Understanding the Drop
Drop and Impact Analysis
A Complete Engineering Guide
Companion Reader
Joseph P. McFadden Sr.
The Holistic Analyst
Combating Engineering Mind Blindness
McFaddenCAE.com | McFadden@snet.net
April 2026 | Version 20.2
This companion reader accompanies the audiobook of the same title. The words are the same — no break tags, no pauses, just the conversation on the page. Sit down with it. Read it at your own pace.
Part One — Why Drop Analysis?
I want to start by talking about something that every engineer who works on portable products understands at a gut level. Products get dropped. Not sometimes. Not occasionally. Constantly.
Consumer electronics, medical handhelds, industrial scanners, ruggedized field equipment — every product that leaves a facility and enters human hands will eventually be dropped. The question is never if. The question is how many times, from what height, onto what surface, and in what orientation. And the product had better survive.
The business case for simulation-driven drop analysis is not subtle. A housing crack discovered at drop test qualification — late in the program, after tooling is cut, after the product architecture is frozen — is the most expensive kind of engineering failure there is. You are not fixing a design. You are changing a tool. You are scrapping a production schedule. You are delaying market entry. And you are explaining to program management why the simulation did not catch this.
Simulation-driven drop analysis, done correctly and done early, is how you find those failures on a computer screen instead of a test floor. That is the promise.
But there is a counterpoint I want to address directly. Simulation done incorrectly is worse than no simulation at all. A model that predicts survival when the physical product will actually fail gives the engineering team false confidence. They sign off. They ship. The product fails in the field. And now you have warranty returns, customer confidence damage, and potentially regulatory action. The cost of that sequence dwarfs any program schedule savings.
This guide — and the tool it is built around — exists to make sure the simulation is done correctly. Not just mechanically correct in the sense of running without errors. Physically correct. Defensible. Documented. So that when someone asks you in a design review why you believe the product survives the drop, you have an answer that goes beyond, "the simulation said so."
One more thing before we go further. Although this guide uses Abaqus as the solver and the Abaqus INP Analyzer as the review tool, the best practices here are not Abaqus-specific. They are engineering fundamentals. The physics of the impact event — the five phases, the stable timestep, the resonant amplification, the energy balance — are the same whether you are running Abaqus, LS-DYNA, ANSYS Explicit, PAM-CRASH, or any other explicit dynamics solver. The material modeling requirements — density in consistent units, rate-dependent plasticity, viscoelastic adhesives — apply universally. The fragility framework — peak G, velocity change, SRS, jerk — is solver-independent. If you take nothing else from this guide, take the underlying reasoning. The specific keywords will change when you change tools. The physics does not.
Part Two — The Physics. What Abaqus Is Actually Solving.
Before we touch a single keyword, I want to make sure you understand what physically happens during a drop impact. Because the analysis settings you choose — the simulation duration, the element sizes, the mass scaling target, the output request intervals — all flow directly from understanding the physics of the event.
There are five distinct phases to every drop impact. Let me walk you through each one.
Phase One — Free Fall
The product falls from the release point to the impact surface under gravity. The physics is simple kinematics. The velocity at impact is determined entirely by drop height. The formula is v equals the square root of two times G times h. At 1.0 meter drop height, in a tonne-millimeter-second unit system where G equals 9810 millimeters per second squared, the impact velocity is approximately 4,429 millimeters per second — call it 4.4 meters per second. At 1.5 meters: 5.4 meters per second. At 2.0 meters: 6.3 meters per second.
We do not simulate the free-fall trajectory in Abaqus. The model starts at the moment of impact. The free-fall is replaced entirely by applying the impact velocity as an initial condition to all product nodes. The Abaqus INP Analyzer computes the implied drop height from the velocity in your input file and shows it to you in the Impact/Drop Analysis tool. Use it to verify that the velocity you applied corresponds to the drop height in the specification.
Phase Two — Initial Contact
The first node or element face contacts the floor surface. Contact is detected by the Abaqus explicit contact algorithm. For general contact, every exterior surface pair is checked at every increment. The contact force begins at zero and rises as penetration develops. This phase is also where the timestep is most constrained — the smallest element in the contact zone controls the stable timestep, and that brings us to one of the most important concepts in explicit dynamics.
Phase Three — Load Propagation
Stress waves travel from the contact zone outward through the structure at the speed of sound in the material. The wave speed is c equals the square root of E divided by rho, where E is Young's modulus and rho is density. For steel: approximately 5,000 meters per second. For polycarbonate housing: approximately 1,400 meters per second. For a 150-millimeter tall polycarbonate phone housing, the stress wave traverses the full height in roughly 0.1 milliseconds. That is fast. But it is not instantaneous. Remote parts of the structure do not respond until the wave arrives.
This phase is also where resonant amplification occurs. If the impact pulse duration is close to the natural period of a structural feature — a printed circuit board bending mode, a cantilever boss, a snap feature — that feature will experience resonant amplification. Its peak stress will be significantly higher than the quasi-static estimate from force equals mass times acceleration would suggest. This is why peak G at the housing does not predict solder joint failure. The board has its own dynamics.
Phase Four — Peak Deformation
The structure reaches maximum deformation. Kinetic energy has been converted to strain energy — both elastic and plastic — and dissipated as heat through plastic deformation and damping. The peak stress occurs somewhere in this phase. But here is something that catches people by surprise: the peak stress does not necessarily occur at the contact point. Resonant amplification can place peak stress at features remote from where the impact happens. Your glass display can be the highest-stress component when the impact was on the corner of the housing.
Phase Five — Rebound and Recovery
Stored elastic strain energy drives rebound. For an elastic impact against a rigid steel floor, the coefficient of restitution approaches 1.0 — the product bounces back to nearly the drop height. For inelastic impact, where energy is dissipated through plastic deformation, the rebound height is reduced. Abaqus simulates the full event including rebound. Confirm that your simulation duration is long enough to capture the full contact event, and any secondary impact if the product bounces and re-contacts the floor.
The Stable Timestep
Abaqus explicit time integration uses a conditionally stable central-difference scheme. The timestep must be smaller than the Courant-Friedrichs-Lewy stability limit — CFL for short. The CFL limit is the time it takes a stress wave to traverse the smallest element in the model. The formula is: Δt equals L-minimum divided by the wave speed.
A 1-millimeter steel element has a stable timestep around 0.2 microseconds. A 0.5-millimeter aluminum element: around 0.1 microseconds. A 0.1-millimeter PCB element: around 0.02 microseconds. And here is the one that gets people: a 0.05-millimeter adhesive element with a soft modulus around 100 megapascals has a stable timestep around 0.005 microseconds.
What does that mean for your simulation? For a 1.5-millisecond simulation with a 0.2-microsecond timestep: roughly 7,500 increments. Manageable. For that same simulation with a 0.005-microsecond timestep from your adhesive layer: 300,000 increments. Forty times longer. From one thin layer of adhesive.
This is why minimum element size management is one of the most important concerns in production drop test modeling. And it is why the Abaqus INP Analyzer reports the minimum element size and the estimated stable timestep in the Impact/Drop Analysis tool. When that number is very small — below about 10 to the negative 8 seconds — examine which element is controlling it. It is almost always a thin adhesive layer or a very small transitional element at a mesh density boundary.
Part Three — Mass Scaling. The Double-Edged Sword.
Mass scaling is universally used in production drop test simulations. It is also universally misused, misunderstood, and under-validated. Let me give you the framework for using it correctly.
The principle is simple. The stable timestep is proportional to the square root of density. If you artificially increase the density of a small element, you increase its stable timestep. To achieve a 10-times increase in timestep, you need to increase density by a factor of 100.
The Abaqus keyword is: *Variable Mass Scaling, dt=2.5e-8, type=below min, frequency=1. This tells Abaqus: at every increment, find all elements whose stable timestep is below 2.5 × 10⁻⁸ seconds, and add artificial mass to bring them up to the target.
The problem is obvious. Adding artificial mass changes the model's inertia. It changes its dynamic response. Peak forces are affected. Wave propagation timing is affected. And the results are wrong in proportion to how much mass was added.
The engineering standard for acceptable mass scaling is that the total added mass should not exceed five percent of the total model mass. This is not a physical law. It is an empirically derived guideline that balances computational efficiency against result accuracy. Models that exceed five percent added mass fraction should be considered suspect.
In Abaqus, the added mass fraction can be tracked by requesting DMASS as history output. The Recommendations tab in the Analyzer flags when mass scaling is present and reminds you to monitor this fraction.
My validation workflow: run the simulation twice. First with your production mass scaling target. Second with a target timestep that is half of your production value — significantly stricter. If peak stresses and deformations change by less than five percent between runs, your mass scaling is adequate. If they change more, you have a mass scaling problem, not a mesh problem.
Part Four — Drop Test Standards. What You Are Actually Qualifying To.
Every drop test analysis should be anchored to a specific qualification standard. The standard defines the drop height, the number of drops per orientation, the surface material, and the acceptance criteria. Let me walk you through the most commonly encountered standards.
IEC 60068-2-31 — Free Fall
The primary international standard for product free-fall testing. Drop heights by product mass: under 10 kilograms, 1.0 meter for general goods and 0.5 meters for fragile goods; 10 to 40 kilograms, 0.5 meters; 40 to 100 kilograms, 0.3 meters. The test surface is a smooth, hard horizontal surface — in practice a five-centimeter-thick steel plate on a concrete foundation. In simulation this is modeled as a rigid body using R3D4 element types.
MIL-STD-810H Method 516.8 — Shock and Drop
The US military standard for mechanical shock and drop testing. Procedure IV — Transit Drop — is most relevant to product-level drop simulation. Drop heights from 0.3 to 1.22 meters depending on equipment weight and handling method. Twenty-six drops minimum: one on each face, edge, and corner of a rectangular package. Equipment must remain functional after all drops.
ISTA 6-Amazon — E-Commerce Product Test
Now the most widely applied drop test specification for consumer electronics and packaged goods shipped through e-commerce channels. Drop heights from 0.5 to 1.0 meters for products under 25 pounds. The sequence includes flat drops, edge drops, corner drops, and rotational drops. Both packaged and unpackaged configurations are tested.
JEDEC JESD22-B111 — Handheld Electronics Solder Joint
Specifically addresses solder joint reliability in handheld electronics during drop events. Defines a standardized input acceleration pulse — 1500G peak, 0.5-millisecond half-sine — applied to the PCB. Thirty drops at this level without electrical failure is the requirement. This standard informs PCB-level analysis where solder joint strain is the primary failure mode.
Part Five — Setting Up the Explicit Drop Test. Every Keyword Explained.
Now let us go through the Abaqus INP setup keyword by keyword. I am going to explain not just what each keyword does, but why it is set the way it is, and what goes wrong when it is set incorrectly.
The Step Definition
nlgeom=YES is non-negotiable for a drop test. The non-linear geometry flag tells Abaqus to update the stiffness matrix as the structure deforms. Without it, the geometric stiffness is evaluated once at the start of the step and never updated. For a structure undergoing large rotations and displacements during impact — which is most products — this produces an incorrect stiffness model throughout the event.
The simulation duration of 0.0015 seconds is 1.5 milliseconds. For a 1-meter drop onto a rigid steel floor with a moderately stiff housing, 1.5 milliseconds typically captures the full impact event including initial rebound. For softer impact surfaces or complex multi-bounce behavior, extend to 3 to 5 milliseconds. Confirm that your contact force history returns to zero before the simulation ends — a simulation that ends while the product is still in contact is giving you truncated results.
Bulk viscosity at 0.06 and 1.2 (the Abaqus defaults) damps compressive stress wave artifacts. Never set these to zero. Without bulk viscosity, you will see spurious high-frequency oscillations in your stress histories that have no physical basis.
Initial Conditions
The initial velocity must be applied to every node in the product assembly that is supposed to be falling. Every one. When a new part is added to the assembly late in the program cycle, it is easy to forget to update the node set that receives the initial velocity. That part starts the simulation stationary. It gets accelerated by contact after the impact begins. The dynamics are wrong. The simulation completes. No error is generated.
The BC and Load Viewer in the Analyzer visualizes which parts have velocity markers in 3D. Every product part should have a marker. If you see parts without markers, your node set definition is incomplete.
The wrong degree-of-freedom direction sends the product sideways instead of down. The Impact/Drop Analysis tool flags when the velocity direction does not align with the gravity vector. The wrong velocity magnitude implies the wrong drop height — the Analyzer computes the drop height from your velocity value and shows you the number.
Gravity Loading
Gravity applies a body force to all elements proportional to their density. In tonne-millimeter-second units, G equals 9810 millimeters per second squared. The direction vector must point toward the floor. The unit system trap: in SI units, G equals 9.81 meters per second squared. Using the SI value in a tonne-millimeter-second model means your gravity is 1000 times too small. The product drifts downward in slow motion. The unit system checker in the Analyzer will flag this.
Boundary Conditions
ENCASTRE must be applied to the rigid body reference point — not to mesh nodes on the floor surface geometry. The rigid body reference point is a control node that does not have to be on the floor mesh. Constraining the reference point fixes the rigid body. Constraining random mesh nodes on the floor surface does not. If your ENCASTRE is on a product node instead: the product is fixed to the ground, the floor falls freely.
Contact Definition
General contact with ALL EXTERIOR is the standard approach for drop test models. It automatically detects all exterior surface pairs and applies contact between them without requiring manual surface pair definitions. The friction coefficient in the surface interaction property typically starts at 0.2 for plastic-on-steel contact, but friction has a significant effect on corner and edge drop results. Sensitivity studies across a range from 0.1 to 0.5 are recommended for any model predicting edge or corner drop behavior.
Output Requests
Energy output is mandatory. Every drop test simulation must have ALLKE for kinetic energy, ALLIE for internal energy, ALLAE for artificial strain energy, all viscous dissipation, all frictional dissipation, and ETOTAL for the total energy balance.
ETOTAL must stay flat throughout the analysis. If it rises, energy is being created from nothing. If it drops without corresponding dissipation, energy is being lost. Either condition means the simulation is numerically unstable and the results are not physically meaningful.
ALLAE divided by ALLIE must stay below five percent. Above five percent, hourglass modes are active and corrupting your stress field. The fix is to switch from C3D8R elements to C3D8I in the affected regions.
Reaction forces at the floor reference point (RF1, RF2, RF3) are the total impact force time history — the primary quantity for test correlation with physical instrumentation. Acceleration at sensor locations (A1, A2, A3) must be placed at the exact locations where you would mount physical accelerometers, defined before you run either the simulation or the physical test.
Use number interval or time interval for field output, not FREQUENCY. With variable mass scaling active, the time duration of each increment is not constant. FREQUENCY produces unpredictable temporal spacing of your output frames.
Part Six — Materials. Where the Most Consequential Errors Occur.
Material definition is where the most consequential and hardest-to-detect errors in drop test modeling occur. The geometry might be perfect. The contact might be correct. The boundary conditions might be right. But if the material properties are wrong, every result in the analysis is wrong.
Density — The Unit System Trap
Without density there is no mass, without mass there is no inertia, and without inertia the explicit central difference integration cannot proceed. But the real danger is the unit system.
If you use SI density values in a tonne-millimeter-second model, the results are garbage.
Steel density in SI units is 7,850 kilograms per cubic meter. Steel density in a tonne-millimeter-second model is 7.85 × 10⁻⁹ tonnes per cubic millimeter. If you put 7,850 in the tonne-millimeter-second model, you have declared that steel is 10¹² times denser than it actually is. The model mass will be astronomically wrong. All timesteps will be wrong. The energy output will show numbers so large they do not fit on the screen. That version is easy to catch. The harder version is a mixed model where some materials were entered in SI units and some in tonne-millimeter-second. The unit system detector in the Analyzer cross-references every material's elastic modulus and density against a library of known engineering materials in both unit systems.
Rate-Dependent Plasticity — The Most Commonly Missing Property
Metals and engineering polymers are strain-rate sensitive. Under quasi-static loading at the standard tensile test speed, a material has one yield strength. Under impact loading during a drop test, where local strain rates reach 10 to 1000 per second, the same material may be 20 to 50 percent stronger. This matters: peak contact forces are higher than quasi-static analysis predicts, and energy absorption through plastic deformation is modified, which changes the rebound velocity and total deformation.
If you model a drop test using only the quasi-static stress-strain curve, you are using data measured at approximately 0.001 per second strain rate to predict behavior at 500 per second. For materials with significant rate sensitivity — steels, many polymers, magnesium alloys — this can produce 30 to 50 percent error in peak force. The Abaqus INP Analyzer flags every plastic material in an explicit dynamic model that lacks a rate-dependent definition.
Display Glass — Brittle Material Modeling
Display glass is linear elastic until it fractures. No yield surface, no plastic deformation, no energy absorption through yielding. The glass stores strain energy elastically until the stress reaches the fracture threshold, and then it fails suddenly. For survival assessment, model the glass as linear elastic with E approximately 70 GPa and Poisson's ratio approximately 0.22. Extract the maximum principal stress and compare to the glass fracture strength.
Here is what most analysts do not appreciate: display glass fracture strength has enormous variability. Pristine chemically strengthened glass panels may withstand maximum principal stresses of 700 to 800 megapascals before fracture. The same panel with a five-micron surface scratch from a single key contact in a pocket may fracture at 150 to 200 megapascals. The flaw population on the glass surface controls the failure stress. This is why glass fracture strength follows Weibull statistics, not a single deterministic value.
VHB Adhesive — Viscoelastic Behavior at Impact Rates
Pressure-sensitive adhesives like 3M VHB behave viscoelastically — their stiffness depends on the rate of loading. Under static loading, VHB is relatively compliant. Under impact loading, VHB is significantly stiffer — it has not had time to flow, and it transmits load more like an elastic solid. If you model VHB with static adhesive properties in a drop test, you will overpredict deflection of bonded components and underpredict the load transmitted through the bond. Proper VHB modeling for drop tests requires the Prony series viscoelastic representation with coefficients measured and fitted to data at impact-relevant rates.
Part Seven — Using the Abaqus INP Analyzer for Drop Analysis Review.
Let me walk you through the specific workflow for using the Abaqus INP Analyzer as your pre-submission review tool. The goal is to work through this systematically so that by the time you submit the job, you have verified every critical aspect of the setup.
Impact / Drop Analysis Tool
Open Tools → Impact/Drop Analysis. The tool automatically searches for the rigid floor surface using three detection methods ordered by confidence: parts with *Rigid Body definitions and R3D element types at 100% confidence; parts with FLOOR, RIGID, GROUND, or TARGET in the name at 60–80% confidence; parts with PLATE, BASE, or SURFACE in the name at 40–55% confidence.
What to check: the implied drop height against your specification, the velocity direction and degree of freedom, the velocity-to-floor alignment angle (0° for a flat drop, flagged above 5°), and the nearest node to the floor confirming it is on the expected impact face.
Then click View 3D. Look at the model. Is the product above the floor? Is the correct face pointing down? Are all parts present and in the correct positions? Is the highlighted hit surface on the right face? A product rotated 90 degrees from the intended orientation is immediately visible in this view. It is invisible in the text file.
BC and Load Viewer
Open Tools → BC & Load Viewer. For drop test models, confirm three things. One: the gravity arrow direction must point toward the floor. If the arrow points sideways or up, the gravity setup is wrong. Two: initial velocity markers should appear on every product part — no exceptions. Any part without a marker starts the simulation at rest. This error is common on models where new parts were added late and the node set was never updated. Three: the ENCASTRE constraint marker should be at the floor reference point only.
Recommendations Tab
Work through every flag. For drop test models: missing energy output must always be resolved; rate-dependent plasticity missing for structural metals should be resolved before design decision simulations; C3D8R element usage in bending-dominated regions gets flagged with a proposed conversion to C3D8I; single-point plasticity is flagged for verification. Document your disposition for every flag — either resolved, intentional and accepted, or requires follow-up.
Export Comprehensive Evaluation
Tools → Export Comprehensive Model Evaluation. Save the report. Attach it to your simulation record. This is the formal documentation artifact for the simulation setup: complete parts inventory with mass per part, all material properties, all quality flags with their engineering justification, the unit system analysis, and a timestamp. It is the document that someone can read five years from now and understand exactly what was simulated.
Part Eight — Post-Processing. Reading and Using Your Results.
The simulation has run. You have an ODB file. Let me walk you through a systematic post-processing workflow. And I want to start with the most important check — the one that should happen before you look at a single stress contour.
Check the Energy Balance First. Always.
Before you look at any stress contour or deformation result, open the energy history plots and confirm ETOTAL is flat. This is not optional. This is the gating check.
Here is what you should see in a physically correct drop test simulation. At time zero, ALLKE equals ½mv² — the kinetic energy of the product at impact velocity. As contact develops, ALLKE drops while ALLIE rises — kinetic energy is being converted to strain energy. During plastic deformation, ALLIE grows faster — the additional energy is absorbed by plastic work. During rebound, ALLKE rises again. ALLAE should remain a small fraction of ALLIE throughout. And ETOTAL should be flat. A flat line. The entire time. If it is not flat, do not proceed. Fix the numerical instability first.
If ALLAE divided by ALLIE exceeds five percent: hourglass energy contamination. Results are not physical. Switch to C3D8I elements.
Impact Force Time History
Plot the reaction force in the drop direction at the floor reference point. This is your total impact force time history. The peak force gives you the maximum inertial loading on the product. The contact duration characterizes the nature of the impact: short duration and high peak means hard surface and stiff housing; long duration and lower peak means a compliant surface or foam gasket in the load path.
High-frequency oscillations superimposed on the force history are often numerical. Apply a CFC 1000 filter — Channel Frequency Class 1000 per SAE J211 — before comparing to physical test measurements. Physical instrumentation has bandwidth limits, and the raw simulation signal will always show higher frequency content than any physical sensor.
Stress Results
Von Mises stress is the starting point for identifying potentially yielded regions in ductile metals. But von Mises is not the right metric for every material or failure mode. For ductile metals: use von Mises, failure when von Mises exceeds yield strength, check PEEQ to confirm yielding occurred. For brittle materials — glass, ceramics, brittle polymers: use maximum principal stress, failure when it exceeds fracture strength — von Mises is a yielding criterion, it does not correctly assess crack initiation in brittle materials. For adhesive bonds under peeling: use normal traction or maximum principal stress. For solder joints: use maximum principal strain, failure is ductility-limited.
The Two-Step Analysis Workflow
The most physically accurate drop test workflow uses two sequential Abaqus steps. Step one: gravity preload — an implicit quasi-static step that applies gravity to establish the initial contact state, component seating, and preload in compliant mounting features. Step two: impact — import the stress and deformation state from step one using *Import, State=YES, then apply the initial velocity and run the impact event. Do not use mass scaling in step two when importing from step one.
Many production drop test models skip the preload step and apply initial velocity directly to an unstressed, unseated model. For products with foam gaskets, rubber seals, compliant mounts, or snap-fit assemblies, the preloaded state is significantly different from the unstressed state. The impact dynamics will differ accordingly.
Part Nine — Duration, Jerk and Fragility. The Five-Parameter Framework.
Peak acceleration is the most commonly reported drop test simulation metric. It is also one of the most misleading metrics, taken in isolation.
Let me give you a concrete example. Two drop scenarios, both measured at the product housing. Pulse A: 1.5-meter face drop on a steel table, 1,800G peak, 0.5-millisecond contact duration, dominant frequency content around 2 kHz. Pulse B: 0.75-meter corner drop on ceramic tile, 1,200G peak, 1.8-millisecond contact duration, dominant frequency content around 550 Hz.
Ask a room full of engineers which is more severe. Most say Pulse A. Higher G, more force, more dangerous. In my experience, Pulse B is far more damaging to the internal electronics. And the answer comes down to the fifth parameter that most analysts never formally track — duration, and its relationship to the natural frequencies of your structure.
The dominant frequency of Pulse B at 550 Hz is squarely in the range of printed circuit board first bending modes. A typical 150-millimeter PCB has a first resonance between 300 and 800 Hz depending on thickness and mount configuration. Pulse B dumps all of its energy at exactly the frequency the board wants to resonate at. The board flexes. Solder joints at component corners strain. Ceramic capacitors crack. Components fail — at 33 percent lower peak G than Pulse A.
This is not theoretical. This is the standard observation in any product that has been through physical drop test correlation. Products sometimes survive brutal face drops and fail on seemingly gentle corner drops. The failure is frequency-content driven, not peak-G driven.
"A simulation that reports only peak G is not doing drop analysis. It is doing theater."
The Five-Parameter Fragility Framework
One: Peak acceleration — instantaneous inertial force. Governs heavy component retention: batteries, shields, heavyweight structural attachments.
Two: Velocity change (ΔV) — total momentum exchange. Kinetic energy equals ½mv². Delta-V integrates peak and duration into a single energy metric more fundamental than peak G alone.
Three: Duration — contact duration relative to the natural period of your structural components. This is the parameter that determines which response regime you are in, and the regime determines how severe the dynamic amplification will be. Duration and the SRS are two windows onto the same physics.
Four: Shock Response Spectrum (SRS) — the frequency-domain expression of what duration does to every possible natural frequency. Overlay the SRS with your product's modal frequencies and you immediately see which modes are most severely driven.
Five: Jerk — rate of change of acceleration. Governs stress wave sharpness and rise time. Critical for brittle fracture: glass crack initiation, ceramic capacitor fracture, solder mask delamination. High jerk means a sharp stress wave front — the material sees an instantaneous load with no time for stress redistribution across the cross-section.
Duration and the Three Response Regimes
The ratio that matters is the contact duration divided by the natural period of the structural component you are trying to protect. Natural period is one divided by natural frequency. A PCB with a first bending mode at 500 Hz has a natural period of 2 milliseconds. Compare that to your contact duration and you have the answer.
Impulsive regime (duration much less than the natural period — roughly below 0.3× the period): the structure does not have time to respond during the pulse. The impulse — mass times ΔV — governs. Peak G matters less than the area under the force-time curve. High peak G with very short duration can be less damaging than lower G with longer duration.
Resonant regime (duration in the range of 0.3 to 0.7 times the natural period): maximum dynamic amplification. The danger zone. The SRS peaks here. This is exactly what happened in the corner drop example — 1.8 milliseconds of contact duration corresponds to roughly 550 Hz frequency content, which sat directly on the PCB bending mode.
Quasi-static regime (duration much greater than the natural period — roughly above 3× the period): the structure follows the load almost statically. Dynamic magnification approaches 1. Peak G and static allowables become the relevant metrics. Long, slow impacts on stiff structures fall here.
Calling out duration explicitly gives you an immediate intuitive check before you ever plot an SRS: if your contact duration is in the same ballpark as any component's natural period, you are in the resonant zone, and you need to take that seriously before the simulation even runs.
Extracting Jerk from Your Simulation
Jerk is the time derivative of acceleration. You already have acceleration time histories at every history output node. The procedure is straightforward, but there is one absolute rule:
Always filter before differentiating.
Numerical differentiation amplifies noise in proportion to frequency. A clean acceleration signal at 50 kHz sampling rate can produce completely garbage jerk values if differentiated without filtering first. Apply a Butterworth low-pass filter — typically a 5 to 10 kHz cutoff for drop test applications. The CFC 1000 filter per SAE J211 is appropriate for most drop test post-processing. Apply it to both the simulation output and the physical test data before any comparison.
Part Ten — Common Mistakes and How to Fix Them.
Let me go through the mistakes I see most often in production drop test models, in order from most catastrophic to most subtle.
Wrong density units — SI kg/m³ in a tonne-mm-s model. Results are garbage. Every result. Fix: use tonne/mm³ values. Steel is 7.85 × 10⁻⁹.
Initial velocity applied to wrong node set. Parts outside the set start at rest and are dragged into motion by contact. Wrong dynamics throughout. Fix: verify coverage with the BC and Load Viewer — every product node must have a marker.
Gravity direction does not match velocity direction. Product experiences gravity in a different direction from its motion. Fix: gravity vector and velocity DOF must be parallel and point toward the floor.
ENCASTRE on a product node instead of the floor reference point. Product is fixed to ground. Floor falls freely. Fix: ENCASTRE belongs on the floor rigid body reference point.
Mass scaling fraction above five percent. Artificial inertia changes the dynamics. Peak forces and timing are wrong. Fix: reduce target dt in *Variable Mass Scaling, or eliminate the small elements driving the scaling.
Missing energy output. No quality metric for the simulation. No way to check ALLAE/ALLIE ratio. Fix: always include *Energy Output, variable=ALL.
ALLAE/ALLIE above five percent. Hourglass energy contaminating results. Stress and deformation are not physical. Fix: switch C3D8R to C3D8I, or add *Hourglass Control with ENHANCED formulation.
Simulation duration too short. Contact force has not returned to zero. Peak deformation not captured. Secondary impact missed. Fix: extend simulation time until contact force returns to zero and kinetic energy stabilizes.
No rate dependence for metals. Impact strain rates produce 20–50% higher yield strength than static data. Omitting rate dependence underpredicts peak force and overpredicts deformation. Fix: add *Plastic tables at multiple strain rates.
Static VHB adhesive properties in an impact model. VHB is viscoelastic. At impact rates it is far stiffer than under static load. Fix: use Prony series viscoelastic definition from validated material data.
nlgeom=NO on the explicit step. Large deformations during impact are not correctly handled. Fix: always include nlgeom=YES on explicit dynamic steps.
Using FREQUENCY for field output. With variable mass scaling, increment duration is not constant and temporal output spacing is unpredictable. Fix: use number interval or time interval.
Forgetting to run all drop orientations. Qualification standards require drops on all faces, edges, and corners. A single-orientation analysis that passes does not mean qualification passes. Fix: run the full drop matrix or perform orientation sensitivity analysis first.
Part Eleven — The Pre-Submission Checklist and Closing Thoughts.
Work through these in order for every drop test model before the job is submitted.
1. Parse and baseline check. Load and parse the INP file. Confirm analysis type shows as DYNAMIC EXPLICIT. Confirm part count matches BOM. Check total assembly mass versus physical product weight. Any discrepancy greater than ten percent demands explanation before proceeding.
2. Run the Impact/Drop Analysis Tool. Confirm rigid floor detected with high confidence. Verify implied drop height matches specification. Confirm velocity direction and degree of freedom. Check velocity-to-floor alignment angle. Confirm nearest node is on the expected impact face. Review the 3D visualization.
3. BC and Load Viewer. Gravity arrow toward floor. Velocity markers on every product part. ENCASTRE on floor reference point only.
4. Materials review. Correct material names. All materials have density. Density values in correct units. Rate dependence flags resolved.
5. Recommendations tab. Every flag resolved or documented.
6. Output requests verified. Energy output present. Reaction forces at floor reference point. Acceleration at sensor locations. Stress at critical locations. Field output frame count appropriate.
7. Export Comprehensive Evaluation. Attach to simulation record. Document dispositions for all open items.
Drop test simulation is one of the most valuable things a CAE organization can do for a product program. It is also one of the easiest to do superficially and one of the hardest to do rigorously.
The gap between a simulation that runs without errors and a simulation that correctly predicts physical product behavior is filled with the details covered in this guide. Material rate dependence. Mass scaling validation. Energy balance monitoring. The five-parameter fragility framework — peak acceleration, velocity change, duration, SRS, and jerk. Proper boundary condition verification. Test correlation methodology.
The Abaqus INP Comprehensive Analyzer was built to make the rigorous approach practical. The pre-submission review workflow takes perhaps thirty minutes on a model that has been set up carefully. It has, in my experience, prevented more program-level problems than any other single practice I have implemented in my career.
Use the tool. Use this guide. Do the work to understand what the simulation is telling you. And when someone in a design review asks you why you believe the product survives the drop, you will have an answer that starts with: "Let me show you the energy balance."
Joseph P. McFadden Sr.
The Holistic Analyst | 45 Years in CAE/FEA Engineering
McFaddenCAE.com | McFadden@snet.net
Abaqus INP Comprehensive Analyzer V20.2 — Free at McFaddenCAE.com
