**2. Methodological approach**

materials. Sinmazçelik et al. [2] justify this combination by considering that metals are isotropic materials, have high strength and impact resistance and are easy to repair, while full composites generally present high strength and stiffness and excellent fatigue characteristics. According to Salve et al. [3], FMLs take positive characteristics from both metals and fiberreinforced composites, resulting in superior mechanical properties compared to conventional lamina processed from fiber-reinforced composites or monolithic metals. Despite exhibiting low weights, according to Seydel and Chang [4], composite materials are very susceptive to impact damage. On the other hand, metals are in general heavier, but might have excellent impact resistance. Thus, very light and impact resistant panels might be produced by combining the low volumetric mass density of a fiber-reinforced polymer with the high impact resistance of ductile metals. As stated by Vlot [5], impact damage zones of FMLs are smaller than those found in fiber-reinforced composites alone. In summary, FMLs are characterized by a balance of low structural weight and high strength and stiffness, when compared with metals. The adhesion between metal and composite is clearly a crucial factor to improve the effectiveness of FMLs and several techniques accomplish it by changing the surface characteristics of the metal part [2]. In this line, hybrid joining approaches have been developed, but specifically aiming at improving the performance of composite-metal joints [6]. These approaches essentially rely on macro-scale metal anchorages on the metal surface to interlock the layers of fiber-reinforced polymer and add strength to the adhesive bond. One of the most recent techniques applies an array of pins deposited on the metal part, before joining, as anchorages (mechanical interlocking) for the composite part. Cold-metal transfer (CMT) PIN, an arc-welding-based process developed and commercialized by Fronius (Austria), would be a relatively low-cost option to be used. The CMT PIN was created from its parent process called CMT, in which metal wire is continuously fed and an electric arc (protected by gas) is open between the wire tip and the material to be welded, as in all gas metal arc welding (GMAW) techniques. But in difference to common GMAW, CMT uses a proprietary torch, which periodically reverses the advancing electrode wire when it touches the weld pool, allowing a smooth transfer of molten material into it (in this instant, current is also considerably reduced, with no short-circuit current peaks) without spattering (loss of material). The heat transferred to the work piece is very low, causing just minor metallurgical changes as well as low levels of distortion and discoloration/oxidation (including in the reverse side) in the base metal. The CMT PIN embarks a program for pin depositions. Following short-circuiting, it causes the wire to stay resting against the weld pool (with no arc) allowing time for cooling and consequent welding of the wire to the base metal. Shortly afterwards, a low current is forced to heat and soften the welded wire near its mid length (between the torch and the work piece), but not enough to break it apart and open the arc. With the reversing motion started, the soft wire undergoes a tensile force and is broken, leaving a pin formed on the base metal. By changing deposition parameters, pins of different sizes and geometries can be formed [7]. Ucsnik et al. [8] have shown that pins deposited by CMT PIN on the metal side improved the strength of metalcomposite joints. According to Graham et al. [9], adhesive bonding with CMT PIN anchorage was able to consistently outperform adhesive bonded only specimens in terms of strength and energy absorption at quasi-static and high loading rates, as well as in terms of damage tolerance

94 Optimum Composite Structures

after impact, environmental durability and mechanical fatigue performance.

The capacity that the CMT PIN process seems to have to enhance the performance of composite-metal hybrid joints might be extended to FMLs. Arrays of pins on the metal sides could perform as anchorages for the composite sides throughout the panels, contributing to the

#### **2.1. Manufacturing of the panels**

The small-sized FMLPs reinforced with metal pins were produced as illustrated in **Figure 1**. The reference FMLPs (without pins) were produced in the same way, with exception of the pin deposition step. The metals parts were composed of AISI 430 stainless steel sheets (350 × 80 × 0.4 mm). For pin deposition, a Fronius TransPuls Synergic 5000 power source was used, connected to a VR7000-CMT wire feeder, a PullMig CMT torch and a RCU 5000i remote control unit, with DB0875 data base selected (synergic line CrNi 19 9 PIN). A welding robot was used to move the torch according to the deposition pattern. An AWS ER309L filler wire with nominal diameter of 1 mm (verified value of 0.98 ± 0.00 mm) was used as pin material and Ar with 4% (verified as 3.7%) of CO<sup>2</sup> gas at a flow rate of 8 L/min was employed to protect the pin deposition. Fronius Contec MD® contact tips were employed, as recommended for CMT PIN. The torch was always kept perpendicular to the metal parts. The metal surfaces were all homogeneously wiped with acetone wetted cloth. Other input parameters, such as contact-tip to work-piece distance (5 mm), metal sheet temperature (room temperature ≈ 27°C) and pin deposition sequence were remained unchanged. The CMT PIN process was then parameterized to get minimum-height (considering the thickness of the FMLs) and small ball-head pins. Ucsnik et al. [8] had shown that the ballhead shaped pins present potential in metal-composite hybrid joining. Pins of 2.50 ± 0.06 mm in height and 1.40 ± 0.03 mm in head diameter were produced, resulting in an average pin weight of 0.023 ± 0.002 g.

The pin deposition process was monitored by sampling the electrical current and voltage data, as exemplified in **Figure 2**. The pin deposition cycle (T) was 5.73 s, consequence of the robot motion between two consecutive deposition points, which was not optimized in this work. As seen, the CMT PIN process works basically by controlling the levels of current applied to the electrode-wire and the dwell time in each level. First, the voltage is at its open-circuit level ready for arc starting. Then, the electrical arc is struck (by short-circuiting the electrode-wire to the base metal) and its tip and base metal below are melted (tarc = 0.04 s). Subsequently, after the electrode-wire forward movement and contact with the base metal, the arc is extinguished and the welding between the electrode-wire and the base metal takes place (tweld = 0.02 s). Finally, the welded electrode-wire undergoes heating, softening and breaking apart at its mid length range by a combination of current flow and electrode-wire backward movement (trup = 0.36 s).

After CMT PIN processing, all metal parts (with and without pins) went through ultrasonic cleaning by immersion in acetone for 8 minutes to remove any process-related or not elements that could impair the adherence of the composite parts (**Figure 3**). For composite lamination, prepregs made of glass fiber (8-Harness Satin Weave) and epoxy resin from Hexcel Corporation (product data 7781-38"-F155) were employed. Each layer of prepreg was cut with dimensions of 360 × 90 mm (in excess of 5 mm measured from each of the metal sheet edges). The prepreg layers were stacked on top of the lower metal parts (with or without pins), always aligning the warp yarns with their length and, consequently, the filling yarns with their width. Next, the upper metal parts were placed aligned to the prepreg layers and lower metal parts. All prepreg handling was executed in a white room, avoiding to the maximum any contamination. All small-sized panels were wrapped with a thin film of release agent (polyamide) and then processed in groups of three in a CARVER® CMG100H-15-C hot-curing press according to the curing cycle displayed in **Figure 4**. After curing, the composite material exciding the edges of each panel was removed by means of band sawing followed by belt

Fiber-Metal Laminate Panels Reinforced with Metal Pins http://dx.doi.org/10.5772/intechopen.78405 97

sanding, ending up in panels of 350 × 80 × 4 mm.

**Figure 3.** Example of metal sheet after pin deposition, where: (a) before; and (b) after cleaning.

**Figure 4.** Curing cycle used for the fabrication of the small-sized FMLPs.

**Figure 1.** Sequence of production of small-sized FMLPs reinforced with metal pins welded by cold-metal transfer (CMT) PIN process.

**Figure 2.** Cycle of pin deposition based on the electrical current and voltage data.

After CMT PIN processing, all metal parts (with and without pins) went through ultrasonic cleaning by immersion in acetone for 8 minutes to remove any process-related or not elements that could impair the adherence of the composite parts (**Figure 3**). For composite lamination, prepregs made of glass fiber (8-Harness Satin Weave) and epoxy resin from Hexcel Corporation (product data 7781-38"-F155) were employed. Each layer of prepreg was cut with dimensions of 360 × 90 mm (in excess of 5 mm measured from each of the metal sheet edges). The prepreg layers were stacked on top of the lower metal parts (with or without pins), always aligning the warp yarns with their length and, consequently, the filling yarns with their width. Next, the upper metal parts were placed aligned to the prepreg layers and lower metal parts. All prepreg handling was executed in a white room, avoiding to the maximum any contamination. All small-sized panels were wrapped with a thin film of release agent (polyamide) and then processed in groups of three in a CARVER® CMG100H-15-C hot-curing press according to the curing cycle displayed in **Figure 4**. After curing, the composite material exciding the edges of each panel was removed by means of band sawing followed by belt sanding, ending up in panels of 350 × 80 × 4 mm.

Finally, the welded electrode-wire undergoes heating, softening and breaking apart at its mid length range by a combination of current flow and electrode-wire backward movement

**Figure 1.** Sequence of production of small-sized FMLPs reinforced with metal pins welded by cold-metal transfer (CMT)

**Figure 2.** Cycle of pin deposition based on the electrical current and voltage data.

(trup = 0.36 s).

96 Optimum Composite Structures

PIN process.

**Figure 3.** Example of metal sheet after pin deposition, where: (a) before; and (b) after cleaning.

**Figure 4.** Curing cycle used for the fabrication of the small-sized FMLPs.

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 was produced twice for replication in the tests.

As shown in **Table 1**, an attempt was made to keep the same thickness (approximately 4 mm) and prepreg layers per panel (13) for all FMLPs types. Pin density was estimated for each pertinent case, by considering the respective number of pins divided by the panel surface area

has only marginal effect, regardless of pin density. The overall average mass density of the

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

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

). However, concerning mass density, usually crucial for FMLPs, the presence of pins

, close to the case of M-C-M (without pins).

**Actual thickness** 

2001 13 4.04 ± 0.03 3.61 3.3

525 13 4.00 ± 0.07 0.94 3.0

1976 13 4.08 ± 0.01 3.57 3.3

518 13 3.96 ± 0.02 0.93 3.1

**Pin density (pin/cm2 )**

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

> **Mass density (g/cm3 )**

99

**(mm)**

(560 cm2

FMLPs with pins was 3.2 g/cm<sup>3</sup>

**Panel type Pins per** 

MPin-C-MPin hexagonal

MPin-C-MPin hexagonal

MPin-C-MPin squared

MPin-C-MPin squared

5 mm

10 mm

5 mm

10 mm

**panel**

**Prepreg layers per panel**

M-C-M (without pins) 0 13 3.90 ± 0.03 0 3.0

**3.1. Modal analysis**

**3. Non-destructive evaluation**

**Table 1.** Characteristics of the small-sized FMLPs.

cost, as well as being a nondestructive analysis.

**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).


**Table 1.** Characteristics of the small-sized FMLPs.

As shown in **Table 1**, an attempt was made to keep the same thickness (approximately 4 mm) and prepreg layers per panel (13) for all FMLPs types. Pin density was estimated for each pertinent case, by considering the respective number of pins divided by the panel surface area (560 cm2 ). However, concerning mass density, usually crucial for FMLPs, the presence of pins has only marginal effect, regardless of pin density. The overall average mass density of the FMLPs with pins was 3.2 g/cm<sup>3</sup> , close to the case of M-C-M (without pins).
