Evaluating the Impact Resistance of Double-layered Aluminum Alloy Plates with Diverse Shapes against Blunt Rigid Projectiles

: Scholars have studied the impact resistance of flat plates quite well. However, in practical applications, the connecting part of the protective end often presents other shapes, and the importance of this part for the protective effect should not be neglected. Therefore, it is valuable to conduct further research on the shape of target plates. This study aims to investigate the impact resistance of combined target plates of 1100 and 7075 aluminum alloys with different shapes. In this study, the failure modes of double-layered aluminum alloy plates under the impact of blunt projectiles were analyzed by changing the order of plate arrangement and the shape of projectiles, and the ballistic limits and energy absorption characteristics were determined. In addition, a 3D finite element model was established using ABAQUS/Explicit software, and after verifying its accuracy, the model of flat nosed projectile impacting other shapes of double-layered target plates was extended using the finite element method. The results show that for plane plates, arched plates and L-square plates, the sequential arrangement of ductile high metal in the front plate and strong high metal in the back plate possesses better ballistic resistance. However, for hemispherical shells, the same arrangement results in weaker impact resistance. Finally, the paper concludes that the shape of the protective ends affects the order of arrangement for optimum impact resistance.


Introduction
In recent years, multi-layered protective structures have been widely used in military and civilian fields, such as in the military field: Metal multi-layer protective structures are widely used in the protection of various military equipment.For example, the external armor of military vehicles such as tanks and armored vehicles is composed of multiple layers of metal sheets, which can effectively absorb and disperse the external impact energy and improve the protection ability of vehicles.In addition, military aircraft, ships, etc., also use metal multi-layer protective structures to improve their protective capabilities.In the field of industry and construction, metal multi-layer protective structures are also widely used in various equipment and facilities for protection and insulation.As one of the most widely used metal materials, aluminum has become the leading research object of multi-layered protective structures because of its advantages of being lightweight, high strength, low price, excellent corrosion resistance, and fire resistanceError!Reference source not found.Error!Reference source not found..In addition, the mechanical properties of different series of aluminum alloys differ significantly.For example, the pure aluminum alloy of the 1xxx series has good toughness but low strength only reaching about 100Mpa, while the high strength aluminum alloy of the 7xxx series can reach more than 600Mpa, but the texture is relatively brittle.
Therefore, if the advantages of different strengths of aluminum alloy can be complementary, it is expected to improve the protective properties of the structure while maintaining low density Error!Reference source not found.Error!Reference source not found..The research on the protection performance of multi-layered target plates can be traced back to Israel's AwerbuchError! Reference source not found., who systematically studied monolithic and layered protection in 1969 and used the velocity drop to describe the bulletproof performance of the target plate.Awerbuch comparing the thickness of the same material from 1.8 mm to 6.0 mm shows that the layered (in-contact type) and monolithic change trend is not apparent, so no exact conclusion has been obtained.However, in the study of different laminated materials (1100-H14 aluminum alloy, mild steel), it was found that the front plate with steel plate (in the order of projectile attack, the front before bullet contact) decreased the velocity more than the front plate with aluminum plate, that is, the ballistic resistance is stronger.When there is a gap, the steel plate in the front will reduce the ballistic resistance.The controlled experiment after changing the material thickness also confirmed this.
Rahman et al.Error!Reference source not found.experimentally and simulate the effects of aluminum-steel laminates on different structures in high-speed impact tests.The results show that AA7075-T6 (7075-T6 aluminum alloy) has better ballistic properties when placed in the rear layer of the double-layered than in the front layer.The ballistic performance of the double-layered target plate is 20% lower than that of the rolled homogeneous armor plate, but the weight is 25% less.At the same time, the combination of Ar500 steel for the front plate and Al7075-T6 for the back plate has better potential performance than the monolithic steel plate with equal surface density.Deng et al.Error!Reference source not found.Error!Reference source not found.used two different shapes of projectile: the ogive and blunt nosed projectile to impact steel-aluminum targets, and also found that the double-layered target plate made of high strength material in the front layer and low strength material in the back layer had a higher ballistic limit.The difference decreases with the increase of initial velocity, and the impact resistance efficiency of the aluminum alloy target per unit mass is higher than that of the steel target.However, the finite element analysis conducted by Flores-Johnson et al.Error!Reference source not found.on the effect of 7.62 mm APM2 projectile on multilayer armor plates shows that the double-layered mixed configuration of 7075-T651 aluminum alloy plate on the front panel and Weldox 700E plate on the rear panel is superior to any other configuration with equal surface density.
At present, most research on the target plate is based on the material, thickness, and laminations, while the research on the target plate's shape is relatively few.The shell structure has been favored by some scholars because of its unique structural properties of in-plane stress and load distribution.
Ning et al.Error!Reference source not found.conducted a numerical and experimental study on the impact of the flat nosed steel bullets on aluminum spherical shells.By studying the target plate's deformation mode and energy absorption characteristics, the results show that the center deflection radius and pit radius decrease with the increase of shell thickness and increase with the increase of shell curvature and impact velocity.While some researchers Error!Reference source not found.Error!Reference source not found.Error!Reference source not found.Error!Reference source not found.conducted experimental and numerical studies on the hemispherical shell target plate made of 1100-H12 aluminum alloy, they concluded that the maximum energy dissipation of the projectile in the hemispherical shell was due to its tangential stretching by comparing the plate experiment.In the study of the stratification of target plates, it is found that the energy absorption of double-layered of the same thickness of the target plate is greater than that of different thicknesses of the target plate, and it is also found that the span has little effect on the energy absorption of the target plate.In addition, they also investigated the effects of impact velocity, eccentric distance, and tilt angle on failure modes, ballistic limits, and energy absorption.
Research workers have studied the penetration resistance performance of flat plates comprehensively.Whether it is the study of target plate layering, material, thickness, resistance to different shapes of projectile impact and other experimental aspects, or the simulation study of the penetration process, target plate energy loss, and failure mode, there has been considerable exploration.However, in terms of target plate shape, there is still a value worth further study.For example, the literature has not addressed the study of shell plates layered with different materials.
In practice, the main part of the protective structure is planar, while the connecting part is of other shapes: hemispheres, arches, angles, and other irregular shapes.Although this part is relatively small, it is often the more critical part, so it is vital to study the anti-impact performance of the joint.
In this paper, the ballistic performance of a double-layer plate consisting of two aluminum alloys, 1100-O and 7075-T651, with a single layer of 2 mm and a total thickness of 4 mm, under the impact of two different blunt projectiles (flat nosed projectile and hemispherical nosed projectile) is experimentally investigated, and the effect of the laminated order on the target plate is analyzed.
After verifying the validity of the finite element simulation model, the elastic resistance of the double-layered target plate with different shapes (hemispherical shell, arch, and right-angle) under the impact of the flat nosed projectile is investigated by finite element expansion.According to the numerical simulation results, the influence of target plate strength, shape and other factors on the protective performance and impact damage characteristics of double-layered aluminum alloy plate is compared and analyzed.

Experimental and numerical methodology 2.1 Experimental methodology
The whole projectile target impact test is completed on a one-stage gas gun equipment, and a high-speed camera is used to video the whole process of target plate intrusion, and the calculation of impact velocity and residual velocity is completed by the built-in software.The whole system is mainly composed of a nitrogen cylinder, pressurized chamber, launch tube, high-speed camera, and target chamber, as shown in Fig. 1.The firing tube of the first stage light gas gun is 2 m long with an inner diameter of 12.7 mm, the gas chamber is filled with high-pressure nitrogen gas, which utilizes gas expansion to accelerate the bullet.The material of the target plates is an 1100-O and 7075-T6 aluminum alloy plate.The size of the target plate is 200 mm ×200 mm, the effective diameter is 180 mm, and the thickness is 2 mm.Mullin et al.Error!Reference source not found.conducted a comparative test of zero pressure and about 133 Mpa when studying the experiment of the blunt nosed projectile impacting the steel-aluminum double-layered target plate.They found that the ballistic limit mainly depended on the order of the target plate and had little relationship with the pressure of the in-contact type, so the bolt link was directly adopted between the target plate and the target chamber.The material of the projectile body is 38CrSi alloy steel after quenching, with a hardness of 53HRC and a nominal mass of 34.5 g.The shape and size of the projectile body and target plate are shown in Fig. 2. To ensure the normal impact of the projectileError!Reference source not found., a hole with a diameter of 6.2 mm and a depth of 10 mm was machined at the back of the projectile to ensure its force balance.

Finite element modeling
The 3D finite element model of four groups of aluminum alloy target plates with hemispherical nosed projectile normal impact was established using commercial finite element software ABAQUS/Explicit, as shown in Fig. 3. Similar to the experiment, the only variable in each set of simulation models is the bullet's impact velocity, which is changed to accurately calculate and determine the ballistic limit of the target plate.Due to the projectile's higher stiffness than the thin aluminum target, it was modeled as a rigid body Error!Reference source not found., which was divided into uniform grids with a size of 1×1×1 mm 3 .The thickness of the target plate is 2 mm, and the free span is 200 mm.The target plate is defined as a plastic deformable body, and the nodes of the outer ring are entirely fixed.The target grid was divided by transitional mesh, and the mesh size of the central region with large plastic deformation was 0.2×0.2×0.2 mm 3 .Both the target plate and the bullet are C3D8R (8-node linearly reduced integration unit with reduced integration with hourglass mode control) mesh generation.Possible Contact between the projectile and the plate and self-contact of the plate material is defined as the General Contact method in ABAQUS.The "Hard" Contact algorithm was adopted to simulate the pressure-interface clearance relationship or normal behavior.Gupta et al.Error!Reference source not found.compared the effects of friction coefficients 0,0.2 and 0.5 on ballistic resistance and found that the change of friction coefficient in high-speed impact had little impact on the results.Set the friction coefficient or tangential behavior between contact surfaces to 0.1.Since the high-speed impact process often accompanies the hightemperature phenomenon, the temperature effect cannot be ignored.Therefore, a predefined temperature field is applied to the model, and the initial temperature is set as room temperatureError!
Reference source not found..For common constitutive fracture, there is a classic JC(Johnson-Cook) constitutive fracture modelError!Reference source not found., and the expression is as follows: Moreover, an improved semi-theoretical and semi-empirical Where eq is Von Mises equivalent stress; eq is equivalent plastic strain; A is the yield strength of the material at the reference strain rate and temperature; B, Q, and n, β are strain hardening coefficient and exponent, respectively;  is the correction coefficient; C, p, and m are strain rate hardening coefficient, temperature softening coefficient and index; * eq eq 0 ε = ε / ε Is the dimensionless strain rate, eq ε is the current strain rate and 0 ε is the reference strain rate;

W=
σ dε £W (10) where: 1 σ is the first principal stress, and 1 σ is the non-negative value of 1 σ , that is, when σ =0 It is obvious that fracture cannot occur in the absence of tensile stress, i.e., 1 σ should be zero in the compressive stress state.Wcr is the failure constant, which can be obtained by integrating the principal stresses over the equivalent plastic strain paths in uniaxial tensile tests.
Under the impact of a hemispherical projectile with a velocity of 167.41m/s, the simulation results of a double-layered AA1100 are shown in Fig. 4 below.By comparing the failure modes, it can be found that the simulation results of JC constitutive +CL fracture are basically consistent with the experimental results.The residual velocity of the simulation is 134.103m/s, and the error between the simulation and the test result is only 5.43%, which is within the acceptable range AA7075 double-layered adopts the MJC constitutive fracture simulation scheme.As shown in Fig. 5, it can be found that the failure mode of the front and rear plates with MJC constitutive fracture is consistent with the test.For the three tests with an initial velocity of about 160m/s, the average residual velocity is 104.11m/s, and the simulated residual velocity is 106.87m/s.It's also within the test error velocity.Therefore, it can be considered that the simulation models of two kinds of aluminum alloy double-layered plates have been successfully established in this paper.Tab. 1 Constants of material models for 1100-O aluminum alloy

Results and discussion
In this paper, the first stage light gas gun system is used to test the impact of nose shapes (flat, hemispherical) on different order targets (1100+1100, 1100+7075, 7075+1100, 7075+7075).The initial-residual velocity curves of flat nosed projectiles and hemispherical nosed projectiles penetrating the four aluminum alloy target plates were plotted based on the measured initial and residual velocity, as shown in Fig. 6.Then, the R-I formulaError!Reference source not found.
proposed by Recht and Ipson was used to fit each series of data points: where bl v is the ballistic limit velocity, a and p are the model parameters, where p p pl a = m / (m +m ) , p m and pl m are the mass of the bullet and the plunger respectively.For blunt projectile, when the target plate is penetrated, it often produces an impact plug, a is less than 1; The value of p is generally between 1 and 3 Error!Reference source not found..The ballistic limit and model parameters of various ballistic impact experiments are obtained by the least square method.

Test results
By comparing the ballistic limits, it is found that the double-layered AA1100 plate has a large difference in the trajectory of different nose shapes.Under the impact of hemispherical shells, it can absorb energy through extrusion deformation, but in the face of the impact of flat nosed shells, it directly takes place tensile tearing, resulting in a small energy absorption capacity, which also indicates that the Al1100 target plate has poor tensile shear capacity.The other target plates did not change the trajectory limit significantly for the change in nose shape.For different aluminum alloy target stacks, the combination of Al1100 and Al7075 has stronger penetration resistance, which is contrary to some scholars' conclusions Error!Reference source not found.Error!Reference source not found.Error!Reference source not found.and similar to Flores-Johnson's conclusion Error!Reference source not found..Although several scholars study the laminated combination of steel and aluminum plates, Flores-Johnson does not adopt the same thickness for different targets but uses the method of equal plane density.However, the double-layered aluminum alloy plate used in this paper has little difference in density between the two kinds of aluminum alloys.Therefore, under equal surface density, the shield structure should be placed with materials with high toughness and low strength on the side facing the bomb to improve the protective ability.

Impact process
In the simulation verification, the impact process simulation is especially important, this paper chooses two more typical impact processes to show.The test and simulation results of the penetration process are shown in Fig. 7, from the test and simulation results, it can be clearly seen that the hemispherical nosed projectile impacted the AA1100 target plate, the first extrusion deformation, and then tearing to produce the punch plug, while the AA7075 target plate is radial cracking.the combination of the punch plugs of the rear panel of the AA7075 is one, while the punch plugs of the rear panel of the AA1100 are two round punch plugs.This was due to the higher strength of the AA7075 target plate, which caused the punch plugs in the front panel to be extruded to fit onto the slug, making the slug head sharper, resulting in radial cracking with large cracks.
In the test and simulation of the flat nosed projectile impacting the double-layered aluminum alloy target plate, a large and a small punch plug appeared in the combination of AA1100+7075, and the large punch plug was caused by the flat nosed projectile's shear plus tensile tearing of the AA1100 target plate, so it started to fracture from the face of the meeting projectile, while the small punch plug was caused by the fracture due to the deformation of the AA7075 target plate being too large, so it started to fracture from the face of the back projectile.After the small plug flew out, the large plug flew out with the bullet due to the pressure of the rear panel.The other target panels are all two same-sized plugs, and a similar phenomenon is simulated in the simulation.

Ballistic curve
Ballistic limit is another important method to validate the simulation, due to the test itself is affected by a variety of factors [26], even at the same velocity and the same structure, under the same warhead, the impact results still have a large difference.Therefore, in this paper, the ballistic curve fitting is adopted to minimize the impact due to individual test errors.From the Fig. 8, it can be seen that the simulation results are overall in the high-speed stage of the fit, while in the ballistic limit near the fit is worse, this point flat nosed projectile impact in the double-layered AA7075 target plate embodiment is more obvious.This is supposed to be due to the fact that the intrinsic fracture model considers the materials to be isotropic, while the anisotropy of 7075 aluminum alloy exists for the reason that, coupled with the fact that, near the ballistic limit, the slightest difference in the target plate may lead to a large change.

Effect of target plate shape on protection performance
Comparing the test results under the impact of the two nosed shape, it is not difficult to see that the simulation of the flat nosed projectile is more consistent with the test results, so the simulation of the flathead projectile impacting the different shapes of the target plate is carried out in this paper.

Penetration process of the target plates of different shapes
The Von Mises stress changes during the double-layered hemispherical shell, arched, and Lsquare plate impacts of the flat nosed projectile are shown in Fig. 10, Fig. 11, and Fig. 12.It can be clearly seen in Fig. 10 that for the hemispherical shell, the Von Mises stress is generally larger when the flat nosed projectile impacts the AA7075 target plate, while the 1100 aluminum alloy target plate with better toughness results in smaller stress because of the larger deformation, and this phenomenon is especially obvious when the 1100 aluminum alloy target plate is the rear panel.This is because when the projectile impacts on the front panel, it is double-layered plate at the same time force; and impact on the rear panel, only one layer of target plate force, and due to the high toughness of 1100 aluminum alloy, the impact, the spherical shell produces a more easy to produce the bending deformation to absorb the energy, When the flat nosed projectile impacts the L-square plate, the 1100 aluminum alloy is flattened due to its lower strength the right-angle structure is flattened, and a round punch plug is eventually formed.When the 1100 aluminum alloy plate breaks and produces a punch plug, the right-angle structure near the punch plug along the direction of the prongs will be concave, and it will eventually be torn and disconnected, and therefore a round concave hole is formed after the impact.The 7075 aluminum alloy target plate is due to the high strength, brittle fracture occurs at impact, along with the surrounding target plate together with the tear, so the punch plug is to target plate right-angled part of the boundary, divided into the upper and lower parts of the target plate, and 1100 aluminum alloy is different from the target plate along the direction of the prismatic direction does not occur in the phenomenon of concavity.The arched target plate experienced a similar phenomenon to the plane plate, and in the AA1100+7075 lamination approach, the AA7075 target plate experienced petal-like cracking, while the double-layered AA7075 target plate produced two circular punch plugs.All four shapes of target plate in AA7075 + 1100 this stacking method appeared in the AA1100 stress is very small (c in the three figures), this is due to the bullet impact 7075 aluminum alloy target plate, 1100 aluminum alloy due to the toughness of the higher and can not provide enough support, resulting in AA7075 target plate stress compared to AA1100 will be very large.Plus, when the bullet penetrates the AA7075, the AA1100 will have less stress compared to the AA7075 due to the larger deformation.
Similarly, due to the presence of the arched structure, the AA1100 target plate undergoes large deformation absorption similar to that of the hemispherical shell structure.

Ballistic limits of target plates with different shapes
The ballistic limits of the three other shapes of target plates and the percentage of lift relative to the plane are shown in Tab. 4. It can be seen that regardless of that arrangement, the ballistic limit of the arched plate and hemispherical shell compared to the flat plate are improved, hemispherical shell on the ballistic limit of the enhancement of the ballistic limit of the double-layered 1100 aluminum alloy ballistic limit of 51.48%, and AA7075 in front of the AA1100 in the back of the aluminum alloy target plate combination of the ballistic limit is improved by 58.76%, and compared to the other sort of the target plate, the double-layered 7075 aluminum alloy target plate hemispherical shell enhancement is not so obvious.It can be seen that the spherical shell shape of the target plate for high toughness, low strength of the metal, the ballistic limit of the enhancement is particularly obvious, for low toughness, high strength of the metal enhancement is relatively poor.Arched plate and L-square plate ballistic limit is in between the plane and hemispherical shell, For the arched plates, the ballistic resistance of the target plate for both combinations of AA7075 in the rear is a bit higher than that of AA1100 in the rear, similar to the test results for the flat plates.Unlike the plane plate, the hemispherical shell structure with high strength and low toughness in the front, and high toughness and low strength in the back of the sequencing is more able to enhance the overall ballistic limit of the target plate.L-square plate in the high-speed state, different sort of target plate ballistic resistance there is a difference, but near the ballistic limit of the near ballistic limit but almost converge together, this may be the structure itself brings the This is probably due to the ballistic resistance of the structure itself, and the material properties of the target plate itself have less influence on it.However, due to the lack of other control tests, the conclusion needs to be further verified.

Energy absorption of target plates with different shapes
In addition to the impact resistance of the target plate being characterized by the ballistic terminal velocity, the energy dissipation can also be used as an evaluation index.By the law of conservation of energy can be obtained before and after the impact of the projectile target plate energy changes: that is, the change in the kinetic energy of the projectile, but also the total energy absorbed by the target plate.The total energy absorbed by the target plate mainly consists of the plastic deformation energy of the target plate, the friction dissipation energy and the kinetic energy of the target plate, while the plastic deformation energy of the target plate is the most important part of the kinetic energy absorbed by the target plate Error!Reference source not found.. Fig. 13 shows the energy absorbed by the plastic deformation of the front and rear panels of the target plate under the impact of a flat nosed projectile with an initial velocity of 180m/s.The two arrangements of the front panels are larger than the rear panels.Overall view in addition to the L-square plate AA7075 in front of the two arrangements is the front panel absorbed energy is greater than the rear panel, and the rest of the front panel is smaller than the rear panel, combined with the impact process, these two groups are the only target plate did not form a circular plug, which shows that the target plate to form a circular plug absorbed by the plastic deformation of the plastic deformation of the energy absorbed is greater than that absorbed by the plastic deformation due to tearing.
The highest plastic deformation of the front plate of hemispherical shell and plane target plate is AA1100+7075 combination target plate, the highest plastic deformation energy of the front plate of L-square plate and arched plate is AA7075+7075 target plate, and the highest plastic deformation energy of the rear plate is AA1100+7075 combination except for the hemispherical shell plate which is AA7075+1100 combination.The absorption of energy by the L-square plate is higher than other target plates.The total energy absorbed by the L-square plate is higher than that of the other target plates, and it is also found in the previous section that the higher the initial velocity, the higher the energy absorption of the L-square plate is than that of the other target plates.The highest plastic deformation of the front panel of the L-square plate and hemispherical shell plate is the AA1100+7075 combination target plate, the highest plastic deformation energy of the front panel of the L-square plate and arched plate is the AA7075+7075 target plate, and the highest plastic deformation energy of the rear panel is the AA1100+7075 combination except for the hemispherical shell plate which is the AA7075+1100 combination.The L-square plate is higher than the other target plates in terms of energy absorption.L-square plate is higher than other target plates, and in the previous section, it is also found that the higher the initial velocity, the higher the energy absorption of L-square plate is than other target plates.
The highest energy absorption of the front plate are L-square plate, the lowest is the panel, and the highest energy absorption of the rear plate, except for the AA7075+1100 laminated method of the hemispherical shell plate and the arched plate is not much difference, the rest are the highest energy absorption of the hemispherical shell plate.The front plate with an L-square plate after the plate with a hemispherical shell of the laminated method may have excellent protection.
In order to clearly visualize the proportion of the plastic deformation energy absorption of the target plate to the total energy absorption, all the energy was divided by the overall energy absorbed by the target plate to make it dimensionless Error!Reference source not found., and Fig. 14

Conclusion
In this paper, the test of a double-layered aluminum alloy target plate impacted by the flat and hemispherical nosed projectile at high speed and the corresponding numerical simulation analysis were carried out, and based on verifying that the model is correct, the influence of the target plate shape on double-layered aluminum alloy target plate was further explored.The effects of the flat plate, arched plate, hemispherical shell plate, and right angle (L-square plate) on the double-layered aluminum alloy plate are analyzed through test and simulation data.Based on the experimental data and numerical simulation results, the following conclusions were obtained: (1) For different aluminum alloy target plate stacks only, the 1100+7075 laminate sequence is more impact resistant.In other words, the protective end should allow the high toughness, low strength material to be placed on the ballistic side of the face to enhance protection.This phenomenon is also found in arched and L-square plates.However, the reverse order is found in hemispherical shell structures, which better enhances the overall protective performance of the target plate.
(2) In the hemispherical nosed projectile high-speed impact of the two metal mixing order of the target plate than double-layered 1100 aluminum alloy ballistic limit is also low, can be seen on the smooth debris impact, the different strengths of the mixed metal will also reduce the protective performance.
(3) hemispherical shell plate for high toughness, low strength metal ballistic limit is particularly obvious, for high toughness, low strength metal should try to make hemispherical shell shape protection.
(4) Plastic deformation of a single layer of the target plate absorbed energy size of the target plate has no decisive effect on the overall energy absorption.The front plate with the highest energy absorption is the L-square plate, and the back plate is the hemispherical shell.If the combination of an L-square plate for the front plate and a hemispherical shell for the rear plate is used, it may have a good protective effect.

Fig. 1
Fig.1 Schematic diagram of ballistic impact test equipment.

Fig. 2
Fig.2 The different nosed shapes of projectiles

Fig. 3 A
Fig. 3 A 3D finite element model of the ballistic impact test In order to simulate the actual situation of the bullet target in this complex situation, this paper combines the experimental results of the target plate intrinsic fracture model, first of all, the doublelayer aluminum alloy target plate of the same material is simulated, and then the laminated layers of different materials are analyzed after getting the verification.Given the extensive research conducted by numerous scholars in the past, which has yielded detailed verification, this study incorporates the JC constitutive model and CL fracture criterion to describe the properties of 1100 aluminum alloy after referring to relevant literature data and conducting testing.Additionally, for 7075 aluminum alloy, the MJC constitutive fracture model was selected.

η
T -T / T -T is the dimensionless temperature, T, Tr, and Tm are the current, reference temperature and the melting point of the material.= σ / σ =(σ +σ +σ ) / (3σ ) Is stress triaxiality, 13 σσ is the first, second, and third principal stress in turn, m σ is hydrostatic pressure, eq σ is von Mises equivalent stress, and D1-D6 are material performance parameters.Parameters D1-D3 related to the stress state are obtained by fitting the fracture strain of quasi-static tensile specimens of round bars with different notch radii at room temperature.The strain rate correlation coefficient D4 can be obtained by fitting the fracture strain of the round bar tensile specimen at different tensile rates.In addition, the temperature-related parameters D5 and D6 can be obtained by fitting the fracture strain of round bar tensile samples at different temperatures.Børvik et al.Error!Reference source not found.and Holmen et al.Error!Reference source not found.have successfully applied the C-L (Cockcroft & Latham)failure criterion to the impact simulation of various metal materials.The empirical and single-parameter C-L fracture criterion Error!Reference source not found.can be expressed by plastic work W per unit volume:

Fig. 7
Fig. 7 Experimental and simulation process diagrams of different projectile impact double-layered aluminum alloy target plate

Fig. 8
Fig.8The initial and residual velocity curves of experimental and numerical

Fig. 9
Fig. 9 Finite element simulation model of 3 target plate shapes In this paper, flat nosed projectile impact simulation is carried out for three kinds of structural target respectively, and the simulation results are shown in Tab. 3.After that this paper compares the effects of four structures, hemispherical shell, plane, arched and right angle, on the impact resistance of double-layered aluminum alloy target plate.

Fig. 10 Fig. 11 Fig. 12
Fig. 10 Variation of stress in the process of flat nosed projectile penetrating double-layered hemispherical shell target plate

Fig. 13
Fig. 13 Plastic deformation energy of target plates of various shapes was obtained.It can be seen from Fig.14that the plastic deformation energy of the L-square plate in proportion to the total energy loss is the highest among all the shapes of the target plate, and the double-layered 1100 aluminum alloy target plate even reaches 84.32%.The lowest percentage of plastic deformation energy is for the double-layered 7075 arched plate, which is only 51.99%, but its target plate kinetic energy percentage is the highest, reaching 11.61%.The percentage of plastic deformation energy of hemispherical shells and arched plates in all arrangement orders is generally low, which should be related to the arched structure.

Fig. 14
Fig.14 The proportion of plastic deformation energy absorption of plates to the overall energy absorption It is worth noting that the target plate's overall plastic deformation energy absorption of the lowest for the plane plate, while the plane plate in the lowest energy absorption for the doublelayered AA1100 target plate, about 87.396J, but its plastic absorption of energy accounted for the proportion of the overall absorption of energy is very high, reached 81.15%.The highest overall plastic energy absorption for the double-layered AA7075 L-square target plate, accounting for the Meanwhile the 7075 in front and 1100 in the rear ordering, the bullet impacted the target plate for the longest time, even more than the double-layered 1100 aluminum alloy target plate.
Tiwari et al.Error!Reference source not found.also found that this phenomenon.
plate enhances the highest double-layered 1100 aluminum alloy target plate, up 31.87%,