HAL Id: jpa-00224754
https://hal.archives-ouvertes.fr/jpa-00224754
Submitted on 1 Jan 1985
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
THE DYNAMIC BEHAVIOUR OF THE INSTRUMENTED CHARPY TEST
R. Mines, C. Ruiz
To cite this version:
R. Mines, C. Ruiz. THE DYNAMIC BEHAVIOUR OF THE INSTRUMENTED CHARPY TEST.
Journal de Physique Colloques, 1985, 46 (C5), pp.C5-187-C5-196. �10.1051/jphyscol:1985524�. �jpa- 00224754�
JOURNAL DE PHYSIQUE
Colloque C5, supplkment au n08, Tome 46, ao0t 1985 page C5-187
THE DYNAMIC BEHAVIOUR OF THE INSTRUMENTED CHARPY T E S T
R.A.W. ~ i n e s * a n d C. R u i z
Oxford U n i v e r s i t y , Department o f E n g i n e e r i n g S c i e n c e , Parks Road, Oxford OX1 3PJ, U . K .
~ 6 s u m 6 - Les caract6ristiques dynamiques des essais Charpy s o n t Q t u d i 6 e s au moyen d'un appareil type Hopkinson et de la photoklasticit6. On trouve que cette mkthode est sup&rieure h celle que l'on utilise normalement dans les laboratoires d'essai de matgriaux, permettant une meilleure interprktation des r6sultats.
Abstract .- The dynamic characteristics of the Charpy test have been studied by means OF a Hopkinson bar apparatus and photoelasticity. It is Found that this technique permits a clear evaluation of the test results, and is preferable to the standard instrumented pendulum test.
I - INTRODUCTION
The fracture toughness of a material can be assessed by direct or indirect methods. Direct measurement, following standard ASTM 399 for example, may require large specimens expensive when not impossible to make. Indirect methods of assessment may be categorized as follows:-
1 - Statistical correlation between fracture toughness and other mechanical properties viz. Charpy V-notch energy (CV), nil ductility temperature (NDT), etc.
2 - Small specimen testing, e.g., instrumented impact testing of Charpy-type specimens, V-notched or fatigue precracked.
The statistical correlation is derived from an extensive number of tests that exhibit large scatter as a result not only of experimental error but also of the inaccuracies inherent in the interpretation of experimental data. An example of this is the oversimplification resulting from the use of a single parameter to describe brittle fracture and ductile tearing, two extreme modes of failure that often coexist and interact in a given test. The alternative of small specimen testing has been gaining popularity in recent years, most of the effort having been directed towards the development of the
instrumented Charpy test with V-notched and fatigue precracked specimens. There now exists an extensive literature on the interpretat ion of the test in terms of fracture toughness parameters /l/
*NOW with the University of Liverpool, ,P.O. Box 147, Liverpool L69 3BX
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985524
C5-188 JOURNAL DE PHYSIQUE
I1 - DERIVATION OF FRACTURE TOUGHNESS FROM INSTRUMENTED CHARPY TESTS The approach adopted by ASTM f o r a proposed s t a n d a r d /2/ assumes t h a t q u a s i - s t a t i c e q u i l i b r i u m , w i t h n e g l i g i b l e i n e r t i a l f o r c e s , p r e v a i l s i n t h e specimen. The f r a c t u r e toughness Kid is d e r i v e d from t h e s t a t i c c a l i b r a t i o n f u n c t i o n , t h e aim being t o measure t h e load a p p l i e d t o t h e specimen a t any g i v e n t i m e , P ( t ) , and from t h i s t o d e r i v e t h e i n s t a n t of c r a c k i n i t i a t i o n and t h e corresponding load.
A t y p i c a l example
a/ ~0.52 is shown i n F i g .
W 1 , b u t a wide
v a r i e t y of forms of response, w i t h v a r y i n g number of o s c i l l a t i o n s is found i n t h e l i t e r a t u r e
depending on t h e mechanics of t h e t e s t , t h e impact v e l o c i t y and t h e
i n s t r umen-
t a t i o n s . In t h i s c a s e t h e change i n average s l o p e F i g . 1 - T y p i c a l load t r a c e from an Instrumented
Charpy T e s t
i n d i c a t e s t h a t some y i e l d i n g o c c u r s a t A and t h u s a n e l a s t o - p l a s t i c a n a l y s i s would be r e q u i r e d f o r t h i s t e s t . Note t h a t t h e t r a c e has been f i l t e r e d . The q u e s t i o n is: a r e t h e o s c i l l a t i o n s i n t h e t r a c e d i r e c t l y r e l a t e d t o t h e a p p l i e d load o n l y o r a r e t h e y t h e r e s u l t of t h e dynamic response of pendulum and i n s t r u m e n t a t i o n ? Note t h a t once t h e specimen has broken t h e r e is ' r i n g i n g ' i n t h e pendulum of period approximately e q u a l t o 125 fis. The p e r i o d of a p p a r e n t o s c i l l a t i o n of t h e specimen, 7 , d e r i v e d e m p i r i c a l l y f o r a r a t i o of t h e c r a c k l e n g t h - to-width, a/W - 0 . 5 is 45 fis. Thus, t h e coupling between t h e mechanical and e l e c t r o n i c c o n s t i t u e n t s of t h e system cannot be d i s c o u n t e d i n t h i s c a s e . The performance of t h e e n t i r e measurement system needs t o be f u l l y d e f i n e d b u t even t h e n t h e s t r a i n gauge on t h e
impacting weight w i l l o n l y g i v e a q u a s i s t a t i c load and any i n e r t i a l o r s t r e s s wave e f f e c t s i n t h e weight w i l l i n f l u e n c e r e s u l t s .
Another q u e s t i o n t o a s k is: does t h e specimen f r a c t u r e a t t h e maximum load? Kalthoff e t a 1 /3/ have shown t h a t t h i s is n o t n e c e s s a r i l y t h e c a s e . In t h e c a s e of a very b r i t t l e m a t e r i a l under high r a t e of l o a d i n g , f r a c t u r e o c c u r s a f t e r t h e peak. Of c o u r s e t h i s may mean t h a t t h e t i m e t o f a i l u r e , t f , exceeds 37, v i o l a t i n g one of t h e ASTM
-
c r i t e r i a / 2 / b u t l i m i t e d p l a s t i c i t y o f t e n d e l a y s o r obgcures t h e i n s t a n t of c r a c k i n i t i a t i o n i n precracked Charpy V specimens of notch- tough m a t e r i a l s . The independent measure of c r a c k i n i t i a t i o n , e s p e c i a l l y when some p l a s t i c deformation o c c u r s , is problematic. A s t r a i n gauge on t h e s u r f a c e of t h e specimen a d j a c e n t t o t h e c r a c k does n o t r e g i s t e r t u n n e l l i n g of t h e c r a c k and o p t i c a l t e c h n i q u e s s u f f e r from t h e same weakness. The p o t e n t i a l d r o p t e c h n i q u e has n o t been s u c c e s s f u l e i t h e r 4 t h u s t h e p o i n t of c r a c k i n i t i a t i o n has t o be e s t i m a t e d from load t r a c e s .
A f u r t h e r s o u r c e of e r r o r is t h e d e r i v a t i o n of t h e dynamic i n i t i a t i o n f r a c t u r e toughness, K I d , by means of a c a l i b r a t i o n f u n c t i o n based on
the static stress analysis of the specimen. The tt 2 37 rule seeks to
-
reduce the error by ensuring that a state of quasi-static equilibrium has been reached when fracture occurs. After 37 a sufficient number of stress wave reflections have taken place on the boundaries of the specimen to justify this assumption, but inertial loading may still play a significant role, as will be shown later in this paper. In any case, the quasi-static analysis restricts the impact velocity and hence compromises the high strain rate that should be used to achieve brittle fracture in notch-tough materials, particularly when the specimen is not fatigue pre-cracked.
1 1 1 - PHOTOELASTIC ANALYSIS OF THE TEST
A single spark dynamic photoelasticity facility was developed /5/, the model material used being Araldite CT 200 with a longitudinal wave speed of 1703 m/s. Fig. 2 shows the loading system and specimen. A 105 mm weight on a falling pendulum impacts an intermediate tup at a velocity oE 3.5 m/s. The intermediate tup in turn impacts the specimen causing it to deflect. The loading system differs from the standard instrumented pendulum or drop weight; the configuration was selected for ease of trig-
ger ing the single spark
source. The spark was of +
a duration of 0.1 j~s and could be E ired at a pre- selected time with an
accuracy of 1 ,us. A lens a A - A
polar iscope, set to dark field, was used to obtain dynamic isochromatic fringes.
F ig - 2 Gener a1 arrangement of apparatus for photoela- stic analysis of Charpy test, (a) penduhm, (b)
intermediate tup, (c)
specimen Dim.in m m
An idealized one dimensional stress wave analysis of the impacting weight and intermediate tup, without the specimen, illustrates the stress wave behaviour of the system and gives an estimate of the rate at which the pointer is driven into the specimen. A qualitative description is as follows. When the. pendulum weight impacts the intermediate tup, compression stress waves emanate from the point of impact at approximately 5 kms-l. The waves travel down the weight and tup, and reflect as tension at the free ends. These reflected tension unloading waves travel in opposite directions to the original compressive waves. Due to the difference between the length of the weight and of the tup, the stress wave behaviour is essentially assymmetric up until the time at which separation occurs between the weight and the tup due to particle velocity mismatch. The stress waves are then trapped in the tup, travelling back and forth. Such a simplified analysis agrees closely with the stresses measured on the tup during an actual test. The analysis gives a pointer displacement that is essentially a ramp, i.e. the pointer is steadily driven into the specimen.
From the dynamic photoelastic fringes the growth of stress in the vicinity of the crack and in the rest of the specimen can be studied.
Fig. 3 gives the growth of fringes. In 3-A the loading from the tup is in the form of coaxial loops, distorted due to dynamic effects. A mode I stress intensity factor has already appeared at the notch tip,
C5-190 JOURNAL DE PHYSIQUE
and flexural waves move out towards the supports. In 3-B these flexural waves have dissipated, the tup is still loading the specimen and there is a large stress gradient in the central section of the beam. In 3-C the tup has lost contact and the beam bends under its own, distributed, inertia. The pointer could still be travelling at the same speed but with the Araldite beam accelerating away from it.
The beam continues to bend under its own inertia (3-D) and the tup regains contact (3-E). The applied load is increasing just prior to failure (3-F) and the stress distribution is similar to a specimen loaded statically. The situation may be
Fig. 3 Isochromatic fringes in the specimen under impact three point bending
different when testing a relatively large specimen, when the inertial load is comparable to the point load impacted by the pendulum. The time of fracture, measured using a strain gauge adjacent to the notch tip was 359 ps after impact. Using the empirical formula for the derivation of 7/1/, tf 1.55 7, violating the proposed 37 criterion.
The well developed mode I stress intensity loops at the notch tip in Fig. 3 mean that stress intensity factors can be derived. In a singularity dominated field the stress distribution in the vicinity of a dynamically loaded stationary crack is the same as for a statically loaded crack. Hence the static linear slope method of Ruiz /6/ was used to extract stress intensities from the fringe patterns. The growth in KI(t) and the variation of kI(t) are given in Fig. 4 . The growth in K T ( t ) is influenced by the loss of contact of the tup and kI tends to the static value just prior to failure. This loss of contact at the impacting weight, and possibly at the supports, has been discussed by Kalthoff 8 Such behaviour complicates the dynamic analysis of the system. The conclusions from this dynamic photoelastic analysis are that the loading system described can be analysed in
terms of one-dimensional s t r e s s wave behaviour and t h a t such an a n a l y s i s provides
K~ i n s i g h t i n t o t h e
~ ~ a f i dynamic behaviour
of t h e system.
The load ing
2 . 8 -
system should be
improved by
lengthening t h e impacting weight and i n t e r m e d i a t e t u p s o t h a t t h e r e
2 - a r e l e s s s t r e s s
wave r e f l e c t i o n s p r i o r t o f a i l u r e . To t h i s end t h e 1.6 -
Hopkinson
p r e s s u r e bar method of l o a d i n g
1.2 - was used. This
t e c h n i q u e is d e s c r i b e d i n /7/.
F i g . 4 - Growth of k I ( t ) and KI ( t ) i n t h e p h o t o e l a s t i c specimen
0 30 90 150 210 270 310 ys
IV - THE HOPKINSON PRESSURE BAR
F i g . 5 g i v e s - a schematic diagram of t h e Hopkinson P r e s s u r e Bar arrangement. Impact bar A c o l l i d e s w i t h bar B , which is h e l d s t a t i o n a r y i n c o n t a c t w i t h t h e specimen. A one dimensional system of compression waves emanate from t h e p o i n t of c o n t a c t C . T h i s s t r e s s wave behaviour is i l l u s t r a t e d by t h e Lagrang i a n 'diagram given i n Fig .
5. The s t r e s s p u l s e p a s s e s s t r a i n gauges S 1 i n bar B g i v i n g a s i g n a l Associated w i t h t h i s s t r e s s l e v e l is a p a r t i c l e v e l o c i t y v I , r e l a t e d by t h e one dimensional wave formula:-
The s t r e s s f r o n t is t h e n i n c i d e n t on s t r a i n gauges S2, and t h e growth i n s t r e s s is t h e same a s f o r S 1 f o r a time tl, a f t e r t h e p u l s e impinged on S2. A t t h i s time t h e s t r a i n gauges S2 unload due t o t h e r e f l e c t i o n s from t h e specimen. The r e a d i n g of S2 is now a combination of i n c i d e n t p u l s e , e I , and r e f l e c t e d p u l s e , Thus S 1 minus S2 g i v e s t h e r e f l e c t e d wave. Two wave systems can now be thought t o e x i s t i n t h e bar - +I and E
R '
In t h e c a s e of s t r a i n gauges S2, t h e t r a c e can be r e c o n s t r u c t e d from and eR. Next c o n s i d e r a p o s i t i o n c l o s e r t o t h e p o i n t e r - t h e time t l between and reduces, dependent on t w i c e t h e time it t a k e s f o r a wave t o t r a v e l from t h i s new p o s i t i o n t o t h e p o i n t e r . In t h e l i m i t i n g c a s e , i . e . a t t h e p o i n t e r , e1 and eR s t a r t a t t h e same time.
This n e g l e c t s t h e e f f e c t of t h e p o i n t e r . Thus i f a s t r a i n gauge was
C5-192 JOURNAL DE PHYSIQUE
' p l a c e d ' a t t h e p o i n t e r t h e r e s u l t of e I -- f R would be t h e s t r e s s a t t h a t p o i n t - t h i s can t h e r e f o r e be r e l a t e d t o t h e a p p l i e d f o r c e /7/:-
where A is t h e c r o s s s e c t i o n a l a r e a of t h e b a r s .
t- I L C , L12 L -1 X > d
A
Fig. 5 ( a ) S t r e s s wave diagram of H . P . B . and specimen, ( b ) c o n s t r u c t i o n of P ( t ) and v ( t ) curves
The r e s u l t i n g p a r t i c l e v e l o c i t y from e I and eg is d e r i v e d a s f o l l o w s r -
Knowing v ( t ) a t any time t h e displacement a t t h e s e c t i o n i n q u e s t i o n is o b t a i n e d by i n t e g r a t i o n . The above one dimensional wave a n a l y s i s breaks down a t t h e p o i n t e r and s o P ( t ) and v ( t ) v a l u e s r e f e r t o s e c t i o n XY g i v e n i n F i g u r e 5.
The p o i n t e r should be included w i t h t h e specimen i n t h e c a l c u l a t e d r e s u l t s . Using t h i s a p p a r a t u s and d a t a r e d u c t i o n , Charpy V notch
specimens in a chain steel and fatigue precracked Charpy V notch specimens in a pressure vessel steel were loaded to failure.
V - IMPACT ENERGIES FROM CHARPY V-NOTCH SPECIMENS
From these results, the conclusion is that energies derived using the two different impact machines are similar for given material properties. Fatigue precracking the specimen increases local constraint, promoting brittle fracture and hence enabling the derivation of fracture toughness values from tests.
V-notch specimens were prepared to ASTM-E23 from DIN 17115-1-6753 chain steel with two heattreatments, 'as used' in mining chain and 'toughness trough'. All tests were carried out at room temperature.
From P(t) and v(t), the energy required to completely fracture the specimens would be calculated and compared with the impact energies from simple Charpy tests. Two impact velocities were selected for each heattreatment varying from 5 ms-I to 8 ms-l. Fig. 6 compares
E n e r g y ( J 1 the impact
energies from the
V1 - FRACTURE TOUGHNESS VALUES FROM PRECRACKED CHARPY V-NOTCH SPEC IMENS
160
120
80
4 0 0
V-notch spec imens E23, and fatigue pr Amsler Vibrophore .
- s imple Char py
tests with energies from the
- Hopkinson
pressure Bar.
Failures in the
- spec imens were
dominated by material flow and so fracture
- toughness values
could not be deduced from the
I I I I tests.
of a pressure vessel steel were prepared to ASTM
,ecracked in line with established practice using an Tests were carried out at four temperatures,
300 380 460 H V
Fig. 6 - Effect of hardness on impact energy to fracture Charpy V- notch specimens
namely -40°c, - ~ O C , 2 4 O ~ and 6s0c. For each temperature the impact velocity was varied to study specimen responses to differing strain
r a l e s . Fig. 7 gives the P(t) curves for temperature - ~ O C . At vimp -
- 1
1 ms the response is quas istatic with inertial oscillations decaying to a ramp, and with yielding occurring. Increasing the impact velocity to 1.5 ms-I gives larger inertial oscillations, and a mean ramp which is more steep. The specimen response changes markedly at t = 138 &S. Does crack initiation occur at this point? The interpretation of the trace is difficult, and from the study of the fracture surfaces after the test, crack initiation has not occurred
- 1 right across the specimen. Increasing the impact velocity to 2.5 m s
JOURNAL DE PHYSIQUE
g i v e s a s e v e r e unloading a t t =
138 ,us. The
response of t h e
specimen changes
completely and it is p o s s i b l e t o assume t h a t c r a c k i n i t i a t i o n o c c u r s a t t h i s time. A f r a c t u r e
toughness K I d is d e r i v e d from t h e
maximum load,
t a k i n g a s t a t i c c a l i b r a t i o n f u n c t i o n .
S i m i l a r p(t-1 c u r v e s were o b t a i n e d f o r t h e o t h e r
t e m p e r a t u r e s .
Values of t h e f r a c t u r e
toughness
parameter K I J ,
proposed t o
c h a r a c t e r i s e F i g . 7 - P ( t ) c u r v e s f o r t = - 8 * ~
f r a c t u r e i n t h e e l a s t o p l a s t i c regime were a l s o o b t a i n e d . ( F i g . 8 ) . The r e s u l t s a r e compared t o t h e s t a t i s t i c a l K I d (mean) curve quoted
i n t h e Marshal1 Report /g/.
Fig. 8 - Comparison between f r a c t u r e toughness parameters o b t a i n e d f o r s t a t i c , instrumented PCVN and HPB t e s t s /9/
There is good agreement between the KId derived from the HPB test and the mean curve, while KIJ always overestimates the fracture toughness.
V11 CONCLUSIONS
The advantages of the HPB method of loading over the conventional Charpy pendulum are:
- the load P(t) is applied to the specimen for a duration of 350 ~ s , depending on the length of the two bars. It is accurately measured by the strain gauges.
- Fracture occurs before stress waves are reflected at the face ends of the bars and therefore a simple one dimensional wave analysis is sufficient to characterise the dynamic behaviour of the system.
- the specimen deflection can be derived. it is therefore possible to obtain the energy absorbed at any given time more accurately than with the conventional pendulum.
- since the load is maintained over a time exceeding tf, there is no bouncing of the specimen on its anvil, thus eliminating a source of non-linear behaviour.
Thus using the HPB a more fundament analysis of the Charpy specimen can be achieved-From the tests described in this paper the P(t) traces can be viewed with high confidence. The measurement of the point of crack initiation remains problematic P(t) traces are open to interpretation. Thus an independent measure of crack initiation should be developed. The departure of the specimen from static equilibrium at fracture could be investigated using a dynamic finite element program. Given progress in these three areas, fracture toughness values derived would be accurate and the geometry dependence or independencg of KId could be investigated experimentally . The
occurrence of plasticity complicates the analysis of an instrumented impact test given the complex interaction between cleavage, ductile tearing and applied strain rate. Thus althoughthe relation between the static fracture toughness under plane strain conditions and the fracture toughness derived from a pre-cracked instrumented Charpy test is complex, the test does give a lower bound to the fracture toughness.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the support provided by the National Coal Board and the UKAEA.
REFERENCES
/l/ CSNI Specialist meeting on instrumented precracked Charpy testing, EPRI-NP 2102 LD, 1981
/2/ Server, W.L., J. Test. Eval. (ASTM) - 6 (1978) 29
/3/ Kalthoff, J.F., Winkler, S., Beinert, J., Int. J. Fracture Mech., 13 (1977) 105
-
/4/ Kobayashi, T., Eng. Fracture Mech., 19 (1984) 49
/5/ Mines, R.A.W., The use of a single spark source to investigate the dynamic photoelastic behaviour of notched bars, 16th Int.
C5-196 J O U R N A L DE PHYSIQUE
Conf. High Speed Phography and Photonius, Strasbourg, 1984
/6/ Ruiz, C., Experimental determination of stress distribution around notches and slits in pressure vessls, Proc. 4th Int. Conf.
Experimental Stress Analysis, I. Mech. E. , Cambr idge ( 1970) /7/ Nicholas, T. in Impact Dynamics, Zukas, J.A., et al, ed., Wiley
(1980) 277
/8/ Kalthoff, J. F., in Workshop on Dynamic Fracture, Knausset, W.G.
et al, ed., California Inst. Techn. (1983) 11
/9/ Marshall, W., An assessment of the integrity of PWR pressure vessels, UKAEA, 1982
