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THE FACTORS OF THE INFLUENCE ON BALLISTIC PROPERTIES OF TARGETS OF
Al-ALLOYS
Xian Mengmei
To cite this version:
Xian Mengmei. THE FACTORS OF THE INFLUENCE ON BALLISTIC PROPERTIES OF TARGETS OF Al-ALLOYS. Journal de Physique Colloques, 1985, 46 (C5), pp.C5-357-C5-362.
�10.1051/jphyscol:1985545�. �jpa-00224776�
J O U R N A L
DEPHYSIQUE
Colloque C5, suppICment au n08, Tome 46, aoQt 1985 page C5 -35 7
THE FACTORS OF THE INFLUENCE ON B A L L I S T I C PROPERTIES OF TARGETS OF A1-ALLOYS
Nei Mongol I n s t i t u t e of Metals Research, P.O. Box 4 , Baotou, China
RESUME
- L'analyse mktallographique d u cratere d e cibles e n alliage d'alumi- nium ayant differentes propriktks balistiques est prksentke. I 1 est trouvk que l a propriktk balistique d e l a cible est relike aux cisaillements adiaba- tiques. La courbe effort-dkformation dynamique de deux alliages dlaluminium est dktermin6e 2 la barre dlHopkinson (6=102s-l). L1effet d u comportement dynamique sur les proprietks balistiques est analysk et compare aux conditions critiques d e deformation et d e vitesse d e dkformation, pour deux alliages.ABSTRACT - The metallographic analysis for the craters of Al-alloy targets wi:h different ballistic properties is presented. It is discovered that the ballistic property of the target is connected with adiabatic shear. Dynamic stress - strain curves of two Al-alloys are determined at strain rate of one hundred persecond (&=102s-' ) by one dimensional split Hopkinson Bar. The effect of dynamic behavior on the ballistic properties is analysed and compared with critical conditions of strain and strain rate, where yield scresses of two alloys are used.
I INTRODUCTION
Al-alloy targezs have the advantage of lighter weight and higher specific strengrh.
It is discoved thac the targets have different fracture characteristic when they are penetrated by 7.6 mm calibre steel projectiles. The statical strength of carget A is the same as that of target B, but the ballistic limit velocity of target B is 100 m/s higher than that of target A. Target C which has higher static strength appears brittle fracture. Mechanical properties and special fracture of the targets are shown in table 1.
In order to research the effective factors of ballistic behaviour of Al-alloy targets, the observation of craters after penetration is made by optical meCalloscopy.
It has been discovered that the ballistic behaviour is connected wi-h the adiabailic slhear.
Table 1. P.lechanica1 properties and fracture of Al-alloy targets
Alloy Number Fracture form
Crater and plug
Crater and little crack at back
Brittle crac!~
Cracer and Plug -
Limit ~elocity(m/s)
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985545
C5-358 JOURNAL
DE
PHYSIQUEDynamic stress-strain curves of two Al-alloys are measured at & =10 /s by the dimensional 2 split Hopkinson Bar. Their dynamic yield stresses are used as critical condition of strain and strain rare of adiabatic shear. The influence of dynamic behaviour on the ballistic properties and adiabatic shear are analysed and compared.
I1 METALLOGRAPHIC OBSERVATION OF CRATER SECTION
Eleven craters on four Al-alloy targets are analysed. Every crater is cut in half, longitudinal section area is observed.
The shape of craters on targets A and D is the same as the shape of deformed projectile. After ground and pol.ished, the section is etched by mixed acid. Then it is observed by optical microscope.
The results are following:
(1) The limit velocities of target A and D are lower than that of target B. The fracture appears as pluging. There are distinct plastic deformation regionand strong deforma"con band abou'c 2mm around the crater (Fig 1). Their microstruceures is the s2me as that of the matrix, bui :heir crys-als are elonsa,:ed and the transformed shear band with uns:able structure is not formed.
F'g 1. DeformatLon regLon of targe: Fig 2. Shear Sand of zarget A neai the
2 near 'he crater Tluging
The adiabatfc shear band (widih abou: 20-30 pix
>
Is loca-ed a; two sides of :he?lung on cacge: A. The nlcioha;-dness of ban< is higier ,ban :ha_ of matrix (mtrix 1b97, shear band iJm=138). The crack is connecied w;th ihe shear hand (shown In Fig 2).
(2) There is shear band around the crater at an angle of 45O to the wall of the crater on target B. The width of shear band is 70-80
nrn
and the m i c r o ~ s s i s 150-176 (matrix Hm=138). This band crosses over the ductile crack which forms little crack on the back face of the target and propagates along rolling direction. The projectile shot target B is mushroomed. The grains of the projectile head are elongaLed.( 3 ) The fracture of target C is brittle. The brittle belt phase which has been
observec; in t&e crack (Fig 3 ) analysed by means bf Energy Dfspersive Analysis of X-Pay and consists of Ti and Zr. The brittle cracks initia~e at the belt and propagate along the grain boundary until damage.
Comparing micro structure and fracture pattern of the craters of four targets, it is shown that ballistic behaviour of targets is related to the dynamic fracture and adiabatic shear. When steel projectile wlth 500-800 m/s velocity- impacts to Al-alloy target, the target produces plastic deformation. Because of high strain rates, the heat produced can not be timely propagated to neighbour. So the temperature of narrow localized deformational region rises quickly. When softening rates exceedstrain hard races, rhe ~lastic: deformation would rake unstable propagation and forrh ehe s h ~ r band in the locgl region.
.
. ' * . ,..
.
,
.
Fig 3. Brittle belt phase and the Fig 4. The melt spheres at the surface.of the plug crack in target C
Previously the phenonemon was discovexed hy Jener and Hollomen") in steel. In past severat2jecades many researchers were engaged in this project and published a lot of works. It is noted in present experiment that shear band appears at the begining of penetration stage on target B. The band is called first shear band. But second shear band surronding the plug at the bottom of crater on target A and D is formed.
Comparing target A,
B
and D, it can be observed:(1) There are deformation bands at two side of craters on target A and D, but there are clear shear bands at two side of crater on target B. The microhardness of the shear band is higher than that of the deformation band.
(2) The shear band on target B appears indication of recrystalization. It is illustrated that the temperature of shear band on target B is higher than that oftargets A and D.
(3) The target B makes the projectile mushroomed, so that interreaction -.bemen projectele and target becomes wider. The material beneath the crater and near back support of target B is loaded by tensile stress and it is resulted in ductile fracture of targetB.The projectile energy expended by ductile fracture is more than energy expended by propagation of cracks along the shear band. Because of the listed items above theballistic property of target B is increased. Some melt spheres with 0.8 pm diameter are observed on the surface of the plug by scanniq electron microscope(Fig4).
It is possible that the spheres are formed by shear band. It is explained when pluging the temperazure a r d the plug raise over melting point.
111 TESTS BASED.ON THE SPLIT HOPKINSON BAR
Dynamic stress-strain curves are measured by the split Hopkinson Bar for AL-alloys A and B. T$$s schematic diagram of the rest is shown in.(3). A detailed reporthas been presented'
.
The length of the cylindric specimen, which lie between two bars is 8 mm and :he diameter 14.5 mm. Radial stress and circular stress are measured by stress gauges sticked on the specimen. The strain wave of the incidenting, the reflecting and tians- mitiing are recorded by two strain gauges sticked on the two sgltt bars. After the
specimen is centred strictly the end of the bar, a cylindrical projectile is impacted to the incident bar by powder gun. The velocity of profecrile strain waves in the bar and the specimen are measured instantaneously.
If &
C
&, representes instantaneous amplitudes of strain of inciden~, reflec-ted and transmitted respectively, Ao representes sectional area of-the splitbar;
A, representes cross sectional area of the specimen, according to (3) the fundamental relations of stress and strain are following:
JOURNAL
DE
PHYSIQUEWhere g E,
.&
show stress, straln and strain rate of ,,specimen respectively.C = the velocity of elastic wave, L = initial length of thk>specimen. The reading ig ~ a k e n once every 3 ,us at every cu:ve. After the stress and the strain at the same moment are calculated,the curves ace drawn (Fig 5). The static curves of two Al-alloys are determined by Instron machine at strain rate of 10-~/s ( F ? ~ 5).
The comparison of static and dynamic strength two Al-alloys is shown in table 1.1.
Table 11. Dynamic and static strongch of 2wo Al-alloys Dynamic Strength
do., ( ~ g . m m - ~ )
Fig 5. Al-alloys6-€curves at different Fig 6. The plots of tensile strength vs.
strain rates temperature of alloys A and B.
Alloy A
Alloy B
It is obvious that their static strength and Q-E curves are identical, but in dynamic test flow stress and yield stress of alloy B are higher than Chat of alloy A . Dynamic strength of alloy B increases by 25% as compared with static, but alloy A increases by 12.5%.
IV DISCUSSION ---
1. Although static 1nachLne properties of A and B alloys are Identical, bu? the 1
o - ~
- --- .- -
153
170 42
42.5
- -
4 8
-.
5 5
limit velocity of target B is 100 m/s higher than that of target A. It is discovered in examination of the craters that t!~ey !lave different failure forms. Deformation hardness of alloy B is more serious than that of alloy A. Target A appears pluging failure, but target B pezal failure, because the projectile is mushroomed and the micro cracks are formed ac the back of target B. Although both craters sections of two targets have adiabatic shear bands, but their location, hardness and width are different.
2. The failure formsoftargets A and Bareconnected with shear band. At high rate deformation, stress change is related not only to strain but also to strain rate(4).
Under high deformation rate dislocations cannot simultanueously slip, so themterial produces hardening which is called high strain rate hardening. When the heat of plastic deformation produced by high strain rate can not immediatelydisperse out, thenthe temperature increases and material softening is called heac softening.
The stress is related to
d r R. S. Culver (4) according to condition of producing adiabatic shear band =0, derived critical strain condition.
where n T = material hardening exponent,
P
=materra1 density, C =specific heat, J =mechanical equivalent of heat,u, = static strength,ud= dynamic strength.Alloy A and B have egual
9 ,
C. If their static strengths are identical then the compa- ,rijon of critical sl,rain condition is relative to conipsrison of ,,I=(
- * I ~ T I
& i ~-
-- - ~-* (6)
E ie % A
-p
where the f0otnote.A or B resjected alloys A or B. The strengths of alloys A and B are measured at different temperature and cJ -T curves are ploted in Fig 6. It is noted tha: two curves are iden.tica1 so the ratio of their critical strains equals the reciprocal ratio of strengths =
L.
Under tile same strain rate U,,& '
.
ad^is higher than a,, so critical cond1?t1on of alloy A Is higher than that of alloy B. The result is correspond with metallographic examination of the craters. The shear band of target B appears at first stage of impact and the deformation of two sides of the band of target B isn't as apparant as that of target A. Although the deformation of two sides of the crater is apparant buz it doesn't become adiabatic shear band.
( 3 ) R. F. ~ e c h t ( ~ ) had taken another conduction heat design. Under condition
(=) =O and had given critical strain rate condition.
dE a
where L = Length of the specimen, 7: = Shear strength, K = Coefficien'; of hea'; condi- tion, C = Specific heat, & = Total strain,
L,=
Strain of yeild initiation,P
=~ e n s i t ~ y 3 = Mechanical equivalent of heat.
If the K,
p
. C , . L and J of two kinds of aluminiuin alloys are considered to be equal, the strain hardening term of alloy B is greater than A one, therefore its critical strain rate is also higher. This resulied in the limit velocity of target B to be higher than target A. I: agrees with experiment one.(4) Tine fracture form of the target is related to ?tie deformation of the pro- jectiles. Because the projectile was upset by target B hardened, so :hat the re- sistance of taggel Fo projectile increased further. The strain rate obtained is about
l o 2
- 10 s In one-dimensional Hopkinson Bar 'est. In actual pro essE Sf
Penetration the dynamic strength will be increased under strain rate 10 - 10 1s.
JOURNAL
DE
PHYSIQUEThe reason theof target B is higher than the target A on the limit velocity isexplained by above several respects. Tinis is not explained by the static properties.
V CONCLUSION
1. The ?recess of impact of a projectile on a target is one of high velocity, high pressure and high strain rate. Under high velocity impact the shear band is produce6 from an instability localization. The ballistic properties of material depend on behavior of the adiabatic shear band. When the projectiles impaci on Al-alloys with different compositions, there are different shear characters, therefore the ballistic properties are different.
2. At strain-rate 10' s-' the dynamic strengths of Al-alloy A and B are determined. The result is shown that alloy A increases by 12.5 percent and B alloy by 25 percent. This results are substituted for critical strain and strain rate. It is shown that to produce shear band the critical strain rate of alloy B is high and i t s critical strain is lower. The result of the high critical strain rate agrees with the ballistic property of alloy B. The low critical strain agrees with
indentation observed.
3. The phenomenon of adiabatic shear band and high temperature which excess melting point on the lug are considered that the ballistic properties of the targets depend on the dynamicmechanical properties and thermostatic properties. There still is much quantitative investigationtobe done on the influence ofthemechanicalproperties and physical properties to ballistic properties.
REFERENCES
/I/ Jener, C. Hollomen J. H., J. Appl. Phys. 15 (1944).
/2/ Hargreaves, C. R., Werner. L., Adiabatic shear
-
An Annotated Bibliography, AD-A006490 (1974) 502./ 3 / Duan, Zhu-ping et al., Mechanical Development (In Chinese)
10
(1980) pl./ 4 / Culver, X. S., Metallurgical EffecCs at High Strain Rates (1973) 2569.
151 Rechc, D. X., J. Appl. Mech.
