Philipp Weißgraeber

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• Professor
• Professor at University of Rostock

We determined the compressive strength of weak layers of faceted crystals and depth hoar using artificially grown samples with a wide range of microstructural morphologies in a cold laboratory setup. Micro-computed tomography ( µ CT) imaging showed that the microstructures of the artificial samples were comparable to that of natural depth hoar. We...

Weak interface models are a potent tool to capture both deformations, stresses, and energy release rates for adhesives or other weak layers between stiffer joints. While the energy release rate for both modes I and II is rendered well, a fast and efficient model, capturing mode III loading of the weak layer is missing. The present study addresses t...

Introduction Subtalar joint arthrodesis is primarily indicated for advanced osteoarthritis, hindfoot deformity, and/or instability. During the first 6-10 weeks after surgery, there is an intermediary structurally weaker state before complete bony fusion of the calcaneus and talus occurs. Loading of the foot can lead to mechanical stresses and relat...

In this work, the failure of rubber-like materials by cavitation is investigated. Under local triaxial loading states, as they can occur in confined geometries of adhesive bonds, the instantaneous formation of voids can be observed. Analytical results of cavitation in a commercial silicone adhesive are provided. The results show that cavitation voi...

Slab fracture plays a crucial role in the release of dry-snow slab avalanches, significantly impacting the size of the release area. However, there is a lack of fracture toughness experiments for well-bonded snow that cover the entire mode interaction regime, from pure tensile loading (mode I) to pure shear (mode II). We propose a field test capabl...

When porous materials are subjected to compressive loads, localized failure chains, commonly termed anticracks, can occur and cause large-scale structural failure. Similar to tensile and shear cracks, the resistance to anticrack growth is governed by fracture toughness. Yet, nothing is known about the mixed-mode fracture toughness for highly porous...

Understanding crack phenomena in the snowpack and their role in avalanche formation is imperative for hazard prediction and mitigation. Many studies have explored how structural properties of snow contribute to the initial instability of the snowpack, focusing particularly on failure initiation within weak snow layers and the onset of crack propaga...

Fiber-reinforced composites are susceptible to failure by delamination. Hence, transverse reinforcements have been thoroughly researched, including the technology of z-pinning, with the insertion of pins in the thickness direction of laminate layups before curing. Such reinforcements act as strong crack bridging elements and enhance the interlamina...

When porous materials are subjected to compressive loads, localized failure chains, commonly termed as anticracks, can occur and cause large-scale structural failure. Similar to tensile and shear cracks, the resistance to anticrack growth is governed by fracture toughness. Yet, nothing is known about the mixed-mode fracture toughness for highly por...

Understanding the creep behaviour of materials is crucial in structural design, since assessing their durability and long-term performance is essential for ensuring the safety of the structures. Experimental testing allows to gather data on the creep behaviour of materials, as well as observe the damage mechanisms and dependence on environmental ef...

The triggering mechanisms of dry snow slab avalanches are subjected to a mixed-mode loading scenario driven by different dynamic loading conditions and the natural inclined terrain. However, standard propagation saw tests, which are common to determine the fracture toughness of weak layers, almost exclusively cover the slope-normal regime (mode I)...

The adhesion of pressure-sensitive adhesives (PSAs) is a complex phenomenon that can be understood through the characterization of different properties, including viscoelastic, mechanical, and fracture properties. The aim of the present paper is to determine the viscoelastic behaviour of an acrylic PSA and place it in the viscoelastic window, as we...

We propose a closed-form analytical model for the mechanical behavior of stratified snow covers for the purpose of investigating and predicting the physical processes that lead to the formation of dry-snow slab avalanches. We represent the system of a stratified snow slab covering a collapsible weak layer by a beam composed of an arbitrary number o...

In the present paper, an exploratory study on the creep behavior of a pressure sensitive adhesive (PSA) is performed. After the determination of the quasi-static behavior of the adhesive for bulk specimens and single lap joints (SLJ), SLJs were subjected to creep tests at 80%, 60%, and 30% of their respective failure load. It was verified that the...

The aim of this work is to analyze the difference between silicone/composite and silicone/metal interphases, both in terms of water diffusion behavior and failure of the aged joints. For that, silicone joints with two different suhbstrates were prepared. The substrates were polybutylene terephthalate with 30% of short glass fiber (PBT-GF30) and 608...

For a slab avalanche to release, we need sustained crack propagation in a weak snow layer beneath a cohesive snow slab – a process we call dynamic crack propagation. Field measurements on crack propagation are very scarce. We therefore performed a series of crack propagation experiments, up to 9 m long, over a period of 10 weeks and analysed these...

We propose a closed-form analytical model for the mechanical behavior of stratified snow covers for the purpose of investigating and predicting the physical processes that lead to the formation of dry-snow slab avalanches. We represent the system of a stratified snow slab covering a collapsible weak layer by a beam composed of an arbitrary number o...

For a slab avalanche to release, the system, consisting of a weak layer below a cohesive slab, must facilitate crack propagation over large distances – a process we call dynamic crack propagation. Field measurements on crack propagation at this scale are very scarce. We therefore performed a series of propagation saw test experiments, up to ten met...

This work aims to analyze the mechanical and chemical effects that contaminants can have prior to curing of the silicone adhesive, if they are in the substrate or surrounding environment at adhesive application, as well as after curing when they appear during the service life of the joint. In this case, the contaminants are water and a surfactant p...

Adhesives are used to join and seal electronic packaging. In these application, they are often used to bond aluminum components and, during the process of machining aluminum, oils and lubricants are used to facilitate the process. To remove these fluids, detergents such as surfactants are often used. However, if the surfactant itself is not removed...

The housing of automotive electrical components are commonly manufactured from polybutylene terephthalate with 30% of glass fiber (PBT-GF30). These housings can be subjected to severe conditions of temperature and humidity that can catalyze the degradation of their properties. In order to connect the different parts of the housing, a silicone adhes...

With the aim of understanding failure modes in the peeling of silicone-based adhesive joints and, in particular, the occurrence of adhesive or cohesive failure, an experimental campaign has been conducted by considering plastic substrates with different surface roughness. A flexible strip has been bonded onto such substrates using a silicone adhesi...

This work describes why additive manufacturing is a key technology for efficient design iterations and rapid production ramp-up with large-scale manufacturing technologies. Laser cutting, injection moulding and folding were used to increase the production capacity of face shields for health care workers during the COVID-19 pandemic. We applied syst...

Short fiber reinforced polymers are widely used in the construction of electronic housings, where they are often exposed to harsh environmental conditions. The main purpose of this work is the in-depth study and characterization of the water uptake behavior of PBT-GF30 (polybutylene terephthalate with 30% of short glass fiber)as well as its consequ...

The present contribution focuses on the effect of adherend surface roughness on the strength of adhesive joints, which are particularly cost-effective and extensively applied in a wide range of industrial applications. However, the reliability of such solutions is a critical concern for the integrity of commercial products. To gain a deeper underst...

In this paper, both location and energy of impacts on an anisotropic carbon fiber reinforced plate (CFRP) are detected with the help of deep learning. We introduce sparse low-cost sensor array integration in CFRP plates that allows for structural monitoring of lightweight structures. Using a resin transfer moulding process microelectromechanical sy...

With the aim of understanding failure modes in the peeling of silicone-based adhesive joints and, in particular, the occurrence of adhesive or cohesive failure, an experimental campaign has been conducted by considering plastic substrates with different surface roughness. A flexible strip has been bonded onto such substrates using a silicone adhesi...

Adhesives play an important role in many key industrial sectors, such as the automotive industry, enabling the construction of lightweight, multi-material structures, combining polymers, composites and metals. However, adhesives are usually polymeric materials, which can be affected by environmental and working conditions, such as moisture and cont...

The present contribution focuses on the effect of adherend surface roughness on the strength of adhesive joints, which are particularly cost-effective and extensively applied in a wide range of industrial applications. However, the reliability of such solutions is a critical concern for the integrity of commercial products. To gain a deeper underst...

Using the analytical model presented in Part 1 of this two-part paper, a new conceptual understanding of anticrack nucleation in weak layers is proposed. To obtain a sufficient condition for onset of failure, two necessary conditions must be satisfied simultaneously: (i) the weak layer must be overloaded in terms of stress and (ii) the initiating c...

Dry-snow slab avalanche release is preceded by a fracture process within the snowpack. Recognizing weak-layer collapse as an integral part of the fracture process is crucial and explains phenomena such as whumpf sounds and remote triggering of avalanches from low-angle terrain. In this two-part work we propose a novel closed-form analytical model f...

Dry-snow slab avalanche release is preceded by a fracture process within the snowpack. Recognizing weak layer collapse as an integral part of the fracture process is crucial and explains phenomena such as whumpf sounds and remote triggering of avalanches from low-angle terrain. In this two-part work we propose a novel closed-form analytical model f...

Using the analytical model presented in part I of this two-part paper, a new conceptual understanding of anticrack nucleation in weak layers is proposed. To obtain a sufficient condition for onset of failure two necessary conditions must be satisfied simultaneously: i) The weak layer must be overloaded in terms of stress and ii) the initiating crac...

To improve modeling of slab avalanche release we propose a new mechanical snowpack model and a novel criterion for anticrack nucleation. The model allows to analyze skier-loaded slopes as well as PST experiments. We obtain closed-form analytical expressions for shear and normal stresses in the weak layer as well as for the energy release rate of cr...

In this work, an enhanced eXtended finite element method (XFEM) implementation is outlined. It allows for modeling two-dimensional crack growth including potential crack deflection at significantly tougher constitutents of multi-material continua. At such material interfaces a user-defined crack deflection criterion is utilized that allows for crac...

In this work, crack initiation in adhesive lap joints of arbitrary joint configuration is studied by means of a finite fracture mechanics approach. The analysis is based on a general stress solution for adhesive joints combined with a coupled stress and energy criterion. The instantaneous formation of a crack of finite size is predicted if a stress...

An application of the finite fracture mechanics concept to open-hole plates subject to combined tensile and bending loading is presented. In finite fracture mechanics, the simultaneous satisfaction of both, a stress and an energy criterion, is enforced as a condition for crack initiation. Efficient modeling and closed-form expressions for the depen...

Abstract Plates notched with open-holes that are subject to combined tensile and in-plane bending loading are studied in this analysis of crack onset. A finite fracture mechanics approach is employed to study the onset of asymmetric crack patterns in the notched structure. The stress field and the energy release rates required to solve the coupled...

The present work provides a new model for the stress analysis of functionally graded adhesive lap joints with composite adherends. It is applicable to various joint configurations such as single lap joints, L-joints, T-joints, reinforcement patches, corner joints or balanced double lap joints. The modelling approach follows the concept of general s...

Using an efficient numerical approach, a model rendering the initiation of asymmetric crack patterns in open-hole plates subject to combined tensile and in-plane bending loading is presented. In the framework of finite fracture mechanics a stress and an energy criterion are enforced simultaneously and used as a coupled condition for the formation o...

In the present work a generally applicable failure model for the assessment of crack onset in adhesive lap joints which is based on finite fracture mechanics is presented. The approach combines a general sandwich-type model with a coupled stress and energy criterion that requires only two fundamental failure parameters: the strength and the toughne...

The failure behaviour of structural situations depends on the shape of the energy release rate function of possible crack configurations. Depending on the structural situation, the energy release rate function can decrease (negative geometry) or increase (positive geometry) with increasing crack size. In this work, three structural situations exhib...

Crack initiation in brittle materials is not covered by classical fracture mechanics that deals only with the growth of pre-existing cracks. In order to overcome this deficiency, the Finite Fracture Mechanics concept assumes the instantaneous formation of cracks of finite size at initiation. Within this framework, a coupled criterion was proposed a...

Abstract In this work an efficient general Finite Fracture Mechanics model for crack initiation in adhesively bonded joints is presented which is applicable to various joint configurations with shear flexible laminate and metallic adherends. The mechanical behaviour of the adhesive joints is modeled by a general sandwich-type model for the analysis...

In this work, crack initiation in elliptically notched plates under uniaxial tension is studied by means of a Finite Fracture Mechanics analysis. The stress concentration factor changes with the aspect ratio of the half axes: from unity for slender ellipses in loading direction to infinity for crack-like ellipses normal to the loading. The closed-f...

In this work crack initiation at elliptical holes in plates under uniaxial tension is studied by means of a closed form analytical Finite Fracture Mechanics approach. To allow for a detailed study of crack initiation, stress intensity factors available in literature are discussed and compared to extensive numerical results. Based on physical ration...

Brittle failure of adhesively bonded single lap joints is considered in this paper. A coupled stress and energy criterion in the framework of finite fracture mechanics is employed to study crack initiation by means of a numerical model presented by Hell et al. (Eng Fract Mech 117:112-126, 2014). Two different formulations of the coupled criterion a...

To study crack initiation at weak stress singularities classical Fracture Mechanics approaches do not suffice. In this work weak singularities are discussed and the obstacles occurring in fracture analysis are outlined. The concept of Finite Fracture Mechanics and the use of coupled criteria is explained. A brief discussion of coupled criteria is g...

Efficient analyses of adhesive joints are very important for pre-dimensioning or optimization processes. In this work a new model for adhesive joints is presented that can be applied to various joint configurations, as e.g. single lap joints, T-joints, L-joints or reinforcement patches. This is done by means of a two-dimensional, plane-strain model...

In this work, a new approach for the prediction of the crack initiation loads of single lap joints (SLJ) is presented. The failure model for crack initiation in brittle adhesive layers is based on Finite Fracture Mechanics and makes use of a coupled stress and energy criterion. It requires only two fundamental material parameters: the tensile stren...

Coupled stress and energy criteria in the framework of Finite Fracture Mechanics have been widely used to study crack initiation at stress concentrations. In many cases, the incremental energy release rates of the considered cracks of finite size increase monotonically with increasing crack sizes. But in certain structural situations, referred to a...

Up to now the failure load assessment of bonded joints is still not fully understood. This work provides a new approach for assessing the crack initiation load of bonded joints. A failure model for single lap joints is proposed that is based on Finite Fracture Mechanics. Only two basic fracture parameters are required: the tensile strength and the...

This work is concerned with the analysis of failure of adhesive joints. Typically adhesive joints fail due to cracking of the adhesive with cracks originating from the reentrant corner of the adherend and the adhesive. A failure model based on a coupled stress and energy criterion settled in the framework of Finite Fracture Mechanics (FFM) is propo...

A deeper understanding of load transfer in the human hip joint is of great importance as there is strong evidence that the mechanical loading of the hip has a major effect on the onset and progression of osteoarthritis. In this work, a biomechanical model of the human hip joint is developed which takes the lateral rotators into account. On the basi...

In this paper a linear, closed-form analysis of the buckling behavior of an orthotropic plate with elastic clamping and edge reinforcement under uniform compressive load is presented. This is a typical structural situation found in aerospace engineering for instance as stiffeners in wings or the fuselage. All governing equations are transformed in...

In this paper a finite fracture mechanics approach for calculating failure loads of adhesively bonded joints is presented. The approach based on classical linear elasticity solutions for adhesively bonded single lap joints and uses a combined energy and stress criterion. Parameter studies are performed and it is shown, that the effects of the major...

In this work, crack formation and the corresponding failure load of bonded lap joints is analyzed. The analysis is based on linear elasticity solutions for bonded lap joints and makes use of the finite fracture mechanics. A hybrid criterion is applied that states the spontaneous formation of a crack of finite size if a stress and an energy criterio...

In this work the determination of the properties of an edge reinforcement of a composite plate stiffener is treated. A minimum stiffness criterion for the edge reinforcement on the basis of a closed-form buckling analysis of a composite plate with edge reinforcement and elastic clamping is given. The minimum stiffness criterion is given in explicit...

For the widespread use of adhesive joints an exact and reliable prediction of the strength is mandatory. In this work, a new approach to assess the strength of single lap joints is presented. The approach is based on the hybrid criterion as postulated by Leguillon in the framework of finite fracture mechanics. It strictly combines a consideration o...

In many structural situations, e.g. in aerospace engineering, composite plates with edge reinforcements are used, for example as stiffeners of a fuselage section of an airplane. Based on an explicit analysis of the buckling behavior of the laminate plate, a minimum stiffness criterion is presented in this contribution. A transformation of the gover...