8.fatigue test

8/13/2019 8.Fatigue Test

1/19

Fatigue Test

ME 251: Fall 2013

Materials Engineering Lab

Wichita State University


2/19

Fatigue

Fatigueis the progressive, localized andpermanent structural damage that occurs when a

material is subjected to cyclic or fluctuating

loading.

The maximum stress values are less than the

ultimate tensile stress limitand may bebelow the

yield stress limitof the material.

It is estimated that 90% of all metal failure occur

from fatigue.



3/19


4/19

Fatigue Failure

Fatigue cracks are generally initiated at flaws ona free surface. Once initiated, cracks grow at an

accelerating rate.

Eventually, a crack becomes unstable and

propagates through the remaining cross-sectional

area of the part during single load application

causing sudden catastrophic failure.



5/19

Fatigue Failure, Cont.

The process of fatigue failure can be divided into the

following stages:

1.Cyclic plastic deformation prior to fatigue crack

initiation.2.Nucleation (Initiation of fatigue cracks).

3.Short crack or small crack phase.

4.Crack propagation or growth rates that can be

characterized by either linear-elastic fracturemechanics, elastic-plastic fracture mechanics or

fully plastic fracture mechanics.

5.Final instability or failure.Wichita State University


6/19


The following basic factors are necessary tocause fatigue fracture:

Applied stress of sufficient magnitude

Large variation or fluctuation in applied stressSufficiently large number of cycles of the

applied stress

Note: With frequent stress fluctuations, the

material may fail at a stress level far below itsstatic ultimate tensile strength or even static yield

strength of the material.



7/19




8/19

Fatigue Failure, Cont. A practical exampleof low-cycle fatigue would be the

bending of a paperclip. A metal paperclip can be bent

past its yield point (i.e., bent so it will stay bent)

without breaking, but repeated bending in the same

section of wire will cause the material to fail. Fatigue is an important design consideration in any

machine such as automotive & aerospace vehicles,

material handling equipments, power generators,

process plant equipments etc.

Note: Fatigue is a stochastic (random) process, often

showing considerable scatter even in controlled

environments.Wichita State University


9/19


10/19


11/19

Designing Against Fatigue

1. Design to keep stress below threshold of fatiguelimit (infinite lifetime concept).

2. Design (conservatively) for a fixed life after which

the user is instructed to replace the part with a new

one (a so-called lifed part, finite lifetime concept, or"safe-life" design practice);

3. Instruct the user to inspect the part periodically for

cracks and to replace the part once a crack exceeds

a critical length. This approach usually uses thetechnologies of nondestructive testing and requires

an accurate prediction of the rate of crack-growth

between inspections. This is often referred to as

damage tolerant design or "retirement-for-cause".Wichita State University


12/19

Infinite Lifetime Concept(S-N Curve)



13/19

Infinite Lifetime concept(S-N Curve),

Cont.

Safe-life design based on the infinite-life criterion reflectsthe classic approach to fatigue.

It is generally categorized as a "high cycle fatigue"methodology.

This methodology is one where the influence of steelseems virtually overwhelming.

The "infinite-life" aspect of this approach is related to theasymptotic behavior of steels, many of which display afatigue limit or "endurance limit"at a high number of cycles

(typically >106) under benign environmental conditions.

Most other materials do not exhibit this response, insteaddisplaying a continuously decreasing stress-life response,even at a great number of cycles (106to 109), which is

more correctly described by a fatigue strength at a givennumber of cycles.Wichita State University


14/19

Finite Lifetime Concept (-N

Curve) With more advanced and highly loaded components,

it became obvious that stress-based techniques alonewould not be sufficient to handle the full range ofproblems that needed to be addressed usingcontinuum assumptions.

The occurrence of plasticity, for example, and theaccompanying lack of proportionality between stressand strain in this regime led to the use of strain as acontrolling quantity.

Strain-life criteria is primarily intended to address the"low-cycle" fatigue area (e.g., from approximately 102to 106cycles).

Where the stress is high enough for plasticdeformation to occur, the account in terms of stress is

less useful and the strain in the material offers aWichita State University


15/19


16/19

Damage Tolerance Concept (da/dN

vs. K)

The S-Nand -Ntechniques are usually appropriate forsituations where a component or structure can beconsidered a continuum (i.e., those meeting the `no cracks'assumption).

In the event of a crack-like discontinuity, however, they offerno support.

1Damage toleranceis a property of a structure relating toits ability to sustain defects safely until repair can beeffected or the structure replaced.

1The approach to engineering design to account fordamage tolerance is based on the assumption that initialflaws exist in any structure and such flaws propagate withusage

An alternative concept and controlling quantity must beused. That quantity is stress intensity, a characterizationWichita State University


17/19

Damage Tolerance Concept,

Cont.

a) Specimen

andloading

b) Measured

data

c) Rate data



18/19

Types of Fatigue Testing

Tension Compression (reversed stress)

Tension Tension (repeated stress)

Irregular or random

Sheet and plate bending (flexural fatigue)



19/19

Lab Experiment: Fatigue Lifetimes of

Smooth and Notched Specimens


8.fatigue test

Documents