• Jul 1, 2016

It Can Groove, But Does It Jive?

B.A. Liechty, Research and Development Engineer
R.C. Dillion, Engineering Technician

In August of 2015 progress towards making wire with microgrooves in the guidewire size range was summarized in A New Take on Wire Geometry – Functional Grooves.  Now the focus has shifted to comparing the mechanical performance to that of solid round wire.

Five straightened 304V (ASTM A313) stainless steel wires were tested: three grooved (described by nominal OD and groove shape), and two round.  Actual OD refers to each wire’s round diameter as measured on a bench micrometer, while the Equivalent OD refers to the round wire diameter that has an equivalent cross sectional area to that of each grooved wire.

Sample Identification
Sample Equivalent OD Approx. Grooove Area
  in. %
0.0134 Deep and Narrow 0.01304 6
0.0137 Shallow and Wide 0.01334 6
0.0134 Deep and Wide 0.01270 11
0.012 Round N/A N/A
0.015 Round N/A N/A
General Groove Proportions

Thus far, much of the grooved work has been focused on spring temper 304V.  Tensile strengths higher than 325 ksi are achievable when the missing cross section due to the groove is accounted for.

Tensile Testing Results
    Computed using Actual OD Computed using Equiv. OD
Sample Elong. Tensile Str. Yield Str. Modulus Tensile Str. Yield Str. Modulus
  % ksi ksi Msi ksi ksi Msi
0.0134 Deep and Narrow 2.2 310 271 26.6 327 286 28.2
0.0137 Shallow and Wide 2.6 323 243 26.2 344 259 27.9
0.0134 Deep and Wide 2.7 293 224 24.1 328 251 27.0
0.012 SLT 2.5 337 257 28.6      
0.015 SLT 2.6 299 229 27.7      

Rotary beam fatigue testing was performed in order to see if the grooves significantly impact fatigue life.  There was a large amount of overlap between the grooved and round wire results.

Fatigue Testing Results @ 0.75% Strain

Fully reversed (R = -1) rotary beam fatigue @ 60Hz


The grooves necessarily spiral around the OD of the wire, so torsion testing was performed to see if the handedness of the groove (left hand helix vs. right hand helix) significantly impacts torque characteristics.

Torsion Testing Results
Sample Groove
Test Direction Revolutions
to Fracture
  in.     ksi ksi Msi
0.0134 Deep and Narrow 0.63 Clockwise 191 148 241 9.3
Counter Clockwise 210 121 240 8.8
0.0137 Shallow and Wide 0.50 Clockwise 255 119 235 9.1
Counter Clockwise 217 124 227 9.0
0.0134 Deep and Wide 0.47 Clockwise 249 120 240 7.9
Counter Clockwise 265 111 234 6.8
0.012 Round N/A Clockwise 274 120 177 5.5
Counter Clockwise 327 133 208 6.2
0.015 Round N/A Clockwise 200 155 235 9.7
Counter Clockwise 233 151 232 9.8

Differences in bending stiffness were anticipated, depending on how the groove was oriented relative to the point of contact with the pin the wire was being bent around.

Bend Moment Testing Results
Sample Orientation AVG Yield Load AVG Ultimate Load
    N•cm N•cm
0.0134 Deep and Narrow A 1.01 1.41
B 1.05 1.47
C 1.01 1.40
0.0137 Shallow and Wide A 1.10 1.56
B 1.16 1.63
C 1.07 1.53
0.0134 Deep and Wide A 0.89 1.25
B 0.98 1.41
C 0.88 1.23
0.012 Round N/A 0.75 1.07
0.015 Round N/A 1.45 2.06

All specimens achieved 30 bends without fracture; each result is the average of five (5) tests.  Each test was run using a 7 N•cm torque?type load cell with a 0.3 cm moment arm.  Each test specimen was subjected to 90° bends; if no fracture was detected, the test was terminated after 30 bends.  Each grooved sample was tested in three (3) orientations, such that the groove was oriented at the fulcrum as shown:

Bend Moment Testing Sample Orientation

The yellow arrow indicates the bending direction, while the red arrows/letters indicate various groove orientations:

  • Orientation A - groove facing pin at point of contact
  • Orientation B - groove facing directly away from pin at point of contact
  • Orientation C - groove facing off to the side relative to the pin at point of contact

Finally, dynamic cast resistance (DCR) testing was ran to explore the groove’s impact on the wire’s resistance to taking a permanent set when bent over a known radius under tension.

DCR Testing Results
Sample Arc Height Arc Height
  in. mm
0.0134 Deep and Narrow 0.49 12.5
0.0137 Shallow and Wide 0.83 21.0
0.0134 Deep and Wide 0.94 24.0
0.012 Round 0.48 12.1
0.015 Round 1.05 26.7


The basic DCR test method is as follows:

  1. Anchor one end of the wire.
  2. Pull the wire over a 1.38” pulley at a speed of 10”/min while a 12 ounce weight hangs from the free end of the wire.
  3. Remove the specimen from the test machine and remove the weight.
  4. With the specimen straightened, cut a 12” section from the area that has been run around the pulley.
  5. Measure the maximum height of the cast as shown below:
DCR Test Specimen Measurement

Ultimately, some mechanical performance is sacrificed by having the groove. However, the grooved samples all performed within a reasonable range that was bracketed out by the round wire test data. Future trials will try to optimize the mechanical properties and push strength levels towards 400 ksi. Stay tuned!


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Disclaimer: Our monthly highlights are sneak peeks of what our R & D department is working on. This does not mean we have what is referenced above ready for manufacturing.