Within the framework of project Z2-50081, protective composite materials with shape memory were developed. They were designed by embedding metallic reinforcements made of Nitinol, a shape memory alloy, into a base polymer matrix. Such composites combine the advantages of porous geometry on one side and load-bearing reinforcements made of shape memory materials on the other. This combination enabled properties that cannot be achieved with conventional materials, such as the possibility of attaining a negative Poisson’s ratio and the capability of (self-)restoring the geometry of products. In addition to 3D CAD models and derived virtual simulations, experimental physical specimens were also manufactured and tested at testing facilities.

During the course of the project, two independent concepts were developed in this direction, including:

i) the fabrication and testing of the reactive response of a hybrid cellular structure with the ability of cell transformation, designed in diamond-shaped silhouettes – see Figure 1, and

ii) the development and fabrication of an auxetic composite capable of utilizing the shape memory effect of wires for the regeneration of damaged parts – see Figure 2.

Figure 1: Hybrid cellular structure designed in diamond-shaped silhouettes.

In both cases, the regeneration of induced surface damage was based on exploiting the shape memory effect of wires made of Nitinol alloy. The obtained results were presented in two separate original scientific articles (COBISS.SI-ID 254349571 and 254324483).

Figure 2: Auxetic composite with the capability of regenerating damaged parts.

Advanced composite specimens were 3D modelled and their response simulated. Subsequently, they were also experimentally manufactured and physically tested, as shown in Figure 3. The simulations and experimental tests show very good agreement of results.

Figure 3: Testing of the real response of an auxetic cellular structure with shape regeneration capability

The presented and developed multifunctional structures with cell transformation capability exploit favourable properties of porosity, such as the ability to tailor the response with a desired positive or negative Poisson’s ratio, as well as noise and vibration damping. Consequently, the effects of pore shape, size, and distribution on material durability were also investigated. For this purpose, statistical tools based on multivariate Gaussian functions and the in-house developed REBMIX code were used. The obtained pore classification represents an important basis for designing new forms of cellular structures. In parallel, the influence of surface roughness on service life was also investigated, as shown in Figure 4. The results of this pore analysis were presented in a separate scientific article (COBISS.SI-ID 238058499).

Figure 4: Analysis of the influence of pore shape, size, orientation, and surface roughness on material service life

In addition to lifetime prediction methods, a customized XFEM code based on the extended finite element method was developed. This method enables the identification of critical locations and the simulation of real crack initiation in currently overloaded mechanical elements and structures. The new method was presented at a conference (COBISS.SI-ID 182089475).

In the final phase of the project, the research focused on the development of a protective panel concept with a dual objective: achieving high protective performance while minimizing structural mass. A special protective composite was developed, incorporating a cellular core with auxetic properties (see Figure 5). The enhanced protective effect was achieved by adding a Kevlar layer to the front surface of the structure. Preliminary tests demonstrated very good performance in impact energy dissipation during collision of a hazardous object with the protective layer, indicating a clear direction for further research.

Figure 5: Concept of an active cellular composite with enhanced protective performance

All four planned comprehensive work reports have been prepared and entered into the COBISS system in accordance with the project timeline. Their brief summaries are as follows:

Milestone M1: Definition of basic cell geometries of the composite and the layout of SMA reinforcements.
In the initial phase, existing known forms of closed-cell structures and pores were identified, their advantages and disadvantages evaluated, and two separate concepts of cell design were developed. The first included diamond-shaped cell silhouettes, while the second involved auxetically designed cells. In this phase, the first experimental specimens were produced using 3D printing and preliminarily tested using Nitinol wire activation. In this way, the basic material properties required for FEM simulations were obtained. For more details see: COBISS.SI-ID 194560515.

Completion rate: 100%

Milestone M2: Experimental characterization and reverse engineering of the base material.
In the second phase, cell geometries were refined in detail, with most effort devoted to optimizing cell connections to facilitate rotation during deformation. At the same time, appropriate integration of Nitinol wire reinforcement into the composite matrix was carried out. Static response tests were performed on a servohydraulic MTS testing system, where yield strength and tensile strength of the composite assembly were determined. For more details see: COBISS.SI-ID 216843267.

Completion rate: 100%

Milestone M3: Definition of optimal final geometries of new protective structures with cell transformation capability for experimental testing.
In the third phase, experimental tests under controlled loading conditions were conducted, followed immediately by regeneration tests of deformed geometries. Temperature activation of the embedded Nitinol reinforcement was thoroughly examined in two different environments (hot air and water immersion). The advantages and disadvantages of both activation methods were analysed. For more details see: COBISS.SI-ID 223202563.

Completion rate: 100%

Milestone M4: Comprehensive synthesis of research results and dissemination.
In the final phase, a complete synthesis of all obtained research results was carried out, along with their dissemination into academic and industrial environments. Four original scientific articles were published in high-impact Q1 journals: Composites Part B: Engineering (IF = 14.2), Materials & Design (IF = 7.9), Journal of Materials Research and Technology (IF = 6.6), and Trees, Forests and People (IF = 2.9), with a cumulative impact factor exceeding IF > 31. All key articles were translated into Slovenian to support terminology transfer into the national environment. The results are publicly available via the Repository of the University of Ljubljana (RUL) in accordance with open science principles. The research was also integrated into the educational process within the course Machine Elements, where two students successfully completed a master’s practical project and a bachelor’s thesis on the topic of efficient cellular structure design for improved energy dissipation. For more details see: COBISS.SI-ID 254353667.

Completion rate: 100%

During the investigation of applications of cellular structures with cell transformation capability, an additional potential application area was identified, namely forestry and tree protection. In 2024, a special auxetic clamping protective structure was developed, which deforms according to a negative Poisson’s ratio, meaning that it automatically adapts to tree growth. The structure shown in Figure 6 protects against bark damage caused by wildlife or human activity during logging.

Figure 6: Development and testing of a new tree protection structure

Based on the obtained results, an upgraded version of the structure was developed, for which both Slovenian and European patent applications have been filed. The upgraded structure includes a variable cell geometry enabling adaptation to conical sections of the tree trunk near the root. It also incorporates safety mechanisms for automatic release in cases where stress concentrations exceed allowable limits, thereby preventing potential bark damage or restriction of tree growth. The developed solution thus combines protective functionality with long-term harmlessness to the tree and represents an innovative contribution in the field of forest and tree-line protection.

Figure 7: Concept design of the tree protection structure for patent applications

Award for outstanding contribution in the research category (Figure 8 – left):
Tomažinčič D., Klemenc J. Adapted XFEM technique for crack detection in gears and other overloaded load-bearing structures. Akademija strojništva: Engineering – Integration for Sustainable Breakthrough, 2023, vol. 12 [COBISS.SI-ID 182089475]

Award for best contribution of the academy of mechanical engineering 2024 (Figure 8 – right):
Tomažinčič D., Klemenc J. Development of an active cellular composite utilizing shape memory wires for regeneration of dents and other damage. Akademija strojništva: Engineering – Innovative Sustainable Development with High Added Value, 2024, vol. 13 [COBISS.SI-ID 222143491]

Figure 8: Awards obtained within the fundamental postdoctoral project Z2-50081

ORIGINAL SCIENTIFIC ARTICLES:

[1] D. Tomažinčič, J. Kajbič, J. Klemenc, Development and fabrication of an auxetic composite with the shape memory effect for the regeneration of damaged parts, Composites Part B: Engineering 309 (2026) 113034. [COBISS.SI-ID 254324483] https://doi.org/10.1016/j.compositesb.2025.113034

[2] (translated into Slovenian) D. Tomažinčič, J. Kajbič, J. Klemenc, Development and fabrication of an auxetic composite with the shape memory effect for the regeneration of damaged parts, Composites Part B: Engineering 309 (2026) 113034. https://repozitorij.uni-lj.si/IzpisGradiva.php?id=175237

[3] D. Tomazincic, J. Kajbič, J. Klemenc, Fabrication and Response Testing of a Hybrid Cellular Structure with the Ability to Transform Cells Formed into Diamond-Shaped Silhouettes, Materials & Design 259 (2025) 114822. [COBISS.SI-ID 254349571] https://doi.org/10.1016/j.matdes.2025.114822

[4] (translated into Slovenian) D. Tomazincic, J. Kajbič, J. Klemenc, Fabrication and Response Testing of a Hybrid Cellular Structure with the Ability to Transform Cells Formed into Diamond-Shaped Silhouettes, Materials & Design 259 (2025) 114822. https://repozitorij.uni-lj.si/IzpisGradiva.php?id=175242

[5] D. Tomažinčič, J. Klemenc, Development of a special self-adaptive auxetic structure for protecting tree trunks from external damage, Trees, Forests and People 20 (2025) 100860. [COBISS.SI-ID 237990915] https://doi.org/10.1016/j.tfp.2025.100860

[6] (translated into Slovenian) D. Tomažinčič, J. Klemenc, Development of a special self-adaptive auxetic structure for protecting tree trunks from external damage, Trees, Forests and People 20 (2025) 100860. https://repozitorij.uni-lj.si/IzpisGradiva.php?id=169542

[7] D. Tomažinčič, B. Panić, M. Nagode, J. Klemenc, The fatigue life estimation of the porous AlSi9Cu3 alloy based on the classification of pore geometry using multivariate probability distribution density, Journal of Materials Research and Technology 36 (2025) 10039–10054. [COBISS.SI-ID 238058499] https://doi.org/10.1016/j.jmrt.2025.05.076

[8] (translated into Slovenian) D. Tomažinčič, B. Panić, M. Nagode, J. Klemenc, The fatigue life estimation of the porous AlSi9Cu3 alloy based on the classification of pore geometry using multivariate probability distribution density, Journal of Materials Research and Technology 36 (2025) 10039–10054. https://repozitorij.uni-lj.si/IzpisGradiva.php?id=170837

PUBLISHED SCIENTIFIC PAPERS AT CONFERENCES:

[9] D. Tomažinčič, A. Škrlec, J. Klemenc, Segment zaščitnega panela z odzivno geometrijo in samopopravljivimi lastnostmi, Akademija strojništva 14 št. 1/6 (2025) 158–159. [COBISS.SI-ID 261692675] https://doi.org/10.62020/svet.str.as2025058.

[10] D. Tomažinčič, J. Klemenc, Razvoj aktivnega celičnega kompozita s sposobnostjo koriščenja oblikovnega spomina žic za regeneracijo udrtin in drugih poškodb, Akademija strojništva 13 št. 1/6 (2024) 74–75. [COBISS.SI-ID 222143491] https://doi.org/10.62020/svet.str.as2024016.

[11] D. Tomažinčič, J. Klemenc, Razvoj posebne žične avksetične strukture za namestitev poškodovanega lubja nazaj okoli debla prizadetega drevesa iglavca, Akademija strojništva 13 št. 1/6 (2024) 78–79. [COBISS.SI-ID 222170883] https://doi.org/10.62020/svet.str.as2024018.

[12] D. Tomažinčič, J. Klemenc, Prilagojena tehnika XFEM za odkrivanje razpok v zobnikih in drugih preobremenjenih nosilnih strukturah, Akademija strojništva 12 št. 3/6 (2023) 114–115. [COBISS.SI-ID 182089475] https://plus-legacy.cobiss.net/cobiss/si/sl/bib/182089475#full.

PROFESSIONAL ARTICLES:

[13] D. Tomažinčič, Avksetična mreža – samoprilagodljiva zaščita, ki raste z drevesom. Bistriški odmevi 135 (2025) 46–47. [COBISS.SI-ID 267891459] https://www.ilirska-bistrica.si/objava/1159588.

INDEPENDENT SCIENTIFIC RESULTS (SSRN):

[14] D. Tomazincic, J. Kajbič, J. Klemenc, Fabrication and Response Testing of a Hybrid Cellular Structure with the Ability to Transform Cells Formed into Diamond-Shaped Silhouettes, SSRN. http://dx.doi.org/10.2139/ssrn.5096746

STUDENT THESES AND MASTER’S INTERNSHIPS:

[15] M. Štukelj, Zasnova 3D tiskanih struktur z oblikovnim spominom iz PLA materiala: diplomsko delo Visokošolskega strokovnega študijskega programa I. stopnje Strojništvo – Projektno aplikativni program. [COBISS.SI-ID 247244035] https://plus-legacy.cobiss.net/cobiss/si/sl/bib/247244035#full

[16] J. Simončič, Simulacija preboja sendvič kompozitov in analiza vpliva različnih oblik 3D tiskanih struktur na prebojnost kompozita: Projektni praktikum – MAG, magistrski študijskega programa II. stopnje Strojništva

ELABORATE AND STUDIES:

[17] M1-bazična oblikovanja struktur celičnega kompozita. [COBISS.SI-ID 194560515] https://plus-legacy.cobiss.net/cobiss/si/sl/bib/194560515#full

[18] M2 – preizkus modeliranja in izdelave različnih izvedb aktivnih celičnih struktur : ARRS-RPROJ-JR-Prijava-2023/584 : Z2-50081. [COBISS.SI-ID 216843267] https://plus legacy.cobiss.net/cobiss/si/sl/bib/216843267#full

[19] M3 – testiranje reakcijskih odzivov različnih izvedb aktivnih celičnih struktur : ARRS-RPROJ-JR-Prijava-2023/584 : Z2-50081. [COBISS.SI-ID 223202563] https://plus legacy.cobiss.net/cobiss/si/sl/bib/223202563#full

[20] M4 – končna predstavitev rezultatov raziskave aktivnih celičnih struktur : ARRS-RPROJ-JR-Prijava-2023/584 : Z2-50081. [COBISS.SI-ID 254353667] https://plus-legacy.cobiss.net/cobiss/si/sl/bib/254353667#full