of a machine having the lowest safety factor is the critical

element. In C-clamp the curved portion is the critical element as it is subjected to bending stress

and has highest bending moment. Doubling or tripling the strength at the jaws or anvils will not

improve the clamp as a whole. A balanced design without excessive over strengt

the safety factor for every component in the machine were the same.

The above figure shows two cantilever beams both of which have the highest bending

moments at the fixed ends which are the critical

fixed ends are less than the strength of the material, the beams will not fail. Due to their smaller

stresses, other elements in the uniform beam have larger safety factors than the critical element.

The uniform beam is, therefore not balanced in stress and strength. The width of the tapered

beam decreases correspondingly with the bending moment which decreases linearly from the

fixed end to the point of application of the force. The tapered beam with uniform sa

thus a balanced design. They can bear same maximum external load. Yet, the tapered beam saves

half the volume of the material required for uniform beam.

Similarly in case of a punch, balanced design to increase the yield strength of the mat

towards its tip is usually achieved by heat

strength Sy is made inversely proportional to cross sectional area. i.e;



in the member. However, a member with excessive strength wastes material. E.As far as



the curved portion is the critical element as it is subjected to bending stress


improve the clamp as a whole. A balanced design without excessive over strengt

the safety factor for every component in the machine were the same.


moments at the fixed ends which are the critical elements of the beams. If the stresses at the



m beam is, therefore not balanced in stress and strength. The width of the tapered


fixed end to the point of application of the force. The tapered beam with uniform sa


half the volume of the material required for uniform beam.

Similarly in case of a punch, balanced design to increase the yield strength of the mat

towards its tip is usually achieved by heat-treating the tip of the punch. By doing this the yield

is made inversely proportional to cross sectional area. i.e; Sy = �

�


er with excessive strength wastes material. E.As far as



the curved portion is the critical element as it is subjected to bending stress


improve the clamp as a whole. A balanced design without excessive over strength would result if


elements of the beams. If the stresses at the



m beam is, therefore not balanced in stress and strength. The width of the tapered


fixed end to the point of application of the force. The tapered beam with uniform safety factors is


Similarly in case of a punch, balanced design to increase the yield strength of the material

treating the tip of the punch. By doing this the yield

�

�


STRENGTH BASED DESIGN:

The criterion is that the strength of the material must be larger than the induced stress, σ,

anywhere in the machine, especially in the critical elements. The design for strength means same

as designing against yielding and fracture.

To prevent yielding, Sy > σ

To prevent fracture, Sult > σ

Where Sy is the yield strength and Sult the ultimate strength.

RIGIDITY BASED DESIGN:

This design criterion is applied in machine tool design and instrument design. The stress level

involved is very low and there is no danger of failing by fracture. Cast iron is used extensively in

machine tools because of a high modulus-to-strength ratio.

Rigidity in structures is important in the prevention of vibration. It is an important factor in the

design of consumer products. This factor favors the use of low-strength, high-modulus materials.

IMPACT BASED DESIGN:

When load is applied at short instant of time on an object is called as impact load. During

impact can create momentarily peak stress in the bodies. Impact design and shock proofing is

most important in the case of automobile bumpers, automobile safety devices, power tools,

watches, electrical and electronic products. Impact is also the foremost factor in designing boxes

and containers for packaging.

In an impact situation, the controlling characteristics of a structure is product of strength and

deflection i.e. energy storage capacity also known as resilience. Impact Energy is given as one-

half the product of the maximum force and maximum deflection

Impact Energy = ½ Fmax * δmax

Material toughness: RESILIENCE:

The energy that a unit volume of material absorbs is given by area under the stress-strain

curve. The elastic part this energy capacity is called the modulus of resilience, R. Mathematically

expressed as ��

�

�∗� . The energy capacity up to the fracture the fracture point of a material is

the toughness. An impact with more energy than the toughness of the material causes fracture.


Impact strength plays important factor while choosing composite materials.

glass beads to plastics will increase the ultimate

toughness of the material may be

where impact strength is needed

DESIGNING FOR UNIFORM STRENGTH

Uniform distribution is usually the ideal stre

balanced and the material is fully

object is equal to the toughness

end connection sand geometric arrangements

DESIGN FOR TENSION AND COMPRESSION STRENGTH:

Many materials are stronger in compression. Glass rods and concrete bars will fail in tension

if bending loads are applied. Th

iron; 5 for glass; and 10 for concrete. Noticeable exceptions are materials with oriented grain or

molecular structures such as steel cables, nylon fibers, natural fibers, and glass fibers. Such

materials are stronger in tension than in compression.

Stability is the major advantage of tension members. A slightly curved member will straighten

out under a tensile load. Compression members are unstable and may buckle. Long members ate

unstable and may buckle. The stability of long and slender compression members depends o

elastic modulus, E, rather than on the. Material yield strength, S

strength steel members is not superior to low

A concave end-plate for pressure cylinder experiences compressive stresses and may fail due

to buckling. A convex end-plate is under tensile stress and may fail if the material in the

has a low tensile strength. Therefore, a steel end plate should be made convex.


plays important factor while choosing composite materials.

glass beads to plastics will increase the ultimate strength and decrease the elongation. The

toughness of the material may be lowered. Such a material is not suitable

where impact strength is needed.

DESIGNING FOR UNIFORM STRENGTH:

is usually the ideal stress distribution. In such a case, the design is

the material is fully utilized. If the stress is uniform, the energy capacity of the

object is equal to the toughness of the material multiplied by volume. In most practical situations,

sand geometric arrangements prevent perfectly uniform stress distribution

SIGN FOR TENSION AND COMPRESSION STRENGTH:

are stronger in compression. Glass rods and concrete bars will fail in tension

bending loads are applied. The ratio of compressive strength to tensile strength is: 3.5, for cast

glass; and 10 for concrete. Noticeable exceptions are materials with oriented grain or

tructures such as steel cables, nylon fibers, natural fibers, and glass fibers. Such

stronger in tension than in compression.

ajor advantage of tension members. A slightly curved member will straighten


unstable and may buckle. The stability of long and slender compression members depends o

rather than on the. Material yield strength, Sy, As far

steel members is not superior to low-strength steel members.

FIG: Design of end plates

of steel and cast iron.

plate for pressure cylinder experiences compressive stresses and may fail due

plate is under tensile stress and may fail if the material in the

has a low tensile strength. Therefore, a steel end plate should be made convex.

plays important factor while choosing composite materials. Adding micro

strength and decrease the elongation. The

Such a material is not suitable for an application

ss distribution. In such a case, the design is

uniform, the energy capacity of the

In most practical situations,

stress distribution.

are stronger in compression. Glass rods and concrete bars will fail in tension

e ratio of compressive strength to tensile strength is: 3.5, for cast

glass; and 10 for concrete. Noticeable exceptions are materials with oriented grain or

tructures such as steel cables, nylon fibers, natural fibers, and glass fibers. Such

ajor advantage of tension members. A slightly curved member will straighten


unstable and may buckle. The stability of long and slender compression members depends on the

far as stability goes high

Design of end plates

of steel and cast iron.

plate for pressure cylinder experiences compressive stresses and may fail due

plate is under tensile stress and may fail if the material in the plate

has a low tensile strength. Therefore, a steel end plate should be made convex. If cast iron is


used, the concave shape is preferable as shown in. The choice of the shape depends on the

strength-to-stiffness (elastic modulus, E) ratio of the material.

DESIGNING FOR STIFFNESS:

In addition to being strong enough to resist the expected service loads, there may also be the

added requirement of stiffness to ensure that deflections do not exceed certain limits.

When an initially straight beam is loaded, it becomes curved as a result of its deflection. As

the deflection at a given point increases, the radius of curvature at this point decreases. The

radius of curvature, r, at any point on the curve is given by the relationship:

r = E*I/M

Above equation shows that the stiffness of a beam under bending is proportional to the elastic

constant of the material, E, and the moment of inertia of the cross-section, I. Selecting materials

with higher elastic constant and efficient disposition of material in the cross-section are essential

in designing beams for stiffness. Placing material as far as possible from the neutral axis of

bending is generally an effective means of increasing I for a given area of cross-section.

The stiffness of components under bending is proportional to the elastic constant of the

material (E) and the moment of inertia of the cross-section (J). Selecting materials with higher E

and efficient disposition of material in the cross-section are essential in designing such

components for stiffness. Placing material as far as possible from the neutral axis of bending is

generally an effective means of increasing I for a given area of cross-section.

CONCLUSION:

A successful design should take into account the function, material properties, and

manufacturing processes. The relationship between material properties and design is complex

because the behavior of the material in the finished product can be quite different from that of

the stock material.

REFERENCE:

Product Design and Manufacturing by A.K.Chitale and R.G.Gupta, Prentice Hall India, 1st

Edition, Second Print,1999.

Material Selection For Engineering Design by Mahmoud.M.Farag, Prentice Hall, 1st

Edition,1997.

http://engineers.ihs.com/products/standards/standards-design-engineer.htm

strength consideration in product design, shiva,deepak

Documents