EP0016205A1 - Method of determining the fatigue threshold value of a material - Google Patents

Method of determining the fatigue threshold value of a material

Info

Publication number
EP0016205A1
EP0016205A1 EP79901048A EP79901048A EP0016205A1 EP 0016205 A1 EP0016205 A1 EP 0016205A1 EP 79901048 A EP79901048 A EP 79901048A EP 79901048 A EP79901048 A EP 79901048A EP 0016205 A1 EP0016205 A1 EP 0016205A1
Authority
EP
European Patent Office
Prior art keywords
load
crack
value
test specimen
values
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP79901048A
Other languages
German (de)
French (fr)
Inventor
Leif Jilken
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
STANGAKONSULT
Original Assignee
STANGAKONSULT
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by STANGAKONSULT filed Critical STANGAKONSULT
Publication of EP0016205A1 publication Critical patent/EP0016205A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0014—Type of force applied
    • G01N2203/0016—Tensile or compressive

Definitions

  • the present invention relates to a method of determining the fatigue threshold value of a material, whereby a test specimen of the material which has a crack notch is subjected to a time dependent alternating load causing crack propagation which by a gradually decreasing load mean value is retarded and whereby it is ensured that the distance between two points, one on each side of the crack notch, alternates between two values.
  • the invention also relates to a device for carrying out this method.
  • a disadvantage of the method described in the essay is that the ratio between the minimum, P min , and the maximum, P max , values of the cyclic load variesduring the reduction, of the load, cf Fig 1a, which means that the measured threshold value will depend on said alternating ratio.
  • the object of the present invention is to remove the above men tioned disadvantage when determining the fatigue threshold value of a material. This is carried out by the method and the device accounted for in the characteristic part of the appended claims.
  • Fig 1a shows the course of a time dependent alternating load according to the initially mentioned essay.
  • Fig 1b shows the output signal from a positioning gauge by which is detected the variation of the distance between two points one on each side of a crack notch (CGD) in a fatigue test specimen.
  • Fig 1c depicts a time dependent alternating load according to the present invention.
  • Fig 2 shows one embodiment of a conventional test specimen used in fatigue testing.
  • Fig 3 shows schematically an arrangement for fatigue testing of a test specimen according to Fig 2.
  • a test specimen according to Fig 2 is subjected to a time dependent alternating load P.
  • the test specimen has a crack notch 1 which propagates on condition that the alternating load is of sufficient magnitude.
  • the load P will decrease as shown in Fig 1a.
  • the ratio P min /P max in the following designated as the R-value, alternates during the test as a conse ⁇ uence of the constant value of P min . This means that the threshold value will be determined for a final value of R which can not be predicted at the start of the fatigue test.
  • the numeral 4 is a loading device by which a test specimen 5, of the same kind as the one shown in Fig 2, clamped in the loading device, is subjected to a time dependent alternating load P.
  • the loading device 4 contains a load frame 7 and two jaws 8, 9 for clamping of the test specimen 5.
  • the upper jaw 8 is via a load transducer 10 connected to a spindle 11 threaded into the load frame.
  • the spindle 11 has a hand wheel 12 by which the distance between the jaws can be adjusted.
  • the lower jaw 9 is connected to item 13 which is slidably mounted in the load frame 7 and operatively connected 'to a hydraulic cylinder unit 14, shown dashed in the figure where as well the piston-rod 15 of the unit is shown.
  • the control system 6 contains two calculation loops, one of which has the task of producing a signal corresponding to the instantaneous P min -value of the load.
  • the other is arranged to produce a control signal for controlling the hydraulic cylinder unit 14 with regard to the instantaneous P max - and P min -values of the load.
  • the first mentioned calculation loop consists of a calculation unit 16 connected to a manually adjustable set point unit 17 for setting a predetermined R-value, and the previously mentioned load transducer 10.
  • the other calculation loop consists of a control unit 18 connected to the hydraulic cylinder unit 14.
  • To the control unit are connected the computation unit 16, a set point unit 19 for manual adjustment of P max and a positioning gauge 20 arranged to the test specimen.
  • the positioning gauge 20 may be optical or mechanical and serves to produce a signal corresponding to the change in the distance (CGD) between the points 2, 3 in Fig 2.
  • the set point units 16 and 19 are manually preset to given values of E and P max respectively.
  • the signals from the load transducer are manually preset to given values of E and P max respectively.
  • the control unit 18 produces a signal which causes the hydraulic cylinder unit 14 to subject the test specimen to a load which changes according to the control signal .
  • the ratio between the minimum and maximum value of the load will thus be kept constant and equal to the preset R-value.
  • the maximum and minimum values of the load are made to assume such values that the distance (CGD) between the points 2, 3 is kept constant in spite of the crack propagation.

Landscapes

  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
  • Burglar Alarm Systems (AREA)
  • Metal Rolling (AREA)

Abstract

Lorsqu'on determine la valeur du seuil de fatigue d'un materiau, un echantillon d'essai (5) fabrique a partir de ce materiau est soumis a une charge (P) alternativement dependante du temps. L'echantillon presente une amorce de crique et la valeur maximale de la charge est choisie afin que la crique se propage. En diminuant graduellement la valeur maximale, la propagation de la crique est retardee et s'arretera a une certaine valeur de la charge. L'ouverture de la crique augmente successivement au fur et a mesure que cette crique se propage. Ainsi, il est possible de diminuer graduellement l'etendue de la charge pendant la propagation de la crique si, en meme temps, la variation de l'ouverture de la crique est gardee constante. Afin d'obtenir une valeur reproductible du seuil de fatigue, on maintient constant le rapport entre les valeurs minimale et maximale de la charge alternative. Pour chaque valeur choisie pour ce rapport, la valeur minimale de la charge decroitra ainsi comme la valeur maximale.When determining the value of the fatigue threshold of a material, a test sample (5) made from this material is subjected to a load (P) alternately dependent on time. The sample presents a crack initiation and the maximum value of the load is chosen so that the crack propagates. By gradually decreasing the maximum value, the propagation of the crack is delayed and will stop at a certain value of the load. The opening of the crack increases successively as this crack spreads. Thus, it is possible to gradually decrease the extent of the load during the propagation of the crack if, at the same time, the variation in the opening of the crack is kept constant. In order to obtain a reproducible value of the fatigue threshold, the ratio between the minimum and maximum values of the alternative load is kept constant. For each value chosen for this report, the minimum value of the load will decrease as the maximum value.

Description

Method of determining the fatigue threshold value of a material
The present invention relates to a method of determining the fatigue threshold value of a material, whereby a test specimen of the material which has a crack notch is subjected to a time dependent alternating load causing crack propagation which by a gradually decreasing load mean value is retarded and whereby it is ensured that the distance between two points, one on each side of the crack notch, alternates between two values. The invention also relates to a device for carrying out this method.
Concepts relating to fracture mechanics are gradually receiving increasing attention in analysis and design of how constructions should be shaped to preclude the initiation of cracks, or should cracks have originated, to prevent propagation of the same due to fatigue. A quantity of considerable importance in the mathematical formulation of the growth of a crack is the so called stress intensity factor. This quantity is further described in an essay titled "System for Determining the Critical Range of Stress Intensity Factor Necessary for Fatigue-Crack Propagation" by K Jerram and E K Priddle. The essay was published in Journal of Mechanical Engineering Science, Volume 15 , number 4, 1973, pp 271-273. In the essay is also described a method and a system for automatic gradual reduction of a cyclic load on a test specimen until a fatigue crack stops growing, by which the threshold value of the range of the stress intensity factor can be determined.
A disadvantage of the method described in the essay is that the ratio between the minimum, Pmin, and the maximum, Pmax, values of the cyclic load variesduring the reduction, of the load, cf Fig 1a, which means that the measured threshold value will depend on said alternating ratio.
The object of the present invention is to remove the above men tioned disadvantage when determining the fatigue threshold value of a material. This is carried out by the method and the device accounted for in the characteristic part of the appended claims. The invention will be further explained below with reference to the appended drawing on which the previously mentioned Fig 1a shows the course of a time dependent alternating load according to the initially mentioned essay. Fig 1b shows the output signal from a positioning gauge by which is detected the variation of the distance between two points one on each side of a crack notch (CGD) in a fatigue test specimen. Fig 1c depicts a time dependent alternating load according to the present invention. Fig 2 shows one embodiment of a conventional test specimen used in fatigue testing. Fig 3 shows schematically an arrangement for fatigue testing of a test specimen according to Fig 2.
In conventional fatigue tests a test specimen according to Fig 2 is subjected to a time dependent alternating load P. The test specimen has a crack notch 1 which propagates on condition that the alternating load is of sufficient magnitude. One can define two points 2, 3 on the test specimen positioned one on each side of the crack notch 1. During the test one tries to let the distance (COD) between the points 2 and 3 alternate between two values under the influence of the load variation, see Fig 1b. As a consequence of this variation and the propagation of the crack, the load P will decrease as shown in Fig 1a. The ratio Pmin/Pmax, in the following designated as the R-value, alternates during the test as a conseαuence of the constant value of Pmin. This means that the threshold value will be determined for a final value of R which can not be predicted at the start of the fatigue test.
According to the invention, one makes sure that the R-value is constant during the fatigue test.
This is effected by selecting an R-value and by choosing the maxi mum value, Pmax, of the load P in such a way that propagation occurs. It follows that Pmin is then also determined. As before the distance between tha points 2, 3 is made to alternate between two values as shown in Fig 1b.
In Fig 3, the numeral 4 is a loading device by which a test specimen 5, of the same kind as the one shown in Fig 2, clamped in the loading device, is subjected to a time dependent alternating load P. A control system 6, accounted for below, controls the load causing it to change with time as shown in Fig 1c.
The loading device 4 contains a load frame 7 and two jaws 8, 9 for clamping of the test specimen 5. The upper jaw 8 is via a load transducer 10 connected to a spindle 11 threaded into the load frame. The spindle 11 has a hand wheel 12 by which the distance between the jaws can be adjusted. The lower jaw 9 is connected to item 13 which is slidably mounted in the load frame 7 and operatively connected 'to a hydraulic cylinder unit 14, shown dashed in the figure where as well the piston-rod 15 of the unit is shown.
The control system 6 contains two calculation loops, one of which has the task of producing a signal corresponding to the instantaneous Pmin-value of the load. The other is arranged to produce a control signal for controlling the hydraulic cylinder unit 14 with regard to the instantaneous Pmax- and Pmin-values of the load. The first mentioned calculation loop consists of a calculation unit 16 connected to a manually adjustable set point unit 17 for setting a predetermined R-value, and the previously mentioned load transducer 10. The other calculation loop consists of a control unit 18 connected to the hydraulic cylinder unit 14. To the control unit are connected the computation unit 16, a set point unit 19 for manual adjustment of Pmax and a positioning gauge 20 arranged to the test specimen. The positioning gauge 20 may be optical or mechanical and serves to produce a signal corresponding to the change in the distance (CGD) between the points 2, 3 in Fig 2.
When determining the fatigue threshold value applicable to the material of the test specimen 5 the following procedure is used.
The set point units 16 and 19 are manually preset to given values of E and Pmax respectively. The signals from the load transducer
10 and the set point unit 17 are processed in the calculation unit 16 and a value Pmin corresponding to the preset value Pmax is received. With the aid of the output signals from units 16 and 20 as well as an exterior control signal suggested in Fig 3 as arrow 21, which for instance can be a sinus-, square- or saw-tooth signal, the control unit 18 produces a signal which causes the hydraulic cylinder unit 14 to subject the test specimen to a load which changes according to the control signal. The ratio between the minimum and maximum value of the load will thus be kept constant and equal to the preset R-value. The maximum and minimum values of the load are made to assume such values that the distance (CGD) between the points 2, 3 is kept constant in spite of the crack propagation.
The invention must not be considered to be limited to the above described specific application. It has a number of applications which all fall within the scope of the invention. It should be obvious that such conditions which must normally be satisfied in similar determinations of threshold values, for instance the condition of constant temperature, must be satisfied as well when determining the fatigue threshold value according to the present invention.

Claims

Claims
1. Method of determining the fatigue threshold value of a material whereby a test specimen (5) of the material which has a crack notch (1), is subjected to a time dependent alternating load (P) causing crack propagation which by a gradually decrea sing load mean value is retarded and whereby it is ensured that the distance between two points (2, 3) one on each side of the crack notch (l), alternates between two values, c h a r a c t e r i z e d in that the ratio (R) between minimum and maximum values of the load is kept constant during the determina tion.
2. Device for carrying out the method according to patent claim 1 comprising a loading device (4), in which a test specimen (5) having a crack notch (1) is clamped, said device being arranged to subject the specimen (5) to a load (P), a positioning gauge (20) arranged to produce a signal which unambiguously corresponds to the distance between two points (2, 3) one on each side of the crack notch (1), means (18) arranged to control the load according to the said signal in such away that this load, starting from an initial value sufficient to cause crack propagation, obtains a gradually decreasing mean value which in the test specimen (5) gives a decelerated and finally ceasing crack propagation, and in such a way that the distance between the points (2, 3) changes between two values, c h a r a c t e r i z e d in that the control means (18) is arranged to control the loading device (4) in such a way that the ratio (R) between the minimum and maximum values of the load is kept constant during the determination.
EP79901048A 1978-08-30 1980-03-25 Method of determining the fatigue threshold value of a material Withdrawn EP0016205A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE7809119A SE413438B (en) 1978-08-30 1978-08-30 SET AND APPARATUS FOR EXECUTING THE SET TO DETERMINE THE MINIMUM LOAD SCOPE OF A MATERIAL FOR CRACK GROWTH, IT WILL SAY IT THROUGH TIRE
SE7809119 1978-08-30

Publications (1)

Publication Number Publication Date
EP0016205A1 true EP0016205A1 (en) 1980-10-01

Family

ID=20335687

Family Applications (1)

Application Number Title Priority Date Filing Date
EP79901048A Withdrawn EP0016205A1 (en) 1978-08-30 1980-03-25 Method of determining the fatigue threshold value of a material

Country Status (6)

Country Link
EP (1) EP0016205A1 (en)
DE (1) DE2953044T1 (en)
GB (1) GB2050630B (en)
NO (1) NO144940C (en)
SE (1) SE413438B (en)
WO (1) WO1980000493A1 (en)

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3314181A1 (en) * 1983-04-19 1984-10-25 Kraftwerk Union AG, 4330 Mülheim METHOD FOR MONITORING THE FATIGUE OF COMPONENTS, e.g. IN NUCLEAR POWER PLANTS
JP2952594B1 (en) * 1998-09-28 1999-09-27 株式会社ビーエムシー Sensor for detecting fatigue damage of structural material and method of mounting the same
CN103743636B (en) * 2014-01-16 2015-11-18 清华大学 A kind of method predicting welding joint threshold in fatigue crack propagation

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4003246A (en) * 1975-08-08 1977-01-18 Mts Systems Corporation Specimen crack stress intensity control loop for test device
US3983745A (en) * 1975-08-08 1976-10-05 Mts Systems Corporation Test specimen crack correlator

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO8000493A1 *

Also Published As

Publication number Publication date
WO1980000493A1 (en) 1980-03-20
DE2953044C2 (en) 1987-09-17
GB2050630B (en) 1983-02-09
NO792806L (en) 1980-03-03
SE413438B (en) 1980-05-27
NO144940C (en) 1981-12-09
SE7809119L (en) 1980-03-01
DE2953044T1 (en) 1980-12-18
GB2050630A (en) 1981-01-07
NO144940B (en) 1981-08-31

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Inventor name: JILKEN, LEIF