Fracture behavior

Fracture behavior determines how components and materials fail under mechanical loading. For industrial case systems, transport containers and foam inlays it is a key factor for safety, service life and functional reliability. Whether aluminium cases, plastic cases, X-PCK backpack cases or the mobile work table in a case – the interplay of material, geometry, fastening technology and operating conditions decides whether a crack initiates, how it propagates and whether failure is brittle or ductile. KKC Koffer GmbH, based in Stemwede-Levern, relies on customizable solutions in specialized manufactory production, where fracture behavior is considered already during design to meet the requirements of professional applications.

Definition: What is meant by fracture behavior?

Fracture behavior describes the mechanical response of a material or component from the first loading through crack initiation to crack propagation and failure. This includes properties such as toughness, impact toughness, ductility, notch sensitivity and fatigue strength. Fracture-mechanical parameters (e.g., energy release rate during crack growth or notched impact energy) describe how cracks initiate and propagate. The behavior is strongly influenced by temperature, humidity, aging, type of loading (tension, compression, bending, shear, torsion), loading rate, geometry, notches, surface condition and the type of joint. For cases and transport containers, impact and drop loads, vibrations, stacking pressures and point loads are additionally relevant.

Relevance of fracture behavior for industrial case systems

Different load spectra act in demo cases, equipment cases, sample and presentation cases, transport cases as well as industrial and special cases. Measuring equipment, medical technology or electrical engineering require reliable protection against shock and vibration. Professional cases for tradespeople and professional machine cases must withstand repeated drop events, edge impacts and lever forces at handles and straps. Fracture behavior is decisive for whether the outer shell tears, a hinge breaks out, a latch shears off, or whether energy is distributed and damped within the system. Through adaptable housings, targeted reinforcements, suitable foam inlays and coordinated branding, the risk of fracture-critical states can be significantly reduced.

Materials and their fracture-mechanics characteristics

Aluminium cases

Aluminium alloys offer a favorable combination of strength, toughness and low weight. Profile frames and corner connectors distribute edge impacts. Dents are often purely plastic and not necessarily relevant for fracture. Critical are sharp notches at corners, holes or cut-outs as well as corrosion in combination with vibrational loading. Surface finishes such as anodizing or powder coating protect and influence crack initiation behavior and friction at contact surfaces. Riveted, bolted or bonded joints should be designed with sufficient edge distances and generous radii to minimize notch stresses.

Plastic cases

Thermoplastics such as PP, ABS or PC/blends allow ductile behavior at moderate temperatures and can dissipate shocks through material flow. At low temperatures, the tendency to brittle fracture increases; at high loading rates likewise. Notch sensitivity is a central topic: sharp transitions, thin webs and hard add-on components cause stress concentrations. Ribbing, load-introduction zones with large radii and uniform wall thickness transitions improve the behavior. Chemicals and cleaning agents can lead to environmental stress cracking – this is particularly important to consider in cases for medical technology and electrical engineering.

X-PCK backpack case

Carrying systems add cyclic, reversing loads. Strap mounts and fastening points are fatigue-critical. Load paths must be guided into the case structure to prevent tear-outs. Damping interlayers between hardware and housing reduce stress concentrations. Abrasion at the contact with the back or with tools can create microscopic notches, which is why edge radii and robust surfaces are advisable.

Transport containers

In large-volume transport containers, stacking pressures, deflection of large panels and lever effects during handling dominate. Sandwich constructions, honeycomb structures or ribbed panels increase bending stiffness and shift critical stress zones. Forklift pockets and lashing points are highly stressed introduction areas that must be structurally secured against peel and shear forces.

Influence of foam inlays on fracture behavior

Foam inlays are central elements for shock absorption and for avoiding fracture-critical peak loads. PE or PU foams, optionally ESD-executed, distribute contact forces, limit relative movements and prevent local notch effects caused by sharp-edged components. The selection of density and dynamic compressive stiffness determines whether energy is predominantly absorbed or transmitted. CNC-milled cutouts, waterjet cuts or grid concepts allow contour-accurate supports that decouple edges and reduce crack initiation at the inner case wall. Suitable foam inlays for shock damping in the case further limit stress peaks. Inserts that are too hard can cause rebound and secondary impacts; inserts that are too soft risk the contents bottoming out.

Fracture behavior in the product: Mobile Work Table in a Case

In the Mobile Work Table in a Case, functional loads act in addition to transport loads: slides, folding mechanisms, latches and the tabletop must absorb torsion, point loads and vibrations. Test laboratories, IT service providers, service technicians, mobile tradespeople, measurement technology manufacturers as well as users from HVAC, facilitation, consulting, trade fair equipment, electrical systems engineering, electrical installation and mechanical engineering require repeatable stability. Fatigue at hinge pins, wear at bearing points and peel forces at fastening surfaces determine service life. Generous radii, material-appropriate bearings, defined end stops and stiffened load paths from the tabletop into the case structure reduce crack growth. Non-slip yet notch-insensitive surfaces on the work surface reduce local pressure peaks.

Fatigue, drop and vibration: load spectra in practice

In cases for measuring instruments and electrical engineering, vibrations and high-frequency excitations dominate; in professional cases for tradespeople, shocks, drop events and edge impacts occur. Medical technology additionally requires chemical resistance and cleaning cycles. Repeated load cycles lead to micro-crack formation, which can propagate at notches, screw bosses, frame corners, hinges and latches. Practice-oriented drop tests from representative heights, impact tests on edges and vibration tests with realistic spectra provide robust statements on fracture behavior.

Design measures for crack prevention

• Large transition radii and uniform wall thicknesses reduce notch stresses.
• Design ribs so that load paths are guided smoothly and stress peaks are avoided.
• Equip screw joints with washers or inserts (e.g., threaded bushings) to distribute bearing pressure.
• Configure hinge and latch mounts with backing to limit peel loading.
• Orient fiber directions or profile directions (for aluminium profiles) in the load direction.
• Use surface finishing and edge deburring to minimize micro-notches.

Joints and assembly

Riveted, screwed and bonded joints significantly influence fracture behavior. Mechanical joints are robust against temperature but create notch effects at the hole edge. Adhesive bonds distribute stresses over an area but require suitable surface preparation. Mixed-material designs (metal hardware on plastic housings) benefit from elastic interlayers that alleviate stiffness jumps.

Branding without impairing the structure

Branding elements such as prints, embossing, engravings or applied logos must be positioned so that they do not create critical notches. Solvent-based processes should be chosen conservatively for stress-sensitive plastics. Mechanically attached emblems should be fastened with large-area, rounded contours so that no crack starters arise.

Environmental conditions and aging

Temperature cycles, UV radiation, humidity and chemicals change fracture behavior. Plastics can become brittle or susceptible to environmental stress cracking; aluminium can corrode under unfavorable conditions. Protective coatings, UV-stable materials and chemically compatible cleaning processes extend service life. For cases in medical technology and outdoor use, material selection with a view to aging mechanisms is particularly important.

Testing and validation

Practice-oriented tests combine drop, impact, vibration, temperature and humidity. Drop tests on face, edge and corner reveal brittle- and ductile-dominated failure modes. Vibration tests identify loose joints and crack-critical natural modes. Leak-tightness tests ensure that crack formation does not lead to loss of function. Visual inspections for stress whitening, hairline cracks and breakouts at fasteners complement the mechanical tests.

Material and design adaptation in the specialized manufactory

KKC Koffer GmbH adapts material thicknesses, profile geometries, ribbing and load-introduction areas to the required load spectrum. Foam inlays are matched in density, Shore hardness and contouring to the goods to be protected. For the X-PCK backpack case, carrying points, padding and load distribution can be configured. In the Mobile Work Table in a Case, mechanisms, latches and supports are designed against local stress peaks. This results in case systems that are aligned with the respective fracture behavior.

Typical damage patterns and diagnosis

• Cracks at screw and rivet holes due to peel and shear forces.
• Stress whitening at sharp transitions in plastic cases.
• Breakouts at hinges and latches after lateral impact.
• Dents and buckles on aluminium surfaces with possible crack formation at notches.
• Compression damage to foam inlays with strike-through to the case wall.

Practical recommendations for remediation

1. Increase radii, widen webs, execute rib roots with fillets.
2. Use washers, rivet nuts or inserts to distribute loads.
3. Provide elastic interlayers under hardware.
4. Match foam hardness and geometry to the device weight.
5. Place branding in low-stress areas and choose notch-insensitive contours.

Application-oriented design for industries and use cases

Demo cases, sample and presentation cases require robust edges and impact-resistant latches so that frequent opening/closing does not generate crack starters. Equipment cases, cases for measuring instruments as well as cases for electrical engineering benefit from vibration-damping inserts and stiff yet notch-insensitive housings. Cases for medical technology require chemically compatible plastics and surfaces to avoid stress cracking. Professional cases for tradespeople and professional machine cases require reinforced handle zones and frame profiles against edge impacts. Transport cases and transport containers should be designed for stacking pressure and point lifting forces at lashing points. For special cases, early consideration of the load paths is advisable to purposefully control fracture behavior from the outset.