Chapter 7 - Mechanical Properties of Solids

Question 1

Brittle Materials

Answer 1

do not exhibit plastic deformation regardless of how high the applied stress becomes. Only exhibit elastic strain behavior.
- fracture at much lower strains.
- often have relatively large Young’s moduli and ultimate stresses.
- fail suddenly and without much warning.
Glass and cast iron fall in the class of Brittle Materials.

Question 2

Charpy Test

Answer 2

is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material’s notch toughness and acts as a tool to study temperature-dependent ductile-brittle transition.

Question 3

Columns

Answer 3

Structural Elements designed to tolerate compressive loads (i.e compression)

Question 4

Combined Loading States

Answer 4

When two or more types of loads contribute to the stress state at a point. machine parts and structural members are simultaneously subjected to both normal stresses (axial and bending) and shear stresses (due to shear forces and torques).

Question 5

Creep

Answer 5

crystalline metals and ceramics undergo continued elongation/strain at temperatures above 0.4 Tm. (Tm = absolute melting temp in degrees Kelvin or R). and 0.5 Tm, . Tests to measure the creep behavior of metals or ceramics typically apply a series of constant stresses over long periods of time at a series of elevated temperatures.

Question 6

Creep Strength

Answer 6

To resist unacceptable permanent deformation over time under sustained loads or forces at elevated temps. (relative to a material’s absolute melting point).

Question 7

Ductile

Answer 7

Ductile materials will withstand large strains before the specimen ruptures.

• often have relatively small Young’s moduli and ultimate stresses.
• exhibit large strains and yielding before they fail.
• Steel and aluminum usually fall in the class of Ductile Materials

Question 8

Ductility

Answer 8

gives a measure of the ability of a material to plastically deform without fracturing (i.e. copper wire) It is important to both to the easy processing of a material by means of deformation and to the ability of a material (or structural element) to resist cracking or fracturing from processing forces or service stresses.

Question 9

Dynamic Loading

Answer 9

or forces are those that are planned by the designer to be present and are known to vary with frequency or may not be planned to be present all the time or very often but can reasonably be expected to occur within the life of the device or structure. (i.e. wind gusts, seismic earthquake loads, ballistic impacts (on military vehicles).

Question 10

Elastic Behavior

Answer 10

permanent plastic deformation never occurs. Solid objects will deform when adequate forces are applied on them. If the material is elastic, the object will return to its initial shape and size when these forces are removed.

Question 11

Elastic Limit

Answer 11

indicates the the stress level at which the material will permanently or plastically deform, or yield; that is, the beginning of the plastic region of the curve and plastic behavior.

Question 12

Elastic Range

Answer 12

is defined by the linear portion of the stress-strain curve. The slope of this line is defined by the modulus of elasticity. Remember that E= stress/strain, which is the same equation that defines the slope of this line (m=y/x). If a material is stretched only in this region and then the force is released, then the material follows the same line down while being unloaded. The material thus returns to its original dimensions. Again, this is seen with a spring; when it is stretched and then released, it returns to its original configuration.

Question 13

Engineering Stress

Answer 13

is defined as the load or force (F) per unit area, based on the initial load-bearing area (Ao), as: F/Ao

Question 14

Engineering Strain

Answer 14

is defined as the elongation per unit length, based on the initial length (Lo) of the test specimen or structural element, as: deltaL / Lo.

Question 15

Fatigue

Answer 15

is the result of repeated cyclic loading. The material quite literally becomes tired, with complete failure by fracture being the ultimate result. It is a form of failure that occurs in structures subjected to fluctuating loads that repeat over time. (i.e. bridges, aircraft, cars, machine components).

Question 16

Fatigue Strength

Answer 16

To resist cyclic loads or forces over the service lifetime required. The highest stress that a material can withstand for a given number of cycles without breaking

Question 17

Flexural Strength

Answer 17

is the stress in a material just before it yields in a flexure test. It is the maximum stress at the outermost fiber on either the compression or tension side of the specimen. Flexural modulus is calculated from the slope of the stress vs. strain deflection curve

Question 18

Fracture Stress (strength, strain)

Answer 18

Fracture strength is the stress when a specimen fails or fractures. In a tension test, true strain is less than engineering strain. This stress is called the tensile stress because every part of the object is subjected to tension.

Question 19

Fracture Toughness

Answer 19

fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for many design applications.

Question 20

Hardness

Answer 20

is a measure of the ability of a material to resist scratching, indentation, or penetration.

Question 21

Hooke’s Law

Answer 21

is the relationship between stress and strain: = E time e.. It says that the strain (deformation) of an elastic object or material is proportional to the stress applied to it.

Question 22

Impact Strength

Answer 22

to resist sudden shock loads or impulse forces without fracture.

Question 23

Percent Elongation

Answer 23

reported in a tensile test is defined as the maximum elongation of the gage length divided by the original gage length.

Question 24

Percent Reduction in Area

Answer 24

is the proportional reduction of the cross-sectional area of a tensile test piece at the plane of fracture measured after fracture.

Question 25

Plastic Behavior

Answer 25

plasticity describes the deformation of a (solid) material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the material itself. In engineering, the transition from elastic behavior to plastic behavior is called yield.
Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, foams, bone and skin

Question 26

Plastic Limit

Answer 26

The yield point is a point on stress-strain curve which indicates the limit of elastic behavior and the beginning of plastic behavior.

Question 27

Proportional Limit

Answer 27

Highest stress at which stress is directly proportional to strain. It is the highest stress at which the curve in a stress-strain diagram is a straight line. Proportional limit is equal to elastic limit for many metals.

Question 28

Shafts

Answer 28

structural elements designed to resist twisting and tolerate torsion.

Question 29

Stiffness

Answer 29

to resist unacceptable elastic deflection or distortion, to absorb and store energy (often to be returned), or to resist buckling under compressive loads or forces

Question 30

Strain

Answer 30

Engineering strain is defined as the amount of deformation in the direction of the applied force divided by the initial length of the material. This results in a unit-less number. For example, the strain in a bar that is being stretched in tension is the amount of elongation or change in length divided by its original length.

Question 31

Strain to Fracture

Answer 31

is given by the projection of the end of the stress-strain curve onto the strain axis just before or at fracture, once the specimen actually fractures, the amount of elastic recovery and permanent or plastic strain (or deformation) are obtained by drawing a straight line with a slope parallel to the initial portion of the stress strain curve.

Question 32

Strength

Answer 32

is a material’s ability to withstand an applied load without failure or plastic deformation.

Question 33

Stress

Answer 33

The term stress (s) is used to express the loading in terms of force applied to a certain cross-sectional area of an object. From the perspective of loading, stress is the applied force or system of forces that tends to deform a body. From the perspective of what is happening within a material, stress is the internal distribution of forces within a body that balance and react to the loads applied to it.

Question 34

Stress Relaxation

Answer 34

in polymers causes the initial stress created by an applied and sustained load to drop as time goes on. (i.e rubber bands, after a long time the rubber band becomes slack (less tight).

Question 35

Ties (Struts)

Answer 35

structural elements designed to tolerate tensile loads (tension).

Question 36

Toughness

Answer 36

the ability of a material to absorb energy without fracturing.

Question 37

True Strain

Answer 37

is the amount that a material deforms per unit length in a tensile test. Is obtained by the relationship ln(Li / Lo)

Question 38

True Stress

Answer 38

Is defined by the applied force (F) divided by the actual (true), instantaneous area (ai) as F/Ai

Question 39

Ultimate Tensile Strength (Stress)

Answer 39

is the capacity of a material or structure to withstand loads tending to elongate. In other words, tensile strength resists tension (being pulled apart)

Question 40

Viscoelastic Creep

Answer 40

When subjected to a step constant stress, viscoelastic materials experience a time-dependent increase in strain. At a time t0, a viscoelastic material is loaded with a constant stress that is maintained for a sufficiently long time period. The material responds to the stress with a strain that increases until the material ultimately fails. When the stress is maintained for a shorter time period, the material undergoes an initial strain until a time t1 at which the stress is relieved, at which time the strain immediately decreases (discontinuity) then continues decreasing gradually to a residual strain.

Question 41

Yield Strength

Answer 41

refers to an indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of the elastic limit.
In engineering structural design, yield strength is very important. For example, when designing a component, it must support the force incurred during use, and the component must not deform plastically. Therefore, a material with sufficient yield strength should be selected.

Question 42

Young’s Modulus (Modulus of Elasticity)

Answer 42

describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress to tensile strain. It measures the resistance of a material to elastic (recoverable) deformation under load. A stiff material has a high Young’s modulus and changes its shape only slightly under elastic loads (e.g. diamond). A flexible material has a low Young’s modulus and changes its shape considerably (e.g. rubbers).

Question 43

0.2% Offset Yield Strength (Stress)

Answer 43

the stress given by the intersection of this offset line with the stress strain curve