A thermoviscoplastic model with damage for simultaneous hot/cold forging analysis

A constitutive model is presented for simultaneous hot/cold forming processes of steels. The phenomenological material theory is based on an enhanced rheological model and accounts temperature dependently for nonlinear hardening, thermally activated recovery effects, an improved description of energy storage and dissipation during plastic deformations, and damage evolution as well. A thermomechanically consistent treatment of dissipative heating due to inelastic deformations, recovery processes and damage mechanisms is applied. The constitutive model is implemented into a commercial FE-code. The material parameters of the effective model response are identified for a low alloyed steel and validated by means of a simultaneous hot/cold forging process

On the Generalization of Uniaxial Thermoviscoplasticity with Damage to Finite Deformations Based on Enhanced Rheological Models

The enhanced concept of rheological models, as proposed in Br¨ocker and Matzenmiller (2013), is generalized systematically to finite deformations. The basic bodies are defined individually for large deformations, and a rheological network of thermoviscoplasticity is assembled, representing nonlinear isotropic and kinematic hardening as well as an improved description of energy storage in metal plasticity. The constitutive equations are deduced in an analogous procedure as for the uniaxial model in Br¨ocker and Matzenmiller (2013). Furthermore, damage evolution is additionally accounted for

Intraply fracture of fiber-reinforced composites: microscopic mechanisms and modeling

The fracture behavior parallel to the fibers of an E-glass/epoxy unidirectional laminate was studied by means of three-point tests on notched beams. Selected tests were carried out within a scanning electron microscope to ascertain the damage and fracture micromechanisms upon loading. The mechanical behavior of the notched beam was simulated within the framework of the embedded cell model, in which the actual composite microstructure was resolved in front of the notch tip. In addition, matrix and interface properties were independently measured in situ using a nanoindentor. The numerical simulations very accurately predicted the macroscopic response of the composite as well as the damage development and crack growth in front of the notch tip, demonstrating the ability of the embedded cell approach to simulate the fracture behavior of heterogeneous materials. Finally, this methodology was exploited to ascertain the influence of matrix and interface properties on the intraply toughness

A Thermodynamically-Based Mesh Objective Work Potential Theory for Predicting Intralaminar Progressive Damage and Failure in Fiber-Reinforced Laminates

A thermodynamically-based work potential theory for modeling progressive damage and failure in fiber-reinforced laminates is presented. The current, multiple-internal state variable (ISV) formulation, enhanced Schapery theory (EST), utilizes separate ISVs for modeling the effects of damage and failure. Damage is considered to be the effect of any structural changes in a material that manifest as pre-peak non-linearity in the stress versus strain response. Conversely, failure is taken to be the effect of the evolution of any mechanisms that results in post-peak strain softening. It is assumed that matrix microdamage is the dominant damage mechanism in continuous fiber-reinforced polymer matrix laminates, and its evolution is controlled with a single ISV. Three additional ISVs are introduced to account for failure due to mode I transverse cracking, mode II transverse cracking, and mode I axial failure. Typically, failure evolution (i.e., post-peak strain softening) results in pathologically mesh dependent solutions within a finite element method (FEM) setting. Therefore, consistent character element lengths are introduced into the formulation of the evolution of the three failure ISVs. Using the stationarity of the total work potential with respect to each ISV, a set of thermodynamically consistent evolution equations for the ISVs is derived. The theory is implemented into commercial FEM software. Objectivity of total energy dissipated during the failure process, with regards to refinements in the FEM mesh, is demonstrated. The model is also verified against experimental results from two laminated, T800/3900-2 panels containing a central notch and different fiber-orientation stacking sequences. Global load versus displacement, global load versus local strain gage data, and macroscopic failure paths obtained from the models are compared to the experiments

Modelling low velocity impact induced damage in composite laminates

The paper presents recent progress on modelling low velocity impact induced damage in fibre reinforced composite laminates. It is important to understand the mechanisms of barely visible impact damage (BVID) and how it affects structural performance. To reduce labour intensive testing, the development of finite element (FE) techniques for simulating impact damage becomes essential and recent effort by the composites research community is reviewed in this work. The FE predicted damage initiation and propagation can be validated by Non Destructive Techniques (NDT) that gives confidence to the developed numerical damage models. A reliable damage simulation can assist the design process to optimise laminate configurations, reduce weight and improve performance of components and structures used in aircraft construction

Modelling low velocity impact induced damage in composite laminates

The paper presents recent progress on modelling low velocity impact induced damage in fibre reinforced composite laminates. It is important to understand the mechanisms of barely visible impact damage (BVID) and how it affects structural performance. To reduce labour intensive testing, the development of finite element (FE) techniques for simulating impact damage becomes essential and recent effort by the composites research community is reviewed in this work. The FE predicted damage initiation and propagation can be validated by Non Destructive Techniques (NDT) that gives confidence to the developed numerical damage models. A reliable damage simulation can assist the design process to optimise laminate configurations, reduce weight and improve performance of components and structures used in aircraft construction

A Solid-Shell Element with Enhanced Assumed Strains for Higher Order Shear Deformations in Laminates

In structural analysis the three-dimensional standard 8-node brick element has become one of the most favourable elements, especially due to its numerical efficiency, its versatility and its wide range of applicability to many types of mechanical problems. However, low-order elements, based on the irreducible displacement model, suffer from different kind of locking effects. For instance, shear locking occurs in the finite element analysis of thin plates or shells under flexural deformations. In this case pure bending modes can not be represented by the standard linear element due to the parasitic shear deformation. By using higher order elements, these locking phenomena are reduced, but the computational effort becomes larger. To overcome this deficiency, numerous investigations have been undertaken for finding a high accurate, low-order element without increasing the computational cost significantly. Based on previous research, our approach aims towards the analysis of laminated structures with solid-shell elements, combined with the higher order shear deformation theory to improve the transverse shear behaviour, necessary for stress based failure criteria in the analysis of inter-laminar fracture in multi-layer shells. As a side effect shear-locking is alleviated by incorporating the higher order transverse shear interpolation instead of using assumed natural strain formulations. The solid-shell element is used for the analysis of delamination in layered composite shells, where the onset of fracture in mode II and III critically depends on the transverse shear stress distribution in the cross section of the shell