PV_3973_EN

Issue 2010-11 Class. No.:

55161

Descrip Descriptor tors: s:

elasto elastomer mer,, O-rin O-ring, g, sealing sealing ring, ring, tensile tensile strengt strength, h, elonga elongatio tion n at at tear tear,, stre stress ss valu value, e, O-ring O-ring

Determining Tensile Strength, Elongation at Tear and Stress Values in the Tensile Test

PV 3973: 2010-09

The following changes have been made compared with PV 3973: 2010-09:  –

Typographical Typographical mistake in German German version corrected corrected

This Testing Specification (PV) serves to determine the t ensile strength, elongation at tear, as well as the stress values of elastomer o-rings subjected to deformation at constant speed.

The following definitions apply when using this standard:

The tensile strength σmax (N/mm2) is the quotient of the maximum measured force Fmax and the initial cross section A0.

Page 1 of 6 Technical responsibility GQL-LP/4

Peter Schulz

GQL-LP

Dr. Roger Hillert

Standards Department Tel.: +49 5361 9-24365

EKDV/4 Ute Hager-Süß

EKDV

Tel.: +49 53 5361 99-49035

Manfred Terlinden

VWNORM-2010-08e

Page 2 PV 3973: 2010-11

The elongation at tear εR is the quotient of change in length measured at the moment the specimen tears LR - L0 and the initial length of the specimen L 0. For ring-shaped specimens, L0 is the inner  perimeter of the ring.

The stress value σi (modulus) is the quotient of the tensile force F i at hand when a specific elongation is reached and the initial cross section A0.

Description example for tensile strength: 2

Description example for elongation at tear:

 A thickness gage acc. to DIN ISO 23529, method A, must be used (e.g., thickness gage acc. to PV 3330). The national preface of DIN ISO 23529 must be taken into consideration in this context.

 A measuring device acc. to DIN ISO 23529, method D, must be used. The national preface of  DIN ISO 23529 must be taken into consideration in this context. A multi-sensor coordinate measuring device with an accuracy of ± 5 µm (e.g., Scope-Check® by Werth) may also be used to measure the inner diameter of the o-rings.

The force indicator range of the tensile test machine (e.g., by Zwick) must correspond to class 1 acc. to DIN EN ISO 7500-1. The tensile test machine must be capable of recording stress-strain diagrams or force-length change curves. It must be possible to move the test mandrels into a position in which they are close enough to allow the rings to be placed on them without being stretched.

Table 1 lists the permissible combinations of o-rings and test mandrels. Combinations deviating from the ones given below may be used in justified cases. This must be noted in the test reports.

Page 3 PV 3973: 2010-11

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Silicone oil, e.g., Wacker® AP 150, is to be used for lubrication.

This Testing Specification refers exclusively to elastomer o-rings.

 According to Technical Supply Specification (TL), but at least n = 3.

Friction is a variable of unknown quantity with a direct, yet uncontrollable, influence on the accuracy of the test results. This disadvantage can, in general, be completely avoided by using a lubricant (silicon oil, see Section 4.1.5) applied to the entire surface of both the o-ring and the test mandrels prior to the test.

The tensile test is to be conducted at room temperature (23 ± 2 °C). The lubricated o-ring is first placed onto the lubricated test mandrels without being stretched (Figure 1). The test mandrels must be mounted to the traverse beam such that the o-ring is subjected to a load acting parallel to the tensile axis.

Page 4 PV 3973: 2010-11

The inner diameter and the thickness of the o-rings must be specified in the tensile test program. The distance of the mandrel axes is used in the tensile test performed on ring-shaped specimens to measure the elongation (Figure 1). The required preparatory distance L V (= mandrel distance at which both side portions of the o-ring are parallel) is calculated using equation (1). The speed at which the mandrels are moved to said distance is 50 mm/min. The subsequent tensile test is conducted at a traverse speed of 200 mm/min. Once the preparatory mandrel position has been reached, the tensile test machine will automatically start the test using the aforementioned traverse speed. The data recorded during the test are transferred online to a load-displacement curve. LV =

π  (di - dR) 2

(1)

di = Inner o-ring diameter in mm dR = Test mandrel diameter in mm

Since, in tensile tests performed on o-rings, two portions of the specimen are simultaneously sub‐  jected to a tensile load, the tensile strength is calculated using equation (2): σmax = σmax = Tensile strength in N/mm2  A0 = Cross section area in mm 2

Fmax 2 A0

(2)

Page 5 PV 3973: 2010-11

Fmax = Maximum force in N The cross section area A0 is calculated using equation (3).  A0 =

π  × ds2 4

(3)

ds = Ring thickness in mm

The following applies to the elongation at tear: εR =

LR - L0 L0

 × 100

(4)

εR = Elongation at tear in % LR = Length of o-ring at the moment it tears in mm L0 = Initial o-ring length in mm The initial o-ring length L 0 is calculated using equation (5). L0 = 2π ×

di

(5)

2

di = Ring thickness in mm The length of the o-ring at the moment of tearing LR is calculated via the distance of the test mandrels at the moment of tearing DR using equation (6). LR = 2 × DR + U

(6)

DR = Distance of the test mandrels at the moment of tearing U = Mandrel perimeter 

The following must be specified in the test report: minimum value, maximum value, average value or  median. For specimen numbers n ≥ 5, the standard deviation as well as the confidence interval of  the average value including the significance level must be specified as well The following applies to the specification of the characteristic values determined:  –

Tensile strength σmax in MPa or N/mm2 rounded to 0,1 MPa or 0,1 N/mm2

 –

Elongation at tear εR in % rounded to whole numbers

 –

Stress value σi in MPa or N/mm2 rounded to 0,1 MPa or 0,1 N/mm2

The following must be specified as well:

Page 6 PV 3973: 2010-11

 –

Deviations from the specifications set forth in this Testing Specification

 –

Date and name (generally included in the EDP measuring report of the tensile test machine)

 –

Diameter of the test mandrel used

NOTE 1 The dimension of the specimens must refer to the condition prior to aging. A reference sample may be used to determine the thickness of the o-rings. The arithmetic mean calculated from six individual thickness values determined on different sections of the o-ring is to be entered into the computer. This must be noted accordingly in the test reports. The inner diameter of the o-ring, which is required for calculating the elongation at tear, is not to be determined until after aging in media. Tests on o-rings with smaller diameters (< 6 mm) are also permissible, depending on the laboratory equipment available. This must be agreed upon and noted accordingly. NOTE 2 The determination of the stress values is based on the mean o-ring perimeter.

The following documents cited in this Standard are necessary to its application. Some of the cited documents are translations from the German original. The translations of German terms in such documents may differ from those used in this Standard, resulting in terminological inconsistency. Standards whose titles are given in German may be available only in German. Editions in other  languages may be available from the institution issuing the standard. PV 3330

Elastomer O-Ring Seals; Compression Set (Permanent Deformation)

DIN EN ISO 7500-1

Metallic materials - Verification of static uniaxial testing machines - Part 1: Tension/compression testing machines - Verification and calibration of the force-measuring system

DIN ISO 23529

Rubber - General procedures for preparing and conditioning test pieces for physical test methods