Post on 18-Sep-2020
Mini-Almen Strips: Applications and Use
Jeff DerdaElectronics, Inc.
Kelly McClurgAvion Solutions, Inc.
Lecture Outline• Introduction to Mini Almen Strips• Why use a Mini Strip• Desirable Applications• Adhesion of Mini Strips• Mini Almen Strip Gauge• Verifying Mini Strip Performance• Creating a Saturation Curve and Correlation
chart for Mini Strips
What is a Mini- Strip?SPECIFICATIONS
• Size: 25.4 mm x 3.18 mm (1” x .125 “ )
• Hardness: 44-50 HRc. (SAE J442)
• Material: SAE 1070 (SAE J442)
• Thickness: A strip = 1.30 mm (.051”) N strip = 0.79 mm (.031”)
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Description
Standard Almen Strips
Mini N Mini AN A C
Mini Almen Strips
Why Use a Mini-Strip?
The Almen Mini-strip was developed to provide a means of more accurately verifying the shot peening intensity in areas that are inaccessible with standard size Almen strips
and holders
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Advantages of using Mini-Almen strips
• Pre-cut strips ensure consistency– Possible variation in masking of shaded strips
• Adhere directly to component after correlation, eliminating the need to create a test fixture
• Calculated mini strip intensity value, rather than a single point, arc-height correlation
Current Applications
• Areas smaller than a Standard Almen Strip– Example: Limited access to a small area on a 10-ft
assembly
Current Applications
• Across Complex Geometry– Tail Rotor Blade Pitch Horn
– Dove tail slots
– Pitch Varying Housing
Current Applications
• Inner Diameter of Holes or Springs
Adhesion
• There are many different adhesion methods for using Mini Almen strips
– Double-Sided tape
– Contact Cement
– Screws
AdhesionImportant: The adhesion method must remain constant when developing a correlation chart and testing intensities on the component in question
• Different bonding properties for each adhesion method
• Check for any de-bonding of the strip after peening– If de-bonding is present a stronger adhesive may be
needed to obtain accurate arc-height measurements
Mini-Almen Strip Gauge
• Different design from standard Almen strips
• The entire length of a Mini Strip is used for arc-height measurements
• Mini Strips rest on 2 Notched holders instead of 4 round balls
• 2 Support pins in the back
Mini-Almen Strip Gauge• Must be zeroed before use (just as the
standard strip gauge) , using Mini Strip Zero Block (as shown)
• Mini Strips should rest on the lower surface of the end pins to get an accurate reading
• Due to small values of resultant arc-heights, pre-bow measurements are required!
• All adhesive material should be removed before obtaining an arc-height reading
Verifying the Mini-strip Performance
• In order to determine the effectiveness and accuracy of the Mini-strips, a series of tests was conducted to compare the performance of the Mini-strip to the performance of a standard Almen strip.
• Tests were performed on both the A and N strips.
• Tests were conducted to determine the following:1. Compare saturation curves for Almen strips and Mini-strips.2. Determine the arc height relationship between the Almen strips and
the Mini-strips.3. Compare the arc height consistency of the Mini-strip4. Determine the conditions and limits for which the Mini-strips may be
used.
• All tests were performed in a specially designed blast cabinet with a rotary table capable of peening 26 Almen strips in the same test.
• The Mini-strips were mounted onto the Almen holders, next to the Almen strip, using double sided tape.
Test Methods
• The strips were peened using two sets of conditions.
• Test Method #1: Generate saturation curves for both the Almen strips and the Mini-strips.
• Test Method #2: Peen multiple strips at T1 times to compare the resultant arc heights and distributions.
NOTE:• Prebow compensation was employed on all of the tests in
order to accurately determine the change in arc height.
Arc Height & Prebow Measurement• Arc heights and prebow for Almen strips were measured on an
Almen gage. (Fig. 1)• Arc heights and prebow for Mini-strips were measured on a
Mini-strip gage. (Fig. 2)
Fig. 1 Fig. 2
Test Method 1 – A Strips
• Compare saturation curves for A Almen strips and A Mini Strips.
• Target intensities:1. 12A (0.30 mm / .012 in)2. 16A (0.40 mm / .016 in)
Saturation CurvesAlmen A Strip & A Mini-Strip
11.7 (0.29 mm / .0017 in)
6.2(0.16 mm / .0062 in)
0
2
4
6
8
10
12
14
16
0 10 20 30 40 50 60
Arc
Hei
ght (
.001
in)
Peening Time (Revolutions)
A Strip Saturation CurveA Mini Saturation CurveA Strip Saturation IntensityA Mini Saturation Intensity
16.3(0.41 mm / .0163 in)
7.6(0.19 mm / .0076 in)
0
2
4
6
8
10
12
14
16
18
20
0 10 20 30 40 50 60
Arc
Hei
ght (
.001
in)
Peening Time (Revolutions)
A Strip Saturation CurveA Mini Saturation CurveA Strip Saturation IntensityA Mini Saturation Intensity
Almen A Strips vs. Mini StripsIntensities of 3A, 6A, 12A, 16A, & 20A
0
5
10
15
0 5 10 15 20 25
A M
ini (
.001
in)
A Strip (.001 in)
Mean Arc Heights
Linear (Mean Arc Heights)
Test Method 1- N Strips
• Compare saturation curves for N Almen strips and N Mini Strips.
• Target intensity:1. 12N (0.30 mm / .012 in)
Saturation CurvesAlmen N Strip & N Mini-Strip
11.4(0.29 mm / .0114 in)
5.5(0.14 mm / .0055 in)
0
2
4
6
8
10
12
14
16
0 5 10 15 20 25 30 35
Arc
Hei
ght (
.001
in)
Peening Time (Revolutions)
N Strip Saturation CurveN Mini Saturation CurveN Strip Saturation IntensityN Mini Saturation Intensity
Correlation & Distribution at 6A Intensity
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Mean
LSL USL3sp3sp
0
2
4
6
8
10
Samples: 12 Cp: 12.36 (Scale: inches x .001)Mean: 5.95Std Dev: .06742
ALMEN "A" STRIP
2 3 4 5 6 7
Mean
LSL USL3sp3sp
02468
101214161820
Samples: 36 Cp: 8.765 (Scale: inches x .001)Mean: 2.46944Std Dev: .095077
"A" MINI STRIP
2 3 4 5 6 7
Correlation & Distribution at 12A Intensity
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Mean
LSL USL3sp3sp
0
2
4
6
8
Samples: 10 Cpk: 10.12 (Scale: Inches x .001)Mean: 12.08 Cp: 28.99Std Dev: .063246
ALMEN "A" STRIP
2 4 6 8 10 12 14
Mean
LSL USL3sp3sp
0
2
4
6
8
Samples: 10 Cpk: 6.054 (Scale: Inches x .001)Mean: 5.68 Cp: 12.42Std Dev: .14757
"A" MINI STRIP
2 4 6 8 10 12 14
Correlation & Distribution at 16A Intensity
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Mean
LSL USL3sp3sp
0
1
2
3
4
5
Samples: 13Mean: 15.7923Std Dev: .13821
Cpk: 4.323Cp: 4.824
ALMEN "A" STRIP
6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19
Mean
LSL USL3sp3sp
0
1
2
3
4
5
Samples: 33Mean: 8.71818Std Dev: .41642
Cpk: 1.375Cp: 2.802
"A" MINI STRIP
6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18
Correlation & Distribution at 6N Intensity
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Mean
LSL USL3sp3sp
0
2
4
6
8
10
Samples: 12 Cp: 19.42 (Scale: inches x .001)Mean: 6.44167Std Dev: .051493
ALMEN "N" STRIP
1 2 3 4 5 6 7 8
Mean
LSL USL3sp3sp
02468
101214161820
Samples: 36 Cp: 18.04 (Scale: inches x .001)Mean: 2.44167Std Dev: .05542
"N" MINI STRIP
1 2 3 4 5 6 7 8
Correlation & Distribution at 12N Intensity
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Mean
LSL USL3sp3sp
0
2
4
6
8
10
Samples: 12 Cp: 11.75 (Scale: inches x .001)Mean: 12.5Std Dev: .1206
ALMEN "N" STRIP
5 6 7 8 9 10 11 12 13 14
Mean
LSL USL3sp3sp
02468
101214161820
Samples: 36 Cp: 6.652 (Scale: inches x .001)Mean: 6.325Std Dev: .21297
"N" MINI STRIP
5 6 7 8 9 10 11 12 13 14
Summary of Test Results• The Mini-strip can be used to accurately verifying the shot
peening intensity in areas that are inaccessible with standard size Almen strips and holders.
• Double sided tape will effectively hold the strip for peening.• The effective working range of the mini-strip is:
.003 - .007 in (0.08 - 0.18 mm).• The working range in terms of standard Almen strips is:
N Strip Intensity = .006 - .012 in (0.15 – 0.30 mm) A Strip Intensity = .006 - .015 in (0.15 – 0.38 mm)
Calculating a Mini-Intensity
• The arc height is not the intensity value
• A saturation curve using Mini-Almen strips is used to derive the Mini Strip peening intensityfor each set of peening parameters– The Mini-Intensity is not a true intensity– The saturation curve is developed from multiple arc
heights (data points) obtained by peening a series of Mini strips
– Exposure may be time or incremental-based
Calculating a Mini-Intensity
• Just as with Standard Almen strips…– The arc-height value on the curve that increases
by 10% when the exposure time is doubled is declared to be the intensity
– The same saturation curve solver can be used to find the Mini-Intensity
Creating a Saturation Curve using Mini Strips
• Just as with Standard Almen Strips…– A minimum of 4 data points is required to
construct a saturation curve
– All strips must have different exposure times without changing machine parameters
– Measure the arc-heights on a Mini-strip Almen Gauge
Creating a Saturation Curve using Mini Strips
• Plot data on graph and draw a fitted curveusing the data points
• This gives us an infinite number of arc-heights with corresponding exposure times to use when looking for the 10% increase in arc height for the doubling of time
Important Differences in Mini-Almen Strip Saturation Curves
• The Mini-Intensity is not a true intensity– It must be correlated to obtain the true intensity
• Mini-Almen strips will have a unique T1 time– When running a T1 validation
strip, the correct Mini StripT1 time should be used to getaccurate results
Mini Strip Correlation Chart
• Correlation between Standard and Mini Almen Strips must be established before the intensity in small and hard-to-reach locations can be determined
• Correlation is conducted on a standard Almen Block in an open environment– Testing is conducted on the component
Developing a Mini Strip Correlation Chart
1. Run a saturation curve using standard Almen strips at the upper intensity limit.
2. Run a saturation curve using Mini Almen strips (using double-sided tape or contact cement) and expose it under the same parameters for upper intensity limit.
Developing a Mini Strip Correlation Chart
3. Run a saturation curve using standard Almen strips at the lower intensity limit.
4. Run a saturation curve using Mini Almen strips (using double-sided tape or contact cement) and expose it under the same parameters for the lower intensity limit
Developing a Mini Strip Correlation Chart• Plot the standard strip intensity values along one axis and
the corresponding Mini Strip intensity values along the other axis
• A correlation between the mini-strip intensities and the full-size strip intensities has been defined
Using the Mini Strip Correlation Chart
The correlation chart provides an acceptable mini-strip intensity range for the tested true intensity values
Important to Remember During Correlation and Testing
• The adhesion method used to create the correlation between Mini strips and Standard strips must remain consistent during testing
• Due to low arc-height readings, Pre-bow measurements are required to get accurate readings with the Mini-Almen strips
• Adhesive material must be completely removedbefore measuring resultant arc-height– Avoid causing damage to the Mini-Strip by scraping or
hacking away adhesive material
Questions?