**3. Non-destructive evaluation**

#### **3.1. Modal analysis**

Two basic types of small-sized FMLPs were made (**Figure 5**): conventional FMLPs (without pins), i.e., metal-composite-metal (M-C-M); and FMLPs with pins, i.e., pined metal-composite-pined metal (MPin-C-MPin). The MPin-C-MPin type was produced using a combination of two levels of pin separation (5 and 10 mm) and two deposition patterns (hexagonal and squared). The pins of the upper metal parts were deposited with an adequate displacement in relation to the pins of the lower metal parts, avoiding contact between the upper and lower pins and providing homogenous distribution. Each one of the five small-sized FMLPs types

**Figure 5.** Types of small-sized panels fabricated with respective cross sections and schematic overlapping of pins on the upper and lower metal sheets, where M-C-M stands for metal-composite-metal, MPin-C-MPin for pined metalcomposite-pined metal for hexagonal and squared pattern and S for distance separating the pins (panel width ≈ 80 mm; panel length ≈ 350 mm; the dot-like marks on the panels with pins are due to stainless steel heat-induced oxidation right

under where the pins were deposited—This esthetic effect could be avoided with inert gas back purging).

was produced twice for replication in the tests.

98 Optimum Composite Structures

Modal analysis is a tool largely used to determine dynamic properties (natural frequencies, damping factors and vibration modes) of mechanical structures by imposing vibrations [11]. According to Rao [12], any movement of a flexible structure that repeats itself after a time interval is called vibration. This movement can be inferred instantaneously or continuously. The use of modal analysis is often applied due to the ease of implementation, relatively low cost, as well as being a nondestructive analysis.

According to Bolina et al. [13], the natural frequencies (fn) indicate the rate of free oscillation of the structure, after ceasing the force that caused its movement. In other words, it represents how much the structure vibrates when there is no longer a force applied to it. It is worth recalling that the value of the natural frequency of a structure depends on its stiffness and mass. In a structure several natural frequencies can be observed because it can vibrate freely (after being excited by a force) in several directions and modes. In practice, higher values of natural frequency indicate that elastic stresses are preponderant to inertial forces. Moreover, whenever a structure oscillates with a frequency equal to one its natural frequencies, a phenomenon called resonance occurs. The resonance implies high amplitudes of vibration, and can cause structural failures as, for example, in the breaking of a crystal glass of wine due to sound energy. In this case, when the frequency of vibration caused by the source (sound wave) coincides with the natural frequency of the crystal glass, the amplitude of oscillation of this body reaches high values, because the source progressively gives energy to the body, and the crystal glass might break if the strain extensions exceed the levels supported by the material. However, vibration levels during a resonance can be attenuated if there are dissipation mechanisms that have high damping factors present in the structure, for example, shock absorbers (holding tightly the crystal glass in one's hand avoid the resonance-induced breakage). A more damped structure at a certain natural frequency can attenuate vibration levels more quickly. The damping factors (ξn) represent the damping levels of a structure, which in turn are characterized for each of its natural frequencies.

• The shaker is a model K2007E01 from manufacturer Modal Shop Inc. with the function of programming time and frequency of vibration to induce in the object of analysis (panel) through a connection tube. In this work, the shaker was programmed to produce a white noise type signal (random signal with equal intensity at different frequencies) comprising the frequency range 0–800 Hz in order to find the dynamic properties of the structure. The choice of this frequency range was made from a preliminary study, in which the first four modes of vibration were found. This frequency band determines the composition of the

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• The load cell is a model 208C03 (S/N LW34448), glued to the panel at a given position, as

• The data acquisition system is capable of obtaining and analyzing the accelerometer and load cell signals. This system was configured to obtain a frequency resolution (Δf) of 0.25 Hz and an analysis band between 0 (fmin) and 800 (fmax) Hz. The acquisition rate (temporal resolution) of the data is then defined automatically by the acquisition system. • The software is from Brüel and Kjær, from the manufacturer Vibrant Technology®, for acquisition and post-processing to identify the dynamic properties parameters of the panels.

**Figure 6.** Illustration of a modal analysis, where: (a) experimental assembly; (b) response of the plate in the time domain; (c) response of the plate in the frequency domain (FRF); (d) overlap of the responses and example of the modal plate

**Figure 7.** Experimental bench with the equipment used for perform modal analysis of the small-sized FMLPs, where: 1—panel (object of analysis); 2—nylon line glued to the edges of the FMLP; 3—accelerometer; 4—shaker; 5—stinger (connection tube) between the load cell and shaker; 6—load cell (bonded to the panel); 7—acquisition data system; 8—

white noise that will be exerted by the electromagnetic exciter.

forms corresponding to each natural frequency (resonance) (after [15]).

computer and software for data processing.

shown in **Figure 8**.

By analyzing, for a given natural frequency (periodic movement) the relationship between the points that discretize the structure, one obtains the natural modes of vibration. That is, the modes of vibration determine the way the structure vibrates at a certain natural frequency. In practice, by knowing the dynamic properties of a structure, it is possible to determine how the vibration oscillations will be at different measurement positions. Therefore, the objective of applying experimental modal analysis in the FMLPs was to evaluate, through the respective natural frequencies (fn) and damping factors (ξn), whether the pins inside the FMLPs would be able to modify the dynamic properties of the panels. The modal forms of the FMLPs were not evaluated in this work.

According to Lundkvist [14], structural dynamic properties or modal parameters in practice are identified from the Frequency Response Function (FRF). To obtain the FRF, the data that are in time domain are transformed to frequency domain, using the Fourier transformation, as exemplified in **Figure 6**. At the same time, from the response of the system vibrating under a condition, the modal properties are determined. It can be said that from the identified modal properties, the dynamic temporal behavior of the structure is predicted under any excitation conditions. As shown in **Figure 6(d)**, a temporal response of the structure contains the participation of its modes of vibration, each mode of vibration contemplating a natural frequency, damping factor and modal form. In order to apply the modal analysis in the small-sized panels, the experimental bench shown in **Figure 7** was used.

Some details of the experimental bench assembly are given below:


• The shaker is a model K2007E01 from manufacturer Modal Shop Inc. with the function of programming time and frequency of vibration to induce in the object of analysis (panel) through a connection tube. In this work, the shaker was programmed to produce a white noise type signal (random signal with equal intensity at different frequencies) comprising the frequency range 0–800 Hz in order to find the dynamic properties of the structure. The choice of this frequency range was made from a preliminary study, in which the first four modes of vibration were found. This frequency band determines the composition of the white noise that will be exerted by the electromagnetic exciter.

(sound wave) coincides with the natural frequency of the crystal glass, the amplitude of oscillation of this body reaches high values, because the source progressively gives energy to the body, and the crystal glass might break if the strain extensions exceed the levels supported by the material. However, vibration levels during a resonance can be attenuated if there are dissipation mechanisms that have high damping factors present in the structure, for example, shock absorbers (holding tightly the crystal glass in one's hand avoid the resonance-induced breakage). A more damped structure at a certain natural frequency can attenuate vibration levels more quickly. The damping factors (ξn) represent the damping levels of a structure,

By analyzing, for a given natural frequency (periodic movement) the relationship between the points that discretize the structure, one obtains the natural modes of vibration. That is, the modes of vibration determine the way the structure vibrates at a certain natural frequency. In practice, by knowing the dynamic properties of a structure, it is possible to determine how the vibration oscillations will be at different measurement positions. Therefore, the objective of applying experimental modal analysis in the FMLPs was to evaluate, through the respective natural frequencies (fn) and damping factors (ξn), whether the pins inside the FMLPs would be able to modify the dynamic properties of the panels. The modal forms of the FMLPs were

According to Lundkvist [14], structural dynamic properties or modal parameters in practice are identified from the Frequency Response Function (FRF). To obtain the FRF, the data that are in time domain are transformed to frequency domain, using the Fourier transformation, as exemplified in **Figure 6**. At the same time, from the response of the system vibrating under a condition, the modal properties are determined. It can be said that from the identified modal properties, the dynamic temporal behavior of the structure is predicted under any excitation conditions. As shown in **Figure 6(d)**, a temporal response of the structure contains the participation of its modes of vibration, each mode of vibration contemplating a natural frequency, damping factor and modal form. In order to apply the modal analysis in the small-sized pan-

• The small-sized FMLP is hung by means of a nylon line glued to the side edges of it, forming a 60 mm arrow, as shown in **Figure 8**. It is called a "free-free" boundary condition when there is no rigid support or fixation of the structure. In this way, FMLP would be, by this experimental setup, in a "free-free" boundary condition, which was chosen because it is

• The positions of analysis of the dynamic response of each panel were based on a mesh of 51

• The piezoelectric accelerometer is a model 352С22 (S/N LW181487) from the manufacturer PCB Piezotronics®, fixed in a position of the small-sized FMLP by means of a specific wax, that allows the propagation of the vibrations between the panel and the

which in turn are characterized for each of its natural frequencies.

els, the experimental bench shown in **Figure 7** was used.

relatively simple to perform and quite informative.

points, as also indicated in **Figure 8**.

acceleration.

Some details of the experimental bench assembly are given below:

not evaluated in this work.

100 Optimum Composite Structures


**Figure 6.** Illustration of a modal analysis, where: (a) experimental assembly; (b) response of the plate in the time domain; (c) response of the plate in the frequency domain (FRF); (d) overlap of the responses and example of the modal plate forms corresponding to each natural frequency (resonance) (after [15]).

**Figure 7.** Experimental bench with the equipment used for perform modal analysis of the small-sized FMLPs, where: 1—panel (object of analysis); 2—nylon line glued to the edges of the FMLP; 3—accelerometer; 4—shaker; 5—stinger (connection tube) between the load cell and shaker; 6—load cell (bonded to the panel); 7—acquisition data system; 8 computer and software for data processing.

that the pins, individually or group, acts as a power dissipation mechanism. Thus, the oscillation amplitudes are attenuated more quickly with the presence of the pins. This quality could be exploited in practice, for example, aiming at structures less susceptible to acoustic problems and to high amplitudes of vibration at low frequencies. Other patterns of deposition and/or pin density could be explored in the sense of improving acoustic insulation and damping characteristics

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**Figure 9.** Amplitude (above) and phase (below) curves evaluated in a point of the MPin-C-MPin hexagonal 10 mm panel type (similar behavior for the other points), where solid lines are FRF and dashed lines are adjustment performed by the RFP method in the analysis band (ω) between 0 and 800 Hz (note that the adjustment overlapped the experimental

**Figure 10.** (a) Average results of the four natural frequency modes (fn) and (b) average results of the four modes of

damping factors (ξn) obtained through modal analysis of the small-sized FMLPs.

of vibrations in structures based on metal-composite laminates.

curve for the amplitude case).

**Figure 8.** 51 mesh sequential point positioning of the accelerometer in a small-sized FMLP MPin-C-MPin hexagonal 10 mm type (≈350 × 80 × 4 mm).

**Figure 9** shows the FRF curve typical of the amplitude and phase relationship (angular lag) along the frequency domain (0–800 Hz) between the output signals (measured acceleration) and input (applied force), from which the dynamic properties of the panels were determined. In this case, the phase between the responses of the accelerometer (acceleration) and the load cell (force) has a role to confirm if a resonant peak actually occurred along the frequency domain. Rao [12] mentions that resonance occurs when simultaneously a peak in the positive signal of FRF amplitude is observed and when the phase is near or passes through 90°. As shown in **Figure 9**, in the four resonance peaks (modes) the phase signals clearly changed their values (downwards), thus, by confirming that the resonance peaks actually occurred. The shape of the peaks (resonances) also indicates the level of damping, since the more "oval" peaks are associated with high damping effect. The Rational Fraction Polynomial (RFP) method [11, 16] was used to accurately determine the natural frequencies (fn) and damping factors (ξn) of each mode. The RFP method consists in adjusting a ratio of polynomials to FRF curves (amplitude and phase) obtained experimentally. For this purpose, a routine was devised in MATLAB® environment.

The dashed lines of **Figure 9** point to the appearance (typical for all panels analyzed) of the adjustment made by the RFP method in FRF (amplitude and phase) experimental curves using a 7th degree polynomial. With the adjustment performed in all positions, 51 modal parameters of each panel, namely the natural frequencies (fn) and damping factors (ξn), were determined. The solid lines of **Figure 9** show the FRF curves of the amplitude and phase evaluated in a given position of the panel (similar behavior for the other points of the mesh). It is possible to note the presence of four peaks (frequencies around 200, 480, 560 and 650 Hz) from the amplitude plot of **Figure 9**, indicating the resonant frequencies of the panel.

Parts (a) and (b) of the **Figure 10** present average results of four modes of the natural frequency (fn) and damping factor (ξn), respectively, measured at the 51 points of the measurement mesh. To facilitate the analysis, **Table 2** was organized to display the results of the four modes of vibration in terms of natural frequency and damping factor for all types of pinned small-sized panels in relation to the conventional one (without pins). Regarding the natural frequency, which represents the relationship between the stiffness and mass of a structure, it is possible to notice that the pinned panels did not have a significant variation of the natural frequencies of the vibration modes compared to the panels without pins. This implies that the pins have little effect on this dynamic property. Possibly, in spite of the mass increase provided by the deposition of the pins, the increase in stiffness takes place in a proportional way so that the natural frequency remains unchanged (in relation to the panel without pins). In contrast, the presence of the pins in the FMLPs significantly increased the damping factor of the FMLPs between two and five times (pinned panels with 10 mm spacing and hexagonal deposition pattern) depending on the vibration mode, compared to conventional panel (without pins). This was probably due to the fact that the pins, individually or group, acts as a power dissipation mechanism. Thus, the oscillation amplitudes are attenuated more quickly with the presence of the pins. This quality could be exploited in practice, for example, aiming at structures less susceptible to acoustic problems and to high amplitudes of vibration at low frequencies. Other patterns of deposition and/or pin density could be explored in the sense of improving acoustic insulation and damping characteristics of vibrations in structures based on metal-composite laminates.

**Figure 9** shows the FRF curve typical of the amplitude and phase relationship (angular lag) along the frequency domain (0–800 Hz) between the output signals (measured acceleration) and input (applied force), from which the dynamic properties of the panels were determined. In this case, the phase between the responses of the accelerometer (acceleration) and the load cell (force) has a role to confirm if a resonant peak actually occurred along the frequency domain. Rao [12] mentions that resonance occurs when simultaneously a peak in the positive signal of FRF amplitude is observed and when the phase is near or passes through 90°. As shown in **Figure 9**, in the four resonance peaks (modes) the phase signals clearly changed their values (downwards), thus, by confirming that the resonance peaks actually occurred. The shape of the peaks (resonances) also indicates the level of damping, since the more "oval" peaks are associated with high damping effect. The Rational Fraction Polynomial (RFP) method [11, 16] was used to accurately determine the natural frequencies (fn) and damping factors (ξn) of each mode. The RFP method consists in adjusting a ratio of polynomials to FRF curves (amplitude and phase) obtained experimentally.

**Figure 8.** 51 mesh sequential point positioning of the accelerometer in a small-sized FMLP MPin-C-MPin hexagonal

The dashed lines of **Figure 9** point to the appearance (typical for all panels analyzed) of the adjustment made by the RFP method in FRF (amplitude and phase) experimental curves using a 7th degree polynomial. With the adjustment performed in all positions, 51 modal parameters of each panel, namely the natural frequencies (fn) and damping factors (ξn), were determined. The solid lines of **Figure 9** show the FRF curves of the amplitude and phase evaluated in a given position of the panel (similar behavior for the other points of the mesh). It is possible to note the presence of four peaks (frequencies around 200, 480, 560 and 650 Hz)

from the amplitude plot of **Figure 9**, indicating the resonant frequencies of the panel.

Parts (a) and (b) of the **Figure 10** present average results of four modes of the natural frequency (fn) and damping factor (ξn), respectively, measured at the 51 points of the measurement mesh. To facilitate the analysis, **Table 2** was organized to display the results of the four modes of vibration in terms of natural frequency and damping factor for all types of pinned small-sized panels in relation to the conventional one (without pins). Regarding the natural frequency, which represents the relationship between the stiffness and mass of a structure, it is possible to notice that the pinned panels did not have a significant variation of the natural frequencies of the vibration modes compared to the panels without pins. This implies that the pins have little effect on this dynamic property. Possibly, in spite of the mass increase provided by the deposition of the pins, the increase in stiffness takes place in a proportional way so that the natural frequency remains unchanged (in relation to the panel without pins). In contrast, the presence of the pins in the FMLPs significantly increased the damping factor of the FMLPs between two and five times (pinned panels with 10 mm spacing and hexagonal deposition pattern) depending on the vibration mode, compared to conventional panel (without pins). This was probably due to the fact

For this purpose, a routine was devised in MATLAB® environment.

10 mm type (≈350 × 80 × 4 mm).

102 Optimum Composite Structures

**Figure 9.** Amplitude (above) and phase (below) curves evaluated in a point of the MPin-C-MPin hexagonal 10 mm panel type (similar behavior for the other points), where solid lines are FRF and dashed lines are adjustment performed by the RFP method in the analysis band (ω) between 0 and 800 Hz (note that the adjustment overlapped the experimental curve for the amplitude case).

**Figure 10.** (a) Average results of the four natural frequency modes (fn) and (b) average results of the four modes of damping factors (ξn) obtained through modal analysis of the small-sized FMLPs.


radially. In the panels without pins, areas of the same size were chosen. For statistical purposes, three samples of surfaces for each type of panel, chosen randomly, were analyzed. **Figure 12(a)** and **(b)** show the typical waviness profiles of the surfaces of panels with and without pins, respectively, and illustrates the associated measured region. The quantification of the waviness was made from the maximum distances between peak and valley [ΔZ of **Figure 12(a)**] of each marking. It has been observed that the peak of the surface corrugation profile is located in the center of the pin deposited on the opposite surface of the metal sheet and the two minimum peaks appear around the pin, as shown in **Figure 12(a)**. Since the pins are made with the same parameters, the values of ΔZ are close, resulting in an average value of 26.92 ± 2.35 μm. For comparison purposes, the same criterion

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The column graph of **Figure 13** shows the average (Ra), maximum (Rz) and total (Rt) roughness of the sampled regions of the panels. Recalling that Ra is the arithmetic mean

**Figure 11.** Example of the dark marks on the metal surface of the small-sized FMLP (MPin-C-MPin hexagonal 5 mm with

**Figure 12.** Typical surface profiles of the small-sized panels, where: (a) with pins (showing sharp waviness); (b) without

was applied in FMLPs without pins, resulting in average ΔZ of only 1.40 ± 0.40 μm.

dimensions ≈350 × 80 × 4 mm) caused by pin deposition process.

pins (without waviness).

**Table 2.** Percentage change of vibration modes of the natural frequency (Δfn) and damping factor (Δξn) of the panels with pins in relation to the conventional panel (without pins).

#### **3.2. Cosmetic characterization**

In literature [17–19], the major part of Fiber-Metal Laminates (FMLs), due to their advantages, is used in modern aircraft parts, such as fuselage, wings, etc. FMLPs are used as manufactured or after paint application. In this context, the quality of the surface becomes a relevant factor. Thus, the objective of this section was to evaluate the influence of the deposition of the pins on the metal surface of the FMLP. A general evaluation of the cosmetic characteristics of the different FMLPs was made by measuring the waviness and roughness of the metal surfaces opposite to the pinned regions.

**Figure 11** shows a small-sized FMLP reinforced with pins, where it is possible to perceive the dark marks on the metal surface, which represent the pins deposited on the opposite side. These marks are characterized by a localized deformation (peak and valley) and a heat-induced oxidation due to the welding of the pins (the cosmetic effect of the oxidation could be avoided with the application of purging with inert gas or reducing agent), that is, it is taken as a thermomechanical effect. The localized deformation is due to the operational mode of the CMT PIN, which deposits the pins by arc process (thermal effect), creating a small weld pool, but not reaching the opposite side. The tip of the electrode wire then penetrates into the weld pool, possibly reaching its bottom and pushing the surface of the metal sheet, deforming it (mechanical effect), characterizing the peak in the center of the marks (out of the panel). Then the current is turned off and the wire solidifies on the surface of the metal sheet. Next, current flows through the welded wire and metal sheet, promoting heating by Joule effect and wire softening, and a retraction movement of the wire breaks it apart, leaving behind a pin. It is believed that this retraction of the wire deforms the metal sheet, creating valleys concentric to the peak in the future panel. Concurrently, some expansion and contraction effects may also occur, but in a secondary way.

The measurements of surface waviness and roughness was performed with a Form Talysurf Intra profilometer from Taylor Hobson® with a resolution of 16 nm and range measurement of 1 mm. The measurement parameters were set as measuring speed equal to 0.25 mm/s, measuring length equal to 10 mm and ambient temperature of 20 ± 2°C (ABNT NBR ISO 1 [20]) and value of cut-off sample length equal to 0.8 mm (according to ISO 4288 [21]). Recalling that the cut-off value represents a segment of measurement length used in some representations of roughness measurements. It is important to note that, for the panels with pins, the tip of the profilometer was positioned to measure only some of the regions of marks left by the deposition of the pins crossing them radially. In the panels without pins, areas of the same size were chosen. For statistical purposes, three samples of surfaces for each type of panel, chosen randomly, were analyzed. **Figure 12(a)** and **(b)** show the typical waviness profiles of the surfaces of panels with and without pins, respectively, and illustrates the associated measured region. The quantification of the waviness was made from the maximum distances between peak and valley [ΔZ of **Figure 12(a)**] of each marking. It has been observed that the peak of the surface corrugation profile is located in the center of the pin deposited on the opposite surface of the metal sheet and the two minimum peaks appear around the pin, as shown in **Figure 12(a)**. Since the pins are made with the same parameters, the values of ΔZ are close, resulting in an average value of 26.92 ± 2.35 μm. For comparison purposes, the same criterion was applied in FMLPs without pins, resulting in average ΔZ of only 1.40 ± 0.40 μm.

The column graph of **Figure 13** shows the average (Ra), maximum (Rz) and total (Rt) roughness of the sampled regions of the panels. Recalling that Ra is the arithmetic mean

**3.2. Cosmetic characterization**

104 Optimum Composite Structures

with pins in relation to the conventional panel (without pins).

surfaces opposite to the pinned regions.

In literature [17–19], the major part of Fiber-Metal Laminates (FMLs), due to their advantages, is used in modern aircraft parts, such as fuselage, wings, etc. FMLPs are used as manufactured or after paint application. In this context, the quality of the surface becomes a relevant factor. Thus, the objective of this section was to evaluate the influence of the deposition of the pins on the metal surface of the FMLP. A general evaluation of the cosmetic characteristics of the different FMLPs was made by measuring the waviness and roughness of the metal

**Table 2.** Percentage change of vibration modes of the natural frequency (Δfn) and damping factor (Δξn) of the panels

**Panel type 1st mode (%) 2nd mode (%) 3rd mode (%) 4th mode (%)**

MPin-C-MPin hexagonal 5 mm 0.02 −6.32 4.46 54.68 0.14 63.90 4.48 165.22 MPin-C-MPin hexagonal 10 mm 0.26 235.09 2.90 276.44 −3.70 355.89 0.96 586.97 MPin-C-MPin squared 5 mm 0.23 12.37 4.12 252.64 −0.46 17.80 0.89 8.75 MPin-C-MPin squared 10 mm 1.28 31.80 0.98 16.29 −0.85 5.67 0.21 26.07

**Δfn Δξn Δfn Δξn Δfn Δξn Δfn Δξn**

**Figure 11** shows a small-sized FMLP reinforced with pins, where it is possible to perceive the dark marks on the metal surface, which represent the pins deposited on the opposite side. These marks are characterized by a localized deformation (peak and valley) and a heat-induced oxidation due to the welding of the pins (the cosmetic effect of the oxidation could be avoided with the application of purging with inert gas or reducing agent), that is, it is taken as a thermomechanical effect. The localized deformation is due to the operational mode of the CMT PIN, which deposits the pins by arc process (thermal effect), creating a small weld pool, but not reaching the opposite side. The tip of the electrode wire then penetrates into the weld pool, possibly reaching its bottom and pushing the surface of the metal sheet, deforming it (mechanical effect), characterizing the peak in the center of the marks (out of the panel). Then the current is turned off and the wire solidifies on the surface of the metal sheet. Next, current flows through the welded wire and metal sheet, promoting heating by Joule effect and wire softening, and a retraction movement of the wire breaks it apart, leaving behind a pin. It is believed that this retraction of the wire deforms the metal sheet, creating valleys concentric to the peak in the future panel. Concurrently, some expansion and contraction effects may also occur, but in a secondary way. The measurements of surface waviness and roughness was performed with a Form Talysurf Intra profilometer from Taylor Hobson® with a resolution of 16 nm and range measurement of 1 mm. The measurement parameters were set as measuring speed equal to 0.25 mm/s, measuring length equal to 10 mm and ambient temperature of 20 ± 2°C (ABNT NBR ISO 1 [20]) and value of cut-off sample length equal to 0.8 mm (according to ISO 4288 [21]). Recalling that the cut-off value represents a segment of measurement length used in some representations of roughness measurements. It is important to note that, for the panels with pins, the tip of the profilometer was positioned to measure only some of the regions of marks left by the deposition of the pins crossing them

**Figure 11.** Example of the dark marks on the metal surface of the small-sized FMLP (MPin-C-MPin hexagonal 5 mm with dimensions ≈350 × 80 × 4 mm) caused by pin deposition process.

**Figure 12.** Typical surface profiles of the small-sized panels, where: (a) with pins (showing sharp waviness); (b) without pins (without waviness).

**4.1. Drop-weight testing**

9.81 m/s<sup>2</sup>

The FMLPs reinforced with pins, as conceived in this work, and the reference panels [M-C-M (without pins)] were submitted to impact damage by drop-weight testing. The small-sized FMLPs produced were cut in half their length, resulting in specimens of ≈175 × 80 × 4 mm each, and thus two specimens were used for each FMLP type. The aim with this test was to verify whether the pins would have positive or deleterious effects on the FMLPs concerning their capacity to absorb impact energy. Based on the ASTM D7136 standard [24], a rig to impose free fall (from around 1850 mm of height) of a constant mass (2.326 kg) over the small-sized panel surface was devised (**Figure 14(a)**). This mass was composed of a 28.5 mm spherical head made of hard steel attached to a plain carbon steel cylinder (50 mm of diameter and 150 mm of length). The rig included a latching device for ensuring no mass bouncing (single impact). A commercial highspeed camera filming at 2000 frames per second with 90 mm f/2.5 macro lens and frontal lighting was employed to quantify the energy (based on mass speed) involved in the impacts. The free fall height aimed a potential energy sufficient for causing apparent damage at impact, which resulted in 10.5 J per each millimeter of panel thickness (gravitational acceleration considered as

Fiber-Metal Laminate Panels Reinforced with Metal Pins http://dx.doi.org/10.5772/intechopen.78405 107

). **Figure 14(b)** shows the upper and lower surfaces of all types of panels after impact.

High-speed images were used for determination of the falling/raising mass velocities immediately before/after impact, based on displacements (visualized from frame to frame) of its spherical head lower surface and respective time lapses, as seen in **Figure 15(a)**. A fitting curve, taking into account the non-uniform rectilinear motion due to gravity of the falling/raising mass, was figured out for each panel (including replications) and the velocities at the panel upper surface level were estimated by extrapolation. The velocity right at the end of the fall (actual impact) is referred as impact velocity and the velocity right at the beginning of the rebound as return velocity, which resultant average levels varied respectively from 5.81 to 5.96 m/s, as indicated in **Figure 15(b)**. According to Ursenbach et al. [25], the drop-weight test applied was classified as of low impact velocity (between 1 and 10 m/s). Farooq and Myler [26] consider an impact as of low velocity when an object impacts

a target without penetrating it, situation observed for all panels tested in this present work.

show any effect concerning the capacity of the panels to dissipate impact energy.

The velocities involved in the impacts, in turn, were used to calculate the impact and return energies. Impact energy was considered as the kinetic energy of the falling mass just before actual impact (fall height tending to zero–panel upper surface level before impact). Analogously, return energy was taken as the kinetic energy of the raising mass at the beginning of the rebound after the impact (rebound height tending to zero–panel upper surface level before impact). Energy dissipation during impact was assumed as the relative drop in the kinetic energy due to impact. The impact energy quantities were represented by two ways: energy and specific energy (considering the panel mass density), as presented in **Figure 16**. As seen, the impact energy was always around 40 J, being the small fluctuation probably due experimental errors. The return energies and energy dissipations were also similar for all FMLPs types. In general, the presence of pins as anchorages inside the FMLPs did not seem to have any significant effect concerning the capacity of the panels to absorb impact energy. Therefore, the pins, at least for the impact conditions applied, did not make the FMLPs more brittle. In addition, the change in the deposition pattern of the pins, at least for the remaining conditions, did not

**Figure 13.** Results of Ra (mean), Rz (maximum) and Rt (total) obtained on small-sized FMLPs surfaces.

of the absolute values of the roughness peaks in relation to a midline within the measurement length. Rz is defined as the highest value of the roughness (peaks), that is, the roughness measured in segments defined by the cut-off, which is presented in the measuring path. Finally, Rt corresponds to the vertical distance between the highest peak and the deepest valley in the measurement length, obtained within the segments defined by the cut-off. These roughness representations were chosen based on the studies of De Chiffre et al. [22] and De Chiffre [23], which demonstrated the parameters most used in industry. The results show that all types of panels have roughly the same roughness values of Ra, Rz and Rt (considering the mean standard deviation). However, the deposition of the pins promoted an increase of roughness in the regions of the marks, in relation to the conventional panel.

Thus, it was concluded that the deposition of the pins by CMT PIN process, through its thermo-mechanical principle (electric arc with advancement and retraction of the wire), changes the surface profile of a metallic sheet (0.4 mm thickness), but only on a microscopic scale (at slightly less than 30 μm in terms of waviness and at just over 0.20 μm in terms of roughness). In this way, the elimination of staining due to pin deposition already largely removes the cosmetic drawbacks.
