Embed Size (px)
DESCRIPTION
Residual stresses influence the behavior of mechanical components, plastic materials, composite materials and can modify the structural, the dimensional stability, the fatigue and the fracture resistance of components. Residual stresses can be caused by the following main factors: non-uniform heating or cooling of a component during manufacturing and fabricating processes (e.g., casting, welding, molding, heat treatment) machining processes to remove shavings or plastic deformation (e.g., turning and forging) through or surface heat treatments (e.g., tempering, welding or grinding) sand-blasting or shot-peening. The hole-drilling method allows accurate experimental stress analyses at moderate costs by drilling a small hole (typically 1.8 – 2.0 mm) which changes the initial deformation allowing redistribution of the residual stresses locked in a material.
Citation preview
Via Delle Calandre 63
50041 Calenzano (Florence) – Italy
Tel: +39.055.8826-302 – Fax: -303
www.sintechnology.com
RESIDUAL STRESS MEASUREMENTS BY HOLE
DRILLING ACCORDING TO ASTM E837 STANDARD
• 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
2 Residual Stress Hole Drilling
PRESENTATION CONTENTS
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
3
What are residual stresses
Residual stresses therefore limit load
capacity and safety of mechanical
components during operation
Residual Stress Hole Drilling
4
Residual stress causes
Machining process (turning)
Heat treatments (quenching) Surface treatments (shot peening)
Fabrication process (forging, welding)
Residual stresses can be caused by several main factors:
Residual Stress Hole Drilling
5
Residual stress measuring methods with strain gages
Residual Stress Hole Drilling
Hole
Drilling
Slitting
Sectioning /
Layer removal
Ring Coring
Sach’s boring
The hole-drilling strain-gage method is the most widely used method for
measuring residual stresses, for several reasons:
6
The hole-drilling strain-gage method
• 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
Residual Stress Hole Drilling
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)
7
The hole changes the initial strain allowing redistribution of the
residual stresses originally existing in the material
Φ 1.8
mm
2 mm
The hole-drilling strain-gage method
Residual Stress Hole Drilling
8
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
Hole-drilling strain-gage method to ASTM E837
Residual Stress Hole Drilling
THROUGH 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
9
INTERMEDIATE HOLE
Approximate results
The elaboration of the test result is
outside the scope of the ASTM E837-13
standard
THROUGH HOLE - THIN WORKPIECE
Workpiece thickness < 0.2·DGAGE (std. 1mm)
stresses are considered uniform over
the drilling depth
Hole-drilling strain-gage method to ASTM E837-13
Different types of holes, based on the workpiece thickness:
Residual Stress Hole Drilling
INTERMEDIATE HOLE
Workpiece thickness between 0.2·DGAGE e DGAGE (std. between 1mm and 5mm)
UNIFORM STRESSES
Drilling depth: 0.2 · DGAGE (std 1mm)
Stress value over the drilling depth
10 drilling steps of 0.02 · DGAGE (std
0.1 mm)
10
NOT UNIFORM STRESSES
Drilling depth: 0.2 · DGAGE (std 1mm)
Residual stress pattern over the drilling depth
20 drilling steps of 0.01 · DGAGE (std 0.05 mm)
Evolution of the “old” Integral Method
BLIND HOLE (Typical) – THICK WORKPIECE
Workpiece thickness > DGAGE (std. 5mm)
Hole-drilling strain-gage method to ASTM E837
Residual Stress Hole Drilling
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.
11
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
The instrument used for the measurements
12
The instrument: measuring chain
Residual Stress Hole Drilling
13
Higher repeatability of measurement
Higher hole drilling accuracy
Shorter testing time
Fully compliant to ASTM E837 standard
HIGHER PRODUCTIVITY REDUCTION IN COSTS
The instrument: main advantages
Chief advantages in using
an AUTOMATIC system:
Residual Stress Hole Drilling
14
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
CHOICE OF CUTTING AND
RECORDING PARAMETERS
CHOICE OF TYPE OF CUTTER
Air turbine (standard)
Electric motor (plastics, composites)
Feed rate
Rotational speed
Delay time
Tungsten carbide
Tungsten carbide with coatings
Diamond
The instrument: fields of application
Residual Stress Hole Drilling
15
Hole drilling: typical application fields
Residual Stress Hole Drilling
• In 95% of the cases, the stresses are NOT uniform with the depth
• 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 -
• 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
16
Typical measurement results: Automotive
Residual Stress Hole Drilling
• Cracked area
17
Typical measurement results: Automotive
Residual Stress Hole Drilling
• Cracked area
18
Typical measurement results: Aerospace
Residual Stress Hole Drilling
• Stress-relieved welding area
19
Typical measurement results: Aerospace
Residual Stress Hole Drilling
• Stress-relieved welding area
20
Typical measurement results: Oil & Gas
Residual Stress Hole Drilling
• Shot-peened area
21
Typical measurement results: Oil & Gas
Residual Stress Hole Drilling
• Shot-peened area
22
Typical measurement results: Production control
Residual Stress Hole Drilling
• Welding area
23
Typical measurement results: Production control
Residual Stress Hole Drilling
• Welding area
24
Typical measurement results: Production control
Residual Stress Hole Drilling
• Before & after stress-relieving heat treatment
25
Typical measurement results: Production control
Residual Stress Hole Drilling
• Before & after stress-relieving heat treatment
Via Delle Calandre 63- 50041 Calenzano (Florence) – Italy
Tel: +39.055.8826-302 – Fax: -303
www.sintechnology.com
