NMV11AETT03

Characterization of Impact Behavior of Composite Car Components

Giangiacomo Minak1, Zlatan Šoškić2, Ana Pavlović2, Cristiano Fragassa1 1University of Bologna-DIEM Department, Italy

2University of Kragujevac, Faculty of Mechanical Engineering Kraljevo, Serbia

NMV11AETT03 - 1 / 14

Abstract: Impact resistance of composite components is usually investigated through series of experimental probes in which the component is hit by impactor, and the impact resistance is described by forces that cause creation of fractures and perforation in the investigated structure. However, these forces depend on characteristics of impactor and the impact, and therefore, do not inherently represent characteristics of the investigated structure. This paper presents analysis of results obtained by experimental studies of impact tests of air-intake manifold made from composite material PA6.6 with 35% of glass fibers. The analysis consists of calculation of various kinematic and dynamic quantities during the experimental impact, with the ultimate aim to investigate alternative possibilities for description of impact resistance of composite structures. Key words: Composites, impact testing, low-velocity, automotive.

1. Introduction Composite materials consist of two or more materials combined in such a way that the individual materials are easily distinguishable. Most composites have two constituent materials: matrix and reinforcement. The reinforcement is usually much stronger and stiffer than the matrix, and gives the composite its good properties. The matrix holds the reinforcements in an orderly pattern, and because the reinforcements are usually discontinuous, the matrix also helps to transfer load among the reinforcements. Composite materials are widely used in aerospace and automotive sectors both for their material properties (low density, high stiffness and strength, resistance to chemical and environmental agents), structural design potentials (ability to design material with desired anisotropic mechanical properties) and manufacturing advantages (energy saving due to low production temperatures and pressures, complicated shaped components can be molded in one process rather than being assembled from components), successfully replacing not only steel, but also light alloys in mechanical structures. ..

 

NMV11AETT03 - 2 / 14

The application of composite materials in mechanical engineering is limited by poor transverse and shear properties of unidirectional composites, which raise concern about their impact behavior. Being that safety is a major priority in the automotive sector, crash performance of vehicles is one the most important aspects of structural behavior both for manufacturers and consumers, making research of high-velocity impacts of composites relevant for automotive industry. Besides, low-velocity impact behavior of composites is also of interest for automotive industry because of variety of accidental hits to which car parts and components are exposed in exploitation conditions, like it is the case with tool-dropping during manufacturing and maintenance operations. Low-velocity impact behavior has special importance because it was shown that low-velocity impacts may cause internal damage within composite structures which, although invisible, may seriously reduce loading capacity of the material. The improved understanding of impact performance will enable automotive engineers to design and test composite products more cost effectively. The theory of impact behavior of composites is still not developed; there is still no constitutive model capable of describing the transition from an un-damaged to a damaged material and further progression of the damage. Furthermore, in spite of numerous efforts and decades-long experimental work, many aspects of impact behavior of composites are still to be investigated and understood by researchers. Such state-of-the art is caused by complex structure and versatility of composites which require studies of the problem from microscopic, mesoscopic and macroscopic point of view. Aforementioned experimental research ([2],[4]-[13]) revealed that impact damage is a complex phenomenon because impact damage appears in multiple forms of damage mechanisms: 1) matrix mode, where cracking occurs parallel to the fibres due to tension, compression or shear; 2) delamination mode-produced by interlaminar stresses of laminated composites 3) fibre mode, when in-tension fibre breakage and in-compression fibre buckling occur and 4) penetration-when the impactor completely perforates the impacted surface. Matrix damage usually takes the form of matrix cracking but also debonding between fibre and matrix. Matrix cracks occur due to property mismatching between the fibre and matrix, and are usually oriented in planes parallel to the fibre direction in unidirectional layers. A delamination is a crack which runs in the resin-rich area between plies of different fibre orientation and not between lamina in the same ply group. Fiber failure mode generally occurs much later in fracture process than matrix cracking and delamination. It arises due to locally high stresses and indentation effects (mainly governed by shear forces) and, on the non-impacted face, due to high bending stresses. Fibre failure is a precursor to catastrophic penetration mode. It was shown [2] that the presence of matrix micro-cracks does not have a strong effect on the laminate stiffness during the impact. However, matrix cracks are initiation points for delamination and fibre breaks that change the local and global stiffness of the plates and modify the force-displacement curve. Delamination Threshold Loads (DTLs) may be recognized from the load-displacement response as the load level at which a sudden loading drop occurs as a result of significant damage. Unlike the previously described mechanisms, penetration is a macroscopic mode of failure and occurs when the fibre failure reaches a critical extent, enabling the impactor to completely penetrate the composite material. Research of penetration impact has mainly concentrated on the ballistic range; however, some low velocity impact work has been performed. Cantwell and Morton [4] showed that the impact energy penetration threshold rises rapidly with specimen's thickness for carbon fibre-reinforced plastic (CFRP). It is very important to identify the mode of failure because this will yield information not only about the impact event, but also regarding the structure’s residual strength. Interaction between failure modes is also very important in understanding damage mode initiation and propagation.

 

NMV11AETT03 - 3 / 14

The complex nature of impact damage of composites has two important consequences: i) there is no single quantitative descriptor of impact damage and impact resistance measure for composite structures and ii) there is no single established testing technique for detecting and quantifying damage mechanisms in composites. Impact behavior of composite structures is usually quantifiying through impact damage resistance (IDR) and impact damage tolerance (IDT). Impact damage resistance (IDR) generally defines a range of measures describing not only the maximum load a composite structure can sustain without suffering appreciable damage, but also the ultimate load at which the load-bearing capability of a composite structure is reached. In investigation of laminated composites is established that IDR comprise two separate elements, namely, resistance to the onset of delamination and its subsequent propagation, and resistance to fibre shearout. Impact damage tolerance, on the contrary, assesses the ability of damaged composite structures to retain residual compressive strength in terms of a damage measure which is ideally the same as that used in IDR. There are several impact test techniques developed for experimental research of composite impact resistance, like swinging pendulum, drop-weight and rail-supported gun methods that are used as low-velocity test methods. One of the simplest, yet very powerful research methods for investigation of impact behavior of composite structures is drop-weight test, in which impactor of known mass falls from selected height on investigated composite structure, which is supported in an appropriate manner that depends on the aim of the investigation. The impactor is instrumented with force measurement sensors that enable recording of force variation during the impact, and the other quantity that is measured in drop-weight impact test is velocity of the impactor immediately above impacted surface, which enables direct calculation of initial and final energy of the impactor. Impactors are usually made of mild steel, in order to behave during the impacts as rigid bodies, compared to composite structures. The advantages of using an instrumented drop-weight impact test are: (1) the initiation and development of damage during impact may be identified from a recorded impact force-time history curve; (2) several impact parameters can be examined; and (3) wide range of incident kinetic energies may be achieved by changing drop height and impactor mass. The nature and extent of damage mechanisms arising within drop tests are affected by a large number of parameters such as shape and mass of impactor, impact velocity, types of fibre and matrix, interfacial treatment, fibre volume fraction, layup, laminate geometry, boundary condition and even pre-stress of the considered composite structure. Low-velocity drop-weight impacts are considered as quasi-static processes; the criterion for low-velocity impacts was researched and discussed for some time [7], and it is quantitatively most clearly articulated through the request that the whole area of structure below impactor undergoes approximately uniform deformation; the request may be expressed by the equation that the low velocities must satisfy condition Cv c ε<< ⋅ , where c stands for speed of sound, and Cε for failure strain of the material. Drop-weight test is usual industrial method for testing composite structures. The procedure of testing is standardized by international industrial standards, but is in some cases a subject of arrangement between manufacturer and customer. In latter procedures, experimentally determined forces and visual inspection of damage are used as criteria for estimation of impact behavior of the composite structures. It has been shown in research of laminated composites [10] that, during low-velocity impacts, the impact force versus displacement curves allow identification of two thresholds: the first damage force and the maximum force. Their values remain substantially constant with the impact energy if the impact energy is large enough to let the force reach the thresholds. Besides, the same research showed that it is possible to define two parameters of interest for the designer: the saturation impact energy (i.e. the maximum energy bearable by the material without perforation) and the damage degree (i.e. the ratio between the dissipated energy and total energy of the impact). In [14], authors followed the procedures of continuum damage mechanics, [1], [3] and defined a damage parameter, D, which can be estimated from the following expression

 

NMV11AETT03 - 4 / 14

[1]: 1 * /D E E= − , where E* is the current stiffness and E is the initial stiffness, having the value for the undamaged material. This paper presents initial results obtained during research which is being made with intention to improve industrial procedures for low-velocity testing of composite components for car engines. The research has several directions, and the one presented here is investigating variation of mechanical quantities during drop-weight impact test of composite structure with the goal to improve understanding of impact between the impactor and the investigated structure, and the ultimate purpose to determine mechanical quantities that would enable estimation impact behavior of composite structure during design phase.

2. Experiment

2.1. Experimental setup The experiment consisted of series of drop-weight impact tests performed on CAB air-intake manifolds manufactured by Italian manufacturer "Magneti Marelli", which are used in engines of German manufacturer "Audi".

Fig. 1: Air intake manif old CAB (inlets marked by letters A,B,C and D). In top-right corner is visible impactor.

The CAB air intake manifolds (Error! Reference source not found., in the following text just CAB) are manufactured from molding compound Ultramid, representing a composite consisting of PA66 polyamide with 35% of glass fibers. The component is manufactured by injection molding process, and glass fibers are oriented in general direction of flow. The nominal density and tensile modulus of the material at room temperature, held during the experiment, are respectively ρ = 1410 kg/m3 and E = 11,5 GPa, which leads to an estimation of speed of longitudinal mechanical waves in the material /c E ρ= ≈2890 m/s. The material is characterized with comparatively very high brake strain, Cε : 3-5%, which combined with the estimated speed of longitudinal mechanical waves leads to an estimated upper limit for low-velocity impacts of C Cv c ε= ⋅ : 90 m/s. Impact tests were performed on eight different samples of CAB and on each of them was tested impact resistance at four points, denominated as A, B, C and D, located at the end of flat part of four different inlets of air intake manifold, as it is shown in Error! Reference source not found.. Selection of the points was made in accordance with testing procedure defined by manufacturers of the component and the engine, and the influence of geometry of the component is going to be discussed in separate paper, dedicated to the procedure itself; however, it will be noticed here that the results did not reveal any significant difference between the forces measured during impacts performed in points A,B,C and D. It appears that the influence of differences in geometry of surroundings of the points, caused by different positions of fasteners on inlets, is negligible, and that the dominant factor which influences impact resistance of the component is the thickness of the material. Therefore, for the sake of the analysis presented in this paper, points A, B, C and D will be considered equivalent.

 

NMV11AETT03 - 5 / 14

Fig. 2: Drop-weight

machine

The impact tests were carried out using a drop-weight machine (Error! Reference source not found.) equipped with an electro-optic device, for measurement of initial and final velocity of the impactor, and with a piezoelectric load cell attached to the impactor, for measurement of contact force history. The impactor head had the shape of hemisphere with 12.7 mm diameter. Multiple collisions were avoided by means of an electromagnetic braking system. A detailed description of the machine can be found in [15]. The impactor mass was 1.22± 0.01 kg. According to the results described in introduction [10], which show that damage forces do not depend on impact energy, the initial height was kept constant during the experiment at level of h = 0.8 m, corresponding to a nominal potential energy of approximately 10 J, also in agreement with the testing procedure defined by the manufacturers of the component and the engine. For the selected height, the free-fall impact speed is easily calculated to be 0 2v gh= ≈ 3.96 m/s, and being that initial impact speed can be only smaller than free-fall impact speed, it is easy to conclude that the impacts in the experiment may be considered as low-velocity impacts, and treated as quasi-static mechanical processes. The CAB samples were fixed to the specially designed holder by screws, and the holder was in turn fastened to base of the impact test by clamps. Measurement data were acquired at 100 kHz sampling frequency, without additional filtering. The reason for the absence of filtering was that, considering speeds of sound in Ultramid and steel, as well as longitudinal and lateral dimensions of impactor and CAB, it can be roughly estimated

that eigenfrequencies of CAB are to be expected in the range 2.5-25 kHz, while eigenfrequencies of impactor are to be expected in the range 5-50 kHz. That estimation shows that, while significant amount of "ringing" due to impact response of the impactor itself may be expected, it is not possible to use low-pass filters in the considered experiment for separation of response of CAB structure. This conclusion goes in line with the recommendations of [9], although contradicts to procedure applied in [10]. Impacts were repeated with impactor released from the same height until penetration occurred in each of the tested points of the struture. A typical sample of CAB after series of impact tests is shown in Error! Reference source not found.. The damage on the top part of inlets A and D is a consequence of further impact tests, not described here.

Fig. 3: CAB air intake manif old after series of impact tests

3.2. Experimental results Experimental results have shown three characteristic behaviors of the considered CAB structures during impact tests, with typical force histories shown in Error! Reference source not found.. Left side in Error! Reference source not found. shows force history of the first impact with the CAB structure at the considered point. Two sharp drops of force are visible, indicating failures of

 

NMV11AETT03 - 6 / 14

the composite materials. Both are followed with oscillations of contact force with frequency spectra showing presence of vibrations of both impactor and CAB. Generally, the second failure do not have such strong drop of contact force, as the one presented in Fig. 4, but it is present in majority of first shots of recorded force histories of first hit to a certain point. The duration of the impact is approximately 6 ms and the maximal force measured is 1.9-2.1 kN.

Fig. 4. Characteristic f orce histories recorded in experiments

Middle picture in Error! Reference source not found. shows force history of the impacts between the first and the last hit at the considered point. The history is characterized by the same duration and maximal force as in the case of the first hit, but the material does not show any visible sign of failures, and only weak vibrations of CAB structures are detectable by spectral analysis of the measured force. In some cases, failures arise between the first and the second hit, but those failures do not change further impact behavior of the considered structure.

Fig. 5: Comparison of sequence of impacts at the same point of CAB air-intake manif old

Right side in Error! Reference source not found. shows force history of the last impact with the structure at the considered point, the impact that causes penetration of impactor through the CAB structure. The impact lasts between 2 and 3 ms, and the force in the moment of perforation is around 10% smaller than during the previous hits. It is of importance to point out that variations of force during impacts that precede the impact with perforation do not show any sign of reduced strength of the structure that causes structural breakdown during the impact with penetration; furthermore, even the variation of the force during the initial part (approximately lasting between 1 and 2 milliseconds) of the impact with penetration is the same as during the previous hits and, without any precursor of incoming breakdown. The curves in Fig. 5 shows the same force histories as in the Fig. 4, but this time they are shown together to demonstrate matching of force histories between the initial and final failure of the structure. It is visible that the mechanical resistance of the material drops sharply after the first failure, and decreases gradually immediately before the final perforation of the structure. Between those two events, which do not have clear precursors detected, the structure behaves remarkably repeatable. Hence, as it was also noted earlier, the failures recorded between the first and the last failure of material do not change its impact behavior.

 

NMV11AETT03 - 7 / 14

Fig. 6: Comparison of force histories of the f irst and last impact hits to different points of the same CAB samples

In the Fig. 6 are presented force histories of the first and last impact hits to different points of the same CAB samples: while the histories of the first hits to different points overlap completely, there is a small, but noticeable difference between time histories of the penetrating hits: it can be observed that at the points where mechanical resistance of the structure was higher, perforation forces were also a bit higher and perforation occurs generally faster. However, no clear explanation is still found for the observed differences noticed during the penetrating hit. The number of hits needed to cause a perforation of the structure significantly varies from sample to sample and from point to point, although it can be initially guessed that thickness of the sample contributes to it. The occurrences of number of hits until perforation are given in the Error! Reference source not found..

Avg Std Dev A 2.13 1.46 B 1.63 0.74 C 2.00 0.76 D 5.00 4.14

Table 1: Av erage number and standard dev iation of hits bef ore perf oration

The results are illustrating large deviation of experimentally obtained results and the fact that the number of hits needed for perforation of structure in point D is larger than in other points; however, considerations of geometry of CAB structure and even variation of material thickness did not revealed valid explanation for the observed trend.

3. Analysis

3.1. Methodology of the analysis The essence of the research presented in this paper is study of variation of kinematic, dynamic, material and structure properties during impact testing processes with the aim to properly characterize impact process and enable characterization of impact damage and impact resistance of the considered mechanical structure. Therefore, in the process of the data analysis, the goal is to calculate and analyze as many quantities as it is possible, trying to understand their variation during the impact testing procedure, and to establish their relevance and connection to damage caused by the impacts. Initial part of the analysis of the experimental data was calculation of kinematic and dynamic properties describing the motion of impactor. In the analysis were made the following assumptions:

• the mass of the part of the impactor between piezoelectric sensor and contact point is small in comparison with the mass of the impactor, so the force acting upon piezoelectric sensor can be considered equal to the contact force between impactor and the investigated CAB;

• the impactor can be considered as a rigid body, so that the energy of vibrations of impactor can be neglected in comparison to the energy transferred to CAB structure;

 

NMV11AETT03 - 8 / 14

• the mass of the part of CAB structure that is significantly affected by impact is small in comparison with masses of impactor and the CAB, and displacement of the part is small in comparison with initial height of impactor, so that change of gravitational potential energy of CAB structure can be neglected;

• fixing clamps prevent motion of CAB structure as a whole, so that kinetic energy of the CAB as a whole may be neglected during impact;

• the motion of impactor may be considered as rectilinear, and may hence be described by one-dimensional model.

Knowing the mass of the impactor and measured contact force history, Newton's law enables calculation of acceleration of the impactor, the first integration of acceleration gives the velocity, and the second integration gives the displacement of the impactor as function of time. In the text that follows, vertical axes with upward orientation will be used for description of motion, so all vector quantities oriented upwards will be represented with positive sign. The equation of motion can easily be integrated imposing initial conditions. As initial time for the calculation is taken the moment of the initial contact between impactor and CAB structure, while the upper surface of un-deformed CAB structure is taken as the reference level for displacement, with the initial coordinate (0) 0z = . The initial value of the velocity, needed for the first integration, is measured by the electro-optical device that gives the actual impactor velocity immediately before the impact v0. Calculation of velocity as a function of time enables calculation of kinetic energy of impactor in each moment, and the principles of the energy balance in low-velocity impact composite tests are discussed in details in [10]. Essentially, under the assumptions stated above, kinetic energy of impactor is transformed into:

• elastic energy of composite structure that will be transformed back to kinetic energy of rebounded impactor after the impact;

• energy of damped vibrations of the CAB structure that is dissipated through internal friction;

• work against the forces of chemical bonding in processes of material failure, causing fractures and defragmentation of the material.

By calculation of variation of kinetic energy during the impact it is possible to identify and calculate the first part of energy of elastic deformation of the composite structure, because it is equal to the kinetic energy of rebounded impactor. On the other hand, separation and calculation of the latter two forms of energy, which are irreversibly transferred during the impact, is not directly possible from experimental data. The other quantity that may be calculated after integration of force histories is power of contact force during the impact. While not usually calculated in analyses, power, as a product of contact force and velocity of impactor is taking into account effects of strain rate and may reveal its potential effects. Besides the quantities describing the motion of impactor, in this analysis an effort was made to estimate effective elastic properties of CAB during the impacts. It was done by calculating force-displacement curves describing the impacts, followed by calculation of their initial slopes. Using the assumption that low-velocity impact may be considered as quasi-static process, initial slope of the force-displacement curve may be interpreted as effective stiffness.

3.2. Results of the analysis The typical results of calculation of kinematic quantities describing motion of the impactor during the impact are shown in Fig. 7. In the cases of rebounding impactor, the presented diagrams indicate the moment of maximum displacement of composite structure surface by intersection of velocity

 

NMV11AETT03 - 9 / 14

history with abscissa, indicating the moment when the impactor is stopped by the CAB. In the case of perforation, maximum displacement of composite structure surface arises in the end of the displacement history, indicating the moment before penetration.

Fig. 7: Variation of kinematic quantities (acceleration a, v elocity v and displacement z)

describing motion of impactor during impact tests; f orce history is also shown as ref erence for comparison with other diagrams

The typical results of calculation of dynamic quantities describing motion of the impactor during the impact are shown in Fig. 8. It is obvious that the sum of kinetic energy of the impactor and the energy transferred to impacted structure equals initial kinetic energy of impactor. The part of energy that is irreversibly transformed during the impact can be seen in diagrams as the final value of the transferred energy.

Fig. 8: Variation of dynamic quantities (kinetic energy W, transf erred energy Q and contact force power P)

describing motion of impactor during impact tests; f orce history is also shown as ref erence for comparison with other diagrams

The typical results of calculation of quantities that describe elastic properties of the CAB structure are shown in Fig. 9. The force-displacement diagrams represent open curves, which are not returning to coordinate origin. However, when contact force become small, the difference between the force measured by sensor, the contact force and elastic force acting upon structure becomes important, so no definite conclusions are to be made here. However, it is noticeable from the presented diagrams that the stiffness of the structure is significantly reduced after the first material failure, but remains constant during the following failures until the perforation of the investigated structure happens.

3.3. Discussion Dynamic quantities are much more used in literature for description of low-velocity impact tests. First will be discussed dynamic quantities used most frequently in literature: first damage force, maximal force, perforation force, relationship between the aforementioned forces and damage degree.

 

NMV11AETT03 - 10 / 14

Fig. 9: Force-displacement diagram at one point CAB during subsequent impacts

First damage force detected in the impacts varied between 1.62 kN and 1.75 kN, with mean value 1.70 kN and standard deviation 0.04 kN. In additional tests with different initial heights of impactor, first damage force showed independence of impact energy, and therefore, first damage force can be used as lower threshold for occurrence of damage in the considered composite structure. Maximal measured force during the experiments varied between 1.92 kN and 2.03 kN, with mean value 1.99 kN and standard deviation 0.05 kN, while perforation force varied between 1.85 kN and 1.95 kN, with mean value 1.87 kN and standard deviation 0.05 kN. In discussing the results obtained in the analysis, it is very important to point out the observed fact that forces in moment of perforation were smaller than maximal forces recorded during previous impacts. While maximal force measured in the experiment represents upper limit of force that the structure is capable to bear without perforation, it is not representing its bearing capacity, because perforation may occur at lower forces. Initial statistical analysis performed showed negative correlation between first damage force and perforation force (RB1=-0.959) and between first damage force and maximal force (Rmax1=-0.881), while the correlation between maximal force and perforation force is positive and substantially higher (RBmax=0.990). Such correlation suggests that the higher the first damage force is - the larger is the extent of damage and weakening of the structure it caused. From the energetic point of view, first was calculated the damage degree DE, defined as the ratio between transformed energy Q and kinetic energy of impactor before the impact W0, 0/ED Q W= . The value of the quantity has indeed different values for the three cases of impact behavior that were presented in description of experimental results: impacts that caused initial damage had damage degree 0.71±0.01, impacts that did not indicated discontinuities in force history had damage degree 0.68±0.01, and impacts that caused perforation have shown damage degree 0.87±0.07. However, the relevance of the damage degree defined in such a way is limited for characterization of the damage, because it really makes strong distinction only between the cases of non-perforated and perforated structure, while in practice it is more important to differentiate the cases of intact and invisibly damaged structures. From the calculated energies may, however, be derived the following estimation of energy that is dissipated in failures. Let it be postulated that dissipated energy may be represented in the form Q=n·Q1+Qper+Qrest , where Q1 represent the energy dissipated during failure that does not cause perforation, n represents number of such failures during an impact, Qper represents energy dissipated during perforation, and Qrest the dissipated energy through non-indicated-failure mechanisms. Then, the energy dissipated during failure that does not cause perforation, Q1, may be estimated through calculating difference between energy dissipated in impacts that cause only such failures and energy dissipated in impacts do not indicate any failure. Similarly, the energy dissipated during failure that causes perforation, Q1, may be estimated through calculation of the difference between energy dissipated in impacts that cause perforation and energy dissipated in impacts do not indicate any

 

NMV11AETT03 - 11 / 14

failure. Besides, values of energies dissipated in failures may be estimated by comparing energies dissipated in impacts that cause different kinds of failures indicated in force histories. Using this approach, a very consistent estimation of Q1=0.30J±0.05J can be derived, while the estimation of Qperf varies in fairly wide range between 1.4 J and 2.3 J. More detailed analysis of these results is going to follow, with a final intention to establish a simple model of low-velocity impact behavior of the considered composite structure, capable of simulation of experimentally observed phenomena. Another considered dynamic quantity that was not usually analyzed in literature is the power of the contact force during the impact. As explained, this quantity accounts for the speed of impactor during impact, and is not only sensitive to the force that acts on impacted structure, but also to the speed at which the structure moves. Therefore, power histories may have potential of being more distinctive or showing more indications than force histories regarding onset of failures in composites. However, no new indications have been revealed in analysis of power histories during this research, as it is illustrated in Fig. 10. The result goes in line with conclusions of research presented in[10], stating that strain rate does not affect low-velocity impact behavior of composite structures.

Fig. 10. Power history of impact causing initial damage of structure. Force history is also shown f or ref erence.

The only characteristic feature of power histories that could be useful in analysis and modeling of impact behavior of composite structures is that power has reduced sensitivity to failures that occur at small velocities, showing discontinuities only for failures that occur at higher velocities. Therefore, power histories might be useful for distinguishing between failure mechanisms.

Fig. 11: Comparison of displacements of one point of CAB during different phases of impact test

Regarding the histories of kinematic quantities, the most important conclusion that can be drawn is that variations of kinematic quantities during impacts do not show any precursor of reduction of material strength and incoming perforation of the structure that occurs during impact that causes the structural breakdown. Besides, when considering some points of the investigated structure, displacements in moment of perforation were smaller than maximal displacements recorded in previous impacts. The fact is illustrated in Fig. 11, and is similar to the previously established fact

 

NMV11AETT03 - 12 / 14

that the force in moment of perforation was smaller than the maximal forces which acted during previous impacts. This result is opposite to what is presented in [10], where it was found that perforation occurs in the moment of maximal displacement. The ratio ξZ between displacement in moment of perforation zB and maximal displacement of structure in certain point zmax, might be of interest; in the tests performed within this research, it was found that it varied between 0.89 and 0.97, with mean value 0.93 and standard deviation 0.03. The diagrams of histories of kinematic quantities show also the fact that when the impactor rebounds, it happens before the moment when composite structure surface reaches its un-deformed position. Several reasons may contribute to this behavior such as strong damping of CAB structure motion due to internal friction, but they are not going to be discussed here. The last analysis that was performed was calculation of equivalent stiffness of CAB structure at impact points during impact testing procedure. The calculation was performed by computation of the slope of linear part of force-displacement curves derived from force histories. The other interesting possibility, calculation of dynamic stiffness of structure during impacts, has not been performed in present research and remains to be investigated. Considerations and calculations showed that the equivalent stiffness has two important characteristics that make it better indicator of the state of investigated structure than it is the case with previously used damage degree, which was based on calculation of energy variation:

• equivalent stiffness changes only during the onset of new kind of material failure;

• equivalent stiffness behaves like state variable, having clearly distinct values in intact, damaged and broke structure;

• equivalent stiffness is characteristic of structure, not the impact itself or impactor, and may be measured independently, without exhibiting the structure to potentially destructive impact tests;

• equivalent stiffness may be determined in design phase by simulations of structure behavior under external load.

However, equivalent stiffness, as defined and used in this research, does not have capability of being precursor of incoming breakdown of composite structure, as it is also the case with all mechanical quantities analyzed in this research. Aforementioned dynamical stiffness, on the other hand, could be quantity with the potential to be used as the sought for precursor of structural breakdown. Going along the lines of research presented in [14], equivalent stiffness may be used for definition of analogous damage parameter Dk=1-k*/k, where k* represents equivalent stiffness of damaged structure and k equivalent stiffness of intact structure. Intact structure has damage parameter equal to 0, after the first material failure the damage parameter rises to values of 0.35-0.45, and by the definition, the damage parameter of perforated structure equals 1.

5. Conclusions This paper presented an analysis of variation of mechanical quantities during low-velocity impact tests of a composite structure. The experimentally investigated structure was air-take manifold CAB for car engines made of PA66 polyamide with 35% of glass fibers. The experimental setup was designed so that time variation of contact force between impactor and investigated structure were recorded. The experiment revealed three different kinds of impact force histories; the first, with discontinuities that indicated weakening of mechanical resistance of the structure, characteristic for the first hit by impactor; the second, that did not show any discontinuities of force; and the third, with sudden drop of contact force due to penetration of impactor through the structure.

 

NMV11AETT03 - 13 / 14

The experimental data were processed so that time variation of kinematic properties of impactor (acceleration, speed and displacement) and dynamic properties of the impact (contact force, kinetic energy, dissipated energy and contact force power) were calculated, and using these quantities, force-displacement diagrams of the investigated structure at various test stages were drawn and relevant equivalent stiffness in impact points was calculated. The results have shown that equivalent stiffness has the best potential of identifying the state of the structure from the point of view of experimental and theoretical approach. Besides, the results of analysis have confirmed the results of previous research that the force that causes the first failure of structure and force that causes perforation of the structure do not depend on energy of the impact or show strain rate dependence. However, the results of the study did not reveal any precursor of incoming penetration failure in behavior of mechanical quantities except for decrease in stiffness of the structure immediately before the breakdown of the structure. Moreover, it was established that the penetration force and displacement were smaller than the forces and displacements that in previous impacts did not cause critical failure of the structure. Therefore, the research established basis for non-destructive characterization of the state of the impacted structure, distinguishing between intact and damaged structures, but it did not revealed basis for prediction of behavior of damaged structures.

Acknowledgements This research was partially financed by FP7 project No.206929-"SeRViCe" financed by European Commission. Authors wish to acknowledge the valuable comments and suggestions received from Prof. Milosav Ognjanović of the Faculty of Mechanical Engineering of University of Belgrade, as well as help in performing the impact experimental tests provided by ing. Daniele Ghelli from DIEM department of University of Bologna. One of authors (Z.Š.) wishes also to acknowledge the support of the Ministry of Science of Republic of Serbia through project No.37020.

References [1] C.L. Chow, J. Wang, "An anisotropic theory of elasticity for continuum damage

mechanics", International Journal of Fracture, 33, 3 (1987) [2] P.O. Sjöblom, J.T. Hartness and T.M. Cordell, "On low-velocity impact testing of

composite materials", Journal of Composite Materials, 22, 30 (1988)

[3] G.M. Newaz,W.J. Walsh, "Interrelationships of damage and strain in particulate composites", Journal of Composite Materials, 23, 326 (1989)

[4] W.J. Cantwell and J. Morton, "The impact resistance of composite materials—a review", Composites 22, 347 (1991)

[5] S. Abrate, "Impact on laminated composite materials", Applied Mechanics Review, 44, 155 (1991)

[6] S. Abrate, Impact on laminated composites: recent advances, Applied Mechanics Review 47, 517 (1994)

[7] M.O.W. Richardson, M.J. Wisheart, "Review of low-velocity impact properties of composite materials", Composites Part A 27,1123 (1996)

[8] S. Abrate, "Impact on composite structures", Cambridge University Press, Cambridge (1998).

 

NMV11AETT03 - 14 / 14

[9] G. Zhou, "The use of experimentally-determined impact force as a damage measure in impact damage resistance and tolerance of composite structures", Composite Structures, 42, 375 (1998)

[10] G. Belingardi, R. Vadori, "Low velocity impact tests of laminate glass-fiber-epoxy matrix composite material plates", International Journal of Impact Engineering, 27, 213 (2002)

[11] Z. Guan, C. Yang, "Low-velocity impact and damage process of composite laminates", Journal of Composite Materials, 36, 851 (2002)

[12] T-W Shyr, Y-H Pan, "Impact resistance and damage characteristics of composite laminates", Composite Structures, 62,193 (2003)

[13] B.S.Sugun, R.M.V.G.K. Rao, "Low-velocity impact characterization of glass, carbon and kevlar composites using repeated drop tests" Journal of Reinforced Plastics and Composites, 23 (15), pp. 1583-1599. (2004)

[14] W.A de Morais, S.N. Monteiro, J.R.M d'Almeida, "Evaluation of repeated low energy impact damage in carbon-epoxy composite materials", Composite Structures, 67, 307 (2005)

[15] G. Minak, D. Ghelli, Design of a drop-weight machine for composite materials impact testing, In: Proceedings of the ICCSM5 Congress. Trogir/Split (HR), September, 2006

[16] Nuno Ferreira Rilo and Luis Manuel Silva Ferreira, Experimental study of low-velocity impacts on glass-epoxy laminated composite plates, International Journal of Mechanics and Materials In Design, 4, 291, (2008)