Download 02 Glaser From Theory to Reality Imposing Real World

facc Aerostructures
Extend Your Expectations
“From Theory to Reality –
Peter Glaser, Ho Stress
imposing real world requirements on the world of numerics.“
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Abstract
The ever increasing performance of computing tools provides unprecedented
possibilities to evaluate the last single detail of, for example, a structural
product. While this, per se, provides numerous novel and useful possibilities, it
calls for an intelligent choice of when to take advantage of such possibilities
and when they do not make a difference. When input parameters are of coarse
nature, then post processing to a high precision is purely numeric, but non
relevant. For example, if you are going to measure the consumption of a car
over 1 km, you don’t need to measure this distance with an accuracy of a mm.
This presentation will discuss similar aspects in the world of structural analysis.
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Introduction
„Dimensional Analysis“
In the AC stress analysis, the RF threshold of 1.00 separates flightworthy from
non- flightworthy.
Sometimes complete design concepts get cancelled because the minimum RF
on paper remains at a value of e.g. 0.95 and noone can think of a “pencil
sharpening” option.
On other occasions, simple, apparently non-dramatic modeling updates lead to
“critical” Rf changes in excess of 30 percentage points up or down.
This curious fact raises a number of questions. Within the scope of this
presentation, the focus lies on the following: ”Is enough attention paid to the
variation of parameters when computing single number RFs?”
The collection of arguments herein are source for discussion and food for
thought and may in some instances not withstand full examination. They are
not suggested practice at the moment.
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Parameters and Variations
Loads
 Load application accuracy;
Winglet sectional loads;
Spoiler load shapes;
load splits between top and bottom surfaces, etc.
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Parameters and Variations
Manufacturing / Handling Tolerances
 Draping angle influence vs. manual layup accuracy. & 5° positioning
tolerance on dwg.
 Measurement accuracy of defect in BA vs. grid density.
 B-Values “improbable” 95% of time + redistribution…
 Ultimate Factors related to probability of failure,
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Operator Behaviour
FEM Detail
 H/C allowable - 3 D Mesh
 Laminate Allowable coupon size – Shell elm size
 Differences in attention to detail – e.g.: every fastener modeled but contact
phenomena ignored; Multiple elms through laminate radii modeled,…
 Test correlation within +-10% successful
 Bolted joint load share accuracy.
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Sources for Alternatives
Damage Tolerance
 Intentional DCFs
 Inherent DCFs – eg: MLP cases are not required to be assessed in
composite; it is assumed that CFRP is itself an MLP. Maybe some composite
failure could well be allowed without threatening product integrity.
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Sources for Alternatives
Statistics
 B-Values “improbable” 95% of time + redistribution…
 Ultimate Factors related to probability of failure,
 How many pcomp layers in the total quantity of composite elements actually
exhibit RF of 1.0 ? How is the remaining RF distribution?
Even if the entire product is built from material with the properties of the
remaining 5% below the “B-value quality”, will that product fail at 1.0xUL?
 Which features really are critical structural elements? Shouldn’t all others be
treated differently?
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Sources for Alternatives
RF bands with probability of occurence.
 Probability of an RF of 1.0 on paper to be the precise failure point in reality?
Dependent on:
Probability of Loadcase occurrence;
Probabiltiy of raw material performance to exactly the B-Value performance.
Probability of the applied load/stress/strain to be acurately predicted
Probability of RF calculation algortihm to be exact
Probability of correct level of defect in correct vicinity of position under
estimation.
Probability of manufacturing process to deliver exactl yat the lowest end of
the required spectrum.
Etc….
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Sources for Alternatives
RF bands with probability of occurence.
In one element? In the element next to the failed element?... Probabiltiy of two
neighboring elements to be equally un“fortunate“.
RF
?
1.0 -
5%
Probability
50%
95%
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Discussion
Options for the future
 Can probability based RF bands realistically become standard use? OEM position?
 How exactly will the product variability (including defects) with the variability of other parameters
get mixed to produce the RF bands?
How does this get sold into the status quo?
 Are shortcurts available? (eg. Work with nominal geometry, as so far, but determine effect factors
or equations….)
?
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Thank you for your attention
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