**4. Destructive evaluation**

In order to evaluate the influence of the pins as anchorages between metal and composite inside FMLPs, mechanical tests were employed until failure. Drop-weight test (low-speed impact test) was performed to assess the capacity of the FMLPs to dissipate impact energy. Buckling test, after impact (drop-weight test), was used to evaluate the performance of the FMLPs in such loading condition. And shear test (Iosipescu) was carried out to survey the effects that the pins have on the delamination of FMLPs.

#### **4.1. Drop-weight testing**

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

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

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

In order to evaluate the influence of the pins as anchorages between metal and composite inside FMLPs, mechanical tests were employed until failure. Drop-weight test (low-speed impact test) was performed to assess the capacity of the FMLPs to dissipate impact energy. Buckling test, after impact (drop-weight test), was used to evaluate the performance of the FMLPs in such loading condition. And shear test (Iosipescu) was carried out to survey the

conventional panel.

106 Optimum Composite Structures

removes the cosmetic drawbacks.

**4. Destructive evaluation**

effects that the pins have on the delamination of FMLPs.

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 9.81 m/s<sup>2</sup> ). **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.

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

**Figure 14.** (a) Schematic frontal and top views of the drop-weight test rig, where: 1—high-speed camera with lens; 2—small-sized panel; 3—halogen lamp of 1 kW (2 units); 4—high-inertia base with central clearance hole (125 mm long, 75 mm wide and 25 mm deep); 5—flat background, perpendicular to the base; 6—mass guidance tube; 7—PC for image viewing; 8—mass with spherical head; (b) upper and lower surfaces of all types of panels (≈175 × 80 × 4 mm) after impact.

**Figure 15.** (a) Extrapolation of the falling/raising mass velocity at impact and return (MPin-C-MPin hexagonal 5 mm panel, as example); (b) average impact and return velocities for each panel (h = height; v = velocity).

#### *4.1.1. Damage characterization (damage depth profile) after impact*

The through-thickness damage extent, i.e., the depth profile of the impact damage (permanent deformation), was found by measuring the vertical displacement of the central transversal and central longitudinal lines drawn in all panels before drop-weight testing. The edges of the panels, both in length and width, were taken as references without permanent deformation after impact. A commercial manual-floating-type coordinate measuring machine, with 1 μm of resolution, was used for taking the measurements. The damage profile was determined by scanning the upper and lower surfaces of the panels, as exemplified in **Figure 17**. As seen, the measuring mesh was reduced to 5 mm near the damage area, against 10 mm in the rest of the surface of the panels.

The average damage depth profiles of each panel are shown in **Figure 18(a)**. As the profiles were nearly longitudinally and transversally symmetrical, only half of the panel's length and width are represented. In general, the damage profiles found in each surface of the FMLPs with pins were similar, longitudinally as well as transversally, to the profiles found in the conventional

**Figure 17.** General setup for damage depth profile determination (upper panel surface, as example), where: 1—damaged small-sized panel (MPin-C-MPin hexagonal 5 mm panel, as example); 2—work table; 3—3D digital probe (touching

head); 4—support; 5—clamping.

**Figure 16.** Average impact and return energies and consequent energy dissipation during drop-weight testing.

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**Figure 16.** Average impact and return energies and consequent energy dissipation during drop-weight testing.

**Figure 17.** General setup for damage depth profile determination (upper panel surface, as example), where: 1—damaged small-sized panel (MPin-C-MPin hexagonal 5 mm panel, as example); 2—work table; 3—3D digital probe (touching head); 4—support; 5—clamping.

*4.1.1. Damage characterization (damage depth profile) after impact*

surface of the panels.

108 Optimum Composite Structures

The through-thickness damage extent, i.e., the depth profile of the impact damage (permanent deformation), was found by measuring the vertical displacement of the central transversal and central longitudinal lines drawn in all panels before drop-weight testing. The edges of the panels, both in length and width, were taken as references without permanent deformation after impact. A commercial manual-floating-type coordinate measuring machine, with 1 μm of resolution, was used for taking the measurements. The damage profile was determined by scanning the upper and lower surfaces of the panels, as exemplified in **Figure 17**. As seen, the measuring mesh was reduced to 5 mm near the damage area, against 10 mm in the rest of the

**Figure 15.** (a) Extrapolation of the falling/raising mass velocity at impact and return (MPin-C-MPin hexagonal 5 mm

**Figure 14.** (a) Schematic frontal and top views of the drop-weight test rig, where: 1—high-speed camera with lens; 2—small-sized panel; 3—halogen lamp of 1 kW (2 units); 4—high-inertia base with central clearance hole (125 mm long, 75 mm wide and 25 mm deep); 5—flat background, perpendicular to the base; 6—mass guidance tube; 7—PC for image viewing; 8—mass with spherical head; (b) upper and lower surfaces of all types of panels (≈175 × 80 × 4 mm) after impact.

panel, as example); (b) average impact and return velocities for each panel (h = height; v = velocity).

The average damage depth profiles of each panel are shown in **Figure 18(a)**. As the profiles were nearly longitudinally and transversally symmetrical, only half of the panel's length and width are represented. In general, the damage profiles found in each surface of the FMLPs with pins were similar, longitudinally as well as transversally, to the profiles found in the conventional FMLP [M-C-M (without pins)]. However, the presence of pins tends to change the permanent deformation of the panels after impact as complementally shown in **Figure 18(b)**. It is also noted that the upper and lower surfaces exhibited different typical deformation profiles. In all panel types, the depth profile was more concentrated (shorter and shallower) in the upper surface than in the lower surface. The falling mass head might have tended to replicate its spherical contour on the upper surface, with the extent limited by the panel own thickness, which would work as restriction against deformation. The final damage extent at the lower surface, in contrast, is limited by the edges of the central clearance hole of the base of the drop-weight test rig, which are reasonably far away. It is even possible to observe that the change in damage depth profile is more gradual along the central longitudinal line (edges of support 125 mm apart) and more abrupt along the central transversal line (edges of support 75 mm apart). As the conventional FMLPs [M-C-M (without pins)] and FMLPs with pins had equal number of prepreg layers, they behaved similarly, consequently dissipating a close amount of energy during impact (**Figure 16**).

head of the impactor reached the surface region with the deposited pin. In turn, this caused less permanent deformation in the panels with pins in comparison to the others. This indicates the pins acted holding the metal sheet to the composite, restraining delamination and

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According to Ishikawa et al. [27] and De Freitas and Reis [28], the resistance of composite panels to in-plane compressive stresses is strongly impaired by the presence of delaminationtype damages, culminating in expressive reductions in buckling resistance of components. In that way, all types of FMLPs were subjected to buckling test after impact (drop-weight test) as investigation on the influence of pins on damage tolerance. For each type of small-sized FMLP two samples were tested for buckling. An electromechanical universal testing machine was used. Upper and lower supports for the panels were designed and built to allow proper alignment and fixture for the test, as shown in **Figure 20**. In accordance with ASTM D7137 standard [29], the testing speed (upper head moving rate) was always set at 1.25 mm/min. **Figure 21(a)** shows the general evolution of axial displacement *versus* load in the buckling tests of all types of FMLPs after impact damage with compression, buckling and failure phases as detailed by Skhabovskyi et al. [10]. The mean values resulting from two tests of maximum compressive load, failure load, axial displacements at maximum load and failure are shown in **Figure 21(b)**. The axial displacement of the failure is shown as an alternative correlated with the lateral deflection of the panels tested. As seen, pinned FMLPs tolerated a higher maximum load, having close performances with each other, except the MPin-C-MPin squared 10 mm panel, which was able to withstand a smaller maximum load but still larger

**Figure 20.** (a) Image of small-sized FMLP (MPin-C-MPin hexagonal 5 mm, as example) assembled in the testing machine; (b) schematic side view of the buckling set up, where: 1—moving upper head; 2—fixed lower head; 3—upper support (two parts); 4—lower support (two parts); 5—panel; 6—support fixture side plates; 7—auxiliary holding bar;

compared to the conventional panels [M-C-M (without pins)].

external deformation.

**4.2. Buckling after impact**

8—auxiliary clamps (dimensions in mm).

For a better understanding of the results of the least permanent deformed FMLPs, MPin-C-MPin squared 5 mm and MPin-C-MPin hexagonal 10 mm (**Figure 18(b)**), they were analyzed visually. In **Figure 19**, it is possible to notice that, in the case of these FMLPs, the spherical

**Figure 18.** (a) Depth profiles of the damages produced in each panel (average of 2 samples) from 3D digital probing; (b) average maximum permanent deformation in each panel.

**Figure 19.** Zoom of the impact region of the upper surfaces of specific FMLPs after the drop-weight test, where: (a) MPin-C-MPin hexagonal 10 mm; (b) MPin-C-MPin squared 5 mm (the arrow indicates the deepest location of the panel after impact deformation).

head of the impactor reached the surface region with the deposited pin. In turn, this caused less permanent deformation in the panels with pins in comparison to the others. This indicates the pins acted holding the metal sheet to the composite, restraining delamination and external deformation.

#### **4.2. Buckling after impact**

FMLP [M-C-M (without pins)]. However, the presence of pins tends to change the permanent deformation of the panels after impact as complementally shown in **Figure 18(b)**. It is also noted that the upper and lower surfaces exhibited different typical deformation profiles. In all panel types, the depth profile was more concentrated (shorter and shallower) in the upper surface than in the lower surface. The falling mass head might have tended to replicate its spherical contour on the upper surface, with the extent limited by the panel own thickness, which would work as restriction against deformation. The final damage extent at the lower surface, in contrast, is limited by the edges of the central clearance hole of the base of the drop-weight test rig, which are reasonably far away. It is even possible to observe that the change in damage depth profile is more gradual along the central longitudinal line (edges of support 125 mm apart) and more abrupt along the central transversal line (edges of support 75 mm apart). As the conventional FMLPs [M-C-M (without pins)] and FMLPs with pins had equal number of prepreg layers, they behaved similarly, consequently dissipating a close amount of energy during impact (**Figure 16**). For a better understanding of the results of the least permanent deformed FMLPs, MPin-C-MPin squared 5 mm and MPin-C-MPin hexagonal 10 mm (**Figure 18(b)**), they were analyzed visually. In **Figure 19**, it is possible to notice that, in the case of these FMLPs, the spherical

**Figure 18.** (a) Depth profiles of the damages produced in each panel (average of 2 samples) from 3D digital probing; (b)

**Figure 19.** Zoom of the impact region of the upper surfaces of specific FMLPs after the drop-weight test, where: (a) MPin-C-MPin hexagonal 10 mm; (b) MPin-C-MPin squared 5 mm (the arrow indicates the deepest location of the panel

average maximum permanent deformation in each panel.

after impact deformation).

110 Optimum Composite Structures

According to Ishikawa et al. [27] and De Freitas and Reis [28], the resistance of composite panels to in-plane compressive stresses is strongly impaired by the presence of delaminationtype damages, culminating in expressive reductions in buckling resistance of components. In that way, all types of FMLPs were subjected to buckling test after impact (drop-weight test) as investigation on the influence of pins on damage tolerance. For each type of small-sized FMLP two samples were tested for buckling. An electromechanical universal testing machine was used. Upper and lower supports for the panels were designed and built to allow proper alignment and fixture for the test, as shown in **Figure 20**. In accordance with ASTM D7137 standard [29], the testing speed (upper head moving rate) was always set at 1.25 mm/min.

**Figure 21(a)** shows the general evolution of axial displacement *versus* load in the buckling tests of all types of FMLPs after impact damage with compression, buckling and failure phases as detailed by Skhabovskyi et al. [10]. The mean values resulting from two tests of maximum compressive load, failure load, axial displacements at maximum load and failure are shown in **Figure 21(b)**. The axial displacement of the failure is shown as an alternative correlated with the lateral deflection of the panels tested. As seen, pinned FMLPs tolerated a higher maximum load, having close performances with each other, except the MPin-C-MPin squared 10 mm panel, which was able to withstand a smaller maximum load but still larger compared to the conventional panels [M-C-M (without pins)].

**Figure 20.** (a) Image of small-sized FMLP (MPin-C-MPin hexagonal 5 mm, as example) assembled in the testing machine; (b) schematic side view of the buckling set up, where: 1—moving upper head; 2—fixed lower head; 3—upper support (two parts); 4—lower support (two parts); 5—panel; 6—support fixture side plates; 7—auxiliary holding bar; 8—auxiliary clamps (dimensions in mm).

**Figure 21.** (a) Typical evolution of axial displacement *versus* load in buckling tests of all types of panels after impact damaging; (b) maximum average compressive load, average load at failure and correspondent axial displacements after buckling tests of all types of panels after impact damaging.

All types of the FMLPs achieved loads of maximum compression around 1.35 mm of axial displacement. **Figure 21(a)** shows that FMLPs with higher pin density (5 mm spacing) were able to withstand higher loads during deflection (buckling phase), followed by pinned panels in lower spacing patterns (panels MPin-C-MPin squared 10 mm and MPin-C-MPin hexagonal 10 mm in this order). As shown in **Figure 21(a)**, conventional FMLPs (without pins) reached a high load value at failure, still with a high axial displacement value at this moment, compared to FMLPs with pins. However, conventional FMLPs (without pins), after reaching a high value of failed load, showed a rapid unloading after the rupture (catastrophic failure).

ruptured transversely in the middle after the buckling test, including the conventional panel (without pins). But in the case of M-C-M (without pins), the fracture did not cross the entire width of the panel, concentrating closer to the damage region (**Figure 22**). It was also noticed that the ruptures of the metallic sheets happened between the metallic pins (spacings), because the pins acted as anchoring of the metallic sheets and because the tensile tensions were present in the inferior metallic sheets (side opposite to the impact) between the pins. It was noted that the M-C-M type panels (without pins) with impact damage ended the buckling test with the metal sheets slightly separated from the composite. In contrast, the FMLPs with pins exhibited metal sheets-composite debonding notably concentrated around the damage areas. This fact ratifies that the pins effectively anchor the metal sheets to the composite, even after impact damage.

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**Figure 22.** Typical appearance of all types of FMLPs with impact damage after buckling test.

To evaluate the influence of the deposition of the pins in the FMLP on shear and delamination resistance, a shear strength test was used, as proposed for the first time in 1967 by Iosipescu. According to Le Bourlegat [30], the Iosipescu shear test uses a simple test body because it is flat and achieves a pure and uniform shear stress-strain state in the region of the notch imposed on the specimen. **Figure 23(a)** shows a simplified view of this test performed on the universal machine for mechanical testing. **Figure 23(b)** shows the dimensions of the specimen used and defined by ASTM D5379 standard [31]. In addition, every Iosipescu shear test procedure was based on this standard, keeping the movable jaw displacement speed equal to 0.5 mm/min. The test was carried out on five specimens taken from each of the five

**4.3. Iosipescu shear test**

MPin-C-MPin squared 5 mm and MPin-C-MPin hexagonal 10 mm panels supported higher maximum compression and failure loads (**Figure 21(b)**), probably because of their lower maximum permanent deformation (**Figure 18(b)**). It was also noted that MPin-C-MPin squared 5 mm and MPin-C-MPin hexagonal 10 mm panels did not show a good repeatability of their high mean maximum compressive load, as evidenced by a high average standard deviation in **Figure 21(b)**. This probably happened because the spherical head of the impactor hit the pin region in one of two tests performed, thus leading to the smallest deformation. The other FMLPs showed similar permanent deformation, which is supported by good repeatability of the loads and displacements in the buckling test. In this case, the pins possibly tend to retard the propagation of debonding between metal sheets and composite, for anchoring them to each other, and even tend to retard the spread of delamination in the composite, for acting as clamps that hold most of its layers together between the metal sheets and pins ball-heads. That is, the pins tend to delay the propagation of the delamination between the metal sheets and the composite by their anchoring and even tend to delay the propagation of the delamination in the composite, acting as staples that support most of their layers between the metal sheets and ball-head pins.

**Figure 22** shows the images of all types of panels with impact damage after the buckling test. As expected, all types of panels had their lateral deflection going from the upper surface (side of impact) towards the lower surface. All panels collapsed and ended folded with presence of corrugations transversally crossing the areas of damage. Still all FMLPs had the lower metal sheet

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**Figure 22.** Typical appearance of all types of FMLPs with impact damage after buckling test.

ruptured transversely in the middle after the buckling test, including the conventional panel (without pins). But in the case of M-C-M (without pins), the fracture did not cross the entire width of the panel, concentrating closer to the damage region (**Figure 22**). It was also noticed that the ruptures of the metallic sheets happened between the metallic pins (spacings), because the pins acted as anchoring of the metallic sheets and because the tensile tensions were present in the inferior metallic sheets (side opposite to the impact) between the pins. It was noted that the M-C-M type panels (without pins) with impact damage ended the buckling test with the metal sheets slightly separated from the composite. In contrast, the FMLPs with pins exhibited metal sheets-composite debonding notably concentrated around the damage areas. This fact ratifies that the pins effectively anchor the metal sheets to the composite, even after impact damage.

#### **4.3. Iosipescu shear test**

All types of the FMLPs achieved loads of maximum compression around 1.35 mm of axial displacement. **Figure 21(a)** shows that FMLPs with higher pin density (5 mm spacing) were able to withstand higher loads during deflection (buckling phase), followed by pinned panels in lower spacing patterns (panels MPin-C-MPin squared 10 mm and MPin-C-MPin hexagonal 10 mm in this order). As shown in **Figure 21(a)**, conventional FMLPs (without pins) reached a high load value at failure, still with a high axial displacement value at this moment, compared to FMLPs with pins. However, conventional FMLPs (without pins), after reaching a high value of failed load, showed a rapid unloading after the rupture (catastrophic failure).

**Figure 21.** (a) Typical evolution of axial displacement *versus* load in buckling tests of all types of panels after impact damaging; (b) maximum average compressive load, average load at failure and correspondent axial displacements after

buckling tests of all types of panels after impact damaging.

112 Optimum Composite Structures

MPin-C-MPin squared 5 mm and MPin-C-MPin hexagonal 10 mm panels supported higher maximum compression and failure loads (**Figure 21(b)**), probably because of their lower maximum permanent deformation (**Figure 18(b)**). It was also noted that MPin-C-MPin squared 5 mm and MPin-C-MPin hexagonal 10 mm panels did not show a good repeatability of their high mean maximum compressive load, as evidenced by a high average standard deviation in **Figure 21(b)**. This probably happened because the spherical head of the impactor hit the pin region in one of two tests performed, thus leading to the smallest deformation. The other FMLPs showed similar permanent deformation, which is supported by good repeatability of the loads and displacements in the buckling test. In this case, the pins possibly tend to retard the propagation of debonding between metal sheets and composite, for anchoring them to each other, and even tend to retard the spread of delamination in the composite, for acting as clamps that hold most of its layers together between the metal sheets and pins ball-heads. That is, the pins tend to delay the propagation of the delamination between the metal sheets and the composite by their anchoring and even tend to delay the propagation of the delamination in the composite, acting as staples that support most of their layers between the metal sheets and ball-head pins. **Figure 22** shows the images of all types of panels with impact damage after the buckling test. As expected, all types of panels had their lateral deflection going from the upper surface (side of impact) towards the lower surface. All panels collapsed and ended folded with presence of corrugations transversally crossing the areas of damage. Still all FMLPs had the lower metal sheet

To evaluate the influence of the deposition of the pins in the FMLP on shear and delamination resistance, a shear strength test was used, as proposed for the first time in 1967 by Iosipescu. According to Le Bourlegat [30], the Iosipescu shear test uses a simple test body because it is flat and achieves a pure and uniform shear stress-strain state in the region of the notch imposed on the specimen. **Figure 23(a)** shows a simplified view of this test performed on the universal machine for mechanical testing. **Figure 23(b)** shows the dimensions of the specimen used and defined by ASTM D5379 standard [31]. In addition, every Iosipescu shear test procedure was based on this standard, keeping the movable jaw displacement speed equal to 0.5 mm/min. The test was carried out on five specimens taken from each of the five

**Figure 23.** (a) Simplified view of the Iosipescu shear test, where: 1—specimen; 2—fixed jaw; 3—mobile jaw; 4—base; F—applied load; (b) dimensions (in mm) of the specimens used in the Iosipescu shear test (adapted from ASTM D5379 standard [31]); (c) area of the specimen used for shear stress calculations (adapted from Le Bourlegat [30]).

types of small-sized FMLPs. **Figure 23(c)** shows the area (A) of the plane between the notches of the specimen (w ≈ 12 mm and t ≈ 4 mm) along which the load (F) was applied. To calculate the shear stress (τ), Eq. (1) was used.

$$
\pi = \frac{\text{F}}{\text{w} \ast \text{t}} \tag{1}
$$

In general, the panels with pins showed better results of maximum load, demanding a higher value of shear stress to break. Probably, this result was caused because the contact between the metallic sheets and the composite of the pinned specimens are more anchored due to the presence of the pins. In such a way, the greater number of pins (5 mm spacing) provided

**Figure 25.** Typical displacement *versus* load curves for each FMLP type in the Iosipescu shear test.

**Figure 24.** Typical aspect of the five types of FMLPs specimens before (left) and after (right) the Iosipescu shear test, where: (a) M-C-M (without pins); (b) MPin-C-MPin hexagonal 5; (c) MPin-C-MPin hexagonal 10; (d) MPin-C-MPin

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squared 5; (e) MPin-C-MPin squared 10.

**Figure 24** presents the typical appearance of FMLPs specimens before and after the Iosipescu shear test. **Figure 25** shows typical displacement *versus* load curves, respectively, for each type of panel in the Iosipescu shear test. During the tests a good repeatability of the results was observed for each of the five types of FMLPs specimens. **Figure 26** shows column diagrams with average values of maximum load, maximum load displacement, and maximum shear stress. It is observed that the specimens of the panels with the largest number of pins (5 mm spacing), regardless of their deposition pattern, showed the highest average maximum load (≈6.61 kN). The panels with lower pin density (10 mm spacing) exhibited a maximum load a bit lower (≈5.83 kN), regardless of the deposition pattern. Finally, the conventional FMLP specimens [M-C-M (without pins)] showed the lowest maximum load (≈5.04 kN), approximately 25% below the results with the specimens of the panels with spacing of 5 mm.

The specimens of FMLPs with pins reached a maximum load displacement of approximately 2.91 mm (**Figure 26**), except for the specimens of the MPin-C-MPin squared 10 mm panel that showed a lower displacement value at the moment of failure (1.44 ± 0.06 mm). This probably happened because of the pin position inside both notches (**Figure 24**). As there was a pin very close to the edge of the notch, there may have been stress concentration right at this point, thus breaking the specimens of this case in advance and leaving no room for further displacements.

**Figure 24.** Typical aspect of the five types of FMLPs specimens before (left) and after (right) the Iosipescu shear test, where: (a) M-C-M (without pins); (b) MPin-C-MPin hexagonal 5; (c) MPin-C-MPin hexagonal 10; (d) MPin-C-MPin squared 5; (e) MPin-C-MPin squared 10.

types of small-sized FMLPs. **Figure 23(c)** shows the area (A) of the plane between the notches of the specimen (w ≈ 12 mm and t ≈ 4 mm) along which the load (F) was applied. To calculate

**Figure 23.** (a) Simplified view of the Iosipescu shear test, where: 1—specimen; 2—fixed jaw; 3—mobile jaw; 4—base; F—applied load; (b) dimensions (in mm) of the specimens used in the Iosipescu shear test (adapted from ASTM D5379

standard [31]); (c) area of the specimen used for shear stress calculations (adapted from Le Bourlegat [30]).

**Figure 24** presents the typical appearance of FMLPs specimens before and after the Iosipescu shear test. **Figure 25** shows typical displacement *versus* load curves, respectively, for each type of panel in the Iosipescu shear test. During the tests a good repeatability of the results was observed for each of the five types of FMLPs specimens. **Figure 26** shows column diagrams with average values of maximum load, maximum load displacement, and maximum shear stress. It is observed that the specimens of the panels with the largest number of pins (5 mm spacing), regardless of their deposition pattern, showed the highest average maximum load (≈6.61 kN). The panels with lower pin density (10 mm spacing) exhibited a maximum load a bit lower (≈5.83 kN), regardless of the deposition pattern. Finally, the conventional FMLP specimens [M-C-M (without pins)] showed the lowest maximum load (≈5.04 kN), approxi-

mately 25% below the results with the specimens of the panels with spacing of 5 mm.

The specimens of FMLPs with pins reached a maximum load displacement of approximately 2.91 mm (**Figure 26**), except for the specimens of the MPin-C-MPin squared 10 mm panel that showed a lower displacement value at the moment of failure (1.44 ± 0.06 mm). This probably happened because of the pin position inside both notches (**Figure 24**). As there was a pin very close to the edge of the notch, there may have been stress concentration right at this point, thus breaking the specimens of this case in advance and leaving no room for further

<sup>w</sup> <sup>∗</sup> <sup>t</sup> (1)

the shear stress (τ), Eq. (1) was used.

114 Optimum Composite Structures

displacements.

τ = \_\_\_\_ <sup>F</sup>

**Figure 25.** Typical displacement *versus* load curves for each FMLP type in the Iosipescu shear test.

In general, the panels with pins showed better results of maximum load, demanding a higher value of shear stress to break. Probably, this result was caused because the contact between the metallic sheets and the composite of the pinned specimens are more anchored due to the presence of the pins. In such a way, the greater number of pins (5 mm spacing) provided

• CMT PIN process, through its thermo-mechanical working principle, changes the surface profile of the metal sheets on the opposite face in the region of deposition, but only on a microscopic scale. In addition, thermoxidation occurs in these regions, which is not a problem because this inconvenience could be avoided/minimized by the application of purge gas (supplementary protection to that used near the electric arc

Fiber-Metal Laminate Panels Reinforced with Metal Pins http://dx.doi.org/10.5772/intechopen.78405 117

• In terms of impact energy dissipation, all FMLPs with pins exhibited similar performance, generally equivalent to the conventional FMLP (without pins). Besides, the addition of weight by the pins as anchorages does not penalize the capacity of the panels to dissipate impact energy, as all panel types dissipated similar levels of specific energy during impact. Therefore, the pins did not make the FMLPs more brittle and the change in their deposition pattern did not show any significant effect concerning the capacity of the panels to

• Concerning the damage depth profiles caused by drop-weight testing, all FMLPs with pins suffered damages similar to that found in the conventional FMLP (without pins).

• Regarding damage tolerance, the FMLPs with pins exhibited a less catastrophic trend, i.e., achieving significantly higher loads at longer axial displacements in buckling test after impact and the pins tend to retard the debonding propagation between metal sheets and composite, for anchoring them to each other. The pins also hold back the delamination spreading in the composite, for clamping the layers together between the metal sheets and

• The anchoring effect the pins have on the FMLPs was confirmed through the Iosipescu shear test. Generally, the panels with pins exhibited higher shearing loads in relation to the conventional panel (without pins). The FMLPs with the highest number of pins (5 mm spacing), regardless of the deposition pattern, presented the highest maximum loads and

The authors acknowledge the Center for Research and Development of Welding Processes (Laprosolda) and Laboratory of Structural Mechanics Prof. José Eduardo Tannús Reis, both at UFU, and the Department of Materials and Technology, at UNESP-Guaratinguetá, for providing laboratorial infrastructure and support, and ALLTEC Materiais Compostos, for providing composite materials. They also acknowledge the Brazilian agencies CNPq (Procs. 302863/2016-8 and 303224/2016-9), CAPES (PROEX 0309/2015), and FAPEMIG (Proc. 11304),

and inert).

Destructive evaluation:

dissipate impact energy.

ball-heads of the pins.

**Acknowledgements**

for research grants and scholarships.

displacements at the moment of failure.

**Figure 26.** Average results obtained through five measurements of maximum load, displacement at maximum load and maximum shear stress of the Iosipescu shear test.

a better union, consequently showing better results. Possibly, failures in pinned specimens would be less catastrophic after reaching the maximum load (after failure), which should be exploited in future work.
