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ON THE USE OF OPTO-ELECTRONIC
COMPONENTS FOR THE REGISTRATION OF CRACK TIP BEHAVIOUR UNDER DYNAMICAL
LOADING CONDITIONS
K. Kussmaul, C. Zimmermann, T. Demler, D. Kraemer
To cite this version:
K. Kussmaul, C. Zimmermann, T. Demler, D. Kraemer. ON THE USE OF OPTO-ELECTRONIC COMPONENTS FOR THE REGISTRATION OF CRACK TIP BEHAVIOUR UNDER DYNAMI- CAL LOADING CONDITIONS. Journal de Physique Colloques, 1985, 46 (C5), pp.C5-219-C5-225.
�10.1051/jphyscol:1985528�. �jpa-00224758�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au nos, Tome 46, aoQt 1985 page C5-219
ON THE USE OF OPTO-ELECTRONIC COMPONENTS FOR THE REGISTRATION OF CRACK TIP BEHAVIOUR UNDER DYNAMICAL LOADING CONDITIONS
K. Kussmaul, C. Z i m r m a n n , T. Demler and D. Kraemer
StaatZiche MateriaZpriifungsmstalt (MPAI, U n i v e r s i t a t S t u t t g a r t , PfaffenwaZdring 3 2 , 7000 S t u t t g a r t 80, F.R.G.
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La c o n c e p t i o n d ' u n e m a c h i n e d ' i m p a c t u t i l i s a n t u n g r a n d d i s q u e t o u r n a n t , a n 6 - c e s s i t 6 l e d Q v e l o p p e r n e n t d e n o u v e l l e s t e c h n i q u e s p o u r m e s u r e r l e s c o n d i t i o n s d e c h a r g e r n e n t d y n a m i q u e . C e l l e s - c i c o r n p r e n n e n t l ' e n r e g i s t r e r n e n t d e s d Q f o r m a t i o n s p r 6 s d e l a p o i n t e d e l a f i s s u r e p a r u n e cam&-a B c o n v e r s i o n d ' i r n a g e s u l t r a - r a - p i d e e t l ' u t i l i s a t i o n d e f i b r e s c o u p l B e s 2 d e s d i o d e s 2 l a s e r p o u r l a m e s u r e d e 1 ' Q c a r t e m e n t d e f i s s u r e e t d u b a i l l e r n e n t d e s l k v r e s d e l ' e n t a i l l e .
Abstract
The design o f a large rotating disk impact machine induced the development o f various new measurement techniques for dynamic loading conditions. This comprises the registration o f the m a t e r i a l deformation near the crack t i p by an ultrafast image converter camera and the application o f glass f i b r e coupled laser diodes f o r COD and CTOD acquisition.
1. Introduction
The dynamical loading o f large specimens a t constant loading rates is only possible w i t h a testing f a c i l i t y o f appropriate energy storage capacity and stiffness. To this end, a "rotating disk impact machine" consisting o f a 6,5 to. fly-wheel o f 2 m diameter and capable o f 1500 r p m has been installed a t the MPA Stuttgart, cf. /l/.
The 33 M J o f rotational energy which the fly-wheel is able t o store, enable measurements i n dynamic testing which were hitherto n o t possible. The same capacity, however, asks f o r multiple redundant EMP-free steering and tracking sensorics f o r a safe handling o f the machine. This contribution deals w i t h some basic research on dynamic fracture mechanics performed w i t h the aid o f this machine, as w e l l as w i t h several opto-electronic sensors f o r acquisition o f fracture mechanic parameters having emerged f r o m the machine-control sensorics.
2. Basic research: the near-field o f dynamically loaded crack tips
Usually, c r i t i c a l i n i t i a t i o n values l i k e K or JId under dynamic loading conditions are determined by using the standard procedudre f o r s t a t i c loading conditions. However, the a t t r i b u t e o f path-independency o f the J-integral, f o r instance, which makes i t a m a t e r i a l parameter o f static fracture mechanics, has not yet been proved experimentally f o r the case o f dynamical crack t i p loading.
To this end micro-lattices are etched into the surface o f specimen t o be loaded and fotografed during the impact event via a specially designed microscope and an ultra- fast image converter camera. The l a t t i c e w i t h a grid-constant o f 50 p m t o 200 p m is situated r i g h t around the t i p o f the starter crack, thus the crack-tip nearfield can be observed. The. dynamic testing machine, a 2 m r o t a t i n g disk w i t h a double striking nose fixed t o i t s circumference allows the loading o f DENT-specimen or CT 15 speci- men as specified i n Fiq. 1 w i t h loading velocities up t o 150 m/s. The movement o f the specimen (which sticks t o the lower end o f a pendulum) i n t o the course o f the r o t a t i n g
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985528
JOURNAL DE PHYSIQUE
nose is computer controlled. A t t h e s a m e instant t h e c a m e r a and t h e necessary flash units a r e triggered, Fiq. 2. Up t o 1 0 s t r a i n gauges may b e applied additionally, t h e signal of which is recorded by 1 MHz, 1 2 Bit-transient recorders. A f t e r t h e a c t u a l t e s t t h e fotografed consecutive l a t t i c e deformation is fed into a computer digitally by a 1 3 Bit-X-y-Scanner and compared with a 2D-dynamic linear elastic FD-simulation of t h e specimen loading. Fiq. 3 shows a framing sequence a s gained on a DENT-specimen having been pulled a t 16,3 m/s. Exposure t i m e per f r a m e was 2 PS, t i m e between t h e d i f f e r e n t f r a m e s 40 ps, pictured a r e a measures 1,l X 1,3 mm . The appropriate s t r e s s distribution a s calculated f o r this t i m e interval is given in Fig. 4.
Work is under way t o use both t h e measured and t h e calculated deformation field f o r testing t h e constancy of J along random paths around t h e crack tip.
3. Opto-electronic components used for d a t a acquisition in dynamic testing
In connection with t h e installation of t h e "rotating disk impact machine" new EMP- resistant fibre sensorics have been developed. From these a glass-fibre CTOD-detector, a self-calibrating glass-fibre COD-meter, and a f a r range IR-Laser COD-meter emerged. All t h r e e gauges have a frequency response of typically b e t t e r than 1 0 MHz and a local resolution of b e t t e r than 1 . . . 1 0 p m within a range of 1,5 . . . 1 0 mm.
Temperature range is -50 'C t o +360 'C.
The CTOD-meter, fiq. 5 , consists essentially of two glass-fibres introduced into t h e specimen t o b e tested vla a drilling hole of 0,9 mm in t h e vicinity of t h e s t a r t e r fatigue crack. The mutual fibre ends a r e then fixed t o t h e crack flanks. As t h e crack opens, t h e distance between t h e two fibres grows accordingly, thus a light beam transmitted by t h e fibres is attenuated. This attenuation is evaluated by some proper nonlinear electronics. As is shown in Fiq. 6, different from t h e parabolic J-COD relation usually found, the new measuring technique yields a linear relation between J and CTOD, cf. /2/.
The glass-fibre COD-gauge d e t e c t s analogously t h e growing distance of t h e notch-flanks a t t h e load-line position. Parallel t o t h e analogue signal TTL-compatible digital calibration marks a r e given a t 50 pm intervals, Fiq. 7. Both gauges a r e very cheaply replacable, since t h e very sensor head only amounts t o less ,than 2 $, a f e a t u r e which is quite advantageous in dynamic testing.
The IR-Laser COD-meter was ordered by VTT Finland, 141, and especially developed f o r use in their 500 J Charpy-Impact T e s t e r with inverted geometry, .Fig. 8. It is capable of detecting t h e notch opening of standard Charpy-V-notch specimens a s they a r e impact loaded with a local resolution of b e t t e r than 10 pm. Detection of t h e notch opening (COD) is based on t h e registration of t h e light strayed back by t h e specimen notch when a light beam e m i t t e d by t h e laser diode illuminates t h e specimen. Required measuring distance is 280 mm and specimens can be t e s t e d a t t e m p e r a t u r e s between -50 'C and +ZOO 'C, without t h e need of having t h e specimen especially prepared f o r t h e COD-meter. A typical t e s t record a s gained during t h e impact t e s t of a pressure vessel steel-specimen with 5 m/s a t 100 'C is given in Fiq. 9. The change of slope in t h e COD-signal c h a r a c t e r i z e s the crack initiation point.
4. Conclusions
The installation of a 2 m rotating disk impact testing machine gave rise t o new research frontiers in dynamic solid s t a t e physics a s well a s a new generation of opto- electronic sensors which will find their way into a wide field of applications not necessarily confined t o material testing.
References:
/l/ KuClmaul, K., C. Zimmermann, W. Issler:
Dynamic Tensile Testing w i t h a Large Scale 33 M J Rotating Impact Disk Machine,
D Y M A T 1985, Paris
/2/ Zimmermann, C., T. Demler:
On the Measurement o f CTOD Using Glass Fibres SMIRT 1985, Bruxelles
/ 3 / Wellmann, R. et. al.:
3-D Elastic-Plastic F E M Analysis o f Three-Point Bend Specimen, WRC-Bulletin, No. 299, pp. 15-25, 1984
/ 4 / Rintamaa, R., T. Saario, K. Wallin:
Procedures for the Determination o f the Static and Dynamic Fracture Behaviour Using Three-Point Bend Specimens
Internat. Conf. on Fracture, 1984, Madrid
Fiq. 1: Specimen geometries for the 2 m r o t a t i n g disk impact machine
Fiq. 2: 33 M J r o t a t i n g disk impact machine at MPA-Stuttgart
(3-222 JOURNAL DE PHYSIQUE
r-'
to (contact nose-specimen)
Fig. 3: Crack t i p near f i e l d deformation o f a DENT-Specimen loaded a t 16,3 m/s.
G r i d constant 170 p m
Fig. 4: Built-up of stress distribution in the specimen of Fiq. 3 during impact
JOURNAL DE PHYSIQUE
GLASS-FIBRE
LASERDIODE
Cl - SPECIMEN
PIN -0IODE
IR-LIGHT
FATIGUE CRACK
NOTCH
Fig. 5: Glass fibre CTOD-Meter
Fiq. 6: J-CTOD relation as measured -+
w i t h CT-25 specimen for va- rious steels. For comparison purposes numerical calculations by /3/ for an A 508 CI.2-Steel, similar t o 22 NiMoCr 3 7, f o r plane stress and plane strain.
RF -TRANSMITTER RECEIVER
I CT-SPECIMEN I
Fig. 7: Glass f i b r e COD-meter f o r static and dynamic testing applications
COO [mm]
0 0,s 1.0 2P
Fiq. 8: IR-Laser COD-meter for a
~ h a r ~ ~ - ~ m b a c t Tester
Fiq. 9: Force vs. t i m e and COD vs.
t i m e relation a s measured by VTT Finland for a r e a c t o r
pressure vessel s t e e l at 1 0 0 ~ ~
