Langford Analytic · Knowledge Base
Impact, Crash & Explicit Dynamics
Short-duration, highly nonlinear events where structural response is governed by mass, velocity, momentum, kinetic energy, contact duration, wave propagation and rapid material failure rather than by applied force alone. This category is event-focused — covering impact, crash, drop, bird strike and high-energy transient problems using explicit finite element methods. General nonlinear modelling methodology, including quasi-static non-linearity and incremental solution, remains in the Non-Linear & Transient Analysis category. This section explains how impact and crash simulations are constructed, verified, interpreted and correlated with physical behaviour — from impact fundamentals and explicit time integration through momentum, energy and impulse, stress waves, contact, high-rate material models, strain-rate effects, failure and element erosion, stable timestep, mass scaling, hourglassing, energy balance, mesh sensitivity, crashworthiness, SPH and ALE formulations, composite impact damage, test correlation and the complete chain from explicit animation to defensible engineering evidence.
Featured
Fundamentals
Explicit vs Implicit Time IntegrationHow implicit equilibrium iteration and explicit incremental advance differ, and why the choice should follow the physics and numerical character of the problem rather than the desire for a dynamic animation.Impact Mechanics: Momentum, Energy & ImpulseWhy momentum, kinetic energy and impulse are not interchangeable, and why the velocity-squared dependence of kinetic energy makes a single peak force an inadequate measure of impact severity.Stress Waves & Dynamic Load PropagationWhy an impact load does not appear everywhere simultaneously: wave propagation, impedance, reflections, and the mesh resolution needed to represent a disturbance travelling through a structure at a finite speed.
Contact & Material Response
Contact Modelling in Impact AnalysisContact as the mechanism that generates the impact load, the parameters that control it, and the verification that distinguishes a physical contact model from one that is merely numerically convenient.Material Models for High-Rate LoadingChoosing a constitutive model for impact: what each common family represents, what data it demands, and why a simpler model with verified data can be more defensible than a complex model with assumed constants.Strain-Rate Effects in Structural MaterialsWhy impact velocity is not strain rate, why the strain-rate field inside a component is spatially varying, and why assuming a single strain rate for the whole structure misrepresents the material response.Failure, Damage & Element ErosionDistinguishing physical failure from numerical element deletion, why deleting an element is not the same as creating a crack, and why a result that changes with element size may be predicting the mesh rather than the structure.
Numerical Control & Verification
Stable Timestep & Explicit Solution ControlWhy explicit integration is conditionally stable, how a single small or stiff element can govern the cost of the entire model, and how to control the stable timestep without corrupting the physics.Mass Scaling: Use, Abuse & VerificationMass scaling trades computational speed for a change in the physical model. This article sets out how it is used, how it is abused, and the verification that must show the added mass has not changed the engineering conclusion.Hourglassing, Element Formulation & Numerical StabilityReduced-integration elements, zero-energy modes, hourglass control and the artificial energy that can either stabilise a formulation or quietly corrupt the structural answer.Energy Balance & Explicit Model VerificationTracking every energy component in an explicit impact model — kinetic, internal, contact, hourglass, artificial, eroded — and using the energy history to prove the simulation is physically credible before any result is used.Mesh Sensitivity, Localisation & Failure PredictionWhy mesh refinement in a failure analysis is not always "the same answer with more decimal places", how localisation changes the convergence question, and what engineering quantities to compare instead.
Applications
Drop, Slam & Component Impact AnalysisDrop height, initial velocity, orientation, contact stiffness and internal inertia — and why potential energy does not by itself define the local structural load in a drop or slam event.Crashworthiness & Progressive Energy AbsorptionCrashworthiness as controlled energy management — crush zones, progressive folding, force-displacement behaviour, and why the strongest structure is not necessarily the best crash structure.Bird Strike Analysis: Lagrangian, SPH & ALE MethodsBird strike as a highly deformable soft-body impact problem — Lagrangian, SPH and ALE approaches, the bird as part of the load definition, and whether the test fixture belongs in the FE model.SPH, ALE & Alternative Impact FormulationsA deeper comparison of Lagrangian, Eulerian, ALE and SPH formulations for severe-deformation impact — mesh behaviour, interface treatment, numerical parameters and how to choose based on how the material moves.Composite Impact, Delamination & DamageMatrix cracking, fibre failure, delamination and barely visible impact damage in composite structures — why the external dent can hide significant internal damage and why equivalent shell properties do not capture interlaminar behaviour.
Validation & Engineering Evidence
Test Correlation & Validation of Impact ModelsRigorous correlation of impact FE models against test — measurable quantities, timing alignment, filtering, and why matching the final deformation alone is not correlation.From Explicit Animation to Defensible Engineering EvidenceThe concluding article — the complete chain from physical event to engineering conclusion, the verification checklist, and the principle that the animation is the visualisation, not the evidence.