Damage Tolerance Analysis

What is the root of DTA ? fatigue is the weakening of a material

caused by repeatedly applied loads.

What happen in Fatigue ?

What is the cast of failure ? Physical harm to people or the

environment Loss or destruction of property or

equipment Loss of productivity or use of the failed

“system” or device Damaged reputation

Different Fatigue Design Methodologies Infinite Life Design Safe-Life Design Damage Tolerance Design

Infinite Life Design

Unlimited safety is the oldest criterion. For parts subjected to many millions of

cycles, like engine valve springs, this is still a good design criterion.

This criterion may not be economical or practical in many design situations.

endurance limit of the material is important in this design criterion.

Something is wrong here !!

Safe-life Design Safe-life refers to the philosophy that

the component or system is designed to not fail within a certain, defined period.

The benefit of safe-life designs includes reducing the likelihood of unplanned maintenance and reducing the likelihood of any failure

Z 42 is made with safe life philosophy

Safe life process evaluating the highest operational

stress on the component safety factors are often applied to

ensure that catastrophic failures Comparing  S-N curve It has  infinite life or limited life

S-N Curve

Did I forget Something again ?

In order to overcome this shortcoming of the safe-life approach, the methods were must developed that assume the structure contains initial cracks.

Damage tolerance analysis

Damage tolerance analysiswe have category :

Slow crack growth

structures are designed such that initial damage will grow at a stable, slow rate under service environment

Fail-safe structures are

designed such that propagating damage is safely contained after failing a major load path by load shift to adjacent intact elements

Slow crack growth damage tolerance (and thus safety) is

assured only by the maintenance of a slow rate of growth of damage, a residual strength capacity

sub-critical damage will either be detected at the depot or will not reach unstable dimensions within several design life times.

Fail-safe damage tolerance is assured by the

allowance of partial structural failure the ability to detect this failure prior to

total loss of the structure Fail Safe structure is designed and

fabricated such that unstable rapid propagation will be stopped within a continuous area of the structure prior to complete failure

Usually:

Single load path • Slow Crack Growth

Multiple load path• Slow Crack Growth• Fail Safe

Lug Example of Slow Crack Growth Structure

The lug fitting illustrated here has multiple lug ends at the pinned connection

occurrence and growth of damage at a typical location (B) would render the structure inoperative.

Wing Box Example

a wing box is attached to the fuselage carry through structure by multiple fittings.

A case could be made to qualify this structure as Fail Safe Multiple Load Path.

if the skin was the major bending member with a design stress of sufficient magnitude to result in a relatively short critical crack length.

Damage concept the majority of the life is spent growing

the resultant cracks to failure. analyses of in-service fractures,

cracking instances, etc. have indicated that a major source of cracks is the occurrence of initial manufacturing defects such as sharp corners, tool marks

typical growth behavior for a crack

structural element as it moves from an initial damage size to a damage size that causes structural failure

• Crack increment (Δa)

• number of loading events (ΔN)

• critical value (acr)

when the crack is small, it grows very slowly

Damage growth effectQuality

Note that the shape of the crack growth curve (for a given configuration and loading) remains essentially constant for any given crack growth increment.

The effect of initial crack size is significant.

Damage growth effectload history

The stress history experienced at each location on the aircraft will also differ due to changes in bending moment, twisting moment, shear loading.

The loading spectra for a lower surface location is typically more severe than a corresponding upper surface location.

Damage growth effectmaterial properties

The crack growth rate (Δa/ΔN) can be derived experimentally for each material

the alloy having the slower growth rate characteristics (i.e. 2024-T3) will have a longer life

Damage growth effectStructural Properties

The most complex of the parameters affecting crack growth behaviorare the structural properties.

Life Prediction Methodology(Initial Flaw Distribution)

For predictions of safety limits, the initial cracks larger than detectability limit are of principal concern.

(Initial Flaw Distribution)

NDT ( nondestructive inspection )

Life Prediction Methodology(Usage)

The sum of the load levels that a structure is expected to experience is determined by a projection of the amount of usage expected over the life in the various possible missions

(Usage )

Life Prediction Methodology(material properties)

Crack growth data are generated in the laboratory under constant cyclic loading on simple specimens with accepted characterizing stress intensity factors.

Life Prediction Methodology(Crack Tip Stress Intensity Factor)

The crack tip stress intensity factor(K) interrelates the crack geometry, the structural geometry, and the load on the structure

It defines as : β- geometric termfor structural

configurationσ- stress applied to the structure a- crack length

Damage Size Characterizations

Reference Documat is JSSG-2006 . This approach assumes that cracks are

present in all critical locations. periods between inspections are greatly

influenced by the crack lengths assumed at the beginning of a usage period.

Damage size in steps

Crack Growth-Life Curve after Second Inspection

Residual Strength

The strength of a structure can be significantly affected by the presence of a crack

The basic concept in damage tolerance design is to ensure the safety of the structure throughout the expected service life.

Residual strenght curve

Residual StrengthSlow crack growth Fail-Safe

detection of this failure prior to total loss of the structure

safely within the partial failure prior to inspection

Residual Strength Capability Single Load Path Residual Strength Diagrams

Built-Up Structure Residual Strength Diagrams

In built-up structures, due to the complex geometrical configuration, one or more failure criterion may have to be considered

Built-Up Structure Residual Strength Diagrams

Damage Tolerance Analysis Procedure

Step 1. Determine the stress-intensity factor (K) as a function of crack size for each member

Step 2. derive the stress history for the location under consideration.

Step 3. Obtain baseline crack-growth data (da/dN as a function of ΔK and R) for all the materials

Damage Tolerance Analysis Procedure(continue)

Step 4. Using the results of Steps, 1, 2, and 3, calculate the crack-growth curve for each element

Start with a 0.02 inch flaw

Damage Tolerance Analysis Procedure(continue)

Step 5. By using the results of the residual strength analysis plot the critical crack sizes, aDMC and aLTC

Step 6. For slow crack growth structure I. whether BD is equal to or greater than 2 design

lifetimes. II. whether CE (or C’E) is equal to or greater than ½ design

lifetime.

Step 7. For safe fail I. whether AF is equal to or greater than 1 design lifetime. II. whether CG (or C’G) is equal to or greater than ¼

design lifetime.