ISO 7257:2016

DRAFT INTERNATIONAL STANDARD
ISO/DIS 7257
ISO/TC 20/SC 10 Secretariat: DIN
Voting begins on: Voting terminates on:
2014-07-28 2014-10-28
Aircraft — Hydraulic tubing joints and fittings — Rotary
flexure test
Aéronautique — Joints et raccords pour tubes hydrauliques — Essai de flexion rotative
ICS: 49.080
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ISO/DIS 7257:2014(E)
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NOTIFICATION OF ANY RELEVANT PATENT
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©
PROVIDE SUPPORTING DOCUMENTATION. ISO 2014

ISO/DIS 7257:2014(E)
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ii © ISO 2014 – All rights reserved

ISO/DIS 7257:2014(E)
Contents  Page
Foreword .iv
Introduction .v
1  Scope . 1
2  Normative references . 1
3  Requirements . 1
3.1 Flexure test device . 1
3.2 Flexure test specimen . 1
3.3 Specimen length and deflection requirements . 6
3.4 Method of classification of fittings according to S/N flexure performance . 6
3.5 Method of determining deflection/fatigue strength . 6
4  Procedure. 7
4.1 Preparation for test . 7
4.2 S/N testing . 8
4.3 Deflection/fatigue testing . 9
5 Formula . 9
6  Equipment . 9
ISO/DIS 7257:2014(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International
Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies
casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 7257 was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 10, Aerospace fluid systems and components.
This second edition cancels and replaces the first edition (ISO 7257:1983). The second edition adds
imperial unit equivalents, additional clarifications and three DN sizes.
iv © ISO 2014 – All rights reserved

ISO/DIS 7257:2014(E)
Introduction
This International Standard describes a flexure fatigue test procedure which allows evaluation of
various tube fitting designs or material combinations. This evaluation is performed by fatigue testing
the tube joints over a spectrum of bending stresses and then plotting the cycles to failure. This test
procedure is intended for comparative evaluation of fatigue characteristics only, the qualification test
procedure for tube fittings being specified in ISO 7169. Other test methods may be used as long as they
develop the same data as the rotary flexure test.
DRAFT INTERNATIONAL STANDARD ISO/DIS 7257:2014(E)
Aircraft — Hydraulic tubing joints and fittings — Rotary
flexure test
1  Scope
This International Standard specifies flexure test procedure to determine and classify the fatigue
strengths of reconnectable or permanent hydraulic tube joints.
The procedure is intended for conducting flexure tests of fittings and joints with high strength hydraulic
tubes of various alloys such as corrosion resistant steel, Nimonic, titanium and aluminium alloy hydraulic
tube for use on commercial and military aircraft.
A mean stress is applied by holding system pressure in the specimens and then flexing in a rotary
bending test machine.
2  Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 2964, Aerospace — Tubing — Outside diameters and thicknesses — Metric dimensions
ISO 7169, Aerospace — Separable tube fittings for fluid systems, for 24 degree cones, for pressures up to
3 000 psi or 21 000 kPa — Procurement specification, inch/metric
3  Requirements
3.1  Flexure test device
The test device should be capable of testing in-line or bulkhead union test specimens and other
configurations such as elbows and tees.
The rotary flexure test device should be similar to that shown in Figure 1. Each rotary flexure test
device should be capable of testing one specimen, but several specimens may be mounted on one plate.
The device should be capable of constantly maintaining the required operating pressure during the test.
The test fluid shall be water or system fluid (working fluid) unless otherwise specified by the responsible
authorities. A typical pressurization and automatic shutdown system is shown in Figure 2. The shut-
down should be automatic in the event of failure or pressure drop. The device should be capable of
testing at controlled constant temperature, if specified by the procuring agency. The tailstock of the test
device should be designed to permit alignment during initial installation and specimen mounting, and
to serve as a pressure manifold. The rotating headstock should have a low-friction, self-aligning bearing
and should be designed to permit total deflections of up to 25 mm/1 inch, and a constant rotational
frequency within the range of 1 500 to 3 600 min−1. The base should be of rigid construction.
3.2  Flexure test specimen
The test specimen should be consist of an adapter fitting (headstock end), a section of straight tubing,
and a test fitting at the tailstock end. Typical test specimens are shown in Figure 3. The tubing shall be
of a size and wall thickness as specified by the user or procuring agency.
ISO/DIS 7257:2014(E)
Key
1 Adjustment for deflection 8 Adapter fitting
2 Self-aligning bearing (without torsion on test joint) 9 10 mm/0,400 inch max
3 Test length, L 10 (5 ± 1) mm/(0,200 ± 0,040) inch
4 Tailstock 11 Hydraulic pressure
5 Tailstock alignment bolt 12 Deflection D
6 Dial indicator, alternate for strain gauge (horizontal 13 Strain gauges
and vertical adjustment)
7 Headstock 14 Test jo
...