61-residual_stress_hole_drilling.pdf

Via Delle Calandre 63 50041 Calenzano (Florence) – Italy Tel: +39.055.8826-302 – Fax: -303 www.sintechnology.com [email protected]

RESIDUAL STRESS MEASUREMENTS BY HOLE DRILLING ACCORDING TO ASTM E837 STANDARD

PRESENTATION CONTENTS • What are residual stresses • Residual stress causes • RS measuring methods with strain gages • The hole-drilling strain-gage method • The standard ASTM E837 • The instrument used for the measurements • Typical application fields

• Typical measurement results

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What are residual stresses All stresses that occur in the materials, also without the application of any external load, are termed residual stresses Residual stresses influence a mechanical component’s behaviour as they:  Affect structural and dimensional stability  Reduce fatigue strength and crack resistance

 Encourage surface crack growth Residual stresses therefore limit load capacity and safety of mechanical components during operation

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Residual stress causes Residual stresses can be caused by several main factors:  Fabrication process (forging, welding)

 Machining process (turning)

 Heat treatments (quenching)

 Surface treatments (shot peening)

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Residual stress measuring methods with strain gages

Hole Drilling

Ring Coring Slitting

Sectioning / Layer removal

Sach’s boring Residual Stress Hole Drilling

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The hole-drilling strain-gage method The hole-drilling strain-gage method is the most widely used method for measuring residual stresses, for several reasons: • Very simple • Low cost of the machine (and of the test when using an automatic system) • Possibility to elaborate the results up to at least 1mm in a very fast way • High availability of strain gages and accessories for the measurement • Compliant to ASTM E837, the only complete standard on RS measurements available in the world • Availability of AUTOMATIC MACHINE with very high resolution and accuracy thanks to the complete control of all the measurement parameters

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The hole-drilling strain-gage method The hole-drilling strain-gage method consists in drilling a small hole (approx. 1.8 mm x 2.0 mm) into the centre of a 3-element strain rosette with suitable drill bits and a special air turbine (400,000 RPM) The hole changes the initial strain allowing redistribution of the

residual stresses originally existing in the material Φ 1.8 mm

2 mm

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Hole-drilling strain-gage method to ASTM E837 The hole-drilling strain-gage method is the only method for calculating residual stress that is STANDARDIZED at world level (ASTM E837) The first version of this standard dates back to 1995, the latest upgrade is available from the end of 2013

Standard ASTM E837 specifies:  The number of drilling increments required  The numerical coefficients for determining the value of residual stresses  The data processing method  The measurement-related uncertainty Residual Stress Hole Drilling

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Hole-drilling strain-gage method to ASTM E837-13 Different types of holes, based on the workpiece thickness:

THROUGH HOLE - THIN WORKPIECE Workpiece thickness < 0.2·DGAGE (std. 1mm) stresses are considered uniform over the drilling depth INTERMEDIATE HOLE Workpiece thickness between 0.2·DGAGE e DGAGE (std. between 1mm and 5mm) THROUGH HOLE

INTERMEDIATE HOLE

 Drilling depth: entire thickness  Stresses are assumed to be uniform  Acquisition of a set of 3 strain values once the through hole is completed

 Approximate results  The elaboration of the test result is outside the scope of the ASTM E837-13 standard

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Hole-drilling strain-gage method to ASTM E837 BLIND HOLE (Typical) – THICK WORKPIECE Workpiece thickness > DGAGE (std. 5mm)

UNIFORM STRESSES

NOT UNIFORM STRESSES

 Drilling depth: 0.2 · DGAGE (std 1mm)

 Drilling depth: 0.2 · DGAGE (std 1mm)

 Stress value over the drilling depth

 Residual stress pattern over the drilling depth

 10 drilling steps of 0.02 · DGAGE (std 0.1 mm)

 20 drilling steps of 0.01 · DGAGE (std 0.05 mm)  Evolution of the “old” Integral Method

A “uniform stress” calculation is appropriate when prior information is available, for example, based on workpiece geometry or processing procedure. Another purpose of doing a uniform stress calculation is to determine a representative size of the residual stresses that are present. Residual Stress Hole Drilling

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The instrument used for the measurements The MTS3000 system is the only fully AUTOMATIC and portable instrument in the world for determining residual stress by the hole-drilling strain-gage method

The MTS3000 system consists of:

 A mechanical setup housing the optical system and drilling system  An electronic control unit  A digital strain gage amplifier  Control and back-calculation software Residual Stress Hole Drilling

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The instrument: measuring chain

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The instrument: main advantages

Chief advantages in using an AUTOMATIC system:  Higher repeatability of measurement

 Shorter testing time

 Higher hole drilling accuracy

 Fully compliant to ASTM E837 standard

HIGHER PRODUCTIVITY Residual Stress Hole Drilling

REDUCTION IN COSTS 13

The instrument: fields of application The MTS3000 system can be used to determine residual stresses on a wide variety of materials, such as:    

Standard metals (e.g. Steel, Aluminum, Cast iron) Non-standard metals (e.g. Titanium, High Tensile Steels) Plastics (e.g. ABS, Polycarbonate) Composites

CHOICE OF CONFIGURATION

 Air turbine (standard)  Electric motor (plastics, composites)

CHOICE OF CUTTING AND RECORDING PARAMETERS

 Feed rate  Rotational speed  Delay time

CHOICE OF TYPE OF CUTTER

 Tungsten carbide  Tungsten carbide with coatings  Diamond

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Hole drilling: typical application fields • In 95% of the cases, the stresses are NOT uniform with the depth • It’s difficult to measure residual stresses without a suitable machine because of the lower precision of these instrument in the detection of the starting point and of the step/total depth: this situation can lead to big errors in the post-elaboration of the results • Generally, it’s difficult to predict the residual stress value of a component BUT… • …In some typical application fields, it’s possible to have typical measurement results

- Typical on-site measurements Residual Stress Hole Drilling

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Typical measurement results: Automotive • Cracked area

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Typical measurement results: Automotive • Cracked area

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Typical measurement results: Aerospace • Stress-relieved welding area

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Typical measurement results: Aerospace • Stress-relieved welding area

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Typical measurement results: Oil & Gas • Shot-peened area

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Typical measurement results: Oil & Gas • Shot-peened area

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Typical measurement results: Production control • Welding area

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Typical measurement results: Production control • Welding area

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Typical measurement results: Production control • Before & after stress-relieving heat treatment

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Typical measurement results: Production control • Before & after stress-relieving heat treatment

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Via Delle Calandre 63- 50041 Calenzano (Florence) – Italy Tel: +39.055.8826-302 – Fax: -303 www.sintechnology.com [email protected]