Abstract
Metallic alloys play the leading role in marine engine construction. Yet, under the compelling goal of reducing cost and weights, this industrial sector is in constant need of new, high-performance materials for the production of marine engine non-structural components. In this respect, nano-engineered thermoplastic polymers are ideal alternatives, allowing for additional benefits (e.g., simplified maintenance and inspection operations). The use of these materials in marine engine design requires computer multiscale simulations to tailor-fit their molecular structure in order to achieve the expected performances required by specific, advanced functions. Importantly, replacing metal alloys with plastic-based materials also contributes to environmental sustainability, in terms of both component production process and recyclability. The introduction of non-structural plastic components in marine engines constitutes a major innovation in the field; thus, a specific rule framework must be still defined. Under this perspective, starting from the analysis of the rule framework currently used for metallic alloys, in this paper a certification procedure is proposed and applied to a case study: a four-stroke marine engine plastic cylinder head cover for which the mechanical properties of the new material have been predicted and verified trough multiscale simulations carried out on the relevant model.
Introduction
Since their discovery, nano-engineered thermoplastic polymers (NETPs) materials have found a wide range of industrial applications. A prime example of massive exploitation of these materials is in the construction of automotive components; however, recent studies have highlighted the possibility of extending the usage of advanced plastics also to the design and manufacturing of engine components [1,2,6,7,10–12]. With respect to metal and/or metal alloys, advanced NETPs are characterized by substantially lower volumetric mass density, easier processability, and ultimately lower production costs [4,24]. Besides these economic and practical aspects, the adoption of polymeric materials can contribute to environmental sustainability, in terms of both component production process and recyclability. Indeed, the energy required for NETPs moulding process is drastically lower than that involved in metallic alloys die-casting. Furthermore, the final properties of these plastic-based materials – from their molecular structure to their thermo-physical behaviour – can be predicted and fine-tuned ahead of their synthesis using computer-assisted multiscale molecular design (CAMMD).
Given the elevated performances offered by engineered plastics, and their consequent increasing pervasiveness in disparate productive sectors, the marine engine industry is also moving its first paces towards the application of these materials in the field [5]. In particular, since the green shipbuilding concept [13,20,26] is becoming more and more important worldwide, the possibility of introducing green processes for the construction of marine engine non-structural components is extremely noteworthy. To date, metallic alloys undeniably play the leading role in marine engine construction. Thus, the adoption of nano-engineered plastic materials for the design and production of non-structural marine engine components represents a great point innovation. As such, the current regulatory framework does not provide specific rules about the usage of NETPs in machinery spaces.
In order to fill this normative gap and provide a classification procedure for non-structural plastic components of marine engines, in the initial part of this paper it is reported the analysis on the specific requirements for materials usable in ship’s machinery spaces, essentially as regards fire prevention [8,9,14–16,19,21–23,25]. Next, on the basis of the working conditions of a cylinder head cover of a four-stroke marine engine chosen as a case study, multiscale simulation techniques have been used to predict, optimize and verify the mechanical properties of a polymer nanocomposite as a suitable replacement of the currently adopted metal-based material.
Rule framework
To date, a well-defined regulatory framework gathering all the rules involved in the classification procedure for the application of non-metallic materials in the construction of non-structural components for marine engines is still missing. Several institutions provide fragmented sets of requirements that are complex and not prone to univocal interpretation. These are summarized below.
International Convention for the Safety of Life at Sea, 1974
The International Maritime Organization (IMO) in the “International Convention for the Safety of Life at Sea, 1974 and Its Amendments” (SOLAS) [15], imposes a restricted use of combustible material, which is defined as “any material other than a non-combustible material. A non-combustible material is a material which neither burns nor gives off flammable vapours in sufficient quantity for self-ignition when heated to approximately 750°C, this being determined in accordance with the Fire Test Procedures (FTP) Code”.
The Convention allows the use of combustible materials only in restricted ship areas, like accommodation and service spaces, and cabin balconies in passenger ships. Anyhow, the ignitability value of combustible materials applied must be very low. The use of combustible materials in machinery spaces is not allowed; consequently, any plastic materials chosen to build non-structural marine engine components must belong to the non-combustible material class. Non-combustibility tests have to be carried out in accordance with the requirements provided by the “FTP Code” [16]. Specifically, Annex I of the Code explains the methodology to perform non-combustibility, smoke and toxic gas generation tests.
FTP code – Part 1: Non-combustibility test
The provisions given in Part 1 must be applied when a material is required to be non-combustible. If a material passes the test as specified in the Part, it shall be considered non-combustible even if it consists of a mixture of inorganic and organic substances.
In order for materials to be classified as non-combustible, they must satisfy the following criteria:
the average furnace thermocouple temperature rise does not exceed 30°C;
the average specimen surface thermocouple temperature rise does not exceed 30°C;
the average duration of sustained flaming does not exceed 10 s;
the average mass loss does not exceed 50%.
The fire test is aimed at identifying products which generate only a very limited amount of heat and flame when exposed to temperatures of approximately 750°C.
FTP code – Part 2: Smoke and toxicity test
The provisions given in Part 2 must be applied when a material is required not to be capable of producing excessive quantities of smoke and toxic products or provoke toxic hazards at elevated temperatures.
Within the general introduction of Part 2, the following results are required:
the maximum specific optical density of smoke DS max, which is determined by a procedure properly described in the Code; the concentration of each toxic gas determined as parts per million (ppm) in the test chamber volume at the time when the maximum specific optical density of smoke DS max is reached.
International Association of Classification Societies interpretations
Besides the requirements issued by the IMO, their interpretation given by the International Association of Classification Societies (IACS) must be considered. A significant expression of interest by the shipbuilding industry about metal replacement in the construction of non-structural components of marine engines is provided by the recent IACS publication “Unified Interpretation of the SOLAS Convention”. Specifically, item SC282 gives interpretations about the application of materials other than steel on engine, turbine and gearbox installations [14].
The SC282 asserts that the usage of materials other than steel may be assessed in relation to the risk of fire associated with the component and its installation. Among other applications, the use of different materials is considered acceptable for components that are either subjected only to liquid spray on their inside when the machinery is running (such as machinery covers, rocker box covers, camshaft end covers, inspection plates and sump tanks), or for components attached to that machinery that satisfy fire test criteria according to a standard acceptable to the Administration.
UL Thermoplastics Testing Center
About material classification, in addition to the FTP Code the provisions of the “UL 94, Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing”, released by Underwriters Laboratories (UL) of the United States, can be considered [25].
The UL 94 consists of a plastic flammability standard, which determines the material’s tendency to either extinguish or spread the flame once the specimen has been ignited. In particular, the UL 94 implements 12 flame classifications that are assigned to materials on the basis of the results of specific tests. As for the materials under analysis (i.e., materials commonly used in manufacturing enclosures), 6 classifications are provided on the basis of the results obtained on specimens after specific tests. The tests to establish the flammability propensity of a material can be the following:
Horizontal Burning test (HB classification);
20 mm Vertical Burning test (V-2, V-1, V-0 classifications);
500 W (125 mm) Vertical Burning test (5VB, 5VA classifications).
The specimens have to be tested in either horizontal or vertical position, and exposed to a defined ignition flame source for a specified period of time.
The HB classification is assigned when the material, tested in horizontal position under a single test flame application for 30 seconds, exhibits a specified burning rate depending on the specimen thickness.
The V-2, V-1 or V-0 classification is obtained when the material is tested in vertical position under two applications of a test flame 20 mm high for 10 seconds, and the resulting self-extinguishing times is less than specified values. Moreover, the effect of flaming particles or melted drops dripping on a cotton indicator placed below the specimen is observed (only for V-2 classification the ignition of the cotton is allowed).
A material classified as 5VB or 5VA is subjected to a flame ignition source that is approximately five times more severe than that used in the V-0, V-1, V-2 and HB tests. The 5VB or 5VA classification is obtained when the material is tested in vertical position by five applications of test flame 125 mm high for 5 seconds each, and after such a burning cycle the self-extinguishing time turns to be less than 60 seconds. For both 5VB and 5VA classifications, the cotton indicator placed below the specimen is not ignited by flaming particles or drops; besides, the absence of burn-through (hole) in the specimen is also required for 5VA classification.
Lloyd’s Register
Classification societies are also expected to contribute in regulating the use of non-structural plastic components on marine engines. In this respect, the Lloyd’s Register “Rules for Manufacture, Testing and Certification of Materials”, which gives provisions for manufacture and test procedures regarding thermoplastic polymers, has been considered [19]. In particular, this document provides approval requirements for base materials used in the construction of machinery components to be certified or intended for classification.
The plastic material for the purposes of the rules is defined in “Sect. 1 – General Requirements” as “a ‘plastic material’ is regarded as an organic substance which may be thermosetting or thermoplastic and which, in its finished state, may contain reinforcements or additives.”
There is a set of requirements regarding both pure and reinforced thermoplastic polymers. In particular, data to be provided by the manufacturer for each thermoplastic polymer include:
melting point;
melt flow index;
density;
bulk density;
filler content, where applicable;
pigment content, where applicable;
colour.
Moreover, specimens are to be prepared by moulding or extrusion in accordance with the recommendations of the polymer manufacturer, and the tests to be carried out shall include:
tensile stress at yield and break;
modulus of elasticity in tension;
tensile strain at yield and break;
compressive stress at yield and break;
compressive modulus;
temperature of deflection under load;
determination of water absorption.
About testing procedures, strain measurements must be performed by a suitable extensometer or strain gauge. The number of specimens of each sample to be tested must be in accordance with an appropriate ISO standard; in particular, for mechanical testing a minimum of five is given. All load vs. displacement tables and graphs are to be reported, along with mean value and standard deviation.
Det Norske Veritas – Germanischer Lloyd
In order to provide the most complete analysis possible, the rules issued by another important classification society, the Det Norske Veritas – Germanischer Lloyd (DNV-GL), have been analysed.
In particular, since specific requirements for the application of plastic materials in ships’ engine room are not provided, it has been necessary to contact a DNV-GL surveyor. As the result of such a meeting, it has been learned that some of the requirements issued for electrical installations (Rules for Classification – Ships – Part 4 Systems and components – Chapter 8 Electrical installations [8]) can be applied also to the purpose of the present study. In detail, the plastic material applied for the construction of non-structural components for marine engines has to comply with the following requests:
it has to be auto-extinguishing;
it has to be fire-retardant;
it has not to produce toxic gas and thick smoke;
it has to be halogen free;
it has not to be vulnerable to the fluids with which it may come into contact;
it has to resist to an applied load equal to 2000 N.
In addition to the previous requirements, in the “Rules for Classification – Ships – Part 1 General Regulations – Chapter 3 Documentation and certification requirements, general” [9], the DNV-GL requires a list of documentation in order to assess that relevant rules are complied with. The prescription for non-metallic materials are provided in the “Discipline M – Materials: M030”, and requires a document describing:
scope, references and definitions;
chemical composition;
delivery conditions;
production process;
testing and requirements;
inspection and non-destructive testing;
repair;
dimensions and tolerances;
surface protection;
certification and marking.
For reinforced materials (e.g., glass fibre reinforced plastics) the Discipline M requires also other information; however, the materials under analysis are not interested by these requirements, since they apply only to the conventional fibre-reinforced plastic (FRP).
These additional information consist of:
type of reinforcement and production process for reinforcing material; production process of finished, composite material; inspection and non-destructive testing of finished, composite material; repair; dimensions and tolerances of finished material; surface protection; certification and marking.
Registro Italiano Navale
Since the final aim of the rule framework analysis was the identification of a classification procedure for the non-structural plastic components on marine engines, the Registro Italiano Navale (RINA) has been involved in this work to elaborate the guidelines for the process.
The use of plastic materials in the construction of non-structural marine engine components must be considered a novel technology, since it is a proven technology in a new environment. As such, reference to the “Guide for Technology Qualification Processes”, aiming at providing a systematic approach to the qualification of novel technologies, ultimately ensuring that they are fit for their intended service [21], is a mandatory step.
The Technology Qualification (TQ) is to be carried out through a documented process that includes examination of the design, engineering analyses and testing programs. The classification society assumes the role of third party, which is the organisation in charge of assessing and witnessing the TQ process to confirm compliance with this procedure.
TQ process is to be based on specified safety, availability and reliability criteria, boundary conditions and interface requirements as defined in the Qualification Basis (QB). In fact, in the possible absence of fully relevant codes and procedures, the QB has the purpose of defining the objectives of the novel technology and represents the input for the TQ process. It should include at least the following key items:
description of the technology to be qualified with the system boundaries defining the scope of the TQ;
operational conditions and limitations;
functional requirements;
safety, reliability, availability and maintainability criteria;
codes and standards.
Once the QB is defined, the Technology Assessment (TA) is to be performed, with the purpose of dividing the proposed technology into manageable parts in order to assess those elements that involve aspects of novelty, thereby identifying the main challenges and uncertainties. The level of detail in the subdivision of the technology should be appropriate to focus on the novel or uncertain aspects, which have to be successively considered for the risk assessment. The technology is classified according to its degree of novelty and the relevant rank is as follows:
Class 1 – No new technical uncertainties;
Class 2 – New technical uncertainties;
Class 3 – New technical challenges;
Class 4 – Demanding new technical challenges.
In particular, a technology ranked Class 2 to 4 is defined as novel technology, and for this reason it is likely that no recognised standard for the design exists or is fully applicable.
After the Technology Assessment, the next step implies the selection of the so-called Qualification Methods (QMs), which must address the key issues of the technology to be qualified. Such QMs will likely consist of a proper combination of engineering analyses and test programs, both aimed at increasing the confidence in the novel technology and at reducing the uncertainties. The selected QMs will become mandatory for the TQ process. In order to establish the QMs, a risk assessment of the novel technology is to be performed according with the techniques dealt with in proper standards. In this context, the term “risk” is related to the events that may affect the fitness for service of the novel technology. The risk assessment should be subdivided into the following main tasks: i) hazard identification; ii) risk assessment against the defined acceptance criteria and interfaces; iii) definition of risk control options; and iv) documentation of the study. Besides, the risk assessment has to be based on the “Guide for Risk Analysis” and the “Guide for Failure Mode and Effect Analysis” issued by RINA [22,23]. At the end of QMs selection, the Data Collection and the Functionality Assessment phases have to be carried out, during which evidence of the design, construction, operations and maintenance of the novel technology in its lifetime, as well as a confirmation that the functional requirements and the safety, reliability, availability and maintainability criteria are fulfilled, are to be provided.
The Technology Qualification final result is an official statement of fitness for service. As supporting evidences, a Technology Assessment Report, a Technology Qualification Plan, and a Technology Qualification Report are to be issued: these deliverables must be verified, commented on and approved by the third party. The technical reports have the aim to confirm that the novel technology meets the specified requirements for its intended service, and to issue a relevant certificate.
Material design
The idea of performing computer-assisted multiscale material design (CAMMD) starting from fundamental physical and chemical principles has an obvious appeal as a tool of potential great impact on technological innovation and material design. Briefly, the advantages of considering multiscale molecular modelling as an integral part of material design include, among others:
the reduction of the product development time by alleviating costly trial-and-error iterations;
the reduction of product costs through innovations in materials, products, and process design;
the reduction of the number of costly, large-scale experiments;
the increase of product quality and performance by providing more accurate predictions in response to material design requirements and loads;
the support provided to conceive and develop entirely new materials.
Thus, in this work, the CAMMD concept has been applied to estimate specific thermophysical and mechanical properties of NETP systems suitable to be employed in the fabrication of covers for marine engines as a suitable replacement of the currently used aluminium alloy.
Based on specific material requirements (i.e., maximum working temperature = 110°C, non-flammable or self-extinguishing material (at least V-0 according to UL94), elastic limit > 0.4 MPa, and price comparable to that of the aluminium alloy actually employed) a preliminary literature survey combined with the application of Ashby’s method [3] as implemented in the CES software (www.grantadesign.com/products/ces) identified polyamide-6,6/glass fibre (PA66/GF)-based NETPs as the most suitable systems for the context application.
Next, computational procedure based on molecular dynamics (MD) simulations specifically developed and optimized for polymer-based nanocomposite materials [17,18] were applied to predict the mechanical properties of PA66/GF NETPs as function of GF content.
According to the simulations (Fig. 1), the addition of glass fibres (15–50% wt) to the PA66 matrix results in a remarkable increase of the predicted Young modulus E for the corresponding NETPs (Fig. 1A). The relative increment of E with respect to the pristine PA66 polymer is well evident from the behaviour of the corresponding enhancement factor
Table 1 summarizes the numerical values of the entire set of properties estimated for the PA66/GF (30% wt) NETP system as estimated by CAMMD, together with those for pure PA66 and the reference aluminium alloy for comparison.
As seen from this table, the predicted glass transition temperature (Tg, i.e., the temperature at which the polymeric material transforms from a glassy state to a viscous and rubbery one) of the thermoplastic PA66 (52°C) is practically unaffected by the 30% wt GF dispersion (50°C), as is the value of the strain at break (5% and 4%, respectively). This evidence indicates that the glass fibres can be interspersed within the polymeric matrix without inducing significant perturbations in its molecular structure (Fig. 2). On the contrary, the addition of GF to the PA66 matrix results in an increase of both the maximum stress at break (189 MPa) and the Young modulus (10.2 GPa) values, as a result of the synergy of load transfer from the matrix to the reinforcement nanofibers. Overall, these results support the concept that the selected NETP material is endowed with the features required to serve the purpose of the current application, as furtherly discussed below.
A – Predicted Young modulus E for the pristine PA66 matrix and for the relevant PA66/GF NETPs as a function of fibre content (% wt). B – Enhancement factor
Snapshot from the equilibrated portion of the molecular dynamics simulation of the PA66/GF (30% wt) NETP system. The polymer matrix is shown as transparent red spheres while glass fibres are portrayed in grey sphere representation.
Values of the mechanical and thermophysical properties for the PA66/GF (30% wt) as estimated by CAMMD at room temperature. The corresponding values predicted for the pristine PA66 matrix and for a reference aluminium alloy are also shown for comparison
The cylinder head cover of a four-stroke marine engine (Fig. 3) has been selected to carry out finite element simulations.
Aluminium-alloy cylinder head cover.
This component can be classified as a non-structural element, since its main purpose is to protect mechanical tappets and injectors from external dust and contain any oil losses from the valve mechanisms. Due to the vibration of the cylinder head, the cover is essentially subjected to displacements in which the rigid-body component is the prevailing one. Consequently, very low stress ranges are in general produced, so that the fatigue effects on the component may be considered not important. However, future works should be done to deeply investigate on this specific issue.
The maintenance interval period is 12000 hours, corresponding to 500 days (≈2 years) of continuous running. About the expected lifetime, the cover should be replaced every 60000 hours (≈10 years). This implies that the cylinder head cover must be replaced 3 times during the engine lifetime.
The cover currently installed is constituted of two main parts: the lower part is the anchoring to the engine head through five studs, and prevents the loss of any fluid by means of a gasket, whereas the upper part is the one removable for inspection operations. The joint between the two parts is equipped with two hinges and two hooks. On the external face of the upper part, two tie rods rising from the injector block to the cover lid are screwed by two handles to provide sufficient fastening. The mechanical resistance of the cover is guaranteed by longitudinal and transversal ribs; furthermore, in the central longitudinal line, two bushings absorb the stress generated by the screwing of the handles.
On the basis of the results obtained from CAMMD, the nano-engineered plastic PA66/GF (30% wt) has been selected to perform finite element simulations. Notably, the original geometry of the cylinder head cover had to be modified to account for the different characteristics of the new material.
The finite element analysis has been performed at the engine room temperature (40°C). It is worth noting that if the engine room temperature rises up to a certain value (for instance, 60°C), the Young modulus decreases between 10–15%, and then displacements increase. Anyway, the new values of displacements remain acceptable as for the functionality of the cover.
The purpose of component redesign was twofold: preserving structural strength and simplifying removal procedure while maintaining the same fixing points of the current metallic component. This, in turn, allowed for no modifications of the engine head configuration.
NETP cylinder head cover model.
The new model (Fig. 4) presents two main parts: an aluminium collar plate and a plastic lid. The collar plate serves to fix the cover to the engine through five studs and provides support to the aluminium inspection door leading to the engine hotbox. The cover lid is the element protecting the cylinder mechanisms. The internal face of the lid is equipped with a series of ribs to ensure the required structural rigidity. The ribs run all around the interior surface of the lid, creating an inverted U-shaped grillage, which rests on the collar. Furthermore, in order to simplify cover removal, the model is equipped with two handles on the top of the lid. It is important to note that the load generated by screwing the handles on a small area could create very high local stresses. Accordingly, in order to redistribute such stresses, two elastic bushings have been inserted in correspondence of the handle positions. To this aim, the bushings are formed by three concentric rings properly linked to each other.
In order to properly compare the aluminium-alloy cover currently installed on engines and the proposed NETP cover, the manufacturing process of the two materials, weights, cost, and expected lifetime of both the covers have been examined.
The aluminium-alloy cover is made through a die-casting process, whereas the NETP cover is made through an injection moulding process.
About the masses, the data regarding the total mass of the entire component (cover lid + aluminium collar) have been analysed. For the aluminium-alloy cover, the total mass is ≈ 35 kg (of which ≈ 22.7 kg for the aluminium cover lid and ≈ 12.3 kg for the aluminium collar); whereas, for the NETP cover, the total mass is ≈ 24 kg (of which ≈ 11.7 kg for the NETP cover lid and ≈ 12.3 kg for the aluminium collar).
As for the costs, the reported values take into account all the activities that regard the construction phase of the component, such as the cost of materials, manufacturing process, transportation and mounting. For the aluminium-alloy cover, the total cost is ≈ 700 €, whereas for the NETP cover is ≈ 250 €.
Regarding the expected lifetime of the NETP cylinder head cover, it has been estimated to be less than the lifetime of the aluminium-alloy cover (≈ 10 years). Indeed, the NETP cover could be replaced 4 times during the entire engine lifetime (estimated ≈ 30 years), and then its lifetime turns to be ≈ 7.5 years.
Finite element simulation
The model has been meshed through an unstructured grid employing tetrahedral solid elements. The mesh has been refined in proximity of geometry changes and model curves. The maximum size of a single element has been set equal to the maximum local thickness.
The entire model has been considered fully fixed at the base. On the lower face of the collar plate all degrees of freedom have been constrained. Furthermore, sliding and separation between faces have been avoided by considering all the surfaces in contact.
The model has been tested under the application of a static load. In particular, with the aim of simulating the load during maintenance and inspection operations (including the presence of a person standing on the cover), the load case analysed considers a load applied on the top face of the lid. To mimic such a situation, a vertical load due to the operator (2000 N) has been uniformly applied to the footprints, and the tightening handwheel loads (2 × 6000 N) have been applied on the two bushings (Fig. 5).
Load case on the lid.
The results obtained by finite-element method simulations (Figs 6 and 7) are reported in Table 2. The maximum displacement is small and located at the centre of the cover lid, and the maximum stress and strain results are far lower than the acceptable limits.
Deformation result.
Stress and strain results.
The most important advantages involved in replacing the aluminium alloy by nano-engineered thermoplastic polymers in the manufacturing of a marine engine cylinder head cover are represented by cost savings and lower weights. For instance, rough estimations reveal that this material replacement will result in cost savings of about 65% while the plastic cover will weigh 50% less than the aluminium-based one. Consequently, all maintenance operations will also be significantly simplified, as they could be carried out without using cranes to move. As regard the expected lifetime of the new NETP cover, it is lesser than the 10 years expected for the aluminium-alloy one. However, even if the plastic component should be replaced more often, it remains convenient from both the economical and operational point of view.
In order to make the application of plastic components compliant with the requirements of the classification societies, an analysis of the current rule framework has been carried out. In particular, it has been highlighted that the most important property for a plastic material to be accepted by a classification society is the non-combustibility. Moreover, the plastic material used for the construction of non-structural engine components shall guarantee acceptable mechanical characteristics.
Regarding material identification, attention has been focused on fibre-reinforced polymers, which represent one of the most interesting material class able to ensure both lightweight and high strength. Specifically, computer-based material design has identified PA66 loaded with 30% wt of glass fibres as the most suitable materials for the current application.
Deformation, stress and strain: critical vs analysis result values
The finite element analysis carried out on the component model has offered encouraging results, since deformation, stress and strain magnitudes are far lower than the critical values of the material. This analysis is to be intended as the basis for future developments, where the component could be optimized in terms of production process, material and cost saving.
Footnotes
Acknowledgements
This work was supported by “PLASTICO – Plastic Cover for Marine Engine” research program, funded by the Regione Autonoma Friuli Venezia Giulia with POR-FESR 2014–2020, Asse 1 Azione 1.3.b.
Conflict of interest
None to report.